Key Readings Index
Bone Marrow and Blood Cells
Structure and Function,
724
Dysfunction/Responses to Injury,
730
Portals of Entry/Pathways of Spread,
744
Defense Mechanisms/Barrier Systems,
744
Disorders of Domestic Animals,
744
Disorders of Horses,
758
Disorders of Ruminants (Cattle, Sheep, and Goats),
758
Disorders of Dogs,
759
Disorders of Cats,
759
Lymphoid/Lymphatic System
Thymus
Structure and Function,
761
Dysfunction/Responses to Injury,
763
Portals of Entry/Pathways of Spread,
764
Defense Mechanisms/Barrier Systems,
764
Spleen
Structure,
764
Function,
766
Dysfunction/Responses to Injury,
771
Portals of Entry/Pathways of Spread,
772
Defense Mechanisms/Barrier Systems,
772
Lymph Nodes
Structure,
772
Function,
775
Dysfunction/Responses to Injury,
775
Portals of Entry/Pathways of Spread,
777
Defense Mechanisms/Barrier Systems,
777
Hemal Nodes
Structure and Function,
777
Mucosa-Associated Lymphoid Tissue
Structure and Function,
777
Dysfunction/Responses to Injury,
778
Portals of Entry/Pathways of Spread,
778
Defense Mechanisms/Barrier Systems,
778
Disorders of the Lymphoid/Lymphatic System of Domestic Animals
Thymus,
778
Spleen,
780
Lymph Nodes,
789
Mucosa-Associated Lymphoid Tissue,
796
Disorders of the Lymphoid/Lymphatic System by Species
Disorders of Horses,
796
Disorders of Ruminants (Cattle, Sheep, and Goats),
797
Disorders of Pigs,
798
Disorders of Dogs,
799
Disorders of Cats,
803
E-Glossary 13-1 Glossary of Abbreviations and Terms
AA amyloidosis—Serum amyloid A amyloidosis
ACT—Activated clotting time
ADP—Adenosine diphosphate
AL amyloidosis—Amyloid–light chain amyloidosis
ALL—Acute lymphoblastic leukemia
AML—Acute myeloid leukemia
AP3—Adaptor protein complex
ATALT—Auditory tube–associated lymphoid tissue
ATP—Adenosine triphosphate
BALT—Bronchus-associated lymphoid tissue
BCR-ABL—Breakpoint cluster region-Abelson
BLAD—Bovine leukocyte adhesion deficiency
BLL—Burkitt-like lymphoma
BLP—B lymphocyte progenitor
BLV—Bovine leukemia virus
BNP—Bovine neonatal pancytopenia
BVD—Bovine viral diarrhea
BVDV—Bovine viral diarrhea virus
CalDAG-GEFI—Calcium diacylglycerol guanine nucleotide exchange factor I
CALT—Conjunctiva-associated lymphoid tissue
CBC—Complete blood count
C-bilirubin—Conjugated bilirubin
CD—Cluster of differentiation
CH—Cutaneous histiocytosis
CHS—Chédiak-Higashi syndrome
CLAD—Canine leukocyte adhesion deficiency
CLL—Chronic lymphocytic leukemia
CLP—Common lymphoid progenitor
CML—Chronic myeloid leukemia
CMP—Common myeloid progenitor
CPV-2—Canine parvovirus type 2
CTCL—Cutaneous T lymphocyte lymphoma
DC—Dendritic cell
DIC—Disseminated intravascular coagulation
DLBCL—Diffuse large B cell lymphoma
DNA—Deoxyribonucleic acid
DNA-PKcs—DNA-dependent protein kinase catalytic subunit
2,3-DPG—2,3-diphosphoglycerate
EATCL—Enteropathy-associated T cell lymphoma
EBL—Enzootic bovine leukosis
EBV—Epstein-Barr virus
EHV-1—Equine herpes virus 1
EHV-5—Equine herpes virus 5
EIAV—Equine infectious anemia virus
EMH—Extramedullary hematopoiesis
EMP—Extramedullary plasmacytoma
EP—Erythroid progenitor
Epo—Erythropoietin
FAD—Flavin adenine dinucleotide
FAE—Follicle-associated epithelium
FcaGHV1—Felis catus gammaherpesvirus 1
Fe3+
—Ferric iron
FeLV—Feline leukemia virus
FIV—Feline immunodeficiency virus
FL—Follicular lymphoma
FPV—Feline parvovirus
GALT—Gut-associated lymphoid tissue
GMP—Granulocyte-macrophage progenitor
GP—Glycoprotein
GP—Granulocyte progenitor
G6PD—Glucose-6-phosphate dehydrogenase
Gr.—Greek
GSH—Reduced glutathione
GT—Glanzmann thrombasthenia
H&E—Hematoxylin and eosin
HEV—High endothelial venule
Hgb—Hemoglobin
Hpt—Haptoglobin
Hpx—Hemopexin
HS—Histiocytic sarcoma
HSC—Hematopoietic stem cell
IBD—Inflammatory bowel disease
iDC—Interstitial dendritic cell
Ig—Immunoglobulin
IgA—Immunoglobulin A
IgG—Immunoglobulin G
IgM—Immunoglobulin M
IL—Interleukin
IMHA—Immune-mediated hemolytic anemia
IMTP—Immune-mediated thrombocytopenia
INF—Interferon
IRF4—Interferon regulatory factor 4
LAD—Leukocyte adhesion deficiency
LALT—Larynx-associated lymphoid tissue
LBL—Lymphoblastic lymphoma
LC—Langerhans cell
LGL—Large granular lymphocyte
LYST—Lysosomal trafficking regulator
MAC—Membrane attack complex
MALT—Mucosa-associated lymphoid tissue
MAP—Mycobacterium avium ssp. paratuberculosis
M cell—Microfold cell
MCF—Malignant catarrhal fever
MCH—Mean cell hemoglobin
MCHC—Mean cell hemoglobin concentration
MCL—Mantle cell lymphoma
MCP—Mast cell progenitor
MCT—Mast cell tumor
MCV—Mean cell volume
MDS—Myelodysplastic syndrome
MEP—Megakaryocyte-erythroid progenitor
MetHgb—Methemoglobin
MHC—Major histocompatibility complex
miRNA—MicroRNA
MKP—Megakaryocyte progenitor
MM—Multiple myeloma
MP—Macrophage progenitor
MPV—Mean platelet volume
MUM1—Melanoma-associated antigen (mutated) 1
MZL—Marginal zone lymphoma
NADH—Reduced nicotinamide adenine dinucleotide
NADPH—Reduced nicotinamide adenine dinucleotide phosphate
NALT—Nasal-associated lymphoid tissue
NCI—National Cancer Institute
NI—Neonatal isoerythrolysis
NK cell—Natural killer cell
NKP—Natural killer cell progenitor
nRBC—Nucleated red blood cell
PALS—Periarteriolar lymphoid sheath
PAMS—Periarteriolar macrophage sheath
PARR—Polymerase chain reaction for antigen receptor rearrangement
PCR—Polymerase chain reaction
PCV2—Porcine circovirus type 2
PCVAD—Porcine circovirus–associated disease
PFK—Phosphofructokinase
PHA—Pelger-Huët anomaly
PK—Pyruvate kinase
PL—Persistent lymphocytosis
PMWS—Postweaning multisystemic wasting syndrome
PPP—Pentose phosphate pathway
PRCA—Pure red cell aplasia
PRDC—Porcine respiratory disease complex
PrPSc
—Scrapie prion protein
PRRS—Porcine reproductive and respiratory syndrome
PT—Prothrombin time
PTCL—Peripheral T cell lymphoma
PTT—Partial thromboplastin time
RBC—Red blood cell
REAL—Revised European-American Classification of Lymphoid Neoplasms
rhEpo—Recombinant human erythropoietin
SCID—Severe combined immunodeficiency disease
SFHN—Splenic fibrohistiocytic nodule
SH—Systemic histiocytosis
SPF—Specific pathogen–free
TCRLBCL—T cell–rich large B cell lymphoma
TGF-β—Transforming growth factor-β
TLP—T lymphocyte progenitor
TNF—Tumor necrosis factor
TNKP—T lymphocyte–natural killer cell progenitor
Tpo—Thrombopoietin
TZL—T zone lymphoma
U-bilirubin—Unconjugated bilirubin
vWD—von Willebrand disease
vWF—von Willebrand factor
WHO—World Health Organization
Bone Marrow and Blood Cells2
Structure and Function
Hematopoiesis, from haima (Gr., blood) and poiein (Gr., to make), is the production
of blood cells, including erythrocytes, leukocytes, and platelets. Also known as hemopoiesis,
hematopoiesis first occurs in the blood islands of the yolk sac and then transitions
to the liver and spleen during gestation. After birth the primary hematopoietic site
is the central cavities of bone, termed bone marrow (Fig. 13-1
). Hematopoiesis occurring elsewhere is called extramedullary hematopoiesis (EMH),
which is most common in the spleen.
Figure 13-1
Structure of Bone Marrow.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University; and Dr. J.F. Zachary, College of Veterinary Medicine, University
of Illinois.)
The bone marrow is supported by an anastomosing network of trabecular bone that radiates
centrally from the compact bone of the cortex. Trabecular bone is covered by periosteum,
consisting of an inner osteogenic layer of endosteal cells, osteoblasts, and osteoclasts,
and an outer fibrous layer that anchors the stromal scaffolding of the marrow spaces.
Within the marrow spaces, a network of stromal cells and extracellular matrix provides
metabolic and structural support to hematopoietic cells. These stromal cells consist
of adipocytes and specialized fibroblasts, called reticular cells. The latter provides
structural support by producing a fine network of a type of collagen, called reticulin,
and by extending long cytoplasmic processes around other cells and structures. Both
reticulin and cytoplasmic processes are not normally visible with light microscopy
but are visible with silver reticulin stains (e.g., Gordon and Sweet's and sometimes
with periodic acid–Schiff).
Bone marrow is highly vascularized but does not have lymphatic drainage. Marrow of
long bones receives part of its blood supply from the nutrient artery, which enters
the bone via the nutrient canal at midshaft. The remaining arterial supply enters
the marrow through an anastomosing array of vessels that arise from the periosteal
arteries and penetrate the cortical bone. Vessels from the nutrient and periosteal
arteries converge and form an interweaving network of venous sinusoids that permeates
the marrow. These sinusoids not only deliver nutrients and remove cellular waste but
also act as the entry point for hematopoietic cells into blood circulation. Sinusoidal
endothelial cells function as a barrier and regulate traffic of chemicals and particles
between the intravascular and extravascular spaces. Venous drainage parallels that
of the nutrient artery and its extensions.
Hematopoiesis occurs in the interstitium between the venous sinusoids in the so-called
hematopoietic spaces. There is a complex functional interplay among hematopoietic
cells with the supporting connective tissue cells, extracellular matrix, and soluble
factors, which form the hematopoietic microenvironment. Behavior of hematopoietic
cells is influenced by direct cell-to-cell and cell-matrix interactions and by soluble
mediators, such as cytokines and hormones that interact with cells and with matrix
proteins. Cells localize to specific niches within the hematopoietic microenvironment
via adhesion molecules, such as integrins, immunoglobulins, lectins, and other receptors,
which recognize ligands on other cells or matrix components. Cells also express receptors
for soluble molecules such as chemokines (chemoattractant cytokines) and hormones
that influence cell trafficking and metabolism.
Other components of the marrow include myelinated and nonmyelinated nerves, as well
as low numbers of resident macrophages, lymphocytes, and plasma cells. Of note, the
macrophages play an important role in iron storage and erythrocyte maturation.
The following basic concepts provide a framework for understanding the mechanisms
of injury and diseases presented later in the chapter.
•
Hematopoietic tissue is highly proliferative. Billions of cells per kilogram of body
weight are produced each day.
•
Pluripotent hematopoietic stem cells are a self-renewing population, giving rise to
cells with committed differentiation programs, and are common ancestors of all blood
cells. The process of hematopoietic differentiation is shown in Fig. 13-2
.
Figure 13-2
Classic and Spatial Model of Hematopoietic Cell Differentiation, Canine Blood Smears,
and Bone Marrow Aspirate.
The bone marrow consists of (1) hematopoietic stem cells, pluripotent cells capable
of self-renewal; (2) progenitor cells that evolve into more differentiated cells with
each cell division; (3) precursor cells that can be identified by light microscopy
(not shown, see Fig. 13-3); and (4) mature hematopoietic cells awaiting release into
the blood vasculature. The earliest lineage commitment is to either the common myeloid
progenitor (CMP), which produces platelets, erythrocytes, and nonlymphoid leukocytes,
or the common lymphoid progenitor (CLP), which differentiates into various lymphocytes
and plasma cells. The cell origin of mast cells is unclear, but they may originate
from a stem cell or a myeloid progenitor. Megakaryocytes remain in the bone marrow
and release cytoplasmic fragments, or platelets, into blood sinusoids. T lymphocyte
progenitor (TLP) cells travel from the bone marrow to the thymus during normal T lymphocyte
maturation. During homeostasis, platelets and erythrocytes remain in circulation,
but the leukocytes leave blood vessels to enter the tissues, where they actively participate
in immune responses. In particular, monocytes and B lymphocytes undergo morphologic
and immunologic changes to form macrophages and plasma cells, respectively. Macrophages,
granulocytes, and mast cells migrate unidirectionally into tissues, but lymphoid cells
can recirculate between the blood, tissues, and lymphatic vessels. BLP, B lymphocyte
progenitor; EP, erythroid progenitor; GMP, granulocyte-macrophage progenitor; GP,
granulocyte progenitor; MCP, mast cell progenitor; MKP, megakaryocyte progenitor;
MEP, megakaryocyte-erythroid progenitor; MP, macrophage progenitor; NK cell, natural
killer cell; NKP, natural killer cell progenitor; TLP, T lymphocyte progenitor; TNKP,
T lymphocyte–natural killer cell progenitor.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University; and Dr. J.F. Zachary, College of Veterinary Medicine, University
of Illinois.)
•
Hematopoietic cells undergo sequential divisions as they develop, so there are progressively
higher numbers of cells as they mature. Cells also continue to mature after they have
stopped dividing. Conceptually, it is helpful to consider cells in the bone marrow
as belonging to mitotic and postmitotic compartments. Examples of developing hematopoietic
cells are shown in Fig. 13-3
.
Figure 13-3
Hematopoietic Cell Morphology, Feline (Erythroid and Granulocyte Lineages) and Canine
(Monocyte Lineage) Blood Smears and Bone Marrow Aspirates.
As erythroid cells mature from a rubriblast to a mature erythrocyte, their nuclei
become smaller and more condensed. The nucleus is eventually extruded to form a polychromatophil.
Erythroid cells also become less basophilic and more eosinophilic as more hemoglobin
is produced and as RNA-rich organelles are lost during maturation. (Hemoglobin stains
eosinophilic, and RNA stains basophilic with routine Romanowsky's stains.) As granulocytes
(e.g., neutrophils, eosinophils, and basophils) mature from a myeloblast to their
mature forms, their nuclei become dense and segmented. Granulocytes acquire their
secondary or specific granules during the myelocyte stage and can be morphologically
differentiated starting at this stage. Neutrophils have neutral-staining secondary
granules, eosinophil secondary granules have an affinity for acidic or eosin dyes,
and basophil secondary granules have an affinity for basic dyes. Monoblasts differentiate
into promonocytes with ruffled nuclear boarders and then into monocytes. (Courtesy
Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute and
State University; and Dr. J.F. Zachary, College of Veterinary Medicine, University
of Illinois.)
•
Mature cells released into the blood circulation have different normal life spans,
varying from hours (neutrophils), to days (platelets), to months (erythrocytes), and
to years (some lymphocytes).
•
The hematopoietic system is under exquisite local and systemic control and responds
rapidly and predictably to various stimuli.
•
Production and turnover of blood cells are balanced so that numbers are maintained
within normal ranges (steady-state kinetics) in healthy individuals.
•
Normally the bone marrow releases mostly mature cell types (and very low numbers of
cells that are almost fully mature) into the circulation. In response to certain physiologic
or pathologic stimuli, however, the bone marrow releases immature cells that are further
back in the supply “pipeline.”
The composition of the marrow changes with age. The general pattern is that hematopoietic
tissue (red marrow) regresses and is replaced with nonhematopoietic tissue, mainly
fat (yellow marrow). Thus in newborns and very young animals the bone marrow consists
largely of hematopoietically active tissue, with relatively little fat, whereas in
geriatric individuals the marrow consists largely of fat. In adults, hematopoiesis
occurs primarily in the pelvis, sternum, ribs, vertebrae, and the proximal ends of
humeri and femora. Even within these areas of active hematopoiesis, fat may constitute
a significant proportion of the marrow volume.
Hematopoiesis
Immature hematopoietic cells can be divided into three stages: stem cells, progenitor
cells, and precursor cells. Hematopoietic stem cells (HSCs) have the capacity to self-renew,
differentiate into mature cells, and repopulate the bone marrow after it is obliterated.
Progenitor cells and precursor cells cannot self-renew; with each cell division, they
evolve into more differentiated cells. Later-stage precursors cannot divide. Stem
cells and progenitor cells require immunochemical stains for identification, but precursor
cells can be identified by their characteristic morphologic features (see Fig. 13-3).
Control of hematopoiesis is complex, with many redundancies, feedback mechanisms,
and pathways that overlap with other physiologic and pathologic processes. Many cytokines
influence cells of different lineages and stages of differentiation. Primary growth
factors for primitive cells are interleukin (IL) 3, produced by T lymphocytes, and
stem cell factor, produced by monocytes, macrophages, fibroblasts, endothelial cells,
and lymphocytes. Interleukin 7 is an early lymphoid growth factor. Lineage-specific
growth factors are discussed in their corresponding sections.
Erythropoiesis.
Erythropoiesis—from erythros (Gr., red)—refers to the production of red blood cells,
or erythrocytes, whose primary function is gas exchange; oxygen is delivered from
the lungs to the tissues, and carbon dioxide is transported from the tissues to the
lungs. During maturation, erythroid precursors synthesize a large quantity of a metalloprotein,
called hemoglobin, to facilitate gas transportation. Erythrocytes have secondary functions,
such as blood acid-base buffering.
The dominant regulator of erythropoiesis is a glycoprotein aptly named erythropoietin
(Epo). Other direct or indirect stimulators of erythropoiesis include interleukins
(e.g., IL-3, IL-4, and IL-9), colony-stimulating factors (e.g., granulocyte-macrophage
colony-stimulating factor and granulocyte colony-stimulating factor), and hormones
(e.g., growth hormone, insulin-like growth factor, testosterone, and thyroid hormone).
Epo is synthesized primarily in the kidney and exerts its effects by promoting proliferation
and inhibiting apoptosis of developing erythroid cells. The stimulus for increased
Epo production is hypoxia.
Within the bone marrow, erythroid precursors surround a central macrophage in specialized
niches, termed erythroblastic islands (Fig. 13-4
). The central macrophage, also known as a nurse cell, anchors the precursors within
the island niche, regulates erythroid proliferation and differentiation, transfers
iron to the erythroid progenitors for hemoglobin synthesis, and phagocytizes extruded
metarubricyte nuclei. Although erythroblastic islands occur throughout the marrow,
those with more differentiated erythroid cells neighbor sinusoids, whereas nonadjacent
islands contain mostly undifferentiated precursors.
Figure 13-4
Erythroblastic Island, Canine Splenic Aspirate.
Erythroid precursors surround and adhere to a central macrophage, or nurse cell (arrow),
which regulates the erythroid cell's maturation and iron acquisition.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University.)
Iron is essential to hemoglobin synthesis and function. It is acquired through the
diet and is transported to the bone marrow via the iron transport protein, transferrin.
Central macrophages either store iron as ferritin or hemosiderin, or transfer the
iron to erythroid precursors for hemoglobin synthesis. Hemosiderin is identifiable
in routinely stained marrow preparations as an intracellular brown pigment. However,
Perls's Prussian blue stain is more sensitive and specific for iron detection.
The earliest erythroid precursor identifiable by routine light microscopy is the rubriblast,
which undergoes maturational division to produce 8 to 32 progeny cells. Late-stage
erythroid precursors, known as metarubricytes, extrude their nuclei and become reticulocytes,
and subsequently mature erythrocytes. The normal transit time from rubriblast to mature
erythrocyte is approximately 1 week.
Reticulocytes start maturing in the bone marrow but finish their maturation in the
blood circulation and spleen. Horses are an exception in that they do not release
reticulocytes into circulation, even in situations of increased demand. Unlike mature
erythrocytes, which lack organelles, reticulocytes still contain ribosomes and mitochondria,
mainly to support completion of hemoglobin synthesis. These remaining organelles impart
a bluish-purple cast (polychromasia) to reticulocytes on routine blood smear examination.
The resultant cells are termed polychromatophils. Because older reticulocytes do not
exhibit polychromasia, more sensitive laboratory techniques must be used for accurate
reticulocyte quantification. When a blood sample is incubated with new methylene blue
stain, the reticulocytes' ribosomal RNA precipitates to form irregular, dark aggregates
(Fig. 13-5
). Cats also have a more mature form of reticulocyte, termed punctate reticulocyte,
which is stippled when stained with new methylene blue. Punctate reticulocytes indicate
prior, not active, regeneration and do not appear polychromatophilic on routine blood
smear evaluation.
Figure 13-5
Reticulocytosis, Canine Blood Smears.
A, Reticulocytes (arrows) appear polychromatophilic with routine staining. Wright's
stain. B, Reticulocytes. Precipitated aggregates of RNA are stained blue (arrows)
with new methylene blue.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
In most mammals, mature erythrocytes have a biconcave disk shape, called a discocyte.
Microscopically, these cells are round and eosinophilic with a central area of pallor.
However, the central concavity may not be microscopically apparent in species other
than the dog. Camelids normally have oval erythrocytes, termed ovalocytes or elliptocytes,
which facilitate better gas exchange at high altitudes. The erythrocytes of some animals
are prone to in vitro shape change, including those of cervids, pigs, and some goat
breeds (e.g., Angora).
Erythrocyte size during health depends on the species, breed, and age of the animal.
In dogs, some breeds have relatively smaller (e.g., Akitas and Shibas) or larger (e.g.,
some poodles) erythrocytes. Akitas and Shibas also have a high concentration of potassium,
unlike erythrocytes in other dogs. Juvenile animals may have larger erythrocytes because
of the persistence of fetal erythrocytes, which is followed by a period of relatively
smaller cells before reaching adult reference intervals.
Mature mammalian erythrocytes lack nuclei and organelles and are thus incapable of
transcription, translation, and oxidative metabolism. However, they do require energy
for various functions, including maintenance of shape and deformability, active transport,
and prevention of oxidative damage. Red blood cells generate this energy entirely
through glycolysis (also known as the Embden-Meyerhof pathway). Except in pigs, glucose
enters erythrocytes from the plasma through an insulin-independent, integral membrane
glucose transporter.
Within circulation the erythrocyte mean life span varies between species and is related
to body weight and metabolic rate: approximately 150 days in horses and cattle, 100
days in dogs, and 70 days in cats. When erythrocytes reach the end of their life span,
they are destroyed in a process termed hemolysis. Hemolysis may occur within blood
vessels (intravascular hemolysis) or by sinusoidal macrophages (extravascular hemolysis).
During intravascular hemolysis, erythrocytes release their contents, mostly hemoglobin,
directly into blood. However, during extravascular hemolysis, macrophages phagocytize
entire erythrocytes, leaving little or no hemoglobin in the blood. Normal turnover
of erythrocytes occurs mainly by extravascular hemolysis within the spleen, and to
a lesser extent in other organs such as the liver and bone marrow. The exact controls
are not clear, but factors that likely play a role in physiologic hemolysis include
the following:
•
Exposure of membrane components normally sequestered on the inner leaflet of the erythrocyte
membrane, particularly phosphatidylserine.
•
Decreased erythrocyte deformability.
•
Binding of immunoglobulin G (IgG) and/or complement to erythrocyte membranes. Complement
binding may be secondary clustering of the membrane anion exchange protein, band 3.
•
Oxidative damage to erythrocytes.
Macrophages degrade erythrocytes into reusable components, such as iron and amino
acids, and the waste product bilirubin. Bilirubin is then exported into circulation,
where it is transported to the liver by albumin. The liver conjugates and subsequently
excretes bilirubin into bile for elimination from the body.
Intravascular hemolysis normally occurs at only extremely low levels. Hemoglobin is
a tetramer that, when released from the erythrocyte into the blood, splits into dimers
that bind to a plasma protein called haptoglobin. The hemoglobin-haptoglobin complex
is taken up by hepatocytes and macrophages. This is the major pathway for handling
free hemoglobin. However, free hemoglobin may also oxidize to form methemoglobin,
which dissociates to form metheme and globin. Metheme binds to a plasma protein called
hemopexin, which is taken up by hepatocytes and macrophages in a similar manner to
hemoglobin-haptoglobin complexes. Free heme in the reduced form binds to albumin,
from which it is taken up in the liver and converted into bilirubin.
The concentration of circulating erythrocytes typically decreases postnatally and
remains below normal adult levels during the period of rapid body growth. The age
at which erythrocyte numbers begin to increase and the age at which adult levels are
reached vary among species. In dogs, adult values are usually reached between 4 and
6 months of age; in horses, this occurs at approximately 1 year of age.
Granulopoiesis and Monocytopoiesis (Myelopoiesis).
Granulopoiesis is the production of neutrophils, eosinophils, and basophils, whereas
monocyte production is termed monocytopoiesis. Granulocytic and monocytic cells are
sometimes collectively referenced as myeloid cells. However, the term myeloid and
the prefix myelo- can be confusing because they have other meanings; they may reference
the bone marrow, all nonlymphoid hemic cells (erythrocytes, leukocytes, and megakaryocytes),
only granulocytes, or the spinal cord.
The main purpose of granulocytes and monocytes is to migrate to sites of tissue inflammation
and function in host defense (see Chapters 3 and 5 ). Briefly, these cells have key
immunologic functions, including phagocytosis and microbicidal activity (neutrophils
and monocyte-derived macrophages), parasiticidal activity and participation in allergic
reactions (eosinophils and basophils), antigen processing and presentation, and cytokine
production (macrophages). Neutrophils are the predominant leukocyte type in blood
of most domestic species.
Primary stimulators of granulopoiesis and monocytopoiesis are granulocyte-macrophage
colony-stimulating factor and IL-1, IL-3, and IL-6 (granulocytes and monocytes), granulocyte
colony-stimulating factor (granulocytes), and macrophage colony-stimulating factor
(monocytes). In general, these cytokines are produced by various inflammatory cells,
with or without contribution from stromal cells.
The earliest granulocytic precursor identifiable by routine light microscopy is the
myeloblast, which undergoes maturational division over 5 days to produce 16 to 32
progeny cells (see Fig. 13-3). These granulocytic precursors are conceptually divided
into those stages that can divide, including myeloblasts, promyelocytes, and myelocytes
(proliferation pool), and those that cannot, including metamyelocytes, and band and
segmented forms (maturation pool). Within the neutrophil maturation pool is a subpool,
termed the storage pool, which consists of a reserve of fully mature neutrophils.
The size of the storage pool varies by species; it is large in the dog, but small
in ruminants. In homeostasis mostly mature segmented granulocytes are released from
the marrow into the blood.
The first monocytic precursor identifiable by morphologic features is the monoblast,
which develops into promonocytes and subsequently monocytes (see Fig. 13-3). Unlike
granulocytes, monocytes do not have a marrow storage pool; they immediately enter
venous sinusoids upon maturation. After migrating into the tissues, monocytes undergo
morphologic and immunophenotypic maturation into macrophages.
Within blood vessels there are two pools of leukocytes: the circulating pool and the
marginating pool. Circulating cells are free flowing in blood, whereas marginating
cells are temporarily adhered to endothelial cells by selectins. In most healthy mammals
there are typically equal numbers of neutrophils in the circulating and marginal pools.
However, there are threefold more marginal neutrophils relative to circulating neutrophils
in cats. Only the circulating leukocyte pool is sampled during phlebotomy. The concentration
of myeloid cells in blood depends on the rate of production and release from the bone
marrow, the proportions of cells in the circulating and marginating pools, and the
rate of migration from the vasculature into tissues.
The fate of neutrophils after they leave the bloodstream in normal conditions (i.e.,
not in the context of inflammation) is poorly understood. They migrate into the gastrointestinal
and respiratory tracts, liver, and spleen and may be lost through mucosal surfaces
or undergo apoptosis and be phagocytized by macrophages.
Lymphopoiesis.
Lymphopoiesis—from lympha (Latin, water)—refers to the production of new lymphocytes,
including B lymphocytes, T lymphocytes, and natural killer (NK) cells. B lymphocytes
primarily produce immunoglobulins, also known as antibodies, and are key effectors
of humoral immunity. They are distinguished by the presence of an immunoglobulin receptor
complex, termed the B lymphocyte receptor. Plasma cells are terminally differentiated
B lymphocytes that produce abundant immunoglobulin. T lymphocytes, effectors of cell-mediated
immunity, possess T lymphocyte receptors that bind antigens prepared by antigen-presenting
cells. A component of innate immunity, NK cells kill a variety of infected and tumor
cells in the absence of prior exposure or priming. Main growth factors for B lymphocytes,
T lymphocytes, and NK cells are IL-4, IL-2, and IL-15, respectively.
Lymphocytes are derived from HSCs within the bone marrow. B lymphocyte development
occurs in two phases, first in an antigen-independent phase in the bone marrow and
ileal Peyer's patches (the site of B lymphocyte development in ruminants), then in
an antigen-dependent phase in peripheral lymphoid tissues (such as spleen, lymph nodes,
and mucosa-associated lymphoid tissue [MALT]). T lymphocyte progenitors migrate from
the bone marrow to the thymus, where they undergo differentiation, selection, and
maturation processes before migrating to the peripheral lymphoid tissue as effector
cells.
Unlike granulocytes, which circulate only in blood vessels and migrate unidirectionally
into target tissues, lymphocytes travel in both blood and lymphatic vessels and continually
circulate between blood, tissues, and lymphatic vessels. Also in contrast to nonlymphoid
hematopoietic cells, blood lymphocyte concentrations in adult animals are primarily
dependent upon extramedullary lymphocyte production and kinetics, and not lymphopoiesis
by the marrow.
In healthy nonruminant mammals, lymphocytes are the second most numerous blood leukocyte.
According to conventional wisdom, cattle normally have higher numbers of lymphocytes
than neutrophils in circulation. However, recent studies suggest that is no longer
the case, most likely due to changes in genetics and husbandry. In most species the
majority of lymphocytes in blood circulation are T lymphocytes. The concentration
of blood lymphocytes decreases with age.
Thrombopoiesis.
Thrombopoiesis—from thrombos (Gr., clot)—refers to the production of platelets, which
are small (2 to 4 µm), round to ovoid, anucleate cells within blood vessels. Platelets
have a central role in primary hemostasis but also participate in secondary hemostasis
(coagulation) and inflammatory pathways (see Chapters 2 and 3).
Thrombopoietin (Tpo) is the primary regulator of thrombopoiesis. The liver and renal
tubular epithelial cells constantly produce Tpo, which is then cleared and destroyed
by platelets and their precursors. Therefore plasma Tpo concentration is inversely
proportional to platelet and platelet precursor mass. If the platelet mass is decreased,
less Tpo is cleared, and there is subsequently more free plasma Tpo to stimulate thrombopoiesis.
The earliest morphologically identifiable platelet precursor is the megakaryoblast,
which undergoes nuclear reduplications without cell division, termed endomitosis,
to form a megakaryocyte with 8 to 64 nuclei. As the name suggests, megakaryocytes
are very large cells, much larger than any other hematopoietic cell (Fig. 13-6
; also see Fig. 13-1). Megakaryocytes neighbor venous sinusoids, extend their cytoplasmic
processes into vascular lumens, and shed membrane-bound cytoplasmic fragments (platelets)
into blood circulation. Orderly platelet shedding is partially facilitated by β1-tubulin
microtubules within megakaryocytes.
Figure 13-6
Megakaryocyte, Canine Bone Marrow Aspirate.
Note the cell's very large size, lobulated nucleus, and abundant granular cytoplasm.
Wright's stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
Platelets circulate in a quiescent form and become activated by binding platelet agonists,
including thrombin, adenosine diphosphate (ADP), and thromboxane. Platelet activation
causes shape change, granule release, and relocation of procoagulant phospholipids
and glycoproteins (GPs) to the outer cell membrane. Specific procoagulant actions
include release of calcium, von Willebrand factor (vWF), factor V, and fibrinogen,
as well as providing phosphatidylserine-rich binding sites for the extrinsic tenase
(factors III, VII, and X), intrinsic tenase (factors IX, VIII, and X), and prothrombinase
(factors X, V, and II) coagulation complexes. Platelet GP surface receptors include
those for binding vWF (GPIb-IX-V), collagen (GPVI), and fibrinogen (GPIIb-IIIa), which
facilitate platelet aggregation and adherence to subendothelial collagen. Expansion
of surface area and release of granule contents is aided by a network of membrane
invaginations known as the open canalicular system. This system is not present in
horses, cattle, and camelids.
Methods for Examination of the Bone Marrow
Gross and Microscopic Examination
Bone marrow is not routinely sampled during postmortem examinations. However, indications
for bone marrow evaluation include suspected leukemia, metastatic neoplasia within
bone marrow, or infectious myelitis, as well as cytopenia(s) or hematopoietic dysplasia
of unknown cause. Multimodal evaluation is ideal, including a recent (<24 hours) complete
blood count with bone marrow cytologic and histopathologic examination. However, antemortem
blood analyses are not always available, and interpretation of hematopoietic cytomorphologic
examination results becomes difficult to impossible shortly after death.
Postmortem bone marrow should be collected as soon as possible after death or euthanasia,
preferably within 30 minutes. Samples may be collected from the proximal femur, rib,
sternum or vertebrae. When collecting from the femur, the femoral neck is removed
with a bone saw, or a fragment of the shaft is removed with bone-cutting shears. Cytologic
samples are first collected using the paintbrush technique: gently sample the red
marrow with a clean, dry, natural-bristle brush, and then carefully brush the material
onto a clean glass microscope slide in two to four parallel wavy lines. The brush
should be cleaned and dried before its use on a different animal. The slide is then
air dried, stored away from formalin fumes, and then stained with a routine (Romanowsky)
stain. For histologic evaluation the entire femoral head or femoral shaft or rib fragment
with exposed red marrow is submersed in 10% neutral buffered formalin. For cosmetic
necropsies, samples may be obtained by antemortem techniques, such as needle biopsies
for cytologic examination and core biopsies for histopathologic examination.
Complete Blood Count
The complete blood count (CBC) is the cornerstone for diagnosis of hematologic disturbances
and is often part of a minimum database in sick patients. The CBC includes numeric
data indicating the concentration of different cell types, as well as other estimations
of red blood cell mass (hemoglobin concentration, packed-cell volume, and hematocrit),
red blood cell volume (mean cell volume), and red blood cell hemoglobin content (mean
cell hemoglobin and mean cell hemoglobin concentration). Cell morphologic features
and the presence or absence of hemic parasites are assessed upon microscopic review
of a blood smear and are also included in a CBC report. (note: Some parasites may
infect blood cells, such as Hepatozoon organisms within circulating neutrophils or
monocytes or Bartonella organisms within erythrocytes, but mainly cause disease in
other body systems and are therefore not discussed in this chapter.) Learning to evaluate
blood smears is a valuable skill for any practicing veterinarian. The CBC also may
include the plasma protein concentration, as measured with a refractometer. It is
important to remember that changes in hydration status and in the distribution of
body fluids between the vascular and extravascular compartments affect the concentration
of both cells and proteins in the blood.
Additional Tests
Other tests that may help with evaluation of the hematopoietic system include cell
or tissue biopsies, the direct antiglobulin test, flow cytometry, immunophenotyping,
and polymerase chain reaction (PCR). Aspiration cytology and/or histopathology of
organs other than the bone marrow can be pursued to assess for the presence of EMH,
increased destruction of erythrocytes, neoplasia, or infection. The Coombs test, or
direct antiglobulin test, detects excessive antibody or complement bound to red blood
cells' surfaces and is the standard assay for immune-mediated hemolytic anemia. Flow
cytometry and immunofluorescent antibody tests may also be used to detect autoantibody
bound to erythrocytes or other hematopoietic cells. Immunophenotyping and PCR are
further discussed in the section on Hematopoietic Neoplasia.
Hemostasis Testing
Structural or functional abnormalities of blood vessels, platelets, or coagulation
factors may result in a tendency toward hypocoagulability (bleeding), hypercoagulability
(inappropriate thrombosis), or both. In veterinary medicine there has been a great
deal of work on specific mechanisms of hypocoagulability, whereas mechanisms of hypercoagulability
are less fully characterized. Disorders of primary hemostasis typically result in
“small bleeds” (e.g., petechiation, mild ecchymosis, bleeding from mucous membranes,
bleeding immediately after venipuncture), whereas disorders of secondary hemostasis
typically result in “big bleeds” (e.g., hemorrhage into body cavities/joints, marked
ecchymosis, large hematomas, delayed bleeding after venipuncture). This chapter concentrates
on primary disorders of hemostasis and also covers disseminated intravascular coagulation,
which is the secondary condition. However, it is important to note that coagulation
disorders can also result from other underlying disease processes. For example, advanced
liver disease can lead to abnormal hemostasis through decreased or defective synthesis
of coagulation factors or impaired clearance of fibrinolytic products that inhibit
coagulation reactions and platelet function. Vascular disorders may also result in
a bleeding tendency because of abnormalities of endothelial function or collagen-platelet
interactions. Specific diseases involving abnormal structure or function of hematopoietic
or hemostatic elements are discussed later in this chapter.
The CBC provides basic information about platelets, including numeric values for platelet
concentration and mean platelet volume (MPV), subjective assessment of platelet morphologic
features (size, shape, and granularity), and a rough estimation of platelet numbers
based on examination of a blood smear. Some laboratories measure reticulated platelets
(platelets recently released from the bone marrow), although this test is mostly used
in the research setting at present. Increased MPV and increased numbers of reticulated
platelets tend to indicate increased thrombopoiesis. Bone marrow examination is indicated
with any unexplained cytopenia, including thrombocytopenia, to evaluate production.
Tests to evaluate the components of the hemostatic process are described and listed
in E-Appendix 13-1.
Dysfunction/Responses to Injury
Bone Marrow
Mechanisms of bone marrow disease are summarized in Box 13-1
. Hematopoietic cells' response to injury is dependent upon whether the insult is
on the marrow or within extramarrow tissues. In general, marrow-directed injury or
disturbances result in production of abnormal hematopoietic cells (dysplasia), fewer
hematopoietic cells (hypoplasia), or a failure of hematopoietic cell development (aplasia).
Dysplasia, hypoplasia, and aplasia may be specific for one cell line, such as pure
red cell aplasia, or affect multiple lineages, as seen with aplastic anemia. Accordingly,
decreased blood concentrations of the involved cell types are expected with hypoplasia
or aplasia. Erythroid, myeloid, and megakaryocytic hypoplasia or aplasia causes nonregenerative
anemia, neutropenia, and thrombocytopenia, respectively. Bicytopenia is used to describe
decreased blood concentrations of two cell lines, whereas pancytopenia indicates decreased
blood concentrations of all three cell types. Bicytopenia or pancytopenia may indicate
generalized marrow disease, such as occurs with aplastic anemia or marrow malignancies
(leukemia), necrosis, fibrosis (myelofibrosis), or inflammation (myelitis). Replacement
of hematopoietic tissue within the bone marrow by abnormal tissue, including neoplastic
cells, fibrosis, or inflammatory cells, is termed myelophthisis.
Box 13-1
Mechanisms of Disease in Bone Marrow and Blood Cells
Bone Marrow
Hypoplasia
Hyperplasia
Dysplasia
Aplasia
Neoplasia
Myelophthisis (fibrosis, metastatic neoplasia)
Necrosis
Inflammation
Blood Cells
Increased destruction
Hemorrhage (especially erythrocytes)
Consumption (platelets)
Neoplasia
Altered distribution
Abnormal function
Insults to extramarrow tissues and cells tend to cause increased production of the
involved cell types (hyperplasia) with or without dysplasia. Loss of erythrocytes
from blood vessels (hemorrhage), or premature destruction of erythrocytes (hemolysis)
causes erythroid hyperplasia. Tissue inflammation may cause neutrophilic, eosinophilic,
basophilic, and/or monocytic hyperplasia, depending on the type of inflammation. Megakaryocytic
hyperplasia may occur with increased platelet use during hemorrhage or disseminated
intravascular coagulation (DIC) or with immune-mediated platelet destruction. Exceptions
to these generalizations, such as anemia of chronic disease, iron deficiency anemia,
and anemia of renal failure, are discussed in more detail later.
Endothelial cell response to injury specifically within the marrow is poorly characterized,
but it is likely similar to that of endothelial cells elsewhere, playing active roles
in coagulation and inflammation (see Chapters 2 and 3). However, one potential sign
of marrow sinusoidal injury is the presence of circulating nucleated erythrocytes
in the absence of erythrocyte regeneration, termed inappropriate metarubricytosis.
It is proposed that injured marrow endothelial cells allow premature passage of metarubricytes
into blood circulation during times of stress. However, a conflicting theory proposes
that marrow stress causes decreased metarubricyte attachment to central macrophages,
and subsequent release into circulation. Specific causes of marrow injury–induced
metarubricytosis include sepsis, hyperthermia, malignancies, hypoxia, and certain
drugs and toxins. Inappropriate metarubricytosis may also occur with erythroid dysplasia
and splenic disorders.
In addition to a suspected role in inappropriate metarubricytosis, marrow macrophages
are integral to altered iron metabolism, including anemia of chronic disease and hemosiderosis.
Anemia of chronic disease is a mild to moderate nonregenerative anemia observed in
animals with a variety of inflammatory and metabolic disorders. This anemia is discussed
in more detail later, but briefly, it is primarily a result of iron sequestration
within macrophages. Hemosiderosis is the excessive accumulation of iron in tissues,
typically macrophages. Accumulation of iron in parenchymal organs, leading to organ
toxicity, is termed hemochromatosis. In animals, iron overload due to blood transfusions
or chronic hemolytic anemias may cause marrow hemosiderosis and hemochromatosis.
Myelitis can take different forms. Granulomatous myelitis occurs with systemic fungal
infections (e.g., histoplasmosis) or mycobacteriosis. Acute or neutrophilic myelitis
may occur with lower-order bacterial infections or those with an immune-mediated component.
Dogs and cats with nonregenerative immune-mediated hemolytic anemia (IMHA) often have
myelitis, in addition to myelofibrosis and necrosis. The inflammation is evident as
fibrin deposition, edema, and multifocal neutrophilic infiltrates; immune-mediated
cytopenias may also concurrently occur with bone marrow lymphocytic and/or plasma
cell hyperplasia.
Bone marrow necrosis is the necrosis of medullary hematopoietic cells, stromal cells,
and stroma in large areas of bone marrow. Potential causes include leukemias, extramarrow
malignancies, infection (bovine viral diarrhea virus [BVDV], Ehrlichia canis, and
feline leukemia virus [FeLV]), sepsis, drugs or toxins (carprofen, chemotherapeutic
agents, estrogen, metronidazole, mitotane, and phenobarbital), and irradiation. Direct
hematopoietic or stromal cytotoxicity and altered marrow microvasculature (disseminated
intravascular coagulation) are proposed pathogeneses. Extensive marrow necrosis results
in decreased hematopoiesis and subsequent blood cytopenias, including anemia, neutropenia,
and thrombocytopenia. If the animal survives the initial insult, the marrow may recover
and resume normal hematopoiesis, or it may undergo scar formation, termed myelofibrosis.
Secondary myelofibrosis is the enhanced deposition of collagen within the marrow by
nonneoplastic fibroblasts and reticular cells. Disease pathogenesis is unclear, but
there are two leading theories. First, it may represent scar formation after marrow
necrosis, as previously presented. And second, high concentrations of growth factors
present during times of marrow injury or activation may stimulate fibroblast proliferation.
In particular, stimulated megakaryocytes and macrophages produce fibrogenic cytokines,
including platelet-derived growth factor, transforming growth factor-β, and epidermal
growth factor. Early in disease there is reticulin deposition without reduction of
hematopoietic elements. However, fibrous collagen replaces hematopoietic cells with
disease progression. Histologic identification of reticulin and collagen fibers can
be aided with reticulin silver and Masson's trichrome stains, respectively. In animals,
secondary myelofibrosis occurs most commonly with leukemias, extramarrow malignancies,
and chronic hemolytic anemias, but many cases are idiopathic. Experimental whole-body
gamma irradiation, dietary strontium-90 exposure, and certain drugs and toxins can
also induce myelofibrosis.
The responses of marrow adipocytes to systemic and localized disease are under current
investigation, especially in relation to energy metabolism, inflammation, and bone
trauma. During times of severe energy imbalance, such as cachexia, the marrow may
undergo serous atrophy of fat, also known as gelatinous marrow transformation (E-Fig.
13-1). The pathogenesis of this phenomenon is unknown, but it is characterized by
adipocyte atrophy, hematopoietic cell hypoplasia with subsequent cytopenias, and replacement
of the marrow with extracellular hyaluronic acid–rich mucopolysaccharides. Positive
Alcian blue staining identifies the extracellular material as mucin.
E-Figure 13-1
Serous Atrophy of Fat (Gelatinous Transformation), Bone Marrow, Calf.
Serous atrophy of fat is caused by starvation and/or cachexia and is characterized
by replacement of normal fat and hematopoietic elements by a gelatinous extracellular
matrix.
(Courtesy Department of Pathobiology, College of Veterinary Medicine, University of
Tennessee).
Marrow adipocytes secrete adipose-derived hormones, termed adipokines, including leptin
and adiponectin. In general, leptin is proinflammatory, prothrombotic, and mitogenic
for various cell types, including lymphocytes, hematopoietic progenitors, and leukemic
cells. Conversely, adiponectin has antiinflammatory and growth inhibitory properties.
During times of inflammation and infection, leptin production is increased.
In response to marrow trauma, such as orthopedic surgery, fat may enter the vasculature,
embolize to various tissues, and cause tissue ischemia. The severity of tissue injury
caused by fat embolism is dependent upon the quantity of fat entering circulation
and the tissue's susceptibility to ischemia (see Chapter 2).
Blood Cells
Responses of circulating blood cells to injury include decreased survival (destruction,
consumption, or loss), altered distribution, and altered structure or function (see
Box 13-1). These responses are not mutually exclusive—for example, altered erythrocyte
structure may lead to decreased survival. Often, but not always, these responses result
in decreased concentrations of blood cells in circulation.
Abnormal Concentrations of Blood Cells.
The concentration of blood cells may be decreased, termed cytopenia (from kytos [Gr.,
hollow vessel] and penia [Gr., poverty]) or increased, designated cytosis (from osis
[Gr., condition]). A specific blood cell type is denoted as being decreased by using
the suffix -penia (Table 13-1
). A decreased concentration of erythrocytes is the exception and is termed anemia
(from a [Gr., without] and haima [Gr., blood]). Decreased concentrations of blood
basophils are not recognized in domestic animals because the lower reference interval
is typically zero. An increased blood cell type is denoted with the suffix -osis or
-philia (see Table 13-1). Postmortem quantification of blood cell concentrations is
not possible due to perimortem coagulation. However, a complete blood count (CBC)
with microscopic blood smear evaluation is the foundation for antemortem assessment
of blood cells.
Table 13-1
Terminology for Increases or Decreases in Hematopoietic Cells in Blood
Cell type
Decreased
Increased
Erythrocytes
Anemia
Erythrocytosis
Reticulocytes
Reticulopenia
Reticulocytosis
Leukocytes
Leukopenia
Leukocytosis
Neutrophils
Neutropenia
Neutrophilia
Lymphocytes
Lymphopenia
Lymphocytosis
Monocytes
Monocytopenia
Monocytosis
Eosinophils
Eosinopenia
Eosinophilia
Basophils
Basopenia
Basophilia
Platelets
Thrombocytopenia
Thrombocytosis
Anemia.
Anemia causes clinical signs referable to decreased red hemoglobin pigment (e.g.,
pale mucous membranes), decreased oxygen-carrying capacity (e.g., depression, lethargy,
weakness, and exercise tolerance), and decreased blood viscosity (e.g., heart murmur).
Recumbency, seizures, syncope, or coma may occur with severe anemia. Anemia is confirmed
by identifying a decreased hemoglobin concentration or reduced erythrocyte mass, as
measured by the packed-cell volume, hematocrit, or red blood cell concentration.
The three general causes of anemia are blood loss (hemorrhage), red blood cell destruction
or lysis (hemolysis), and decreased red blood cell production (erythroid hypoplasia).
Classifying anemia as regenerative or nonregenerative is clinically useful because
it provides information about the mechanism of disease; regenerative anemia indicates
hemorrhage or hemolysis, whereas erythroid hypoplasia or aplasia causes nonregenerative
anemia (Table 13-2
).
Table 13-2
Causes of Regenerative and Nonregenerative Anemia
Regenerative Anemia
Nonregenerative Anemia
Hemorrhage
Trauma
Hemostasis defect
Neoplasia
Gastrointestinal ulceration
Parasitism
Phlebotomy
Primary bone marrow disease
Immune-mediated
Infections (e.g., feline leukemia virus)
Myelophthisis (e.g., myelitis, leukemia, myelofibrosis)
Toxicity (e.g., chemotherapeutic agents, estrogen, bracken fern)
Congenital disorders
Hemolysis
Immune-mediated
Infections (e.g., hemoparasitism)
Toxicity (oxidants)
Mechanical fragmentation (e.g., disseminated intravascular coagulation)
Enzymatic (e.g., bacterial phospholipases)
Neoplasia (e.g., hemophagocytic histiocytic sarcoma)
Hypophosphatemia
Congenital disorders
Extramarrow disease
Inflammatory disease
Chronic renal failure
Liver disease or failure
Endocrinopathies (e.g., hypoadrenocorticism, hypothyroidism)
Nutritional deficiency (e.g., iron deficiency, vitamin B12 deficiency, malnutrition)
The hallmark of regenerative anemias, except in horses, is reticulocytosis (i.e.,
increased numbers of circulating reticulocytes [immature erythrocytes]), which is
evident as polychromasia on a routinely stained blood smear (see Fig. 13-5). Reticulocytosis
indicates increased bone marrow erythropoiesis (Fig. 13-7
) and release of erythrocytes before they are fully mature. Reticulocytosis is an
appropriate marrow response to anemia and is often seen with hemorrhage or hemolysis.
On a CBC a strong regenerative response may produce an increased mean cell volume
(MCV) and decreased mean cell hemoglobin concentration (MCHC) because reticulocytes
are larger and have a lower hemoglobin concentration than mature erythrocytes. Horses
are an exception to this classification scheme because they do not release reticulocytes
into circulation, even with erythroid hyperplasia. Horses with a regenerative response
may have an increased MCV and red cell distribution width (an index of variation in
cell size). But definitive determination of regeneration in a horse requires demonstration
of erythroid hyperplasia via bone marrow examination or an increasing red cell mass
over sequential CBCs.
Figure 13-7
Hemopoietically Active Bone Marrow, Femur, Calf.
Note that the bone marrow has a uniform consistency and is red to dark red. These
responses are characteristic of hemopoietically active bone marrow.
(Courtesy Dr. Ramos, Autonomous University of Barcelona; and Noah's Arkive, College
of Veterinary Medicine, The University of Georgia.)
In addition to reticulocytosis there may be increased numbers of nucleated red blood
cells (nRBCs) in circulation with erythrocyte regeneration, termed appropriate metarubricytosis.
When nRBCs are present as part of a regenerative response, they should be in low numbers
relative to the numbers of reticulocytes. However, the presence of circulating nRBCs
is not in itself definitive evidence of regeneration and may signify dyserythropoiesis
(e.g., lead poisoning or bone marrow disease) or splenic dysfunction. These processes
should be suspected when nRBCs are increased without reticulocytosis, or their numbers
are high relative to the degree of reticulocytosis, termed inappropriate metarubricytosis.
In ruminants, reticulocytosis is often accompanied by basophilic stippling (Fig. 13-8
). However, like metarubricytosis, basophilic stippling without reticulocytosis is
concerning for lead poisoning or other causes of dyserythropoiesis.
Figure 13-8
Basophilic Stippling and Polychromasia, Bovine Blood Smear.
Erythrocytes from this cow with regenerative anemia include several cells with basophilic
stippling (arrow) and two polychromatophilic cells (reticulocytes) (arrowheads). Wright's
stain. (Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
Recall that the stimulus for increased erythropoiesis is increased secretion of Epo
in response to tissue hypoxia. Although the action of Epo on erythropoiesis is rapid,
evidence of a regenerative response is not immediately apparent in a blood sample.
One of the main effects of Epo is to expand the pool of early-stage erythroid precursors,
and it takes time for these cells to differentiate to the point where they are released
into circulation. In a case of acute hemorrhage or hemolysis, for example, it typically
takes 3 to 4 days until reticulocytosis is evident on the CBC and several more days
until the regenerative response peaks. The term preregenerative anemia is sometimes
used to describe anemia with a regenerative response that is impending but not yet
apparent on the CBC. Confirming a regenerative response in such cases requires either
evidence of erythroid hyperplasia in the bone marrow or emergence of a reticulocytosis
on subsequent days.
Hemorrhage results in escape of erythrocytes and other blood components, such as protein,
from the vasculature. As a result, a decreased plasma or serum protein concentration,
termed hypoproteinemia, may be evident on a CBC or chemistry panel. If the hemorrhage
is into the gastrointestinal lumen, some of the protein may be resorbed and converted
to urea, resulting in an increased urea nitrogen concentration relative to creatinine
in plasma. Hemorrhage within the urinary tract may cause red urine with erythrocytes
observed in the urine sediment. Causes of hemorrhage include trauma, abnormal hemostasis,
certain parasitisms, ulceration, and neoplasia.
Hemorrhage may be acute or chronic, or internal or external. During acute hemorrhage,
there are ample iron stores within the body for hemoglobin synthesis and erythrocyte
regeneration. However, with chronic external hemorrhage, continued loss of iron may
deplete the body's iron stores. As iron stores diminish, so does erythrocyte regeneration,
eventually leading to iron deficiency anemia. Iron deficiency anemia is either poorly
regenerative or nonregenerative and is discussed in more detail later in the chapter.
Iron deficiency anemia does not occur with chronic internal hemorrhage, such as into
the peritoneal cavity, because iron is not lost from the body and can be reused for
erythropoiesis.
In hemolytic anemia, erythrocytes are destroyed at an increased rate. Whether the
mechanism is intravascular or extravascular, or a combination, depends on the specific
disease process (specific diseases are discussed later in this chapter). Some clinical
indicators of hemolytic anemia and their pathogeneses are summarized in Fig. 13-9
and are further described in the following discussion.
Figure 13-9
Mechanisms of Color Abnormalities of Plasma, Urine, and Feces during Hemolysis.
Intravascular hemolysis: Several initiating processes can cause intravascular hemolysis;
formation of the complement membrane attack complex is pictured. With intravascular
hemolysis, free hemoglobin is release directly into the plasma, where it is scavenged
by haptoglobin and hemopexin. When haptoglobin and hemopexin are saturated, the cell-free
hemoglobin causes red discoloration of the plasma (hemolysis) and is excreted in the
urine (hemoglobinuria; dark red urine). The liver clears haptoglobin-hemoglobin and
hemopexin-methemoglobin complexes from plasma and converts hemoglobin to unconjugated
bilirubin and then conjugated bilirubin. Conjugated bilirubin is normally excreted
in the bile and then converted to urobilinogen (yellow) and subsequently stercobilinogen
(dark brown). However, excessive bilirubin will spill over into the plasma, resulting
in hyperbilirubinemia, icteric plasma (if severe enough), and urinary excretion of
bilirubin (bilirubinuria; icteric urine). Extravascular hemolysis: During extravascular
hemolysis, erythrocytes are phagocytized by macrophages, which digest erythrocytes,
and convert hemoglobin to unconjugated bilirubin. Excessive bilirubin in plasma causes
hyperbilirubinemia with or without icteric plasma. Unconjugated bilirubin is processed
and excreted by the liver (as previously described) and in dogs, the kidney. C-bilirubin,
Conjugated bilirubin; Hgb, hemoglobin; Hpt, haptoglobin; Hpx, hemopexin; MACs, membrane
attack complexes; MetHgb, methemoglobin; U-bilirubin, unconjugated bilirubin.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University; and Dr. J.F. Zachary, College of Veterinary Medicine, University
of Illinois.)
A classic sequela of hemolytic anemias in general is hyperbilirubinemia, which is
an increase in the plasma bilirubin concentration. Bilirubin is a yellow pigment,
which explains why hyperbilirubinemia, if severe enough, causes icterus—the grossly
visible yellowing of fluid or tissues (Fig. 13-10
). Icterus, also known as jaundice, is usually detectable when the plasma bilirubin
concentration exceeds 2 mg/dL. However, it is important to note that hyperbilirubinemia
and icterus are not pathognomonic for hemolysis and may also occur with conditions
of impaired bile flow (cholestasis), such as hepatopathy or cholangiopathy.
Figure 13-10
Icterus, Immune-Mediated Hemolytic Anemia, Subcutaneous Fat, Splenomegaly, Spleen,
Dog.
The marked yellow discoloration of tissues, most strikingly visible in the subcutaneous
fat, is from high concentrations of serum bilirubin produced as a result of the hemolytic
anemia.
(Courtesy Dr. J.A. Ramos-Vara, College of Veterinary Medicine, Michigan State University;
and Noah's Arkive, College of Veterinary Medicine, The University of Georgia.)
In addition to icterus, hemolytic anemia often results in splenomegaly (Fig. 13-11
), which is secondary to extravascular hemolysis and macrophagic hyperplasia within
the spleen, as well as splenic EMH. Splenomegaly may also occur in other conditions,
as discussed elsewhere in this chapter.
Figure 13-11
Splenomegaly, Fatal Hemolytic Anemia, Mycoplasma suis, Pig.
The spleen is extremely enlarged, meaty, and congested.
(Courtesy College of Veterinary Medicine, University of Illinois.)
Intravascular hemolysis is grossly evident as pink-tinged plasma or serum, termed
hemolysis or hemoglobinemia. Hemolysis is not apparent until the concentration of
extracellular hemoglobin is greater than approximately 50 mg/dL. Cell-free hemoglobin
is scavenged by haptoglobin until haptoglobin becomes saturated with hemoglobin at
a concentration of approximately 150 mg/dL. When haptoglobin is saturated, any remaining
free hemoglobin has a low enough molecular weight to pass through the renal glomerular
filter into the urine. This imparts a pink or red discoloration to the urine, called
hemoglobinuria. Thus extracellular hemoglobin can cause gross discoloration of the
plasma, where it is bound to haptoglobin, before becoming grossly visible in urine.
The half-life of haptoglobin is markedly decreased when bound to hemoglobin, so when
large amounts of haptoglobin-hemoglobin complex are formed, the concentration of haptoglobin
in the blood decreases and hemoglobin can pass through the glomerulus at even lower
concentrations. Hemoglobinuria is a contributing factor in the renal tubular necrosis
(hemoglobinuric nephrosis) that often occurs in cases of acute intravascular hemolysis
(see Chapter 11). A similar lesion occurs in the kidneys of individuals with marked
muscle damage and resulting myoglobinuria (see Chapters 11 and 15).
Hemoglobinuria cannot be distinguished grossly from hematuria (erythrocytes in the
urine) or myoglobinuria (myoglobin in the urine), and all three processes cause a
positive reaction for “blood protein” on urine test strips. Comparing the colors of
the plasma and the urine may be informative. In contrast to hemoglobin, myoglobin
causes gross discoloration of the urine before the plasma is discolored. This is because
myoglobin is a low-molecular-weight monomer, freely filtered by the glomerulus, and
does not bind plasma proteins to a significant degree. Hematuria can be distinguished
from hemoglobinuria on the basis of microscopic examination of urine sediment (i.e.,
erythrocytes are present in cases of hematuria).
In addition to red plasma and urine, hemoglobinemia may also be identified by increased
MCH or MCHC values on a CBC. This is because the hemoglobin concentration is measured
by lysing all erythrocytes in the sample and then measuring the total hemoglobin via
spectrophotometry. By this method, hemoglobin that originated within or outside of
erythrocytes is measured together. However, calculations for MCH and MCHC, which include
results for the hemoglobin and red blood cell concentrations, assume that all of the
hemoglobin originated within erythrocytes. In the case of hemoglobinemia, the excess
extracellular hemoglobin may cause an artifactual increase in the calculated MCH and
MCHC. It is important to remember that similar artifactual increases may also occur
with lipemia.
Once hemolytic anemia has been identified, the specific cause for hemolysis should
be investigated based on signalment, clinical history, and microscopic blood smear
evaluation. The most common causes of hemolytic anemia in domestic animals are immune-mediated,
infectious, oxidative, and mechanical fragmentation (i.e., microangiopathic) disorders
(Table 13-3
).
Table 13-3
Four Common Causes of Hemolytic Anemia, and Their Main Hematologic Characteristics
Immune-Mediated
Infectious
Oxidative
Mechanical Fragmentation
Agglutination Spherocytes
Ghost cells
Agglutination
Spherocytes
Ghost cells
Hemoparasites
Heinz bodies
Eccentrocytes
Ghost cells
Methemoglobinemia
Schistocytes
Acanthocytes
Keratocytes
Ghost cells
Spherocytosis and autoagglutination are hallmarks of immune-mediated hemolytic anemia,
either primary (also known as idiopathic) or secondary to infectious disease, drugs/toxins,
or neoplasms. Spherocytes form when macrophages (mainly in the spleen) phagocytize
part of an erythrocyte plasma membrane bound with autoantibody (Fig. 13-12
). The remaining portion of the erythrocyte assumes a spherical shape, thus preserving
maximal volume. This change in shape results in decreased deformability of the cells.
Erythrocytes must be extremely pliable to traverse the splenic red pulp and sinusoidal
walls; spherocytes therefore tend to be retained in the spleen in close association
with macrophages with risk for further injury and eventual destruction. In the dog,
spherocytes appear smaller than normal and have uniform staining (Fig. 13-13, A
), in contrast to normal erythrocytes, which have a region of central pallor imparted
by their biconcave shape. This difference in staining between spherocytes and normal
erythrocytes is not consistently discernible in many other domestic animals (including
horses, cattle, and cats), whose erythrocytes differ from those of the dog in that
they are smaller and have less pronounced biconcavity and therefore less pronounced
central pallor. Autoagglutination occurs because of cross-linking of antibodies bound
to erythrocytes (see Fig 13-12). Autoagglutination is evident macroscopically as blood
with a grainy consistency (see Fig. 13-13, B), and microscopically as clusters of
erythrocytes (see Fig. 13-13, C). Autoagglutination may also result in a falsely increased
MCV and decreased red blood cell concentration when clustered cells are mistakenly
counted as single cells by automated hematology analyzers. When autoagglutination
is present, the packed-cell volume is the most reliable measurement of red blood cell
mass.
Figure 13-12
Pathogenesis of Abnormal Erythrocyte Morphologic Changes in Immune-Mediated Hemolytic
Anemia.
1, Red blood cell (RBC) degradation. Antierythrocyte antibodies bind RBC surface antigens,
resulting in RBC opsonization by immunoglobulins (mainly immunoglobulin G [IgG]) and
complement (primarily C3b). Immunoglobulin- or C3b-bound RBCs are phagocytized and
digested by sinusoidal macrophages. 2, Spherocytes. Spherocytes form when the membrane
of immunoglobulin- or C3b-bound RBCs are phagocytized by macrophages, without removing
the entire RBC from circulation. Compared to normal erythrocytes, spherocytes appear
smaller, more eosinophilic, and lack central pallor. 3, RBC aggregation (agglutination).
RBC aggregation occurs when antierythrocyte immunoglobulins (immunoglobulin M [IgM]
or high concentrations of IgG) bind multiple erythrocytes simultaneously. 4, Ghost
cells. Antierythrocyte antibodies bind RBC surface antigens, resulting in complement
activation and formation of the membrane attack complex (MAC). MACs form membrane
pores, resulting in rupture of RBCs, and the release of hemoglobin into the circulation.
Ghost cells are RBC membrane remnants that lack cytoplasm (hemoglobin).
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University; and Dr. J.F. Zachary, College of Veterinary Medicine, University
of Illinois.)
Figure 13-13
Immune-Mediated Hemolytic Anemia, Canine Blood, Dog.
A, Spherocytosis. Numerous spherocytes, several ghost cells, and one polychromatophil.
Wright-Giemsa stain. B, Macroscopic autoagglutination. Note the grossly visible agglutination.
C, Microscopic agglutination. Note the grapelike cluster of erythrocytes. Wright-Giemsa
stain. (Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic
Institute and State University.)
Ghost cells are ruptured red blood cell membranes devoid of cytoplasmic contents (see
Figs. 13-12 and 13-13, A). They indicate intravascular hemolysis and may be seen with
a variety of hemolytic disorders, including those with immune-mediated, infectious,
oxidative, or fragmentation causes. In the case of immune-mediated hemolytic anemia,
antibody or complement binds to red blood cell membranes and activates the complement
membrane attack complex (see Fig. 13-12). This causes pore formation in the red blood
cell membrane and release of cytoplasmic contents into the plasma. Ghost cells are
eventually cleared from circulation by phagocytic macrophages, mainly within the spleen.
Oxidative damage to erythrocytes occurs when normal antioxidative pathways that generate
reducing agents (such as reduced nicotinamide adenine dinucleotide [NADH], reduced
nicotinamide adenine dinucleotide phosphate [NADPH], and reduced glutathione [GSH])
are compromised or overwhelmed, resulting in hemolytic anemia, abnormal hemoglobin
function, or both. Hemolysis caused by oxidative damage may be extravascular or intravascular,
or a combination. Evidence of oxidative damage to erythrocytes may be apparent on
blood smear examination as Heinz bodies or eccentrocytes or on gross examination as
methemoglobinemia.
Heinz bodies are foci of denatured globin that interact with the erythrocyte membrane.
They are usually subtly evident on routine Wright-stained blood smears as pale circular
inclusions or blunt, rounded protrusions of the cell margin but are readily discernible
on smears stained with new methylene blue (Fig. 13-14
). Cats are particularly susceptible to Heinz body formation and may have low numbers
of Heinz bodies normally. There is no unanimity of opinion, but some clinical pathologists
believe that the presence of Heinz bodies in up to 10% of all erythrocytes in cats
is within normal limits. This predisposition is believed to reflect unique features
of the feline erythrocyte, whose hemoglobin has more sulfhydryl groups (preferential
sites for oxidative damage) than do erythrocytes of other species and may also have
lower intrinsic reducing capacity. It is also possible that the feline spleen does
not have as efficient a “pitting” function (splenic structure and function are discussed
in more detail later in this chapter).
Figure 13-14
Heinz Bodies, Blood Smears.
A, Feline blood smear. With routine staining, Heinz bodies appear as pale circular
intraerythrocytic inclusions that may protrude (arrows) from the margin of the cell.
Wright's stain. B, Canine blood smear. Using a supravital stain, Heinz bodies are
blue inclusions (arrows) and easier to see. New methylene blue stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
Eccentrocytes, evident as erythrocytes in which one side of the cell has increased
pallor (Fig. 13-15, A
), are another manifestation of oxidative damage. They form because of cross-linking
of membrane proteins, with adhesion of opposing areas of the cell's inner membrane
leaflet, and displacement of most of the hemoglobin toward the other side. The fused
membranes may fragment off of the eccentrocyte, leaving a slightly ruffled border;
this cellular morphologic abnormality is called a pyknocyte (see Fig. 13-15, B).
Figure 13-15
Common Erythrocyte Morphologic Abnormalities.
A, Blood from a dog that was administered a continuous rate infusion of propofol.
The dog developed oxidant-induced hemolytic anemia with eccentrocytes. Wright-Giemsa
stain. B, Blood from the same dog as in A, showing a pyknocyte. Note the spherocyte-like
appearance of the pyknocyte, except for a small portion of the red blood cell membrane
that is ruffled. Wright-Giemsa stain. C, A schistocyte in the blood of a dog with
mechanical fragmentation hemolysis from disseminated intravascular coagulation. Wright-Giemsa
stain. D, Blood from a dog with hemangiosarcoma, showing an acanthocyte. Wright-Giemsa
stain. E, A keratocyte, exhibiting what appears to be a ruptured “vesicle” in blood
from a dog. Wright-Giemsa stain. F, Blood from a dog with crenation artifact showing
echinocytes. Wright-Giemsa stain. G, Blood from a dog with iron deficiency anemia.
Note the patient's microcytic and hypochromic cell (left) and the normocytic hypochromic
cell (top), as well as the normocytic normochromic erythrocyte (bottom right) from
a recent blood transfusion. Wright-Giemsa stain. H, Blood from a dog. The center erythrocyte
is a target cell, or codocyte. Wright-Giemsa stain. I, Blood from a dog shows a Howell-Jolly
body, which is round, deeply basophilic remnant of the erythrocyte's nucleus. Wright-Giemsa
stain.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University.)
Oxidative insult may also result in conversion of hemoglobin (iron in the Fe2+ state)
to methemoglobin (iron in the Fe3+ state), which is incapable of binding oxygen. Methemoglobin
is produced normally in small amounts but reduced back to oxyhemoglobin by the enzyme
cytochrome-b
5 reductase (also known as methemoglobin reductase). Methemoglobinemia results when
methemoglobin is produced in excessive amounts (because of oxidative insult) or when
the normal pathways for maintaining hemoglobin in the Fe2+ state are impaired (as
in cytochrome-b
5 reductase deficiency). When present in sufficiently high concentration (approximately
10% of total hemoglobin), methemoglobin imparts a grossly discernible chocolate color
to the blood.
By itself, mechanical fragmentation hemolysis tends to cause mild or no anemia. Mechanical
fragmentation results from trauma or shearing of erythrocytes within blood vessels.
Normal erythrocytes may be flowing through abnormal vasculature, such as with heart
valve defects, intravascular fibrin deposition (e.g., disseminated intravascular coagulation),
vasculitis, or hemangiosarcoma. Alternatively, the red blood cells may be particularly
fragile within normal blood vasculature, as occurs with iron deficiency. In either
instance, microscopic evidence of mechanical fragmentation includes the presence of
erythrocyte fragments (schistocytes [see Fig. 13-15, C]), erythrocytes with irregular
cytoplasmic projections (acanthocytes), erythrocytes with blister-like projections
(keratocytes), or ghost cells (see Figs. 13-13, A, 13-15, D, and 13-15, E). Schistocytes
are the only red blood cell morphologic abnormality specific for mechanical fragmentation
because all other morphologic abnormalities can be seen with other disease processes.
For example, ghost cells may be observed with other types of hemolysis.
Nonregenerative anemia is characterized by a lack of reticulocytosis on the CBC; however,
reticulocytosis does not occur in horses even in the context of regeneration. Most
often this is a result of decreased production in the marrow (i.e., erythroid hypoplasia).
Erythrocytes circulate for a long time, so anemias caused by decreased production
tend to develop slowly.
The most common form of nonregenerative anemia is known as anemia of inflammation
or anemia of chronic disease. In this form of anemia, erythrocytes are decreased in
number but are typically normal in size and hemoglobin concentration (so-called normocytic,
normochromic anemia). It has long been known that patients with inflammatory or other
chronic disease often become anemic, and that this condition results in increased
iron stores in the bone marrow. Sequestration of iron may be a bacteriostatic evolutionary
adaptation because many bacteria require iron as a cofactor for growth. In recent
years, investigators have begun to elucidate the molecular mechanisms underlying anemia
of inflammation. Hepcidin, an acute phase protein and antimicrobial peptide synthesized
in the liver, is a key mediator that limits iron availability. Hepcidin expression
increases with inflammation, infection, or iron overload and decreases with anemia
or hypoxia. Hepcidin exerts its effects by causing functional iron deficiency. It
binds to and causes the degradation of the cell surface iron efflux molecule, ferroportin,
thus inhibiting both absorption of dietary iron from the intestinal epithelium and
export of iron from macrophages and hepatocytes into the plasma (Fig. 13-16
).
Figure 13-16
Mechanisms of Anemia in Inflammatory Diseases.
Inflammatory mediators, including interleukin-1 (IL-1), interleukin-6 (IL-6), interferon
(INF), and tumor necrosis factor (TNF), cause anemia of inflammatory disease due to
oxidative hemolysis, iron sequestration within enterocytes and macrophages, and impaired
erythroid responsiveness to erythropoietin (Epo). During homeostasis the membrane
transport molecule, ferroportin, transports iron from the cytosol to the extracellular
space. The iron is then used for various physiologic processes, including hemoglobin
production within bone marrow erythroid precursors. During times of inflammation the
liver increases production of hepcidin, which binds ferroportin and causes its internalization
and lysosomal degradation. With fewer membrane ferroportin molecules, less iron is
absorbed from the diet and mobilized from macrophages. RBC, Red blood cell.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University; and Dr. J.F. Zachary, College of Veterinary Medicine, University
of Illinois.)
Anemia of inflammation involves factors besides decreased iron availability. Inflammatory
cytokines are likely to inhibit erythropoiesis by oxidative damage to and triggering
apoptosis of developing erythroid cells, by decreasing expression of Epo and stem
cell factor, and by decreasing expression of Epo receptors. In addition, experimentally
induced sterile inflammation in cats resulted in shortened erythrocyte survival, indicating
that anemia of inflammation is likely also a function of increased erythrocyte destruction.
Other causes of decreased erythropoiesis are listed in Table 13-2. Specific examples
of diseases causing nonregenerative anemia by these mechanisms are discussed later
in this chapter.
Neutropenia.
Neutropenia refers to a decrease in the concentration of neutrophils in circulating
blood. Neutropenia may be caused by decreased production, increased destruction, altered
distribution, or a demand for neutrophils in tissues that exceeds the rate of granulopoiesis.
Decreased production is evident on bone marrow examination as granulocytic hypoplasia.
This usually results from an insult that affects multiple hematopoietic lineages,
such as chemical insult, radiation, neoplasia, infection, or fibrosis, but may also
be caused by a process that preferentially targets granulopoiesis. In marked contrast
to erythrocytes, neutrophils have a very short life span in circulation. Once released
from the bone marrow, a neutrophil is in the bloodstream only for hours before migrating
into the tissues. When neutrophil production ceases, a reserve of mature neutrophils
in the bone marrow storage pool may be adequate to maintain normal numbers of circulating
neutrophils for a few days; however, after the bone marrow storage pool is depleted,
neutropenia rapidly ensues.
Immune-mediated neutropenia is a rare but recognized condition in domestic animals.
Bone marrow findings range from granulocytic hypoplasia to hyperplasia, depending
on where the cells under immune attack are in their differentiation programs. Neutropenia
with no evidence of decreased production and in which other causes of neutropenia
have been excluded may be a result of destruction of neutrophils before they leave
the bone marrow, a condition known as ineffective granulopoiesis. Like other forms
of ineffective hematopoiesis, this condition is often presumed to be immune mediated;
in cats this condition may occur as a result of infection of hematopoietic cells with
FeLV.
As presented in the earlier section on Granulopoiesis and Monocytopoiesis (Myelopoiesis),
neutrophils within the blood vasculature are in two compartments: a circulating pool,
consisting of those cells flowing freely in the blood, and a marginating pool, consisting
of those cells transiently interacting with the endothelial surface. (In reality,
neutrophils are constantly shifting between these two pools, but the proportion of
cells in either pool normally remains fairly constant in any given species.) Circulating
neutrophils are part of the blood sample collected during routine venipuncture and
are thus counted in the CBC, whereas marginating neutrophils are not. Pseudoneutropenia
refers to the situation in which there is an increased proportion of neutrophils in
the marginating pool. This may occur because of decreased blood flow or in response
to stimuli, such as endotoxemia, that increase expression of molecules promoting interaction
between neutrophils and endothelial cells. This mechanism of neutropenia is rarely
observed in clinical practice.
Neutropenia may also result from increased demand for neutrophils in the tissue. How
rapidly such a situation develops depends not only on the magnitude of the inflammatory
stimulus but also on the reserve of postmitotic neutrophils in the bone marrow. The
size of this reserve, or storage pool, is species dependent. In dogs this pool contains
the equivalent of 5 days' normal production of neutrophils. Cattle represent the other
extreme in that they have a small storage pool and thus are predisposed to becoming
neutropenic during times of acute inflammation. Horses and cats are somewhere between
the two extremes, closer to cattle and dogs, respectively. It stands to reason that
the clinical significance of neutropenia because of a supply and demand imbalance
is also species dependent. In dogs, neutropenia as a result of inflammation is an
alarming finding because it is evidence of a massive tissue demand for neutrophils
that has exhausted the patient's storage pool and is exceeding the rate of granulopoiesis
in the bone marrow. However in cows, neutropenia is commonly noted in a wide range
of conditions involving acute inflammation and does not necessarily indicate an overwhelming
demand.
Eosinopenia/Basopenia.
Eosinopenia and basopenia are decreased concentrations of blood eosinophils and basophils,
respectively. In many laboratories, CBC reference values for eosinophils and basophils
are as low as zero cells per microliter, precluding detection of eosinopenia or basopenia.
When detectable, eosinopenia is often a result of stress (i.e., glucocorticoid mediated).
Monocytopenia.
Monocytopenia denotes a decreased concentration of monocytes in blood; it is of little
to no pathologic significance by itself.
Thrombocytopenia.
Thrombocytopenia refers to a decrease in the concentration of circulating platelets.
Mechanisms of thrombocytopenia include decreased production, increased destruction,
increased consumption, and altered distribution.
Decreased production may occur because of a condition affecting cells of multiple
hematopoietic lineages, including megakaryocytes, or because of one specifically depressing
thrombopoiesis. In either case, decreased thrombopoiesis is evident as megakaryocytic
hypoplasia upon bone marrow examination. General causes of decreased hematopoiesis
outlined earlier in the sections on anemia and neutropenia also apply to thrombocytopenia.
Increased platelet destruction due to immune-mediated thrombocytopenia (IMTP) is a
fairly common disease in dogs and may also occur in other species. Thrombocytopenia
with immune-mediated thrombocytopenia is often severe (e.g., <10,000 platelets/µL),
resulting in spontaneous multisystemic hemorrhage.
Increased use of platelets occurs with hemorrhage and disseminated intravascular coagulation.
Thrombocytopenia secondary to hemorrhage is often mild to moderate, whereas disseminated
intravascular coagulation may cause mild to severe thrombocytopenia, often with evidence
of mechanical fragmentation hemolysis (e.g., schistocytes). Disseminated intravascular
coagulation is a syndrome in which hypercoagulability leads to increased consumption
of both platelets and coagulation factors in the plasma, with subsequent hypocoagulability
and susceptibility to bleeding. Risk factors for developing disseminated intravascular
coagulation include severe inflammation, such as sepsis or pancreatitis, neoplasia,
and organ failure.
The spleen normally contains a significant proportion of total platelet mass (up to
one-third in some species), and abnormalities involving the spleen may result in changes
in the number of circulating platelets. For example, splenic congestion may result
in platelet sequestration and thrombocytopenia, and splenic contraction may cause
thrombocytosis.
Lymphopenia.
Lymphopenia refers to a decreased concentration of lymphocytes in blood. It is a common
hematologic finding in sick animals. Usually the precise mechanism of lymphopenia
is not clear but is often presumed secondary to endogenous glucocorticoid excess that
occurs with stress. Excess glucocorticoids, either endogenous or exogenous, cause
an altered distribution of lymphocytes; there is increased trafficking of lymphocytes
from blood to lymphoid tissue, and decreased egress of lymphocytes from lymphoid tissue
to blood. At higher concentrations of glucocorticoids, lymphocytes are destroyed.
Other causes of lymphotoxicity include chemotherapeutic agents, radiation therapy,
and some infectious agents. Lymphopenia may occur with various mechanisms, including
loss of lymphocyte-rich lymphatic fluid (e.g., gastrointestinal disease, repeated
drainage of chylous effusions), and disruption of the normal lymphoid tissue architecture
because of generalized lymphadenopathy (e.g., lymphoma, blastomycosis). Some hereditary
immunodeficiencies, such as severe combined immunodeficiency or thymic aplasia, can
cause lymphopenia due to lymphoid aplasia.
Erythrocytosis.
An increase in the measured red cell mass above the normal range is known as erythrocytosis.
The term polycythemia is often used interchangeably with erythrocytosis, but technically
and for the purposes of this chapter, polycythemia refers to a specific type of leukemia
called primary erythrocytosis or polycythemia vera.
Causes of erythrocytosis are either relative or absolute. Relative erythrocytosis
results from a fluid deficit or an altered distribution of erythrocytes within the
body (i.e., the body's total erythrocyte mass is not increased). It occurs most frequently
with dehydration, when the decreased proportion of water in the blood results in hemoconcentration.
It is observed less frequently with epinephrine-mediated splenic contraction, wherein
erythrocytes move from the spleen into peripheral circulation. Erythrocytosis from
splenic contraction occurs to the most pronounced degree in horses and cats, especially
in young, healthy animals.
Absolute erythrocytosis is a true increase in red blood cell mass due to erythroid
neoplasia or hyperplasia and includes causes of primary and secondary erythrocytosis.
Primary erythrocytosis, or polycythemia vera, is a neoplastic proliferation of erythroid
cells with a predominance of mature erythrocytes. Diagnosis is based on a marked increase
in red cell mass (hematocrit in normally hydrated dogs ranges from 65% to >80%), an
absence of hypoxemia, an absence of other tumors, and a normal or decreased plasma
Epo concentration.
Secondary erythrocytosis refers to Epo-mediated erythroid hyperplasia causing an increased
red blood cell mass. The erythroid hyperplasia may be an appropriate response to chronic
hypoxia, such as occurs with right-to-left cardiac shunts or chronic pulmonary disease.
Rarely, an Epo-secreting tumor may cause inappropriately elevated levels of Epo in
the absence of hypoxia.
Absolute erythrocytosis, whether primary or secondary, causes increased viscosity
of the blood, resulting in impaired blood flow and microvasculature distention. Affected
individuals are at increased risk for tissue hypoxia, thrombosis, and hemorrhage.
Clinical signs of hyperviscosity syndrome may include erythematous mucous membranes
(Fig. 13-17
), prolonged capillary refill time, prominent scleral vessels, evidence of thrombosis
or hemorrhage, and secondary signs related to specific organ systems affected (e.g.,
neurologic and cardiovascular signs).
Figure 13-17
Absolute Erythrocytosis, Hyperviscosity Syndrome, Erythematous Mucous Membranes, Cat.
Erythema of mucous membranes is one of the signs associated with hyperviscosity syndrome.
In this case the oral mucous membranes are deeper red (arrows) than normal because
of an abnormally high concentration of erythrocytes and associated sludging of blood.
Hyperviscosity syndrome may also occur as the result of increased plasma immunoglobulin
concentration.
(Courtesy Dr. C. Patrick Ryan, Veterinary Public Health, Los Angeles Department of
Health Services; and Noah's Arkive, College of Veterinary Medicine, The University
of Georgia.)
Neutrophilia.
Neutrophilia, an increased blood concentration of neutrophils, occurs in response
to a number of different stimuli, which are not mutually exclusive. Major mechanisms
of neutrophilia are shown in Fig. 13-18
. Understanding the CBC findings characteristic of these responses is an important
part of clinical veterinary medicine. Inflammation can result in neutropenia, as discussed
earlier, or neutrophilia, as discussed next. However, before moving on to a discussion
of inflammatory neutrophilia and the so-called left shift, it is important to mention
two other common causes of neutrophilia: glucocorticoid excess and epinephrine excess.
Less common causes of neutrophilia, such as leukocyte adhesion deficiency and neoplasia,
are discussed later in the chapter.
Figure 13-18
Mechanisms of Neutrophilic Leukocytosis.
1, Neutrophils and their precursors are distributed in five pools: a bone marrow precursor
pool, which includes mitotically active and inactive immature cells; a bone marrow
storage pool, consisting of mitotically inactive mature neutrophils; a peripheral
blood marginating pool; a peripheral blood circulating pool; and a tissue pool. The
relative size of each pool is represented by the size of its corresponding wedge.
The peripheral blood neutrophil count measures only neutrophils within the circulating
peripheral blood pool, which can be enlarged by (2) increased demargination, (3) diminished
extravasation into tissue, (4) increased release of cells from the marrow storage
pool, and (5) expansion of the marrow precursor pool.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University; and Dr. J.F. Zachary, College of Veterinary Medicine, University
of Illinois.)
Glucocorticoid excess, either because of endogenous production or exogenous administration,
results in a CBC pattern known as the stress leukogram, characterized by mature neutrophilia
(i.e., increased concentration of segmented neutrophils without immature neutrophils)
and lymphopenia, with or without monocytosis and eosinopenia. Mechanisms contributing
to glucocorticoid-mediated neutrophilia include the following:
•
Increased release of mature neutrophils from the bone marrow storage pool
•
Decreased margination of neutrophils within the vasculature, with a resulting increase
in the circulating pool
•
Decreased migration of neutrophils from the bloodstream into tissues
The magnitude of neutrophilia tends to be species dependent, with dogs having the
most pronounced response (up to 35,000 cells/µL) and in decreasing order of responsiveness,
cats (30,000 cells/µL), horses (20,000 cells/µL), and cattle (15,000 cells/µL) having
less marked responses. With long-term glucocorticoid excess, neutrophil numbers tend
to normalize, whereas lymphopenia persists.
Epinephrine release results in a different pattern, known as physiologic leukocytosis
or excitement leukocytosis, characterized by mature neutrophilia (like the glucocorticoid
response) and lymphocytosis (unlike the glucocorticoid response). This phenomenon
is short lived (i.e., <1 hour). Neutrophilia occurs primarily because of a shift of
cells from the marginating to the circulating pool. Physiologic leukocytosis is common
in cats (especially when they are highly stressed during blood collection) and horses,
less common in cattle, and uncommon in dogs.
Of course, neutrophilia may also indicate inflammation, and inflammatory stimuli of
varying magnitude and duration produce different patterns of neutrophilia. A classic
hematologic finding in patients with increased demand for neutrophils is the presence
of immature forms in the blood, known as a left shift. Not all inflammatory responses
have a left shift, but the presence of a left shift almost always signifies active
demand for neutrophils in the tissue. The magnitude of a left shift is assessed by
the number of immature cells and their degree of immaturity. The mildest form is characterized
by increased numbers of band neutrophils, the immediate predecessor to the segmented
neutrophil normally found in circulation. Progressively immature predecessors are
seen with increasingly severe inflammation. A left shift is considered orderly if
the number of immature neutrophils in circulation decreases as they become progressively
immature. The term degenerative left shift is sometimes used to describe cases in
which the number of immature forms exceeds the number of segmented neutrophils. As
with glucocorticoid-mediated neutrophilia, the typical magnitude of neutrophilia caused
by inflammation varies by species, with dogs having the most pronounced response.
It might be useful to think of neutrophil kinetics in terms of a producer-consumer
model in which the bone marrow is the factory, and the tissues (where the neutrophils
eventually go) are the customers. The bone marrow storage pool is the factory inventory,
and the neutrophils in the bloodstream are in delivery to the customer. Within the
blood vessels, circulating neutrophils are on the highway, and marginating neutrophils
are temporarily pulled off to the side of the road. During health, there is an even
flow of neutrophils from the factory to the customer. Thus the system is in steady
state, and neutrophil numbers remain relatively constant and within the normal range.
However, disease states may perturb this system at multiple levels. Decreased granulopoiesis
is analogous to a factory working below normal production level. Ineffective granulopoiesis
is analogous to goods that are produced at a normal to increased rate but are damaged
during manufacturing and never leave the factory. A left shift is analogous to the
factory meeting increased customer demand by shipping out unfinished goods. Cases
of persistent, established inflammation are characterized by bone marrow granulocytic
hyperplasia and mature neutrophilia, analogous to a factory that has had time to adjust
to increased demand and is meeting it more efficiently by increasing its output.
Eosinophilia/Basophilia.
Eosinophilia and basophilia are increased concentrations of blood eosinophils and
basophils, respectively. They may occur with parasitism, hypersensitivity reactions,
paraneoplastic responses (e.g., lymphoma, mast cell neoplasia, or leukemia), and nonparasitic
infectious disease. Eosinophilia has also been documented with hypoadrenocorticism
and rare idiopathic conditions (e.g., hypereosinophilic syndrome). Most cases of eosinophilia
and basophilia are due to eosinophilic and basophilic hyperplasia within the bone
marrow in response to inflammatory growth factors. However, cortisol deficiency is
thought to cause eosinophilia in dogs with hypoadrenocorticism.
Monocytosis.
Monocytosis is an increased concentration of monocytes in blood. It most commonly
occurs with excessive glucocorticoids or inflammation and uncommonly to rarely with
monocytic leukemia, immune-mediated neutropenia, and cyclic hematopoiesis. With excessive
endogenous or exogenous glucocorticoids, monocytes shift from the marginating pool
to the circulating pool. This stress monocytosis is most common in dogs, less frequent
in cats, and rare in horses and cattle. Inflammatory diseases cause monocytosis by
cytokine-mediated monocytic hyperplasia in the bone marrow.
Thrombocytosis.
Thrombocytosis, or an increased concentration of platelets in the blood, is a relatively
common, nonspecific finding in veterinary patients. In the vast majority of cases,
thrombocytosis is reactive—a response to another, often apparently unrelated, disease
process. Examples of conditions having reactive thrombocytosis include inflammatory
and infectious diseases, iron deficiency, hemorrhage, endocrinopathies, and neoplasia.
Factors that may contribute to reactive thrombocytosis include increased plasma concentration
of thrombopoietin, inflammatory cytokines (e.g., IL-6), or catecholamines. Thrombocytosis
may also occur as part of a regenerative response in patients recovering from thrombocytopenia,
as a result of redistribution after splenic contraction, or within the several weeks
after splenectomy. In these cases, thrombocytosis is transient. In the case of splenectomy,
thrombocytosis may be marked but normalizes after several weeks. Because the body's
total platelet mass regulates thrombopoiesis, and a significant portion of the platelet
mass is normally in the spleen, it makes sense that splenectomized animals develop
thrombocytosis. However, the reason that the number of circulating platelets normalizes
in these individuals in the weeks after splenectomy is not clear. There is also a
rare form of megakaryocytic leukemia known as essential thrombocythemia, which is
characterized by marked thrombocytosis.
Lymphocytosis.
Lymphocytosis refers to an increase in the concentration of lymphocytes in blood circulation.
There are several causes of lymphocytosis, including age, excessive epinephrine, chronic
inflammation, hypoadrenocorticism, and lymphoid neoplasia; lymphoid neoplasms are
presented later in the chapter. Young animals normally have higher concentrations
of lymphocytes than older animals, and normal healthy young animals may have counts
that exceed adult reference values. Because this is not pathologic lymphocytosis,
but normal physiologic variation, it is often termed pseudolymphocytosis of young
animals. As discussed earlier in the section on neutrophilia, lymphocytosis is also
a feature of epinephrine-mediated physiologic leukocytosis, resulting from redistribution
of lymphocytes from the blood marginating pool into the blood circulating pool. Epinephrine-mediated
lymphocytosis may be more marked than neutrophilia, particularly in cats (lymphocyte
counts of > 20,000/µL are not uncommon). Antigenic stimulation may result in lymphocytosis,
which may be marked in rare cases (up to approximately 30,000/µL in dogs and 40,000/µL
in cats); however, this is not usually the case, even when there is clear evidence
of increased immunologic activity in lymphoid tissues. In cases of antigenic stimulation,
it is common for a minority of lymphocytes to have “reactive” morphologic features—larger
lymphocytes with more abundant, deeply basophilic cytoplasm and more open chromatin
(Fig. 13-19
). Just as glucocorticoid excess can cause lymphopenia, glucocorticoid deficiency
(hypoadrenocorticism) can cause lymphocytosis, or lack of lymphopenia during conditions
of stress that typically result in glucocorticoid-mediated lymphopenia.
Figure 13-19
Lymphocytosis (B), Lymphocytes, Canine Blood Smear.
A, Small lymphocytes, the predominant type of lymphocyte in the blood under normal
conditions. B, A reactive lymphocyte, characterized by mildly increased size and an
increased amount of basophilic cytoplasm, from a recently vaccinated 16-week-old dog.
Wright's stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
A condition known as persistent lymphocytosis (PL) occurs in approximately 30% of
cattle infected with the bovine leukemia virus (BLV). The condition is defined as
an increase in the blood concentration of lymphocytes above the reference interval
for at least 3 months. This form of lymphocytosis is a nonneoplastic proliferation
(i.e., hyperplasia) of B lymphocytes. In the absence of other disease, cattle with
persistent lymphocytosis are asymptomatic. However, cattle infected with BLV, especially
those animals with persistent lymphocytosis, are at increased risk for developing
B lymphocyte lymphoma.
Secondary Abnormal Structure or Function of Blood Cells.
The preceding section focused on abnormalities in the number of blood cells. There
are also various acquired and congenital conditions involving abnormal structure or
function of blood cells. This section briefly discusses abnormal blood cell structure
or function occurring secondary to other underlying disease. Primary disorders of
blood cells are discussed later in the chapter in the section on specific diseases.
Morphologic abnormalities detected on routine microscopic examination of blood smears
may provide important clues about underlying disease processes. Poikilocytosis is
a broad term referring to the presence of abnormally shaped erythrocytes in circulation.
E-Table 13-1 lists conditions with and mechanisms involved in the formation of a number
of specific types of erythrocyte morphologic abnormalities, and Fig. 13-15 shows some
examples.
E-Table 13-1
Common Erythrocyte Morphologic Abnormalities
Term
Description
Common Causes or Conditions
Macrocyte
Abnormally large
Regenerative anemiaFeline leukemia virus infection (cats)Congenital in some poodle
dogs
Microcyte
Abnormally small
Iron deficiencyPortosystemic shuntsNormal in Akita and Shiba dogs
Polychromasia
Bluish color (see Figs. 13-5 and 13-8)
Reticulocytosis (erythroid hyperplasia)
Basophilic stippling
Fine punctate inclusions (see Fig. 13-8)
Lead toxicityRegenerative anemia (especially ruminants)
Howell-Jolly body
Small, round, blue-black inclusion, usually off center (see Fig. 13-15)
Regenerative anemiaSplenic dysfunctionCongenital in some poodle dogs
Hypochromasia
Increased central pallor (see Fig. 13-15)
Iron deficiency
Heinz body
Pale round inclusion, may bulge from cell margin; stains blue with new methylene blue
(see Fig. 13-14)
Oxidative damage
Poikilocyte
Nonspecific term for shape abnormality—specific types of poikilocytosis listed below
See below
Spherocyte
Appears abnormally small with uniform staining (see Fig. 13-13)
Extravascular hemolysis
Schistocyte
Small, irregular RBC fragment, often crescent shaped (see Fig. 13-15)
Microangiopathies (e.g., disseminated intravascular coagulation, hemangiosarcoma,
vasculitis)Increased fragility (e.g., iron deficiency)
Acanthocyte
Few irregular projections (see Fig. 13-15)
Abnormal lipid metabolism (e.g., liver disease)Microangiopathies (as per schistocytes)Increased
fragility (as per schistocytes)Common normal finding in young goats
Echinocyte
Many relatively uniform projections (see Fig. 13-15)
Usually in vitro artifactNormal in pigsSome envenomations
Eccentrocyte
Eccentric staining (see Fig. 13-15)
Oxidative damage
Leptocyte
Thin, often hypochromic and/or folded
ReticulocytosisIron deficiency
Codocyte (“target cell”)
Type of leptocyte with area of dense staining within central pallor (see Fig. 13-15)
ReticulocytosisLiver disease
Keratocyte
Intact or ruptured “blister” (see Fig. 13-15)
Conditions causing schistocytosis and/or acanthocytosis
Stomatocyte
Type of leptocyte with linear central pallor
Reticulocytosis
RBC, Red blood cell.
The acquired neutrophil morphologic abnormality known as toxic change (Fig. 13-20
) reflects accelerated production of neutrophils as part of the inflammatory response.
Features of toxic change include increased cytoplasmic basophilia, the presence of
small blue-gray cytoplasmic inclusions known as Döhle bodies (often noted incidentally
in cats), and in more severe cases, cytoplasmic vacuolation. Although not causing
impaired neutrophil function, toxic change occurs during granulopoiesis and thus is
technically a form of dysplasia (e.g., Döhle bodies are foci of aggregated endoplasmic
reticulum). Toxic change may accompany any inflammatory response, but in general the
more marked the toxic change, the higher the index of suspicion for infection or endotoxemia.
Other secondary changes to neutrophils may not be evident morphologically. For example,
studies in human beings and dogs have shown that individuals with cancer have abnormal
neutrophil function (including phagocytic activity, killing capacity, and oxidative
burst activity) before initiation of therapy. The clinical significance of this finding
is not clear.
Figure 13-20
Toxic Change, Neutrophils, Canine Blood Smear.
Two band neutrophils with basophilic, foamy cytoplasm indicate toxic change. This
dog also has reactive thrombocytosis. Wright's stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
Platelet function disorders, also known as thrombopathies or thrombopathias, may be
primary or secondary. Many conditions are known or suspected to cause secondary platelet
dysfunction (hypofunction or hyperfunction) by altering platelet adhesion or aggregation
or by mechanisms that are not fully understood. Box 13-2
shows underlying conditions having secondary platelet dysfunction.
Box 13-2
Conditions Known or Suspected to Cause Secondary Platelet Dysfunction in Animals
Secondary Platelet Hypofunction
Underlying Disease
Uremia
Antiplatelet antibodies (also cause immune-mediated thrombocytopenia)
Infection (BVDV, FeLV)
Hyperglobulinemia
Increased fibrinolytic products
Hypoammonemia
Snake envenomation
Drugs or Other Exogenous Agents
Platelet inhibitors
NSAIDs—irreversible (aspirin) or reversible inhibition of cyclooxygenase
Colloidal plasma expanders (e.g., hydroxyethyl starch)
Other drugs and exogenous agents (many)
Secondary Platelet Hyperfunction
Underlying Disease
Infection (heartworm and RMSF in dogs, FIP, pasteurellosis in cattle)
Inflammation
Neoplasia
Taurine deficiency in cats
Nephrotic syndrome
BVDV, Bovine viral diarrhea virus; FeLV, feline leukemia virus; FIP, feline infectious
peritonitis, NSAIDs, nonsteroidal antiinflammatory drugs; RMSF, Rocky Mountain spotted
fever.
Portals of Entry/Pathways of Spread
Invading cells or microorganism gain access to the bone marrow or blood circulation
either hematogenously or by trauma. Trauma may be as obvious as a gaping wound or
as subtle as the bite of an insect. Portals of entry for the bone marrow are summarized
in Box 13-3
. Diseases that arise from the bone marrow, such as leukemia, typically spread to
other tissues hematogenously.
Box 13-3
Portals of Entry into Bone Marrow
Bone Marrow
Hematogenously
Direct penetration (trauma)
Defense Mechanisms/Barrier Systems
The bone marrow is encased by a protective shell of cortical bone, and blood supply
to the marrow provides access to systemic humoral and cellular defenses. Of course,
leukocytes themselves function as an essential part of inflammation and immune function,
as discussed briefly in the section on Granulopoiesis and Monocytopoiesis (Myelopoiesis)
and in greater detail in Chapters 3 and 5.
Biochemical steps in the glycolytic pathway or linked to it generate antioxidant molecules
that enable erythrocytes to withstand oxidative insults throughout their many days
in circulation. In addition to producing energy in the form of adenosine triphosphate
(ATP), glycolysis generates NADH, which helps convert the oxidized, nonfunctional
form of hemoglobin, known as methemoglobin, back to its active, reduced state. Another
antioxidant erythrocyte metabolic pathway, the pentose shunt or hexose monophosphate
shunt, generates NADPH to help maintain glutathione in the reduced state.
Disorders of Domestic Animals
Aplastic Anemia (Aplastic Pancytopenia)
Aplastic anemia, or more accurately aplastic pancytopenia, is a rare condition characterized
by aplasia or severe hypoplasia of all hematopoietic lineages in the bone marrow with
resulting cytopenias. The term aplastic anemia is a misnomer because affected cells
are not limited to the erythroid lineage.
Many of the conditions reported to cause aplastic anemia do so only rarely or idiosyncratically;
more frequently, they cause other hematologic or nonhematologic abnormalities. A partial
list of reported causes of aplastic anemia in domestic animals includes the following:
•
Chemical agents
•
Antimicrobial agents (dogs, cats)
•
Chemotherapeutic agents (dogs, cats)
•
Phenylbutazone (horses, dogs)
•
Bracken fern (cattle, sheep)
•
Estrogen (dogs)
•
Trichloroethylene (cattle, sheep)
•
Aflatoxin B1 (horses, cattle, dogs, pigs)
•
Infectious agents
•
Ehrlichia (Ehrlichiosis [dogs, cats])
•
Parvovirus (dogs, cats)
•
FeLV (cats)
•
Feline immunodeficiency virus (cats)
•
Lentivirus (Equine infectious anemia [horses])
•
Idiopathic (horses, cattle, dogs, cats)
Most of these causes, especially the chemical agents, are directly cytotoxic to HSCs
or progenitor cells, resulting in their destruction. However, another proposed mechanism
is disruption of normal stem cell function because of mutation or perturbation of
hematopoietic cells and/or their microenvironment. This pathogenesis is mostly recognized
in retroviral infections.
Aplastic anemia occurs in both acute and chronic forms. Most of the chemical causes
result in acute disease. Grossly, affected animals may show signs of multisystemic
infection and hemorrhage due to severe neutropenia and thrombocytopenia, respectively.
Severe neutropenia typically develops within 1 week of an acute insult to the bone
marrow, and severe thrombocytopenia occurs in the second week. This sequence is a
result of the circulating life spans of each cell type; in health, neutrophils have
a blood half-life of 5 to 10 hours, whereas platelets circulate for 5 to 10 days.
The development of signs of anemia, such as pale mucous membranes, is more variable.
The presence and severity of anemia depends on how rapidly the marrow recovers from
the insult and the erythrocyte life span of the particular species.
Microscopically, bone marrow is hypocellular with markedly reduced hematopoietic cells.
Hematopoietic cells are replaced with adipose tissue, and there is a variable inflammatory
infiltrate of lymphocytes, plasma cells, and macrophages. In addition, there may be
necrosis, hematopoietic cell apoptosis, and an increase in phagocytic macrophages.
Fig. 13-21
shows bone marrow aspirates from a dog with pancytopenia from acute 5-fluorouracil
toxicosis, before and during recovery.
Figure 13-21
Aplastic Anemia, Canine Bone Marrow Aspirate.
A, Bone marrow aspirate from a dog 8 days after ingestion of a toxic dose of 5-fluorouracil
shows stromal cells but a lack of developing blood cells. B, Bone marrow aspirate
from the same dog 1 week later, after resumption of hematopoiesis. Inset, Higher magnification
of Figure 13-21, B, shows early- and late-stage erythroid and granulocytic precursors.
Wright's stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
Congenital Disorders
Many inherited or presumably inherited disorders of blood cells have been recognized
in domestic animals, including rare or sporadic cases and conditions that are of questionable
clinical relevance. This section and the later sections covering species-specific
disorders are not comprehensive but instead focus on the more common, well-characterized,
or recently reported conditions.
Erythropoietic Porphyrias.
Porphyrias are a group of hereditary disorders in which porphyrins accumulate in the
body because of defective heme synthesis. Inherited enzyme defects in hemoglobin synthesis
have been identified in Holstein cattle, Siamese cats, and other cattle and cat breeds,
resulting in bovine congenital erythropoietic porphyria and feline erythropoietic
porphyria, respectively. Accumulation of toxic porphyrins in erythrocytes causes hemolytic
anemia, whereas accumulation of porphyrins in tissues and fluids produces discoloration,
including red-brown teeth, bones, and urine (see Fig. 1-59). Because of the circulation
of the photodynamic porphyrins in blood, these animals have lesions of photosensitization
of the nonpigmented skin. All affected tissues, including erythrocytes, exhibit fluorescence
with ultraviolet light. Histologically, animals may exhibit perivascular dermatitis,
as well as multisystemic porphyrin deposition, hemosiderosis, EMH, and marrow erythroid
hyperplasia. Cats may show evidence of renal disease, including hypercellular glomeruli,
thickened glomerular and tubular basement membranes, and tubular epithelial lipidosis,
degeneration, and necrosis. Other porphyrias have been diagnosed in cattle, pigs,
and cats but are not known to cause hemolytic anemia.
Pyruvate Kinase Deficiency.
Pyruvate kinase (PK) deficiency is an inherited autosomal recessive condition due
to a defective R-type PK isoenzyme that is normally present in high concentrations
in mature erythrocytes. To compensate for this deficiency, there is persistence of
the M2-type PK isoenzyme, which is less stable than the R-type isoenzyme. The disease
is reported in many dog breeds and fewer cat breeds (e.g., Abyssinian, Somali, and
domestic shorthair). Erythrocyte PK deficiency results in decreased ATP production
and shortened erythrocyte life spans. In dogs the hemolytic anemia is typically chronic,
moderate to severe, extravascular, and strongly regenerative. With chronicity, hemolytic
anemia causes enhanced intestinal absorption of iron and subsequent hemosiderosis,
especially of the liver and bone marrow. Dogs typically die at 1 to 5 years of age
of hemochromatosis-induced liver and bone marrow failure. However, cats with PK deficiency
typically show no clinical signs, have milder anemia, and do not develop organ failure.
Grossly, affected animals have lesions attributed to hemolytic anemia, including splenomegaly,
pale mucous membranes, and rarely icterus. Dogs with end-stage disease have cirrhosis,
myelofibrosis, and osteosclerosis. Dogs with PK deficiency do not necessarily have
the same genetic defect, so mutation-specific DNA-based assays are required. In contrast,
a single DNA-based test is available to detect the common mutation affecting Abyssinian,
Somali, and domestic shorthair cats.
Cytochrome-b5
Reductase Deficiency.
Deficiency of cytochrome-b
5 reductase (Cb5R, also known as methemoglobin reductase), the enzyme that catalyzes
the reduction of methemoglobin (Fe3+) to hemoglobin (Fe2+), has been recognized in
many dog breeds and in domestic shorthair cats. It is probably an autosomal recessive
trait. Affected animals may have cyanotic mucous membranes or exercise intolerance
but usually lack anemia and clinical signs of disease. Life expectancies are normal.
Glucose-6-Phosphate Dehydrogenase Deficiency.
Deficiency of glucose-6-phosphate dehydrogenase (G6PD), the rate-controlling enzyme
of the pentose phosphate pathway (PPP), has been reported in an American saddlebred
colt, its dam, and one male dog. The PPP is an antioxidative pathway that generates
NADPH, which maintains glutathione in its reduced form (GSH). Therefore in animals
with G6PD deficiency, oxidants are not scavenged, and erythrocyte oxidative injury
occurs. The colt with G6PD deficiency had severe oxidative hemolytic anemia with eccentrocytes
on blood smear evaluation. However, the colt's dam only had eccentrocytes, and showed
no hematologic signs of disease.
Leukocyte Adhesion Deficiency.
Leukocyte adhesion deficiency (LAD) is a fatal autosomal recessive defect of leukocyte
integrins, in particular the β2 chain (also known as cluster of differentiation [CD]
18 [CD18]). Disease has been recognized in Holstein cattle (known as bovine leukocyte
adhesion deficiency [BLAD]) and Irish setter dogs (known as canine leukocyte adhesion
deficiency [CLAD]) (see Chapter 3). Without normal expression of this adhesion molecule,
leukocytes have severely impaired abilities to migrate from the blood into tissues.
As a result, animals with leukocyte adhesion deficiency have marked neutrophilia with
nonsuppurative multisystemic infections. Blood neutrophils often have nuclei with
greater than five nuclear segments, termed hypersegmented neutrophils, due to neutrophil
aging within blood vessels (Fig. 13-22
). These animals are highly susceptible to infections and usually die at a young age.
Figure 13-22
Leukocyte Adhesion Deficiency, Canine Blood Smear.
Neutrophils in animals with leukocyte adhesion deficiency cannot migrate into the
tissues, resulting in marked neutrophilia and morphologic signs of aging, such as
nuclear hypersegmentation (arrow). Wright-Giemsa stain.
(Courtesy Dr. K.M. Boes and Dr. K. Zimmerman, College of Veterinary Medicine, Virginia
Polytechnic Institute and State University.)
Pelger-Huët Anomaly.
Pelger-Huët anomaly (PHA) is a condition of hyposegmented granulocytes due to a lamin
B receptor mutation. It has been described in dogs, cats, horses, and rabbits, especially
in certain breeds. In Australian shepherd dogs the mode of inheritance is autosomal
dominant with incomplete penetrance. Most cases of Pelger-Huët anomaly are the heterozygous
form and of no clinical significance. However, skeletal abnormalities, stillbirths,
and/or early mortality may accompany Pelger-Huët anomaly in rabbits and cats, especially
homozygotes. In Pelger-Huët anomaly the nuclei of neutrophils, eosinophils, and basophils
fail to segment, resulting in band-shaped, bean-shaped, or round nuclei. Although
the nuclear shape is similar to that of an inflammatory left shift, healthy animals
with Pelger-Huët anomaly do not have clinical signs or other laboratory findings indicating
inflammation. For example, neutrophils in healthy animals with Pelger-Huët anomaly
have mature (clumped) chromatin and do not show signs of toxicity (Fig. 13-23
). An acquired, reversible condition mimicking Pelger-Huët anomaly, known as pseudo–Pelger-Huët
anomaly, is occasionally noted in animals with infectious disease, neoplasia, or drug
administration.
Figure 13-23
Pelger-Huët Anomaly, Feline Blood Smear.
Eosinophil (A) and neutrophil (B) have hyposegmented nuclei with mature, condensed
chromatin. Wright's stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
Chédiak-Higashi Syndrome.
Chédiak-Higashi syndrome (CHS) is a rare autosomal recessive defect in the lysosomal
trafficking regulator (LYST) protein. The syndrome has been identified in Hereford,
Brangus, and Japanese black cattle, Persian cats, and several nondomestic species.
The defective LYST protein results in granule fusion in multiple cell types, including
granulocytes, platelets, and melanocytes, as well as abnormal cell function. Individuals
with Chédiak-Higashi syndrome have severely impaired cellular innate immunity because
of neutropenia, impaired leukocyte chemotaxis, and impaired killing by granulocytes
and cytotoxic lymphocytes. Platelets lack the dense granules that normally contain
key bioactive molecules involved in hemostasis, including platelet agonists, such
as ADP and serotonin. In vitro platelet aggregation is severely impaired. As a result,
animals with Chédiak-Higashi syndrome exhibit oculocutaneous albinism (due to altered
distribution of melanin granules) and are prone to infection and bleeding. Blood smear
evaluation reveals granulocytes with large cytoplasmic granules.
Glanzmann Thrombasthenia.
Glanzmann thrombasthenia (GT) is an inherited platelet function defect caused by a
mutated αIIb subunit of the integrin αIIbβ3 (also known as glycoprotein IIb-IIIa [GPIIb-IIIa]).
The disorder has been recognized in Great Pyrenees and otterhound dogs and several
horse breeds, including a quarter horse, a standardbred, a thoroughbred-cross, a Peruvian
Paso mare, and an Oldenburg filly. The αIIbβ3 molecule has multiple functions but
is best known as a fibrinogen receptor that is essential for normal platelet aggregation.
Bleeding tendencies vary widely between affected individuals but mainly occur on mucosal
surfaces. The condition is characterized by an in vitro lack of response to all platelet
agonists and severely impaired clot retraction (i.e., whole blood samples without
anticoagulant often fail to clot). Molecular testing is available to detect diseased
or carrier states in dogs and horses.
CalDAG-GEFI Thrombopathia.
Calcium diacylglycerol guanine nucleotide exchange factor I (CalDAG-GEFI) is a molecule
within the signaling pathway that results in platelet activation in response to platelet
agonists. Mutated CalDAG-GEFI has been documented in basset hound, Eskimo spitz, and
Landseer dogs, and Simmental cattle. All reported mutations have a bleeding tendency.
In vitro platelet aggregation responses to platelet agonists, such as ADP, collagen,
and thrombin, are absent or impaired.
von Willebrand Disease
von Willebrand disease (vWD) is the most common canine hereditary bleeding disorder
and has also been described in many other domestic species. The disease actually refers
to a group of inherited conditions characterized by a quantitative or qualitative
deficiency of vWF. This factor is a multimeric glycoprotein that is stored in platelet
α-granules and endothelial cells and circulates as a complex with coagulation factor
VIII. Its primary functions are to stabilize factor VIII and mediate platelet binding
to other platelets and subendothelial collagen. Although not technically a platelet
disorder, von Willebrand disease is often classified as such because it results in
a loss of normal platelet function. Different types of von Willebrand disease vary
in terms of mode of inheritance and severity of clinical disease. Type I von Willebrand
disease is characterized by low plasma vWF concentration but normal multimeric proportions
and a mild to moderate clinical bleeding tendency; it has been reported in many dog
breeds. Type II von Willebrand disease is characterized by low vWF concentration,
absence of large multimers, and a moderate to severe bleeding tendency; it has been
reported in German short-haired pointer and German wirehaired pointer dogs. Type III
von Willebrand disease is characterized by absence of vWF and a severe bleeding tendency;
familial and sporadic cases have been reported in numerous dog breeds. The buccal
mucosal bleeding time is prolonged with von Willebrand disease, often with adequate
concentrations of platelets and normal prothrombin time and partial thromboplastin
time (PTT). However, PTT may be mildly prolonged because vWF stabilizes factor VIII,
and deficiency of vWF results in enhanced factor VIII degradation. Grossly, affected
animals exhibit bleeding tendencies, especially in the form of gingival bleeding,
epistaxis, and hematuria or at sites of injections, venipuncture, or surgery.
Hereditary Coagulation Factor Deficiencies
Inherited coagulation factor deficiencies have been documented in most domestic species,
including deficiencies of prekallikrein and factors I, II, VII, VIII, IX, X, XI, and
XII. Of these disorders, hereditary coagulation factor VIII (hemophilia A) and factor
IX (hemophilia B) deficiencies are most common. Hemophilia A has been recognized in
horses, cattle, dogs, and cats, and hemophilia B occurs in dogs and cats. Both disorders
have an X-linked recessive mode of inheritance, meaning that clinical disease is more
common in males. Affected males have variable tendencies to bleed, depending on the
severity of the deficiency, exposure to trauma, and size and activity level of the
affected individual. Carrier females are usually asymptomatic. Laboratory tests often
reveal adequate platelets, normal prothrombin times, and prolonged partial thromboplastin
times.
Hereditary γ-Glutamyl Carboxylase Defect
Hereditary defects in γ-glutamyl carboxylase, the enzyme required for normal carboxylation
of vitamin K–dependent coagulation factors, have been recognized in a flock of Rambouillet
sheep and two Devon rex cats from the same litter. The genetic defect is not known
in cats, but in sheep it is an autosomal recessive trait that results in a premature
stop codon and truncated γ-glutamyl carboxylase. In sheep there is increased lamb
mortality with excessive bleeding during parturition, especially through the umbilicus
or into subcutaneous tissues.
Toxicoses
Oxidative Agents.
A variety of oxidative toxins cause hemolytic anemia and/or methemoglobinemia in domestic
species. More common or well-characterized oxidants are listed here:
•
Horses—Acer rubrum (red maple)
•
Ruminants—Brassica spp. (cabbage, kale, and rape), copper
•
Dogs—Acetaminophen, propofol, zinc
•
Cats—Acetaminophen, propofol, propylene glycol
•
All species—Allium spp. (chives, garlic, and onions)
In horses, red maple leaves and bark are toxic, especially wilted or dried leaves.
The toxic principle is believed to be gallic acid. Plants that contain high concentrations
of nitrates, such as cabbage, kale, and rape, may cause oxidative injury to erythrocytes;
cattle are more susceptible than sheep and goats. However, sheep are more prone to
copper toxicosis relative to other ruminants. The condition occurs in animals that
have chronically accumulated large amounts of copper in the liver through the diet.
The copper is then acutely released during conditions of stress, such as shipping
or starvation. Continuous rate infusions of the anesthetic propofol may cause oxidative
hemolytic anemia in dogs and cats, but single or multiple single doses are not expected
to cause clinical hemolysis. Zinc toxicosis has been identified in a wide range of
animals; however, it is most common in dogs due to their indiscriminate eating habits.
Common sources include pennies, batteries, paints, creams, automotive parts, screws,
nuts, and coating on galvanized metals. Propylene glycol is an odorless, slightly
sweet solvent and moistening agent in many foods, drugs, and tobacco products. Although
it is “generally recognized as safe” for animal foods other than for cats by the Food
and Drug Administration, it has been banned from cat food since 1996.
Grossly and microscopically, animals show varying signs of oxidative hemolysis and/or
methemoglobinemia, as previously presented in the section discussing anemias (see
Bone Marrow and Blood Cells, Dysfunction/Responses to Injury, Blood Cells, Abnormal
Concentrations of Blood Cells, Anemia). In sheep with copper toxicosis, hemoglobinuric
nephrosis, frequently described as gunmetal-colored kidneys with port wine–colored
urine, is a classic postmortem lesion.
Snake Envenomation.
Hemolytic anemia from snake envenomation has been reported in horses, dogs, and cats.
It is most commonly reported with viper and pit viper envenomations, including those
from rattlesnakes. Hemolysins within viper venom directly injure erythrocytes, causing
intravascular hemolysis. Other mechanisms of hemolysis include the action of phospholipase
A2 on erythrocyte membranes and erythrocyte mechanical fragmentation due to intravascular
coagulation and vasculitis. Nonhemolytic lesions depend on the venom's additional
components and may include hemorrhage, paralysis, and/or tissue edema, inflammation,
and necrosis. On blood smear evaluation, animals with snake envenomation may have
ghost cells, spherocytes, and/or echinocytes (see Figs. 13-13 and 13-15).
Avitaminosis K
Antagonism of vitamin K leads to production of a nonfunctional form of some coagulation
factors and resulting coagulopathy; a similar condition results from vitamin K deficiency.
Conditions with avitaminosis K include poisoning with coumarin-related molecules,
fat malabsorption (vitamin K is a fat-soluble vitamin) caused by primary intestinal
disease or impaired biliary outflow (uncommon), dietary deficiency (rare), and antibiotics
that interfere with vitamin K absorption or usage.
A number of coagulation factors—factors II, VII, IX, and X (collectively known as
the vitamin K–dependent factors), as well as the regulatory molecules protein C and
protein S—must undergo carboxylation to be functional. This posttranslational modification
is catalyzed by the enzyme γ-glutamyl carboxylase, and requires vitamin K as a cofactor.
Vitamin K is oxidized during the carboxylation reaction and is converted back into
its active reduced form by the enzyme vitamin K epoxide reductase. Coumarin-related
rodenticides, such as warfarin, act by inhibiting vitamin K epoxide reductase, resulting
in an absence of vitamin K in its active reduced form (E-Fig. 13-2). This inhibition
lasts until the rodenticide is metabolized and cleared. How long this takes depends
on the half-life of the rodenticide and dose, but it may take many weeks. Second-generation
rodenticides, such as bromadoline and brodifacoum, are more potent than warfarin,
with longer half-lives. Spoiled sweet clover contains dicumarol, which causes coagulopathy
by the same mechanism.
E-Figure 13-2
Mechanism of Anticoagulant Rodenticide Toxicity.
Anticoagulant rodenticides inhibit the enzyme that converts vitamin K back to its
active reduced form.
Laboratory findings include prolonged coagulation times (prothrombin time [PT], PTT,
and activated clotting time [ACT]). Early in the course of rodenticide and related
toxicoses, PT may be the only one of these tests that is prolonged because factor
VII has the shortest half-life of the vitamin K–dependent factors. However, the other
tests become prolonged as nonfunctional forms of the other factors accumulate. In
uncomplicated cases, patients are not thrombocytopenic.
A wide range of hemorrhagic lesions may occur in affected individuals, including ecchymoses,
epistaxis, gingival bleeding, hematomas, hemoptysis, melena or hematochezia, hematuria,
and other forms of hemorrhage. There are also lesions with regenerative anemia, such
as pale mucous membranes and splenomegaly. Histologically, there is hemorrhage, EMH,
and marrow erythroid hyperplasia.
The treatment of cases of rodenticide and related toxicoses is regular administration
of exogenous vitamin K1 until the toxin is cleared (determined by repeat coagulation
testing after withholding treatment).
Nutritional and Metabolic Disorders
Severe malnutrition is probably a cause of nonregenerative anemia in all species attributable
to combined deficiencies of molecular building blocks, energy, and essential cofactors.
By far the most commonly recognized specific deficiency that results in anemia is
iron deficiency. Other specific nutritional deficiencies causing anemia in animals
are uncommon or rare. Acquired cobalamin (vitamin B12) and folate deficiencies are
recognized as causes of anemia in human beings but are rare in animals.
Iron Deficiency Anemia.
Iron deficiency is usually not a primary nutritional deficiency but rather occurs
secondary to depletion of iron stores via chronic blood loss. The most common route
of loss is through the gastrointestinal tract (e.g., neoplasia in older animals or
hookworm infection in puppies). Chronic blood loss may also be caused by marked ectoparasitism
(e.g., pediculosis in cattle or massive flea burden in kittens and puppies), neoplasia
in locations other than the gastrointestinal tract (e.g., cutaneous hemangiosarcoma),
coagulation disorders, and repeated phlebotomy of blood donor animals. Rapidly growing
nursing animals may be iron deficient when compared with adults because milk is an
iron-poor diet. In most cases this has little clinical significance (and in fact is
normal). An important exception is piglets with no access to iron, which may cause
anemia, failure to thrive, and increased mortality. Neonatal piglets are routinely
given parenteral iron for this reason. Copper deficiency can cause iron deficiency
in ruminants and may occur because of copper-deficient forage or impaired usage of
copper by high dietary molybdenum or sulfate. It is believed that copper deficiency
impairs production of ceruloplasmin, a copper-containing enzyme involved in gastrointestinal
iron absorption.
Iron deficiency causes anemia by impaired hemoglobin synthesis. Iron is an essential
component of hemoglobin, and when it is absent, hemoglobin synthesis is depressed.
Because erythrocyte maturation is dependent upon obtaining a critical hemoglobin concentration,
maturing erythroid precursors undergo additional cell divisions during iron-deficient
states. These additional cell divisions result in small erythrocytes, termed microcytes
(see Fig. 13-15, G). However, erythrocytes with low hemoglobin concentrations are
produced when microcyte formation can no longer compensate for iron deficiency. The
classic hematologic picture with iron deficiency anemia is microcytic (i.e., decreased
MCV), hypochromic (i.e., decreased MCHC) anemia. Microcytes and hypochromasia (see
Fig. 13-15, G) may also be discernible on blood smear examination as erythrocytes
that are abnormally small and paler-staining, respectively. Early iron deficiency
anemia is poorly regenerative, whereas continued hemorrhage and iron loss cause nonregenerative
anemia. Additional hematologic changes may include evidence of erythrocyte mechanical
fragmentation (e.g., schistocytes) and reactive thrombocytosis.
Hypophosphatemic Hemolytic Anemia.
Marked hypophosphatemia is recognized as a cause of intravascular hemolytic anemia
in postparturient dairy cows and diabetic animals receiving insulin therapy. In postparturient
cows, hypophosphatemia results from increased loss of phosphorus in their milk. Insulin
therapy may cause hypophosphatemia by shifting phosphorus from the extracellular space
to the intracellular space. In either case, marked hypophosphatemia (e.g., 1 mg/dL
in cows, or ≤ 1.5 mg/dL in cats) is thought to decrease erythrocyte production of
ATP, leading to inadequate energy required for maintenance of membrane and cytoskeletal
integrity. An accompanying decrease in reducing capacity and increase in methemoglobin
concentration have also been noted in experimental studies of hypophosphatemic hemolytic
anemia in dairy cattle, suggesting that oxidative mechanisms may also contribute to
anemia. Affected animals are anemic and hemoglobinuric. Gross postmortem findings
include pallor, decreased viscosity of the blood, and lesions arising from the underlying
metabolic derangement (e.g., discolored pale yellow and swollen liver due to hepatic
lipidosis). Renal tubular necrosis and hemoglobin pigment within the tubules is evident
microscopically.
Infectious Diseases
This section covers infectious agents within the same genus that are recognized to
cause disease in multiple species. Other infectious agents with more limited host
specificity (e.g., cytauxzoonosis in cats, feline and equine retroviruses) are covered
in later sections on species-specific diseases. Throughout both sections, diseases
are organized by taxonomy (protozoal, bacterial and rickettsial, and viral).
Babesiosis (Piroplasmosis).
Babesia spp. and Theileria spp., presented in the next section, are members of the
order Piroplasmida, and are generally referenced as piroplasms. These organisms are
morphologically similar but have different life cycles; Babesia spp. are primarily
erythrocytic parasites, whereas Theileria spp. sequentially parasitize leukocytes
and then erythrocytes. Both are protozoan parasites spread by ticks, but other modes
of transmission are possible (e.g., biting flies, transplacental, and blood transfusions).
Evidence is accumulating that dog fighting also transmits Babesia gibsoni infection.
Babesia organisms are typically classified as large (2 to 4 µm) or small (<2 µm) with
routine light microscopy (Fig. 13-24
). Over 100 Babesia species have been identified, some of which are listed here, along
with their relative microscopic size in parentheses:
•
Horses—Babesia caballi (large)
•
Cattle—Babesia bigemina (large), Babesia bovis (small)
•
Sheep and goats—Babesia motasi (large), Babesia ovis (small)
•
Dogs—Babesia canis (large), Babesia conradae, B. gibsoni (small)
•
Cats—Babesia cati, Babesia felis, Babesia herpailuri (small)
Figure 13-24
Babesiosis, Canine Blood Smear.
A, Small form (arrows) of Babesia (consistent with Babesia gibsonii). B,
Babesia canis (arrow) organisms infecting erythrocytes. Wright's stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
Geographic distributions vary with the species, but most have higher prevalences in
tropical and subtropical regions. For example, equine and bovine babesiosis are endemic
in parts of Africa, the Middle East, Asia, Central and South America, the Caribbean,
and Europe. Both were eradicated from the United States and are now considered exotic
diseases in that country. Of the previously mentioned species, only agents of canine
babesiosis are thought to be endemic in the United States.
Babesiosis may cause intravascular and extravascular hemolytic anemia via direct red
blood cell injury, the innocent bystander effect, and secondary immune-mediated hemolytic
anemia. Infection with highly virulent strains may cause severe multisystemic disease.
In these cases, massive immunostimulation and cytokine release cause circulatory disturbances,
which may result in shock, induction of the systemic inflammatory response, and multiple
organ dysfunction syndromes.
Babesia organisms can usually be detected on a routine blood smear in animals with
acute disease. Infected erythrocytes may be more prevalent in capillary blood, so
blood smears made from samples taken from the pinna of the ear or the nail bed may
increase the likelihood of detecting organisms microscopically. Buffy coat smears
also have an enriched population of infected erythrocytes. PCR-based tests are the
most sensitive assay for detecting infection in animals with very low levels of parasitemia.
At necropsy, gross lesions are mainly related to hemolysis and include pale mucous
membranes, icterus, splenomegaly, dark red or black kidneys, and reddish-brown urine.
The cut surface of the congested spleen oozes blood. The gallbladder is usually distended
with thick bile. Less common lesions include pulmonary edema, ascites, and congestion,
petechiae, and ecchymoses of organs, including the heart and brain. Parasitized erythrocytes
are best visualized on impression smears of the kidney, brain, and skeletal muscle.
Microscopic findings in the liver and kidney are typical of a hemolytic crisis and
include anemia-induced degeneration, necrosis of periacinar hepatocytes and cholestasis,
and hemoglobinuric nephrosis with degeneration of tubular epithelium. Erythroid hyperplasia
is present in the bone marrow. In animals that survive the acute disease, there is
hemosiderin accumulation in the liver, kidney, spleen, and bone marrow. In chronic
cases there is hyperplasia of macrophages in the red pulp of the spleen.
Theileriosis (Piroplasmosis).
Theileria spp. are tick-borne protozoal organisms that infect many domestic and wild
animals worldwide. Numerous Theileria spp. have been documented, but only the more
economically or regionally important species are mentioned here. Diseases with the
greatest economic impact in ruminants are East Coast fever (Theileria parva infection)
and tropical theileriosis (Theileria annulata infection).
•
Horses—Theileria equi (formerly Babesia equi)
•
Cattle—Theileria annulata, Theileria buffeli, T. parva
•
Sheep and goats—Theileria lestoquardi (formerly Theileria hirci)
Like babesiosis, theileriosis is generally restricted to tropical and subtropical
regions, including parts of Africa, Asia, the Middle East, and Europe. Except for
T. buffeli, all previously listed species are exotic to the United States.
Infection is characterized by schizonts within lymphocytes or monocytes, and pleomorphic
intraerythrocytic piroplasms (merozoites and trophozoites). Within host leukocytes
the parasite induces leukocyte cellular division, which expands the parasitized cell
population. Infected cells disseminate throughout the lymphoid system via the lymphatic
and blood vessels. The infected leukocyte may block capillaries, causing tissue ischemia.
Later in infection some schizonts cause leukocyte lysis and release of merozoites.
Merozoites then invade and parasitize erythrocytes, causing hemolytic anemia. Possible
mechanisms of anemia in theileriosis include invasion of erythroid precursors by merozoite
stages and associated erythroid hypoplasia (as occurs with T. parva infection), immune-mediated
hemolysis, mechanical fragmentation because of vasculitis or microthrombi, enzymatic
destruction by proteases, and oxidative damage.
Gross and microscopic lesions are similar to those of babesiosis, except that cattle
with East Coast fever tend not to develop hemolytic anemia. In acute East Coast fever,
lymph nodes are enlarged, edematous, and hemorrhagic. But with chronic cases they
may be shrunken. There is often splenomegaly, hepatomegaly, and hemorrhagic enteritis
with white foci of lymphoid infiltrates (pseudoinfarcts) in the liver and kidney.
Microscopically, infected leukocytes may block capillaries.
African Trypanosomiasis.
Trypanosomes are flagellated protozoa that can infect all domesticated animals. The
most important species that cause disease are Trypanosoma congolense, Trypanosoma
vivax, and Trypanosoma brucei ssp. brucei. Disease is most common in parts of Africa
where the biologic vector, the tsetse fly, exists. However, T. vivax has spread to
Central and South America and the Caribbean, where other biting flies transmit the
parasite mechanically. In Africa, cattle are mainly affected due to the feeding preferences
of the tsetse fly. African trypanosomiasis must be distinguished from nonpathogenic
trypanosomiasis, such as Trypanosoma theileri infection in cattle.
Animals become infected when feeding tsetse flies inoculate metacyclic trypanosomes
into the skin of animals. The trypanosomes grow for a few days, causing a localized
chancre sore, and then sequentially enter the lymph nodes and bloodstream. Trypanosomal
organisms do not infect erythrocytes but rather exist as free trypomastigotes (i.e.,
flagellated protozoa with a characteristic undulating membrane) in the blood (Fig.
13-25, A
) or as amastigotes in tissue. The mechanism of anemia is believed to be immune mediated.
Cattle with acute trypanosomiasis have significant anemia, which initially is regenerative,
but less so with time. The extent of parasitemia is readily apparent with T. vivax
and T. theileri infections because the organisms are present in large numbers in the
blood. This is in contrast to T. congolense, which localizes within the vasculature
of the brain and skeletal muscle. Chronically infected animals often die secondary
to poor body condition, immunosuppression, and concurrent infections.
Figure 13-25
Trypanosomiasis, Bovine (A) and Canine (B) Blood Smears.
A, The trypomastigote life stage of trypanosomes is a flagellated protozoan (arrows)
with an undulating membrane, kinetoplast, and nucleus. They may be identified in a
wet mount made from the buffy coat portion of the packed cells. B,
Trypanosoma cruzi trypomastigotes from a dog with Chagas's disease. Wright-Giemsa
stain.
(A courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.
B courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University.)
Gross examination of animals with acute disease often reveals generalized lymphadenomegaly,
splenomegaly, and petechiae on serosal membranes. An acute hemorrhagic syndrome may
occur in cattle, resulting in lesions of severe anemia (e.g., pale mucous membranes)
and widespread mucosal and visceral hemorrhages. Main lesions of chronic infections
include signs of anemia, lymphadenopathy (e.g., enlarged or atrophied lymph nodes),
emaciation, subcutaneous edema, pulmonary edema, increased fluid in body cavities,
and serous atrophy of fat.
American Trypanosomiasis (Chagas's Disease).
Trypanosoma cruzi is the flagellated protozoal agent of American trypanosomiasis.
Infections have been reported in more than 100 mammal species in South America, Central
America, and the southern United States, but dogs and cats are among the more common
domestic hosts.
Infected triatomine insects, or “kissing bugs,” defecate as they feed on their mammalian
host, releasing infective T. cruzi organisms. The parasite then enters the body through
mucous membranes or breaks in the skin. Like the other trypanosomes described previously,
T. cruzi lives in the blood as extracellular trypomastigotes (see Fig. 13-25, B) and
in the tissues as intracellular amastigotes.
Trypanosoma cruzi primarily causes heart disease. Lesions of acute disease include
a pale myocardium, subendocardial and subepicardial hemorrhages, and yellowish-white
spots and streaks. There may also be secondary lesions, such as pulmonary edema, ascites,
and congestion of the liver, spleen, and kidneys. In chronic disease the heart may
be enlarged and flaccid with thin walls. Microscopically, there is often myocarditis
and amastigotes within cardiomyocytes.
Anaplasmosis, Ehrlichiosis, Heartwater, and Tick-Borne Fever.
Anaplasmosis, ehrlichiosis, heartwater, and tick-borne fever are tick-borne diseases
caused by small, pleomorphic, Gram-negative, obligate intracellular bacteria within
the order Rickettsiaceae, also colloquially known as rickettsias. As a group, rickettsias
primarily infect hematopoietic cells and endothelial cells. Rickettsias that predominantly
infect endothelial cells (e.g., Rickettsia rickettsii [Rocky Mountain spotted fever]),
or cause gastrointestinal disease (e.g., Neorickettsia helminthoeca [salmon poisoning
disease] and Neorickettsia risticii [Potomac horse fever]) are discussed elsewhere
(see Chapters 4 and 7). Less commonly, transmission may occur via blood transfusions
or blood-contaminated medical supplies.
Rickettsias that infect erythrocytes include the following species (the disease name
follows in parentheses):
•
Cattle—Anaplasma marginale, Anaplasma centrale (bovine anaplasmosis)
•
Sheep and goats—Anaplasma ovis (ovine and caprine anaplasmosis, respectively)
Anaplasma marginale and A. ovis have worldwide distributions, but A. centrale is mostly
restricted to South America, Africa, and the Middle East.
Bovine anaplasmosis causes anemia mainly by immune-mediated extravascular hemolysis.
The severity of disease in infected animals varies with age. Infected calves under
1 year of age rarely develop clinical disease, whereas cattle 3 years of age or older
are more likely to develop severe, potentially fatal, illness. The reason for this
discrepancy is not clear. Indian cattle (Bos indicus) are more resistant to disease
than European cattle (Bos taurus). Surviving cattle become chronic carriers (and thus
reservoirs for infection of other animals) and develop cyclic bacteremia, which is
typically not detectable on blood smears. Splenectomy of carrier animals results in
marked bacteremia and acute hemolysis. PCR testing is the most sensitive means of
identifying animals with low levels of bacteremia.
Grossly, acute disease causes lesions of acute hemolytic anemia, including pale mucous
membranes, low blood viscosity, icterus, splenomegaly, hepatomegaly, and a distended
gallbladder. In animals with acute disease it is usually easy to detect A. marginale
organisms on routine blood smear evaluation (Fig. 13-26
) or impression smears from cut sections of the spleen. However, in recovering animals,
the organisms may be difficult to find.
Figure 13-26
Anaplasmosis, Bovine Blood Smear.
Note the darkly stained Anaplasma marginale organisms (arrow), most of which are located
on the edges of the erythrocytes. Anaplasmosis causes anemia mainly by immune-mediated
extravascular hemolysis.
(Courtesy Dr. J. Simon, College of Veterinary Medicine, University of Illinois.)
Rickettsias that infect leukocytes are broadly divided into those that preferentially
infect granulocytes (Anaplasma phagocytophilum [previously Ehrlichia equi, the agent
of human granulocytic ehrlichiosis, and Ehrlichia phagocytophila] and Ehrlichia ewingii),
mononuclear cells (E. canis and Ehrlichia chaffeensis), or both (Ehrlichia ruminantium
[previously Cowdria ruminantium]). Anaplasma platys (previously Ehrlichia platys)
infects platelets. Some of these agents, such as A. phagocytophilum and E. ruminantium,
are proven to also infect endothelial cells. The rickettsias have variable host ranges,
including the domestic hosts listed here (the disease name follows in parentheses):
•
Horses—A. phagocytophilum (equine granulocytic ehrlichiosis)
•
Cattle—A. phagocytophilum (tick-borne fever), E. ruminantium (heartwater)
•
Sheep and goats—A. phagocytophilum (tick-borne fever), E. ruminantium (heartwater)
•
Dogs—A. phagocytophilum, E. ewingii (canine granulocytic ehrlichiosis [E-Fig. 13-3]);
A. platys (canine cyclic thrombocytopenia); E. canis, E. chaffeensis (canine monocytic
ehrlichiosis)
E-Figure 13-3
Granulocytic Ehrlichiosis, Anaplasma phagocytophila, Canine Blood Smear.
The top neutrophil contains an inclusion (arrow) consistent with an A. phagocytophila
morula. Wright's stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
•
Cats—A. phagocytophilum, possibly E. canis (feline ehrlichiosis)
Most of these rickettsias have worldwide distributions. However, E. ewingii has been
reported only in the United States, and E. ruminantium is endemic only in parts of
Africa and the Caribbean. Although A. phagocytophilum has a wide geographic distribution,
strain variants are regionally restricted. For example, A. phagocytophilum causes
disease in ruminants in Europe, but it has not been documented in ruminants in the
United States.
Reservoirs of disease vary, depending upon the rickettsial species. Cattle are the
reservoir host for E. ruminantium, canids are the reservoir host for A. platys and
E. canis, and the other rickettsias have wildlife reservoirs.
Pathogenesis of disease involves endothelial cell, platelet, and leukocyte dysfunction.
Those agents that infect endothelial cells cause vasculitis and increased vascular
permeability of small blood vessels. If only plasma is lost, then there is hypotension
and tissue edema. However, more severe vasculitis causes microvascular hemorrhage
with the potential for platelet consumption thrombocytopenia, disseminated intravascular
coagulation, and hypotension. Infection of platelets may cause thrombocytopenia by
direct platelet lysis, immune-mediated mechanisms, or platelet sequestration within
the spleen. Pathogenesis of leukocyte dysfunction is unclear, but may involve sepsis,
inhibited leukocyte function, endothelial cell activation, and platelet consumption.
Chronic E. canis infection may cause aplastic anemia with pancytopenia by an unknown
mechanism. Some studies indicate that German shepherd dogs with ehrlichiosis are predisposed
to have particularly severe clinical disease. Some breeds of cattle (Bos taurus),
sheep (merino), and goats (Angora and Saanen) are more susceptible to heartwater.
Upon blood smear evaluation, thrombocytopenia is the most common hematologic abnormality;
anemia and neutropenia occur less frequently. In early stages of infection, blood
cells may contain morulae, which are clusters of rickettsial organisms within cytoplasmic,
membrane-bound vacuoles (Fig. 13-27
). Examination of buffy coat smears increases the probability of detecting the organism.
Chronic infection may cause lymphocytosis, particularly of granular lymphocytes. Anaplasma
platys causes recurrent marked thrombocytopenia.
Figure 13-27
Granulocytic Ehrlichiosis, Equine Blood Smear.
The neutrophil contains an inclusion (arrow) consistent with an Anaplasma phagocytophilum
morula. Wright-Giemsa stain.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University.)
In general, more common gross lesions are splenomegaly, lymphadenomegaly, and pulmonary
edema and hemorrhage. More severe cases may also exhibit multisystemic petechiae,
ecchymoses, and edema, cavitary effusions, and effusive polyarthropathy. Hydropericardium
gives heartwater its name but is more consistently found in small ruminants than in
cattle. Chronically infected dogs are emaciated. The bone marrow is hyperplastic and
red in the acute disease but becomes hypoplastic and pale in dogs with chronic E.
canis infection. Equine anaplasmosis is often mild but may cause edema and hemorrhages.
Disease in cats is rare and poorly documented.
Histologic findings include generalized perivascular plasma cell infiltration, which
is most pronounced in animals with chronic disease. Multifocal, nonsuppurative meningoencephalitis,
interstitial pneumonia, and glomerulonephritis are present in most dogs with the disease.
Rickettsial organisms are difficult to detect histologically; examination of Wright-Giemsa–stained
impression smears of lung, liver, lymph nodes, and spleen is a more effective method
for detecting the morulae within leukocytes. Heartwater is often diagnosed by observing
morulae in endothelial cells of Giemsa-stained squash preparations of brain. Rickettsial
diseases are often diagnosed on the basis of serologic testing, but PCR testing is
more sensitive.
Clostridial Diseases.
Certain Clostridium spp. may cause potentially fatal hemolytic anemias in animals;
nonhemolytic lesions are presented elsewhere (see Chapters 4, 7, 8, and 19Chapter
4Chapter 7Chapter 8Chapter 19). Clostridium haemolyticum and Clostridium novyi type
D cause the disease in cattle known as bacillary hemoglobinuria. (The phrase “red
water” has also been used for this disease and for hemolytic anemias in cattle caused
by Babesia spp.) Similar naturally occurring disease has been reported in sheep. In
cattle the disease is caused by liver fluke (Fasciola hepatica) migration in susceptible
animals. Ingested clostridial spores may live in Kupffer cells for a long time without
causing disease. However, when migrating flukes cause hepatic necrosis, the resulting
anaerobic environment stimulates the clostridial organisms to proliferate and elaborate
their hemolytic toxins, causing additional hepatic necrosis. The mechanism of hemolysis
involves a bacterial β-toxin (phospholipase C or lecithinase), which enzymatically
degrades cell membranes, causing acute intravascular hemolysis. Bacillary hemoglobinuria
also occurs with liver biopsies in calves.
Clostridium perfringens type A causes intravascular hemolytic anemia in lambs and
calves—a condition known as yellow lamb disease, yellows, or enterotoxemic jaundice
because of the characteristic icterus. The organism is a normal inhabitant of the
gastrointestinal tract in these animals but may proliferate abnormally in response
to some diets. C. perfringens causes intravascular hemolytic anemia in horses with
clostridial abscesses, and clostridial mastitis in ewes. C. perfringens type A produces
hemolytic α-toxin, which also has phospholipase C activity.
Leptospirosis.
Leptospirosis is recognized as a cause of hemolytic anemia in calves, lambs, and pigs.
Specific leptospiral organisms that cause hemolytic disease include Leptospira interrogans
serovars pomona and ictohaemorrhagiae.
Leptospira organisms are ubiquitous in the environment. Infection occurs percutaneously
and via mucosal surfaces and is followed by leptospiremia; organisms then localize
preferentially in certain tissues (e.g., kidney, liver, and pregnant uterus). Proposed
mechanisms of hemolytic disease include immune-mediated (immunoglobulin M [IgM] cold
agglutinin) extravascular hemolysis and enzymatic (phospholipase produced by the organism)
intravascular hemolysis. Leptospirosis can also cause many disease manifestations
besides hemolysis (e.g., renal failure, liver failure, abortion, and other conditions)
that are not discussed here.
In addition to anemia, common findings in animals with leptospirosis-induced hemolysis
include hemoglobinuria and icterus. On necropsy, renal tubular necrosis, which occurs
in part because of hemoglobinuria (hemoglobinuric nephrosis), may also be present.
Hemotropic Mycoplasmosis (Hemoplasmosis).
The term hemotropic mycoplasmas, or hemoplasmas, encompasses a group of bacteria,
formerly known as Haemobartonella or Eperythrozoon spp., that infect erythrocytes
of many domestic, laboratory, and wild animals. Hemotropic mycoplasmas affecting common
domestic species are as follows:
•
Cattle—Mycoplasma wenyonii
•
Camelids—“Candidatus Mycoplasma haemolamae”
•
Sheep and goats—Mycoplasma ovis
•
Pigs—Mycoplasma suis (E-Fig. 13-4)
E-Figure 13-4
Hemotropic Mycoplasmosis, Porcine Blood Smear.
Blood smear from a splenectomized pig infected with Mycoplasma suis (formerly Eperythrozoon
suis). Note the small oval to ring-shaped organisms attached to the surface of the
erythrocytes and free in the protein of the blood smear. Wright's stain.
(Courtesy Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
•
Dogs—Mycoplasma haemocanis, “Candidatus Mycoplasma haematoparvum”
•
Cats—Mycoplasma haemofelis, “Candidatus Mycoplasma haemominutum,” “Candidatus Mycoplasma
turicensus”
Like other mycoplasmas, hemoplasmas are small (0.3 to 3 µm in diameter) and lack a
cell wall. They are epicellular parasites, residing in indentations and invaginations
of red blood cell surfaces. The mode of transmission is poorly understood, but blood-sucking
arthropods are believed to play a role; transmission in utero, through biting or fighting,
and transfusion of infected blood products are also suspected.
Effects of infection vary from subclinical to fatal anemia, depending on the specific
organism, dose, and host susceptibility. Most hemoplasmas are more likely to cause
acute illness in individuals that are immunocompromised or have concurrent disease.
However, M. haemofelis is an exception and tends to cause acute hemolytic anemia in
immunocompetent cats. Anemia occurs mainly because of extravascular hemolysis, but
intravascular hemolysis also occurs. Although the pathogenic mechanisms are not completely
understood, an immune-mediated component is highly probable, as well as direct red
blood cell injury by the bacteria and the innocent bystander effect. Hemotropic mycoplasmas
induce cold agglutinins in infected individuals, although it is not clear whether
these particular antibodies are important in the development of hemolytic anemia.
When detected on routine blood smear evaluation, the organisms are variably shaped
(cocci, small rods, or ring forms) and sometimes arranged in short, branching chains
(Fig. 13-28
). The organisms may also be noted extracellularly, in the background of the blood
smear, especially if the smear is made after prolonged storage of the blood in an
anticoagulant tube.
Figure 13-28
Hemotropic Mycoplasmosis, Alpaca (A), Porcine (B), Canine (C), and Feline (D) Blood
Smears.
Blood smears from an alpaca with Mycoplasma haemolamae
(A), a pig with Mycoplasma suis
(B), a dog with Mycoplasma haemocanis
(C), and a cat with Mycoplasma haemominutum
(D) infections. Note the small oval to ring-shaped organisms attached to the surface
of the erythrocytes and free in the background of the blood smear. Wright-Giemsa stain.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University.)
In animals dying of acute hemoplasma infection, the gross findings are typical of
extravascular hemolysis, with pallor, icterus, splenomegaly, and distended gallbladder
(Fig. 13-29
). Additional lesions documented in cattle include scrotal and hind limb edema and
swelling of the teats. Microscopic lesions in the red pulp of the spleen include congestion,
erythrophagocytosis, macrophage hyperplasia, EMH, and increased numbers of plasma
cells. Bone marrow has varying degrees of erythroid hyperplasia, depending on the
duration of hemolysis.
Figure 13-29
Mycoplasma haemofelis, Cat.
Note the splenomegaly, hepatomegaly, and icterus caused by infection of erythrocytes
with this hemotropic parasite. Splenomegaly and icterus are the result of increased
destruction (extravascular hemolysis) of infected erythrocytes.
(Courtesy College of Veterinary Medicine, University of Illinois.)
Immune-Mediated Disorders
Immune-Mediated Hemolytic Anemia.
Immune-mediated hemolytic anemia is a condition characterized by increased destruction
of erythrocytes because of binding of immunoglobulin to red blood cell surface antigens.
It is a common, life-threatening condition in dogs but also has been described in
horses, cattle, and cats. Immune-mediated hemolytic anemia may be idiopathic (also
called primary immune-mediated hemolytic anemia or autoimmune hemolytic anemia) or
secondary to a known initiator, termed secondary immune-mediated hemolytic anemia.
Although the cause of idiopathic immune-mediated hemolytic anemia is unknown, certain
dog breeds (e.g., cocker spaniels) are predisposed to developing disease, suggesting
the possibility of a genetic component. Causes of secondary immune-mediated hemolytic
anemia include certain infections (e.g., hemoplasmosis, babesiosis, and theileriosis),
drugs (e.g., cephalosporins, penicillin, and sulfonamides), vaccines, and envenomations
(e.g., bee stings). Immune-mediated hemolysis directed at nonself antigens, such as
in neonatal isoerythrolysis, is presented later.
In most cases of idiopathic immune-mediated hemolytic anemia, the reactive antibody
is IgG, and the hemolysis is extravascular (i.e., erythrocytes with surface-bound
antibody are phagocytized by macrophages, mainly in the spleen). IgM and/or complement
proteins may also contribute to idiopathic immune-mediated hemolytic anemia. Complement
factor C3b usually acts as an opsonin that promotes phagocytosis and extravascular
hemolysis. However, formation of the complement membrane attack complex on red blood
cell surfaces causes intravascular hemolysis; this mechanism more commonly occurs
with IgM autoantibodies. Most immunoglobulins implicated in immune-mediated hemolytic
anemia are reactive at body temperature (warm hemagglutinins). A smaller portion,
usually IgM, are more reactive at lower temperatures, causing a condition known as
cold hemagglutinin disease. This results in ischemic necrosis at anatomic extremities
(e.g., tips of the ears), where cooling of the circulation causes autoagglutination
of erythrocytes and occlusion of the microvasculature. Typically immune-mediated hemolytic
anemia targets mature erythrocytes, causing a marked regenerative response. However,
as discussed earlier in the chapter, immune-mediated destruction of immature erythroid
cells in the bone marrow may also occur, resulting in nonregenerative anemia.
Pathogenesis of secondary immune-mediated hemolytic anemia is dependent upon the cause.
Erythrocytic parasites may cause immune-mediated hemolysis by altering the red blood
cell surface and exposing “hidden antigens” that are not recognized as self-antigens
by the host's immune system. Alternatively, the immune attack may be directed at the
infectious agent, but erythrocytes are nonspecifically destroyed because of their
close proximity—this is called the innocent bystander mechanism. Certain drugs, such
as penicillin, may cause immune-mediated hemolytic anemia by binding to erythrocyte
membranes and forming drug-autoantigen complexes that induce antibody formation, termed
hapten-dependent antibodies. Other proposed mechanisms include binding of drug-antibody
immune complexes to the erythrocyte membrane, or induction of a true autoantibody
directed against an erythrocyte antigen.
Hematologic, gross, and histopathologic abnormalities are typical of those of hemolytic
anemia, as presented in the earlier section on Bone Marrow and Blood Cells, Dysfunction/Responses
to Injury, Blood Cells, Abnormal Concentrations of Blood Cells, Anemia). In brief,
there may be spherocytes and autoagglutination on blood smear evaluation, icterus
and splenomegaly on gross examination, and EMH, erythrophagocytic macrophages, and
hypoxia-induced or thromboemboli-induced tissue necrosis on histopathologic examination.
Dogs with immune-mediated hemolytic anemia also frequently develop an inflammatory
leukocytosis and coagulation abnormalities (prolonged coagulation times, decreased
plasma antithrombin concentration, increased plasma concentration of fibrin degradation
products, thrombocytopenia, and disseminated intravascular coagulation). Intravascular
hemolysis plays a relatively insignificant role in most cases of immune-mediated hemolytic
anemia, but evidence of intravascular hemolysis (e.g., ghost cells, red plasma and
urine, dark red kidneys) is noted occasionally, presumably in those cases in which
IgM and complement are major mediators of hemolysis.
Neonatal Isoerythrolysis.
Neonatal isoerythrolysis (NI) is a form of immune-mediated hemolytic anemia in which
colostrum-derived maternal antibodies react against the newborn's erythrocytes. It
is common in horses (Fig. 13-30
) and has been reported in cattle, cats, and some other domestic and wildlife species.
In horses, neonatal isoerythrolysis occurs as a result of immunosensitization of the
dam from exposure to an incompatible blood type inherited from the stallion (e.g.,
transplacental exposure to fetal blood during pregnancy or mixing of maternal and
fetal blood during parturition). A previously mismatched blood transfusion produces
the same results. Some equine blood groups are more antigenic than others; in particular,
types Aa and Qa are very immunogenic in mares. In cattle, neonatal isoerythrolysis
has been caused by vaccination with whole blood products or products containing erythrocyte
membrane fragments. Neonatal isoerythrolysis has been produced experimentally in dogs,
but there are no reports of naturally occurring disease. In cats the recognized form
of neonatal isoerythrolysis does not depend on prior maternal immunosensitization
but on naturally occurring anti-A antibodies in queens with type B blood. Affected
animals are young (hours to days old) with typical gross and microscopic changes of
immune-mediated hemolytic anemia.
Figure 13-30
Neonatal Isoerythrolysis, Foal.
Note the enlarged spleen (S) (also liver [L]) and icterus. The newborn foal had colostrum-derived
maternal antibodies, which reacted against its own erythrocytes. Macrophages in the
splenic red pulp remove erythrocytes whose membranes have bound antibody.
(Courtesy College of Veterinary Medicine, University of Illinois.)
Pure Red Cell Aplasia.
Pure red cell aplasia (PRCA) is a rare bone marrow disorder characterized by absence
of erythropoiesis and severe nonregenerative anemia. Primary and secondary forms of
pure red cell aplasia have been described in dogs and cats. Primary pure red cell
aplasia is apparently caused by immune-mediated destruction of early erythroid progenitor
cells, a presumption supported by the response of some patients to immunosuppressive
therapy and by the detection of antibodies inhibiting erythroid colony formation in
vitro in some dogs. Administration of recombinant human erythropoietin (rhEpo) has
been identified as a cause of secondary pure red cell aplasia in dogs, cats, and horses,
presumably caused by induction of antibodies against rhEpo that cross-react with endogenous
Epo. Experimentation with the use of species-specific recombinant Epo has produced
mixed results. Dogs treated with recombinant canine Epo have not developed pure red
cell aplasia. However, in experiments reported thus far involving cats treated with
recombinant feline Epo, at least some animals have developed pure red cell aplasia.
Parvoviral infection has been suggested as a possible cause of secondary pure red
cell aplasia in dogs. Infection with FeLV subgroup C causes secondary erythroid aplasia
in cats, probably because of infection of early-stage erythroid precursors. Grossly,
animals with pure red cell aplasia have pale mucous membranes without indicators of
hemolysis (e.g., icterus). Microscopic examination of the bone marrow shows an absence
or near absence of erythroid precursors with or without lymphocytosis, plasmacytosis,
and myelofibrosis; production of other cell lines (e.g., neutrophils and platelets)
is normal or hyperplastic.
Immune-Mediated Neutropenia.
Immune-mediated neutropenia is a rare condition that has been reported in horses,
dogs, and cats. This disease is characterized by severe neutropenia from immune-mediated
destruction of neutrophils or their precursors. The range of causes is presumably
similar to that of other immune-mediated cytopenias (e.g., immune-mediated hemolytic
anemia, pure red cell aplasia, and immune-mediated thrombocytopenia). Affected animals
may have infections, such as dermatitis, conjunctivitis, or vaginitis, which are secondary
to marked neutropenia and a compromised innate immune system. Microscopically, there
may be neutrophil hyperplasia, maturation arrest, or aplasia in the bone marrow, depending
on which neutrophil maturation stage is targeted for destruction. Marrow lymphocytosis
and plasmacytosis may be marked (e.g., >60% of nucleated cells). The diagnosis may
be supported by flow cytometric detection of immunoglobulin bound to neutrophils but
is most often made on the basis of exclusion of other causes of neutropenia and response
to immunosuppressive therapy.
Immune-Mediated Thrombocytopenia.
Immune-mediated thrombocytopenia (IMTP) is a condition characterized by immune-mediated
destruction of platelets. It is a fairly common condition in dogs and is less frequent
in horses and cats. The disease is usually idiopathic but may be secondary to infection
(e.g., equine infectious anemia and ehrlichiosis), drug administration (e.g., cephalosporins
and sulfonamides), neoplasia, and other immune-mediated diseases. When immune-mediated
thrombocytopenia occurs together with immune-mediated hemolytic anemia, the condition
is called Evans's syndrome. The thrombocytopenia is often severe (e.g., < 20,000 platelets/µL),
resulting in varying degrees of bleeding tendencies, mainly in skin and mucous membranes.
Microscopically, there are multifocal perivascular hemorrhages in multiple tissues,
and the bone marrow exhibits megakaryocytic and erythroid hyperplasia. Rarely, immune-mediated
destruction of megakaryocytes may cause megakaryocytic hypoplasia, termed amegakaryocytic
thrombocytopenia.
Neonatal Alloimmune Thrombocytopenia.
A form of immune-mediated thrombocytopenia, known as neonatal alloimmune thrombocytopenia,
is recognized in neonatal pigs and foals. The pathogenesis of this disease is virtually
identical to that of neonatal isoerythrolysis as a cause of anemia: a neonate inheriting
paternal platelet antigens absorbs maternal antibodies against these antigens through
the colostrum. In principle, a similar situation may occur after platelet-incompatible
transfusion of blood or blood products containing platelets. Gross and microscopic
changes are similar to those of immune-mediated thrombocytopenia except that the animal
is young (e.g., 1 to 3 days).
Inflammatory Disorders
Hemophagocytic Syndrome.
Hemophagocytic syndrome is a term used to describe the proliferation of nonneoplastic
(i.e., polyclonal), well-differentiated but highly erythrophagic macrophages. The
condition is rare but has been recognized in dogs and cats. Unlike hemophagocytic
histiocytic sarcoma, which is a neoplastic proliferation of phagocytic macrophages,
hemophagocytic syndrome is secondary to an underlying disease, such as neoplasia,
infection, or an immune-mediated disorder. The primary disease process causes increased
production of stimulatory cytokines, which results in macrophage proliferation and
hyperactivation. These activated macrophages phagocytize mature hematopoietic cells
and hematopoietic precursors at an enhanced rate, resulting in one or more cytopenias.
Affected animals usually have lesions of the primary disease, as well as signs of
the anemia (e.g., pale mucous membranes), neutropenia (e.g., bacterial infections),
and thrombocytopenia (e.g., petechiae and ecchymoses). Microscopically, phagocytic
macrophages are found in high numbers in the bone marrow and commonly in other tissues,
including lymph nodes, spleen, and liver. Additional bone marrow findings reported
in animals with hemophagocytic syndrome vary widely, ranging from hypoplasia to hyperplasia
of cell lines with peripheral cytopenias.
Disseminated Intravascular Coagulation.
Disseminated intravascular coagulation is a syndrome characterized by continuous activation
of both coagulation and fibrinolytic pathways and is also known as consumptive coagulopathy.
It is not a primary disease, but rather a secondary complication of many types of
underlying disease, including severe inflammation, organ failure, and neoplasia. It
is included in the section on Inflammatory Disorders because the coagulation cascade
is closely linked to inflammatory pathways.
Disseminated intravascular coagulation is a consumptive coagulopathy resulting from
activation of both coagulation and fibrinolytic pathways. It is a secondary complication
of many types of underlying disease, including many infectious diseases, trauma, burns,
heat stroke, immune-mediated disease, hemolysis, shock, neoplasia, organ failure,
obstetric complications, and noninfectious inflammatory disease, such as pancreatitis.
It is common in critically ill domestic animals. Disseminated intravascular coagulation
involves an initial hypercoagulable phase, resulting in thrombosis and ischemic tissue
damage, and a subsequent hypocoagulable phase as a result of consumption of coagulation
factors and platelets, resulting in hemorrhage (E-Fig. 13-5). The pathogenesis of
disseminated intravascular coagulation typically involves the release of tissue factor
(thromboplastin) and subsequent activation of coagulation pathways and platelets but
may also involve defective normal inhibition of coagulation or defective fibrinolysis.
Classically diagnosis of disseminated intravascular coagulation is based on clinical
evidence of hemorrhage and/or thromboembolic disease and a triad of laboratory findings:
thrombocytopenia, usually moderate (below the lower reference value but above 50,000/µL);
prolonged coagulation times (prothrombin time and/or partial thromboplastin time);
and decreased fibrinogen or increased concentration of plasma fibrin degradation products
or D-dimer. Milder forms of disseminated intravascular coagulation that do not meet
all of the diagnostic criteria also occur. Decreased plasma antithrombin (antithrombin
III) concentration and schistocytosis are other laboratory abnormalities often found
in patients with disseminated intravascular coagulation.
E-Figure 13-5
Pathophysiology of Disseminated Intravascular Coagulation.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University; and Dr. J.F. Zachary, College of Veterinary Medicine, University
of Illinois.)
Hematopoietic Neoplasia
The term hematopoietic neoplasia encompasses a large and diverse group of clonal proliferative
disorders of hematopoietic cells. Historically, numerous systems have been used to
classify hematopoietic neoplasms in human medicine, some of which have been applied
inconsistently to veterinary species (examples include the Kiel classification and
National Cancer Institute Working Formulation). The World Health Organization (WHO)
classification of hematopoietic neoplasia was first published in 2001 (updated in
2008) and is based on the principles defined in the Revised European-American Classification
of Lymphoid Neoplasms (REAL) from the International Lymphoma Study Group. The WHO
classification system is considered the first true worldwide consensus on the classification
of hematopoietic malignancies and integrates information on tumor topography, cell
morphology, immunophenotype, genetic features, and clinical presentation and course.
A veterinary reference of the WHO classification system, published in 2002, was later
validated in 2011 using the canine model of lymphoma. This project, modeled after
the study to validate the system in human beings, yielded an overall accuracy (i.e.,
agreement on a diagnosis) among pathologists of 83%. Currently this classification
system is accepted as the method of choice in both human and veterinary medicine.
The WHO classification broadly categorizes neoplasms primarily according to cell lineage:
myeloid, lymphoid, and histiocytic. This distinction is based on the fact that the
earliest commitment of a pluripotent HSC is to either a lymphoid or nonlymphoid lineage.
Many pathologists and clinicians distinguish leukemias from other hematopoietic neoplasms.
Leukemia refers to a group of hematopoietic neoplasms that arise from the bone marrow
and are present within the blood. Leukemia may be difficult to differentiate from
other forms of hematopoietic neoplasms that originate outside of the bone marrow but
infiltrate the bone marrow and blood. For simplicity, cases of secondary bone marrow
or blood involvement may not be considered leukemia but rather the “leukemic phase”
of another primary neoplasm. It is now recognized that certain lymphomas and leukemias
are different manifestations of the same disease (e.g., chronic lymphocytic leukemia
and small lymphocytic lymphoma), and the designation of lymphoma or leukemia is placed
on the tissue with the largest tumor burden.
Based on their degree of differentiation, leukemias are classified as acute or chronic.
Acute leukemias are poorly differentiated or undifferentiated, meaning that there
are high percentages of early progenitor and precursor cells, including lymphoblasts,
myeloblasts, monoblasts, erythroblasts, and/or megakaryoblasts. In contrast, well-differentiated
cells predominate in chronic leukemias. Because well-differentiated cells also predominate
with nonneoplastic proliferations, chronic leukemias must be differentiated from reactive
processes, such as those cells that occur in chronic and/or granulomatous inflammation.
Diagnosis of chronic leukemia is often made by excluding all other causes for the
proliferating cell type. For example, causes of relative and secondary erythrocytosis
are excluded to be able to diagnose polycythemia vera. Furthermore, the designation
of acute or chronic also refers to the disease's clinical course. Acute leukemias
tend to have an acute onset of severe and rapidly progressive clinical signs, whereas
animals with chronic leukemia typically have indolent, slowly progressive disease.
This classification scheme is summarized in Table 13-4
. Subcategories exist within each of these groups, as discussed further later.
Table 13-4
Basic Classification of Leukemias
Leukemia
Basic Diagnostic Criteria
Acute undifferentiated leukemia
•
No lineage commitment
•
≥20% blasts in bone marrow
Lymphoid leukemia
•
Commitment to lymphoid lineage
Acute lymphoblastic leukemia
•
≥20% blasts in bone marrow
Chronic lymphocytic leukemia
•
<20% blasts in bone marrow
•
Predominantly small lymphocytes in blood, often > 100,000/µL
Multiple myeloma
•
Increased plasma cells in bone marrow
•
Osteolysis
•
Monoclonal gammopathy
•
Light chain (Bence Jones) proteinuria
Myeloid leukemia
•
Commitment to myeloid lineage
Myelodysplastic syndrome
•
<20% blasts in bone marrow
•
Normal or increased marrow cellularity
•
Peripheral blood cytopenias
•
Myelodysplasia
Acute myeloid leukemia
•
≥20% blasts in bone marrow
Chronic myeloid leukemia
•
<20% blasts in bone marrow
•
Markedly increased numbers of mature myeloid cells in blood
Diagnostic Techniques Used to Classify Hematopoietic Neoplasms
Before the discussion of specific diseases, it is worthwhile to describe the diagnostic
techniques required to classify hematopoietic neoplasms that are becoming increasingly
available for routine use in veterinary medicine. Immunophenotyping refers to the
use of antibodies recognizing specific molecules expressed on different cell types
to determine the identity of a cell population of interest. Immunophenotyping on the
basis of these lineage-specific or lineage-associated markers can be performed on
histologic sections (immunohistochemistry [see Fig. 13-86]), air-dried cytologic examination
smears (immunocytochemistry), or by laser analysis of cells in suspension in blood
or buffer solutions (flow cytometry). In cases of lymphoid neoplasia, immunophenotyping
most routinely refers to determination of B or T lymphocyte origin. Clonality assays,
PCR for antigen receptor rearrangement (PARR), can help identify neoplastic lymphoid
proliferations on the basis of clonal rearrangements of genes encoding lymphocyte
antigen receptors. In terms of practical application the PARR assay is most useful
in helping to distinguish lymphoid neoplasms from those nonneoplastic lymphoid proliferations
mimicking neoplasia. Cytogenetic testing has not been routinely used in veterinary
medicine, though several genetic mutations have been identified in dogs. For example,
breakpoint cluster region-Abelson (BCR-ABL) translocations have been identified in
some canine leukemias, including acute myeloblastic leukemia and chronic monocytic
leukemia. Dogs with Burkitt-like lymphoma have a translocation leading to constitutive
c-Myc expression.
Types of Hematopoietic Neoplasia
This section discusses examples of myeloid neoplasms, including myelodysplastic syndrome,
myeloid leukemias, and mast cell neoplasms (technically a form of myeloid neoplasia),
and lymphoid neoplasms, including lymphoid leukemias and multiple myeloma. Other lymphoid
neoplasms, such as the numerous subtypes of lymphoma and extramedullary plasmacytomas
(EMPs), as well as histiocytic disorders are described in the section on Lymphoid/Lymphatic
System, Disorders of Domestic Animals, Neoplasia. Additional discussion of hematopoietic
neoplasia occurs in the species-specific sections at the end of this chapter.
Myeloid Neoplasia
Myelodysplastic Syndrome.
Myelodysplastic syndrome (MDS) most commonly occurs in dogs and cats and may be caused
by FeLV infection in cats. The disease refers to a group of clonal myeloid proliferative
disorders with ineffective hematopoiesis in the bone marrow, resulting in cytopenias
of more than one cell line. Hematopoietic proliferation in bone marrow with concurrent
peripheral blood cytopenias is likely a result of increased apoptosis of neoplastic
cells within the bone marrow, before their release into circulation. Clinical illness
and death often result from secondary manifestations, such as secondary infections
or cachexia, attributable to the effects of cytopenias and/or transformation of the
neoplasm into acute myeloid leukemia. Gross lesions are dependent upon the type and
severity of the cytopenias. However, essential microscopic findings within the bone
marrow are normal or increased cellularity, dysplasia of myeloid cells, and fewer
than 20% myeloblasts and “blast equivalents.”3
Acute Myeloid Leukemia.
Acute myeloid leukemia (AML) is uncommon in domestic animals but most frequently occurs
in dogs and cats. In veterinary species, acute myeloid leukemia is most commonly of
neutrophil, monocyte, and/or erythroid origin, with rare reports of eosinophil, basophil,
or megakaryocytic lineages. It is caused by FeLV infection in cats. Evaluations of
blood smears show many early myeloid precursors, including myeloblasts and blast equivalents
(Fig. 13-31, A
). In dogs the total leukocyte concentration averages approximately 70,000/µL; anemia,
neutropenia, and thrombocytopenia commonly occur. Grossly, animals show lesions attributed
to anemia, neutropenia, and thrombocytopenia, such as pale mucous membranes, secondary
infections, and multisystemic bleeding, respectively. Neoplastic cells often infiltrate
tissues, resulting in splenomegaly, hepatomegaly, and lymphadenomegaly. Microscopically,
myeloid cells efface (replace) the bone marrow and infiltrate extramedullary tissues,
especially lymphoid tissue.
Figure 13-31
Leukemia, Canine Blood Smears.
A, Acute myeloid leukemia. A dog with acute myelomonocytic leukemia has a marked leukocytosis
(52,200 white blood cells/µL) with myeloid blasts (arrows) differentiating into dysplastic
neutrophils (arrowheads) and monocytes (m). Modified Wright's stain. B, Chronic myeloid
leukemia. A dog with chronic myelomonocytic leukemia (arrows) has a marked leukocytosis
(138,300 white blood cells/µL) with a predominance of mature neutrophils (105,108/µL)
and monocytes (26,277/µL). Wright-Giemsa stain. C, Acute lymphoblastic leukemia. Note
the large lymphoid cells with immature (fine) chromatin and nucleoli (arrows). The
dog also had pancytopenia due to neoplastic myelophthisis. Modified Wright's stain.
D, Chronic lymphocytic leukemia (CLL). Small lymphocytes predominate in CLL. The neoplastic
lymphocytes have clumped chromatin and no to rare nucleoli (arrows). Most canine CLLs
are of T lymphocyte origin with a granular phenotype. Modified Wright's stain.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University.)
Chronic Myeloid Leukemia.
Chronic myeloid leukemia (CML), also called chronic myelogenous leukemia or myeloproliferative
neoplasia, is rare in animals. Most reported cases occur in dogs and cats. There are
various subclassifications of chronic myeloid leukemia, including excessive production
of erythrocytes (polycythemia vera), platelets (essential thrombocythemia), neutrophils
(chronic neutrophilic leukemia), monocytes (chronic monocytic leukemia), neutrophils
and monocytes (chronic myelomonocytic leukemia), eosinophils (chronic eosinophilic
leukemia), or basophils (chronic basophilic leukemia). Complete peripheral blood count
analysis often reveals very high concentrations of the neoplastic cells, such as greater
than 50,000 to 100,000 leukocytes/µL (see Fig. 13-31, B) or 2,000,000 platelets/µL.
Cellular morphologic features are often normal, but slight dysplasia may be observed.
Later in the disease there may be cytopenias of nonneoplastic cell types.
Animals with polycythemia vera often have red mucous membranes and lesions of hyperviscosity
syndrome, such as bleeding and dilated, tortuous retinal vessels. Essential thrombocythemia
results in multisystemic bleeding due to dysfunctional platelets, or multisystemic
infarcts from hyperaggregability and excessive platelets. Chronic myeloid leukemias
of leukocytes often result in splenomegaly, hepatomegaly, and lymphadenomegaly because
of infiltration by the neoplastic cells. Histologically, the bone marrow shows proliferation
of the neoplastic cell type characterized by dysplasia and low numbers (e.g., <20%)
of myeloblasts and blast equivalents.
Mast Cell Neoplasia.
Mast cell tumors (MCTs) of the skin and other sites are common in animals (see Chapters
6, 7, and 17Chapter 6Chapter 7Chapter 17), but mast cell leukemia is rare. In cats,
MCTs are the most common neoplasm in the spleen (E-Fig. 13-6). Mast cells normally
are not present in the blood vascular system, but the finding of mast cells in the
blood (mastocytemia) is highly suggestive of disseminated mast cell neoplasia (systemic
mastocytosis) in cats. However, mastocytemia does not necessarily indicate myeloid
neoplasia in dogs. In fact, one study found that the severity of mastocytemia in dogs
was frequently higher in animals without MCTs than those with MCTs and that random
detection of mast cells in blood smears usually is not the result of underlying MCT.
E-Figure 13-6
Feline Splenic Mast Cell Tumor
A, Sheets of neoplastic mast cells fill the splenic parenchyma. H&E stain. B, Astral
blue stain highlights the positively staining (turquoise) metachromatic granules.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Granulocytic Sarcoma.
Granulocytic sarcoma is a poorly characterized extramedullary proliferation of myeloid
precursors, most often of eosinophilic or neutrophilic cell lines. Although rare,
there are reports of granulocytic sarcoma in dogs, cats, cattle, and pigs, and it
may arise in a number of sites, such as lung, intestine, lymph nodes, liver, kidney,
skin, and muscle.
Lymphoid Neoplasia
Lymphoid Leukemia
Acute Lymphoblastic Leukemia.
Acute lymphoblastic leukemia (ALL) is uncommon in dogs and cats, and rare in horses
and cattle. In a recent immunophenotype study of 51 cases of acute lymphoblastic leukemia
in dogs, 47 arose from B lymphocytes and 4 arose from double-negative T lymphocytes
that were immunonegative for CD4 and CD8 markers. In the blood of animals with acute
lymphoblastic leukemia, there are typically many medium to large lymphoid cells with
deeply basophilic cytoplasm, reticular to coarse chromatin, and prominent, multiple
nucleoli (see Fig. 13-31, C). In affected dogs the mean blood lymphoid concentration
is approximately 70,000/µL, but cats with acute lymphoblastic leukemia often have
low numbers of neoplastic cells in the circulation. As with animals with acute myeloid
leukemia, anemia, neutropenia, and thrombocytopenia commonly occur. Gross and microscopic
lesions also are similar to those that occur in cases of acute myeloid leukemia, except
that neoplastic cells may differentiate into morphologically identifiable lymphoid
cells.
Chronic Lymphocytic Leukemia.
Chronic lymphocytic leukemia (CLL) is uncommon in veterinary medicine. It is predominantly
a disease of middle-aged to older dogs but is also documented in horses, cattle, and
cats. Most canine chronic lymphocytic leukemia cases are of T lymphocyte origin, typically
cytotoxic T lymphocytes expressing CD8. In cats the majority of chronic lymphocytic
leukemia cases have a T helper lymphocyte immunophenotype. A CBC often shows very
high numbers of small lymphocytes with clumped chromatin and scant cytoplasm. Proliferating
cytotoxic T lymphocytes frequently contain a few pink cytoplasmic granules when stained
with most methanol-based Romanowsky stains (e.g., Wright-Giemsa). However, these granules
may not be appreciated with some aqueous-based Romanowsky stains (e.g., Diff-Quik).
Although the number of total blood lymphocytes is often greater than 100,000/µL, relatively
mild lymphocytosis (e.g., 15,000/µL) has been reported. Seventy-five percent of affected
dogs also have anemia, and 15% have thrombocytopenia. Autopsy findings depend on the
stage of disease. In advanced cases with marked infiltration of organs with neoplastic
cells, there is often uniform splenomegaly, hepatomegaly, and lymphadenomegaly, and
the bone marrow is highly cellular (E-Fig. 13-7; see Fig. 13-31, D). Other lesions
depend on whether there are concurrent cytopenias, such as anemia, neutropenia, and
thrombocytopenia, and if the neoplastic cells produce excessive immunoglobulin. Lesions
caused by excessive immunoglobulin are further discussed in the section on multiple
myeloma. Histologically, the bone marrow is densely cellular with well-differentiated
lymphocytes. Small lymphocytes infiltrate and often efface in the architecture of
the lymph nodes and spleen. The liver may have dense accumulations of neoplastic cells
in the connective tissue around the portal triad.
E-Figure 13-7
Chronic Leukemia, Hypercellularity, Bone Marrow, Dog.
Grossly, the marrow consists entirely of hematopoietic tissue (red) and no fat (yellow).
In this case, hypercellularity of the marrow is attributable to neoplastic proliferation,
but hyperplasia may cause similar gross findings.
(Courtesy Dr. B.C. Ward, College of Veterinary Medicine, University of Mississippi;
and Noah's Arkive, College of Veterinary Medicine, The University of Georgia.)
Plasma Cell Neoplasia.
Plasma cell neoplasms are most easily categorized as myeloma or multiple myeloma,
which arises in the bone marrow, and extramedullary plasmacytoma, which as the name
implies involves sites other than bone; the latter is discussed in the section on
Lymphoid/Lymphatic System, Disorders of Domestic Animals: Lymph Nodes, Neoplasia,
Plasma Cell Neoplasia.
Multiple Myeloma.
Multiple myeloma (MM) is a rare, malignant tumor of plasma cells that arises in the
bone marrow and usually secretes large amounts of immunoglobulin. The finding of neoplastic
plasma cells in blood samples or smears is rare. Dogs are affected more frequently
than other species, but multiple myeloma has also been reported in horses, cattle,
cats, and pigs. Diagnosis of multiple myeloma is based on finding a minimum of two
or three (opinions vary) of the following abnormalities:
•
Markedly increased numbers of plasma cells in the bone marrow (Fig. 13-32, A
)
Figure 13-32
Multiple Myeloma and Monoclonal Gammopathy.
A, Canine bone marrow aspirate. Many of the neoplastic plasma cells in the bone marrow
aspirate have pink-tinged cytoplasm (arrow), the result of a high concentration of
immunoglobulin. Wright's stain. B, Multiple myeloma, cat. Agarose gels and densitometry
tracings showing results of serum electrophoresis. The serum has a high concentration
of a monoclonal immunoglobulin (the dark band [arrow] on the right of the gel, corresponding
to the tall peak on the right of the tracing). C, Normal cat. Agarose gels and densitometry
tracings showing results of serum electrophoresis. The serum has a normal distribution
of protein fractions, the most abundant being albumin (the dark band [arrow] on the
left of the gel, corresponding to the tall peak on the left of the tracing).
(A courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.
B and C courtesy Dr. S.A. Kania, College of Veterinary Medicine, University of Tennessee.)
•
Monoclonal gammopathy
•
Radiographic evidence of osteolysis
•
Light chain proteinuria
The classic laboratory finding in patients with multiple myeloma is hyperglobulinemia,
which results from the excessive production of immunoglobulin or an immunoglobulin
subunit by the neoplastic cells. This homogeneous protein fraction is often called
paraprotein or M protein. Paraproteins produced from the same clone of plasma cells
have the same molecular weight and electric charge. Therefore they have the same migration
pattern using serum protein electrophoresis, which results in a tall, narrow spike
in the globulins region, termed monoclonal gammopathy (see Fig. 13-32, B). The term
gammopathy is used because most immunoglobulins migrate in the γ-region of an electrophoresis
gel. However, some immunoglobulins, especially immunoglobulin A (IgA) and IgM, migrate
to the β-region. Occasionally, biclonal or other atypical electrophoretic patterns
may be seen with multiple myeloma as a result of protein degradation, protein complex
formation, binding to other proteins, or when the tumor includes more than one clonal
population. It is important to note that monoclonal gammopathy is not specific to
multiple myeloma but has also been reported with lymphoma, chronic lymphocytic leukemia,
canine ehrlichiosis, and canine leishmaniasis. Definitively distinguishing monoclonal
from polyclonal gammopathy requires immunoelectrophoresis or immunofixation using
species-specific antibodies recognizing different immunoglobulin subclasses and subunits.
Occasionally, multiple myeloma cells produce only the immunoglobulin light chain.
An immunoglobulin monomer consists of two heavy chains and two light chains connected
by disulfide bonds. These light chains may deposit in tissues and cause organ dysfunction,
especially renal failure. When the light chains form amyloid deposits, the disease
is called amyloid light chain amyloidosis. But if the light chains deposit as nonamyloid
granules, it is termed light chain deposition disease. Light chains are low-molecular-weight
proteins that pass through the glomerular filter into the urine, wherein they are
also known as Bence Jones proteins. They tend to not react with urine dipstick protein
indicators and are most specifically detected by electrophoresis and immunoprecipitation.
In addition to aiding in the diagnosis of multiple myeloma, paraproteins have an important
role in pathogenesis of disease. These proteins may inhibit platelet function, increase
blood viscosity, deposit in glomerular basement membranes (see Chapter 11; see Figs.
11-27 and 11-28, or precipitate at cool temperatures, which results in bleeding tendencies,
hyperviscosity syndrome, glomerulopathies, and cryoglobulinemia, respectively. Hyperviscosity
syndrome refers to the clinical sequelae of pathologically increased blood viscosity,
which are slowed blood flow and loss of laminar flow. Clinical signs include mucosal
hemorrhages, visual impairment due to retinopathy, and neurologic signs, such as tremors
and abnormal aggressive behavior. Cryoglobulinemia is the condition in which proteins,
typically IgM, precipitate at temperatures below normal body temperature (cold agglutinins).
Precipitation often occurs in blood vessels of the skin and extremities, such as the
ears and digits, and results in ischemic necrosis.
In multiple myeloma the neoplastic proliferation of plasma cells results in osteolysis.
Work with human cell cultures has shown that osteoclasts support the growth of myeloma
cells, and that direct contact between the two cell types increases the myeloma cell
proliferation and promotes osteoclast survival. Increased osteoclast activity causes
osteolysis, but the exact mechanism is not known. Osteolysis often results in bone
pain, lytic bone lesions on radiographs, hypercalcemia, and increased serum alkaline
phosphatase activity. Later in disease, osteolysis may cause pathologic fractures.
Morphologically, myeloma cells tend to grow in sheets that displace normal hematopoietic
cells in the bone marrow. A proposed diagnostic criterion of multiple myeloma is that
plasma cells constitute 30% or more of the nucleated cells in the marrow. Well-differentiated
plasma cells are round with abundant basophilic cytoplasm (due to increased rough
endoplasmic reticulum) and a perinuclear pale zone (enlarged Golgi apparatus for the
production of immunoglobulin); anisocytosis and anisokaryosis are often mild but may
be marked. Some plasma cell neoplasms have a bright eosinophilic fringe due to accumulated
IgA (see Fig. 13-32, A). Nuclei are round with clumped chromatin and often peripherally
placed with the cytoplasm; binucleation and multinucleation are common. Poorly differentiated
myeloma cells may lack and/or display less characteristic features. Osteolysis of
bone may be present microscopically. Common sites of metastasis include the spleen,
liver, lymph nodes, and kidneys.
Disorders of Horses
Congenital Disorders
Flavin Adenine Dinucleotide Deficiency.
Flavin adenine dinucleotide (FAD) is a cofactor for cytochrome-b
5 reductase, the enzyme that maintains hemoglobin in its functional reduced state,
and for glutathione reductase, an enzyme that also protects erythrocytes from oxidative
damage. Reported in a Spanish mustang mare and a Kentucky mountain saddle horse gelding,
erythrocyte FAD deficiency is a result of an abnormal riboflavin kinase reaction,
which is the first reaction in converting riboflavin to FAD. Clinicopathologic changes
include persistent methemoglobinemia of 26% to 46%, eccentrocytosis, a slightly decreased
or normal hematocrit, and erythroid hyperplasia in the bone marrow.
Infectious Diseases
Equine Infectious Anemia Virus.
Equine infectious anemia virus (EIAV), the agent of equine infectious anemia, is a
lentivirus that infects cells of the monocyte-macrophage system in horses (also ponies,
donkeys, and mules). The virus is mechanically transmitted by biting flies, such as
horseflies and deer flies. Less common routes of transmission include blood transfusions,
contaminated medical equipment, and transplacentally. Disease may present in acute,
subacute, and chronic forms and is potentially fatal. After an acute period of fever,
depression, and thrombocytopenia that lasts 1 to 3 days, there is a prolonged period
of recurrent fever, thrombocytopenia, and anemia. In most cases, clinical disease
subsides within a year, and horses become lifelong carriers and reservoirs of EIAV.
EIAV causes anemia by both immune-mediated hemolysis and decreased erythropoiesis.
Hemolysis is typically extravascular but may have an intravascular component during
the acute phase. Decreased erythropoiesis may result from direct suppression of early-stage
erythroid cells by the virus, as well as anemia of inflammation. Thrombocytopenia
likely results from immune-mediated platelet destruction and suppressed platelet production.
Animals dying during hemolytic crises are pale with mucosal hemorrhages and dependent
edema. The spleen and liver are enlarged, dark, and turgid, and they and other organs
have superficial subcapsular hemorrhages. Petechiae are evident beneath the renal
capsule and throughout the cortex and medulla. The bone marrow is dark red as a result
of replacement of fat by hematopoietic tissue; the extent of replacement is an indication
of the duration of the anemia.
The severity of microscopic lesions is dependent on the chronicity of the disease,
and they are most significant in the spleen, liver, and bone marrow. As would be anticipated,
microscopic findings of the spleen are predominantly influenced by the number and
activity of macrophages, which is a reflection of the duration of the disease and
the frequency of hemolytic episodes. Hemosiderin-laden macrophages persist for months
to years; therefore large numbers are consistent with chronicity. Kupffer cell hyperplasia
with hemosiderin stores and periportal infiltrates of lymphocytes are the most significant
changes in the liver. Bone marrow histologic findings vary depending on the duration
of the disease. In most animals the marrow is cellular because of the replacement
of fat by intense, orderly erythropoiesis. Granulocytes are relatively less numerous,
and plasma cells are increased. As in the spleen, hemosiderin-laden macrophages are
present in large numbers in chronic cases. Emaciated animals with chronic disease
have serous atrophy of fat (see E-Fig. 13-1).
Clinical findings with viremic episodes include fever, depression, icterus, petechial
hemorrhages, lymph node enlargement, and dependent edema. Equine infectious anemia
infection is diagnosed on the basis of the Coggins test, an agarose gel immunodiffusion
test for the presence of the antibody against the virus.
Disorders of Ruminants (Cattle, Sheep, and Goats)
Congenital Disorders
Congenital Dyserythropoiesis in Polled Herefords.
A syndrome of congenital dyserythropoiesis and alopecia occurs in polled Hereford
calves. The cause and pathogenesis of this often fatal disease are unknown. Early
in disease there is hyperkeratosis and alopecia of the muzzle and ears, which progresses
to generalized alopecia and hyperkeratotic dermatitis. Histologically, there is orthokeratotic
hyperkeratosis with dyskeratosis, as well as erythroid hyperplasia, dysplasia, and
maturation arrest in the bone marrow. Ineffective erythropoiesis results in nonregenerative
to poorly regenerative anemia.
Erythrocyte Band 3 Deficiency in Japanese Black Cattle.
Erythrocyte band 3 is integral membrane protein that connects to the cytoskeleton
and aids in erythrocyte stability. A hereditary deficiency of this protein has been
identified in Japanese black cattle, resulting in increased erythrocyte fragility,
spherocytosis, intravascular hemolytic anemia, and retarded growth. Affected calves
show lesions consistent with hemolytic anemia, including pale mucous membranes, icterus,
and splenomegaly. Histologically, there are bilirubin accumulations in the liver,
and hemosiderin in renal tubules.
Infectious Diseases
Bovine Leukemia Virus.
Bovine leukemia virus is discussed in the later section on lymphoma (see Lymphoid/Lymphatic
System, Disorders of Domestic Animals: Lymph Nodes, Neoplasia, Lymphoma).
Bovine Viral Diarrhea Virus.
BVDV infection may cause thrombocytopenia in cattle, and a thrombocytopenic hemorrhagic
syndrome has been specifically caused by type II BVDV infection. Investigations of
the mechanism of BVDV-induced thrombocytopenia have resulted in varying, sometimes
conflicting, conclusions. More than one study has shown viral antigen within bone
marrow megakaryocytes and circulating platelets. Evidence of impaired thrombopoiesis
(megakaryocyte necrosis, megakaryocyte pyknosis, and degeneration) and increased thrombopoiesis
(megakaryocytic hyperplasia, increased numbers of immature megakaryocytes) in the
bone marrow has been reported in type II BVDV–infected animals, including concurrent
megakaryocyte necrosis and hyperplasia in some experimental subjects. Calves infected
with type II BVDV also have impaired platelet function.
Cattle with the hemorrhagic syndrome are severely thrombocytopenic and neutropenic
with multisystemic hemorrhages, particularly of the digestive tract, spleen, gallbladder,
urinary bladder, and lymph nodes. Histologic lesions include hemorrhage, epithelial
necrosis of enterocytes, intestinal erosions, crypt proliferation with microabscesses,
and lymphoid depletion of the gut-associated lymphoid tissue, Peyer's patches, and
spleen. Lesions of the bone marrow are variable, as previously described.
Immune-Mediated Disorders
Bovine Neonatal Pancytopenia.
Bovine neonatal pancytopenia (BNP) is caused by alloantibodies absorbed from colostrum,
resulting in a hemorrhagic syndrome in calves. The syndrome was first recognized in
Europe in the early 2000s and has since been experimentally correlated with prior
vaccination of affected calves' dams with a commercial BVDV vaccine (Pregsure BVD;
Pfizer Animal Health). The vaccine has since been voluntarily recalled from the market.
It is thought that vaccination induces alloantibody formation by the dam. The alloantibodies
are ingested by the calf and bind to the calf's hematopoietic progenitor cells, resulting
in functional compromise of those cells. Acutely affected calves are less than a year
of age and have peripheral thrombocytopenia and neutropenia. Death results from thrombocytopenia-induced
hemorrhages or neutropenia-induced secondary infections, including pneumonia, enteritis,
and septicemia. Within the bone marrow there is erythroid, myeloid, and megakaryocytic
hypoplasia.
Disorders of Dogs
Congenital Disorders
Cyclic Hematopoiesis.
Cyclic hematopoiesis (also known as lethal gray collie disease) is an autosomal recessive
disorder of pluripotent HSCs in gray collie dogs. A defect in the adaptor protein
complex (AP3) results in defective intracellular signaling and predictable fluctuations
in concentrations of blood cells that occur in 14-day cycles. The pattern is cyclic
marked neutropenia, and in a different phase, cyclic reticulocytosis, monocytosis,
and thrombocytosis. Production of key cytokines involved in regulation of hematopoiesis
is also cyclic. Neutropenia predisposes affected animals to infection, and many die
of infectious causes. Affected animals have dilute hair coats and lesions with acute
or chronic infectious disease, especially of the lungs, gastrointestinal tract, and
kidneys. Dogs older than 30 weeks of age have systemic amyloidosis, which occurs because
of cyclic increases in concentration of acute phase proteins during phases of monocytosis.
Phosphofructokinase Deficiency.
Inherited autosomal recessive deficiency of the erythrocyte glycolytic enzyme, phosphofructokinase
(PFK), is described in English springer spaniel, American cocker spaniel, and mixed-breed
dogs. There are three genes encoding PFK enzymes, designated M-PFK in muscle and erythrocytes,
L-PFK in liver, and P-PFK in platelets. A point mutation in the gene coding for M-PFK
results in an unstable, truncated molecule. Erythrocytes in PFK-deficient dogs have
decreased ATP and 2,3-diphosphoglycerate (2,3-DPG) production and increased fragility
under alkaline conditions. The disease is characterized by chronic hemolysis with
marked reticulocytosis. The marked regenerative response may compensate for the ongoing
hemolysis; therefore affected animals are not necessarily anemic. However, acute intravascular
hemolytic episodes may occur with hyperventilation-induced alkalemia. Lesions are
typical of hemolytic anemia and include pale mucous membranes, icterus, hepatosplenomegaly,
and dark red urine with microscopic EMH and marrow erythroid hyperplasia. A single
DNA-based test is available to detect the common mutation.
Erythrocyte Structural Abnormalities.
Congenital erythrocyte structural abnormalities may occur with abnormal membrane composition
or defective proteins within the membrane or cytoskeleton. Some of these morphologic
changes occur concurrently with clinical disease, but others do not.
Hereditary stomatocytosis is recognized in Alaskan malamutes, Drentse patrijshonds,
and schnauzers. The specific defects are not known, but they are likely different
in the various dog breeds. However, all affected dogs have stomatocytes on blood smear
evaluation, as identified by their slit-shaped area of central pallor. Erythrocytes
also have increased osmotic fragility and decreased survival. Schnauzers are clinically
healthy and not anemic but do have reticulocytosis, suggesting that the hemolytic
anemia is compensated by erythroid hyperplasia. Mild to marked hemolytic anemia is
documented in Alaskan malamutes and Drentse patrijshonds. Alaskan malamutes have concurrent
short-limb dwarfism, and Drentse patrijshonds have hypertrophic gastritis and polycystic
kidney disease.
Other (presumably heritable) erythrocyte abnormalities in dogs that do not have clinical
signs include elliptocytosis caused by band 4.1 deficiency or β-spectrin mutation,
and familial macrocytosis and dyshematopoiesis in poodles.
Scott's Syndrome.
An inherited thrombopathy resembling Scott's syndrome in human beings, in which platelets
lack normal procoagulant activity, has been recognized in a family of German shepherd
dogs. The specific defect in these dogs has not been identified on the molecular level
but involves impaired expression of phosphatidylserine on the platelet surface. Affected
dogs have a mild to moderate clinical bleeding tendency characterized by epistaxis,
hyphema, intramuscular hematoma formation, and increased hemorrhage with surgery.
Macrothrombocytopenia.
Macrothrombocytopenia is an inherited condition in Cavalier King Charles spaniels
in which there are lower than normal concentrations of platelets with enlarged and
giant platelets. The condition is caused by defective β1-tubulin, which results in
impaired microtubule assembly. Affected dogs are asymptomatic but may have abnormal
platelet aggregation in vitro.
Infectious Diseases
Canine Distemper.
Canine distemper virus preferentially infects lymphoid, epithelial, and nervous cells
and is presented in greater detail in the lymphoid section. Canine distemper virus
may also infect other hematopoietic cells, including erythrocytes, nonlymphoid leukocytes,
and platelets (Fig. 13-33
), and can cause decreased peripheral blood concentrations of neutrophils, lymphocytes,
monocytes, and platelets during viremia. The thrombocytopenia is a result of virus-antibody
immune complexes on platelet membranes and direct viral infection of megakaryocytes.
Figure 13-33
Canine Distemper Viral Inclusions, Canine Blood Smear.
Note the viral inclusions within the erythrocyte and neutrophil (arrows). Inclusions
may also be present within other leukocyte types, or rarely, platelets. Diff-Quik
stain.
(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute
and State University.)
Disorders of Cats
Congenital Disorders
Increased Erythrocyte Osmotic Fragility.
A condition characterized by increased erythrocyte osmotic fragility has been described
in Abyssinian and Somali cats. The specific defect has not been identified, but PK
deficiency (which has been reported in these breeds) was excluded as the cause. Affected
cats have chronic intermittent severe hemolytic anemia and often have other lesions
secondary to hemolytic anemia (e.g., splenomegaly and hyperbilirubinemia).
Infectious Diseases
Cytauxzoonosis.
Cytauxzoonosis is a severe, often fatal disease of domestic cats caused by the protozoal
organism, Cytauxzoon felis. Disease is relatively common in the south central United
States, particularly during summer months. Bobcats (Lynx rufus) and other wild felids
are thought to be wildlife reservoirs of disease. C. felis is transmitted by a tick
vector, Dermacentor variabilis, which is probably essential for infectivity of the
organism.
Cytauxzoonosis has a schizogenous phase within macrophages throughout the body (especially
liver, spleen, lung, lymph nodes, and bone marrow) that causes systemic illness. These
schizont-containing macrophages enlarge and accumulate within the walls of veins,
eventually causing vessel occlusion, circulatory impairment, and tissue hypoxia. Later
in disease, merozoites released from schizonts enter erythrocytes, resulting in an
erythrocytic phase of infection. Infected domestic cats often have nonregenerative
anemia, but the pathogenesis for the anemia is unclear. However, it likely represents
preregenerative hemolytic anemia because erythrocyte phagocytosis is a prominent finding
in many organs. Infected cats often also develop neutropenia and thrombocytopenia,
which likely result from inflammation and disseminated intravascular coagulation,
respectively.
On blood smear evaluation, signet ring–shaped erythrocytic inclusions (piroplasms)
may be observed during the erythrocytic phase of disease (Fig. 13-34
). These inclusions closely resemble small-form Babesia (see Fig. 13-24, A) and some
Theileria organisms. Postmortem examination typically shows pallor, icterus, splenomegaly,
enlarged and red lymph nodes, diffuse pulmonary congestion and edema, and multisystemic
petechiae and ecchymoses. Vascular obstruction may cause marked distention of abdominal
veins. Cavitary effusions are present in some cats. Microscopically, large, schizont-laden
macrophages accumulate within venous and sinusoidal lumens and often completely occlude
the lumens (Fig. 13-35
). Erythrophagocytosis, thrombosis, and histologic changes of ischemia are common,
especially within the spleen, liver, and lungs.
Figure 13-34
Cytauxzoonosis, Feline Blood Smear.
Erythrocytes parasitized by Cytauxzoon felis contain signet ring–shaped inclusions
(arrows). (Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic
Institute and State University.)
Figure 13-35
Cytauxzoonosis, Tissue Aspirate (A) and Biopsy (B), Cat.
A, Lymph node aspirate. A large macrophage (center of figure) is laden with schizonts
of Cytauxzoon felis. Wright's stain. B, Splenic macrophages are filled with Cytauxzoon
organisms. H&E stain.
(A courtesy Dr. D.F. Edwards, College of Veterinary Medicine, University of Tennessee.
B courtesy Dr. A.R. Doster, University of Nebraska; and Noah's Arkive, College of
Veterinary Medicine, The University of Georgia.)
Affected cats typically become acutely ill with fever, pallor, and icterus and usually
die within 2 to 3 days. For many years, cytauxzoonosis was considered to be almost
always fatal. However, a recent report, in which numerous cats from a subregion of
the endemic area in the United States survived infection with an organism with greater
than 99% homology to Cytauxzoon felis, suggests the emergence of a less virulent strain.
Feline Leukemia Virus.
FeLV is an oncogenic, immunosuppressive lentivirus that causes hematologic abnormalities
of widely varying types and severity. Manifestations of disease caused by FeLV infection
vary depending on dose, viral genetics, and host factors, but normal hematopoiesis
is probably suppressed to some degree in all cases.
FeLV infects hematopoietic precursor cells soon after the animal is exposed and continues
to replicate in hematopoietic and lymphatic tissue of animals that remain persistently
viremic. The virus disrupts normal hematopoiesis by inducing genetic mutations, by
other direct effects of the virus on infected hematopoietic cells, or by an altered
host immune system. Hematologic changes include dysmyelopoiesis with resultant cytopenias
or abnormal cell morphologic features, and neoplastic transformation of hematopoietic
cells (leukemia). A notable form of dysplasia is the presence of macrocytic erythrocytes
(macrocytes) and metarubricytosis in the absence of erythrocyte regeneration (inappropriate
metarubricytosis). The relatively uncommon subgroup C viruses cause erythroid hypoplasia,
probably because of infection of early-stage erythroid precursors. FeLV may be detected
in megakaryocytes and platelets in infected cats and may result in platelet abnormalities,
including thrombocytopenia, thrombocytosis, increased platelet size, and decreased
function. Proposed mechanisms of FeLV-induced thrombocytopenia include direct cytopathic
effects, myelophthisis, and immune-mediated destruction. Platelet life span and function
have been shown to be decreased in FeLV-positive cats. Persistently viremic cats are
immunosuppressed and are prone to developing other diseases, including infectious
diseases, bone marrow disorders, and lymphoma.
CBC abnormalities attributed to FeLV infection include various cytopenias, especially
nonregenerative anemia, which may be persistent or cyclical. Regenerative anemia may
also occur with FeLV infection, often because of coinfection with M. haemofelis. Hematopoietic
cell dysplasia or neoplasia may also be evident. Grossly, infected cats are often
pale, but other lesions are dependent upon the presence of other cytopenias or concurrent
disease. Microscopically, the bone marrow is hypocellular, normocellular, or hypercellular.
There may be erythroid hypoplasia, erythroid hyperplasia with maturation arrest, or
acute leukemia.
Feline Immunodeficiency Virus.
Feline immunodeficiency virus (FIV), another feline lentivirus, causes anemia in a
minority of infected cats. Immunosuppressive effects of FIV from thymic depletion
are discussed elsewhere. It is generally accepted that anemia does not result directly
from FIV infection but instead develops because of concurrent disease such as coinfection
with FeLV or hemotropic mycoplasma, other infection, or malignancy. The severity and
type of anemia in FIV-infected cats depends on the other specific disease processes
involved.
Lymphoid/Lymphatic System
The thymus, spleen, lymph nodes, and lymph nodules, including MALT, are classified
as part of both the lymphoid and immune systems. The lymphoid system (also known as
lymphatic system in some texts) is broadly categorized into primary and secondary
lymphoid organs. The main primary lymphoid organs include thymus, bone marrow, and
bursa of Fabricius in birds and are the sites at which the B and T lymphocytes proliferate,
differentiate, and mature. In mammals, lymphocytes arise from HSCs in the bone marrow,
and B lymphocytes continue to develop at this site. Ruminants also have B lymphocyte
proliferation and maturation within their Peyer's patches. Progenitor T lymphocytes
migrate from bone marrow to mature and undergo selection in the thymus. The spleen,
lymph nodes, and lymph nodules are secondary lymphoid organs and are responsible for
the immune responses to antigens, such as the production of antibody and cell-mediated
immune reactions. At these sites, lymphocytes are activated by antigens and undergo
clonal selection, proliferation, and differentiation (see also Chapter 5). In addition,
the spleen and lymph nodes contain cells of the monocyte-macrophage system and thus
also participate in the phagocytosis of cells and materials.
The bone marrow is described in the first section of this chapter. The remaining primary
lymphoid organ, the thymus, is described first in this section, followed by the secondary
lymphoid organs: spleen, lymph nodes, and diffuse and nodular lymphatic tissues.
Dissection and Fixation of Lymphoid/Lymphatic Tissues
Errors from selection of inappropriate sampling sites and artifacts from compression
and incorrect fixation for histopathologic and immunohistochemical examinations are
common in routine veterinary pathologic analysis. The identification and remedies
for these problems are discussed in E-Appendix 13-2.
Thymus
Structure and Function
The thymus is essential for the development and function of the immune system, specifically
for the differentiation, selection, and maturation of T lymphocytes generated in the
bone marrow (see also Chapter 5). The basic arrangement of the thymus in domestic
animals consists of paired cervical lobes (left and right), an intermediate lobe at
the thoracic inlet, and a thoracic lobe, which may be bilobed. The cervical lobes
are positioned ventrolateral to the trachea, adjacent to the carotid arteries, and
extend from the intermediate lobe at the thoracic inlet as far cranially as the larynx.
The intermediate lobe bridges between the cervical and the thoracic lobe. The right
thoracic lobe is usually small or completely absent. The left lobe lies in the ventral
aspect of the cranial mediastinum (except in the ruminant, where it is dorsal) and
extends caudally as far as the pericardium.
Horse—The cervical lobes in foals are small, and the thoracic lobe constitutes the
bulk of the thymus.
Ruminant—The cervical lobes are large. The left and right thoracic lobes are fused
and unlike other domestic animals, lie in the dorsal aspect of the cranial mediastinum.
Pig—The cervical lobes are large.
Dog—The cervical lobes regress very early and thus appear absent. The thoracic lobe
extends caudally to the pericardium.
Cat—The cervical lobes are small, and the thoracic lobe, which forms the majority
of the thymus, extends caudally to the pericardium and molds to its surface.
The thymus is referred to as a lymphoepithelial organ and hence is composed of epithelial
and lymphoid tissue. Formed from the endoderm of the third pharyngeal pouch in the
fetus, the thymic epithelium is infiltrated by blood vessels from the surrounding
mesoderm, resulting in the development of the thymic epithelial reticulum. The lymphocyte
population consists of bone marrow–derived progenitor cells, which fill spaces within
the epithelial network. A connective tissue capsule surrounds the thymus, and attached
thin septa subdivide the tissue into partially separated lobules. Each lobule is composed
of a central medulla and surrounding cortex (Fig. 13-36
).
Figure 13-36
Lobular Organization of the Thymus.
The thymus consists of several incomplete lobules. Each lobule contains an independent
outer cortical region, and the central medullary region is shared by adjacent lobules.
Trabeculae, extensions of the capsule down to the corticomedullary region, form the
boundary of each lobule. The cortex consists of stromal cells, cortical epithelial
cells, macrophages, and developing T lymphocytes (thymocytes). Major histocompatibility
complex class I and II molecules are present on the surface of the cortical epithelial
cells. The characteristic deep blue staining of the cortex in histologic preparation
reflects the predominant dense population of T lymphocytes as compared with the less
basophilic medulla, which contains a lower number of thymocytes.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania;
and Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
The thymic cortex consists mainly of an epithelial reticulum and lymphocytes (Fig.
13-37
). The stellate cells of the epithelial reticulum have elongate branching cytoplasmic
processes that connect to adjacent epithelial cells through desmosomes, thus forming
a supportive network (cytoreticulum). The lymphoid component is composed of differentiating
lymphocytes derived from progenitor (also known as precursor) T lymphocytes in the
bone marrow. The medulla is composed of similar epithelial reticular cells, many of
which are much larger than those in the cortex and have a more obvious epithelial
structure. Some of the epithelial reticular cells form thymic corpuscles, also called
Hassall's corpuscles, which are distinctive keratinized epithelial structures (see
Fig. 13-37). Interdigitating dendritic cells (DCs) are also present within the medulla,
but there are far fewer lymphocytes than in the cortex.
Figure 13-37
Cell Populations in Lobules of the Thymus.
The functional thymus consists of two cell populations: stromal cells and thymocytes.
The stroma consists mainly of epithelial cells present beneath the capsule, lining
trabeculae and blood vessels, and forming the supportive network (cytoreticulum) within
the cortex and medulla; the medulla also contains Hassall's corpuscles. Macrophages
within the cortex and medulla are involved in the removal of apoptotic thymocytes
eliminated during clonal selection.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania
and Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
The progenitor T lymphocytes released from the bone marrow into the blood enter the
thymus in the subcapsular zone of the cortex and begin the differentiation and selection
processes, developing into mature naïve T lymphocytes as they traverse the thymic
cortex to the medulla. In the cortex, T lymphocytes that recognize self-molecules
(major histocompatibility complex [MHC] molecules) but not self-antigens are permitted
to mature by a process called positive selection. Cells that do not recognize MHC
molecules are removed by apoptosis. Those T lymphocytes that recognize both MHC molecules
and self-antigens are removed by macrophages at the corticomedullary junction, a process
called negative selection. Because of the rigid differentiation requirements attributable
to MHC restriction and tolerance (positive and negative selection, respectively),
only a small fraction (<5%) of the developing T lymphocytes that arrive at the thymus
from the bone marrow survive. Mature naïve T lymphocytes exit the thymus through postcapillary
venules in the corticomedullary region, enter the circulation, and recirculate through
secondary lymphoid tissues, primarily located in the paracortex of lymph nodes and
the periarteriolar sheaths of the spleen. In these specialized sites, the mature naïve
T lymphocytes are activated upon exposure to their specific antigens and undergo additional
phases of development to differentiate into effector and memory cells.
The thymus attains its maximal mass relative to body weight at birth and involutes
after sexual maturity; the rate of involution may vary among domestic species. The
lymphoid and epithelial components are gradually replaced by loose connective tissue
and fat, although remnants remain histologically, even in aged animals.
Dysfunction/Responses to Injury
The responses of the thymus to injury and causes are listed in Boxes 13-4
and 13-5
. The most common change is lymphoid atrophy caused by physical and physiologic stresses,
toxins, drugs, and viral infections.
Box 13-4
Responses of the Thymus to Injury
Lymphoid atrophy (see Box 13-5)
Inflammation—rare
Infectious agents (e.g., porcine circovirus type 2)
Hemorrhage and hematomas
Neoplasia
Thymoma
Lymphoma
Box 13-5
General Causes of Lymphoid Atrophy in Lymphoid Organs
Lack of antigenic stimulus
Toxins
For example, halogenated aromatic hydrocarbons, metals (lead, mercury), mycotoxins
Chemotherapeutic agents
For example, azathioprine, cyclophosphamide, cyclosporin A, corticosteroids
Ionizing radiation (+/−)
For example, when lymphoid tissue is present within therapeutic field
Viruses
For example, CDV, canine and feline parvovirus, FIV, BVDV, classic swine fever virus,
EHV-1
Malnutrition and Cachexia
Aging
BVDV, Bovine viral diarrhea virus; CDV, canine distemper virus; EHV-1, equine herpesvirus
1; FIV, feline immunodeficiency virus.
Atrophy.
Because the thymus does not contain any lymphopoietic tissue, it depends on the bone
marrow for the supply of progenitor T lymphocytes. Thus thymic lymphoid atrophy can
be the result of either an inadequate supply of lymphocytes from the bone marrow or
lysis of lymphocytes (lymphocytolysis) in the thymus. Thymic atrophy must be differentiated
from involution, which normally begins at sexual maturity. This distinction is difficult
to make, unless the change is extreme or age-matched control animals are available
for comparison.
Inflammation.
Inflammation of the thymus is rare. Neutrophils and macrophages are often present
within keratinized Hassall's corpuscles during involution and should not be mistaken
for a true thymitis. Thymitis has been reported in salmon poisoning disease of dogs
(see Chapter 7), epizootic bovine abortion (see Chapter 18), and in pigs infected
with porcine circovirus type 2 (PCV2). Necrosis and secondary infiltrates of neutrophils
and macrophages may be seen in other infectious diseases (e.g., equine herpesvirus
1 [EHV-1]).
Hemorrhage and Hematomas.
Thymic enlargement is often the result of hemorrhage, hematomas, or neoplasia and
is discussed further in the section on Lymphoid/Lymphatic System, Disorders of Domestic
Animals: Thymus, Disorders of Dogs.
Neoplasia.
Primary tumors of the thymus are thymomas, arising from the epithelial component,
and lymphomas and are discussed further in the section on Disorders of Domestic Animals:
Thymus.
Portals of Entry/Pathways of Spread
The main portal of entry to the thymus is hematogenous. Portals of entry used by microorganisms
and other agents and substances to access the lymphatic system are summarized in Box
13-6
. These portals include the blood vessels (hematogenous spread by microorganisms free
in the plasma or within circulating leukocytes or erythrocytes), afferent lymphatic
vessels (lymphatic spread), direct penetration, or through M (for “microfold”) cells
and DCs in MALT.
Box 13-6
Portals of Entry into Lymphoid Organs
Thymus
Hematogenous
Spleen
Hematogenous
Direct penetration
Lymph node
Hematogenous
Afferent lymphatic vessels
MALT
Hematogenous
Migrating macrophages
Dendritic cells
M cells (Peyer's patches)
MALT, Mucosa-associated lymphoid tissue.
Defense Mechanisms/Barrier Systems
Defense mechanisms used by the thymus to protect itself against microorganisms and
other agents are the innate and adaptive immune responses, discussed in Chapters 3,
4, and 5Chapter 3Chapter 4Chapter 5. Viruses, bacteria, and particles arriving in
the lymph and blood interact with cells of the monocyte-macrophage system through
phagocytosis and antigen processing and presentation. Hyperplasia of the macrophages
often occurs concurrently. Antigen processing and presentation are followed by an
immune response resulting in proliferation of B lymphocytes, plasma cells, and the
subsequent production of antibody; proliferation of T lymphocytes may also occur.
Spleen
The relationships between anatomic structures and the different functions of the spleen
are complicated. There are also anatomic differences among domestic animal species
and confusion about the correct and up-to-date terminology. The following brief discussion
aims to define the terms used in this chapter. The term splenic sinusoid is used to
describe a vascular structure present in the sinusal spleen (also known as sinusoidal
spleen); dogs are the only domestic animal with true splenic sinusoids. The term red
pulp vascular spaces is used (as opposed to “sinus”) to describe the vascular spaces
in the red pulp of both the nonsinusal and nonsinusoidal spleens of all domestic animals.4
The other terms used here include marginal sinus, marginal zone, periarteriolar lymphoid
sheath (PALS), periarteriolar macrophage sheath (PAMS), and splenic lymphoid follicles.
Structure
The spleen is located in the left cranial hypogastric region of the abdomen, where
it is typically suspended in the gastrosplenic ligament between the diaphragm, stomach,
and the body wall. The exception is in domestic ruminants, where it is closely adhered
to the left dorsolateral aspect of the rumen. The gross shape and size of the spleen
vary markedly among domestic animals, but generally it is a flattened, elongated organ.
Some species, notably birds, demonstrate seasonal variation in splenic shape and size.
The spleen is covered by a thick capsule composed of smooth muscle and elastic fibers,
from which numerous intertwining fibromuscular trabeculae extend into the parenchyma.
These trabeculae and reticular cells form a spongelike supportive matrix for the parenchyma
of the mammalian spleen in all domestic species. In cattle and horses the three muscular
layers of the capsule lie perpendicular to each other, forming a capsule thicker than
that of carnivores. Carnivores, small ruminants, and pigs have interwoven smooth muscle
within the splenic capsule, and pigs also have abundant elastic fibers within the
capsule.
The spleen differs from many other organs in the organization of its parenchyma. Instead
of a cortex and medulla, the spleen is divided into two distinct structural and functional
components: the red pulp and white pulp (Fig. 13-38
). With hematoxylin and eosin (H&E) staining, red pulp appears red-pink because of
the abundance of red blood cells, whereas white pulp appears blue-purple because of
the heavy concentration of lymphocytes. The white pulp consists of splenic follicles,
populated by B lymphocytes; the PALS, inhabited by T lymphocytes; and the marginal
zone at the periphery of follicles. Macrophages, antigen-presenting cells, and trafficking
B and T lymphocytes populate the marginal zone. The radial arteries, branches of the
central artery (also known as central arteriole), and capillaries from both red and
white pulp drain into the marginal sinus of the marginal zone, although the latter
has not been shown to be the case in all species to the same degree (e.g., the cat
has a small marginal sinus but a well-developed PAMS) (Figs. 13-39
and 13-40
). The red pulp consists of cells of the monocyte-macrophage system, PAMS, sinusoids
(dogs, rats, and human beings only), red pulp vascular spaces, and associated stromal
elements such as reticular cells, fibroblasts, and trabecular myocytes. The labyrinth
of the splenic red pulp vascular spaces serves as both a functional and physical filter
for circulating blood cells.
Figure 13-38
Structure of the Spleen—Red and White Pulp.
The spleen is organized into two distinct components. The red pulp consists of cells
of the monocyte-macrophage system, periarteriolar macrophage sheaths, sinusoids (dogs,
rats and human beings only), red pulp vascular spaces, and associated stromal elements
such as reticular cells, fibroblasts and trabecular myocytes. The white pulp is composed
of splenic follicles (B lymphocytes), periarteriolar sheaths (T lymphocytes), and
the marginal zone.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania;
Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee; and Dr.
J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
Figure 13-39
Structure of a Splenic Lymphoid Follicle (White Pulp).
The splenic white pulp is organized into periarteriolar sheaths (PALSs) around central
arteries composed mainly of T lymphocytes, splenic follicles primarily composed of
B lymphocytes, and the marginal zone, which forms the outer rim of the white pulp
nodule. When exposed to antigen, the splenic lymphoid follicles develop germinal centers.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania;
Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee; and Dr.
J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
Figure 13-40
Structure of the Marginal Zone in the Splenic Follicle.
Antigens, bacteria, particles, and other material enter the follicle via the central
arteries and reach the marginal sinus, where they are phagocytized by macrophages
of the marginal zone. Once captured by these macrophages, blood-borne antigens are
processed and presented to the lymphocytes within the white pulp.
The blood circulation of the spleen is particularly suited to enable its functions,
namely, (1) filtering and clearing the blood of particulate matter and senescent cells;
(2) transporting recirculating lymphocytes and naïve B and T lymphocytes to the follicle
and PALS, respectively, to fulfill their specific immune functions; and (3) storage
of blood in some domestic animal species (dog, cat, and horse) (Fig. 13-41
). Phagocytosis is particularly effective in the spleen because blood flows through
areas within the red pulp that are populated with increased concentrations of macrophages,
namely, within the marginal sinuses, in cuffs around the penicillar arteries (PAMS),
diffusely on the reticular walls of the red pulp vascular spaces, and along the sinusoids
in dogs. Trafficking of naïve and recirculating lymphocytes is facilitated by the
proximity of the marginal sinus to the follicular germinal centers and PALS.
Figure 13-41
Major Pathways of Blood Flow in Nonsinusoidal and Sinusoidal Spleens.
A, Nonsinusoidal spleen, all domestic animals except the dog. The splenic artery enters
at the hilus and divides into arteries, which enter the trabeculae. When a trabecular
artery emerges from a trabecula it becomes the central artery and is encased in a
periarteriolar lymphoid sheath (PALS), which is composed of T lymphocytes. It then
enters the splenic follicle and gives off branches—the radial arteries, which supply
the marginal sinus and marginal zone. The central artery emerges from the splenic
follicle to enter the red pulp and branches into the penicillary arterioles, which
are enclosed in a cuff of macrophages—the periarteriolar macrophage sheath (PAMS).
The emerging penicillar arteries branch into arterioles and capillaries that supply
the red pulp vascular spaces (see Fig. 13-43). The red pulp vascular spaces also receive
blood from capillaries draining from the marginal sinus and drain into the splenic
venules and then into the trabecular veins and splenic vein. B, Sinusoidal spleen,
dog. The blood flow is essentially the same but with the additional feature that arterioles
from the marginal sinus drain into the sinusoids and some blood from the red pulp
vascular space passes through slits in the sinusoidal wall to enter the sinusoid (see
Fig. 13-42). This is the site of pitting and erythrophagocytosis. Note that the major
flow in A is sequentially past concentrations of macrophages in the marginal sinus,
PAMS, and red pulp vascular spaces. In B there is the additional route from the marginal
zone into the sinusoids. The figure does not illustrate variations in anatomy in the
different domestic species such as a small marginal sinus in the cat and large PAMS
in the dog, pig, and cattle, or variations in the blood supply, such as the sheathed
arteries emptying into the marginal sinus.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania;
Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee; and Dr.
J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
Maps of the vascular blood flow in sinusoidal and nonsinusoidal spleens are illustrated
in Figure 13-41, Figure 13-42, Figure 13-43
. The celiac artery is the major branch of the abdominal aorta from which the splenic
artery arises. The splenic artery enters the splenic capsule at the hilus, where it
branches and enters the fibromuscular trabeculae as trabecular arteries to supply
the splenic parenchyma. Trabecular arteries become the central arteries of the white
pulp and are surrounded by cuffs of T lymphocytes forming the PALS. The splenic follicles,
populated by B lymphocytes, are eccentrically embedded within or just adjacent to
the PALS. The central arteries send branches—the radial arteries—to supply the marginal
sinus surrounding the splenic follicles. Thus the cells at the circumferences of the
follicles are brought into intimate contact with blood-borne antigens and trafficking
B and T lymphocytes in the marginal sinus. As a result of this pattern of blood flow,
macrophages in the marginal sinus have the first opportunity to phagocytize antigens,
bacteria, particles, and other material before macrophages in the sinusoids (in the
dog) or in the PAMS and red pulp vascular spaces (all other domestic animals). In
the dog the marginal sinus drains into the sinusoids, but in other domestic animals
it drains into the red pulp vascular spaces.
Figure 13-42
Vascular Flow in the Red Pulp of Dogs—Sinusoids.
A, Sinusoidal spleen, dog. Structure and function of red pulp sinusoids. 1, Branches
of the central arteries of the white pulp and vessels from the marginal sinus enter
into the sinusoids. 2, Sinusoids are lined by a discontinuous endothelium, and these
empty into splenic venules, creating a closed system of circulation. 3, The red pulp
of the dog spleen consists of both sinusoids and red pulp vascular spaces. B, Histomorphologic
features of red and white pulp. C, capsule; T, trabeculae.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania;
Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee; and Dr.
J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
Figure 13-43
Vascular Flow in the Red Pulp of Domestic Animals—Red Pulp Vascular Spaces.
Red pulp vascular spaces occur in all domestic animals. 1, Blood travels into red
pulp arterioles to enter red pulp vascular spaces in both types of spleens. 2, Blood
leaves red pulp vascular spaces via red pulp venules and the trabecular veins. Red
pulp vascular spaces are lined by reticular cells, and macrophages attached to these
reticular walls provide constant surveillance of the blood. A, Artery; V, vein; T,
trabecula.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania;
Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee; and Dr.
J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
The central arteries leave the white pulp, enter the red pulp, and branch into smaller
penicillar arterioles. Each arteriole is surrounded by a sheath of macrophages known
as periarteriolar macrophage sheaths (PAMS, previously known as ellipsoids), which
are notably prominent in pigs, dogs, and cats. In horses, cattle, pigs, and cats the
terminal branches of the penicillar arterioles empty into the red pulp vascular spaces
lined by reticular cells. Because the red pulp vascular spaces are not lined by endothelium,
this type of circulation is known as an open system. This system is in contrast to
the sinusoidal spleen of the dog (also of the rat and human beings), where the branches
of the central artery of the white pulp and vessels from the marginal sinus enter
into the sinusoids, which are lined by a discontinuous endothelium, and these empty
into splenic venules. This type of circulation is known as a closed system because
the blood flow is through blood vessels (arterioles, capillaries, sinusoids, and venules),
all of which are lined by endothelium. Although circulation in the red pulp is anatomically
open in nonsinusoidal spleens, under certain conditions (e.g., during splenic contraction)
the circulation is functionally closed, and the blood in the red pulp is diverted
into “channels” lined by reticular cells. Because the dog has both sinusoids and red
pulp vascular spaces, it has both open and closed splenic circulations, which may
allow for both fast and slow flows of blood depending on the physiologic need of the
animal. Blood flowing through the sinusoids or red pulp vascular spaces is under the
surveillance of macrophages. In dogs the pseudopodia of these perisinusoidal macrophages
project into the sinusoidal lumen through the spaces in the discontinuous endothelium.
In all domestic animals, blood in the red pulp vascular spaces is under surveillance
of macrophages attached to the reticular walls. Blood from the red pulp vascular spaces
and sinusoids then drains into the splenic venules, splenic veins, and ultimately
into the portal vein, which empties into the liver.
Function
The spleen filters blood and removes foreign particles, bacteria, and erythrocytes
that are senescent, have structural membrane abnormalities, or are infected with hemotropic
parasites. As a secondary lymphoid organ, its immunologic functions include the activation
of macrophages to process and present antigen, the proliferation of B lymphocytes
and production of antibody and biologic molecules, and the interaction of T lymphocytes
and antigens. In some species the spleen stores significant quantities of blood (Box
13-7
). The functions of the spleen are best considered on the basis of the two main components
of the spleen: the red and white pulp and the anatomic systems contained within them
(monocyte-macrophage system, red pulp vascular spaces, and hematopoiesis in the red
pulp, and the B and T lymphocyte systems within the white pulp).
Box 13-7
Functions of the Spleen
Red Pulp
Filtration (Monocyte-Macrophage System)
Removal (phagocytosis) of foreign material and bacteria
Removal of erythrocytes
Senescent erythrocytes
Damaged erythrocytes (e.g., immune-mediated anemias)
Parasitized erythrocytes (e.g., hemotropic parasites)
Storage (Red Pulp Vascular Spaces)
Storage of blood (in storage spleens)
Hematopoiesis
Extramedullary hematopoiesis
Severe demand (e.g., anemias)
Degenerative/inflammatory conditions without concomitant hematologic disease
Incidental (e.g., within nodules of hyperplasia)
Monocytes within splenic cords
Reserve for generating tissue macrophages in response to inflammation
White Pulp
Immunologic Functions
PALS, splenic lymphoid follicle
Lymphocyte transformation and proliferation
Production of antibodies
Marginal zone
Homing of circulating lymphocytes in the blood
Phagocytosis and processing of antigen
Macrophage activation
PALS, Periarteriolar lymphoid sheath.
Red Pulp
Monocyte-Macrophage System.
Within the red pulp, macrophages are located in the marginal sinus, PAMS, and attached
to the reticular walls of the red pulp vascular spaces. In the dog, macrophages are
also located perisinusoidally. The supportive reticular network of the red pulp vascular
spaces is composed of a fine meshwork of reticular fibers made of type III collagen,
on which macrophages are dispersed. Exactly in which of these concentrations of macrophages
phagocytosis of blood-borne particles takes place depends upon (1) the sequence in
which they are exposed to the incoming blood, (2) the concentration of macrophages
in these areas (e.g., the cat marginal sinus is small and thus not a major site of
clearance; there is a compensatory increase in PAMS for phagocytosis), and (3) the
functions of the macrophages. Some of the macrophages in the marginal sinus and marginal
zone are responsible for phagocytosis of particulate matter and others for the trapping
and ingestion of antigens and antigen-antibody complexes. Macrophages responsible
for phagocytosis of blood-borne foreign material (Fig. 13-44
), bacteria, and senescent and/or damaged erythrocytes (e.g., as seen in immune-mediated
anemias and infections with hemotropic parasites) are also found in the red pulp.
In the dog, sinusoidal macrophages remove entire erythrocytes (erythrophagocytosis),
as well as portions of an erythrocyte's membrane and cytoplasmic inclusions, such
as nuclear remnants like Heinz bodies, by a process called pitting. As such, the presence
of large numbers of nuclear remnants in erythrocytes in canine blood smears may indicate
malfunction of the sinusoidal system. The normal rate of removal of senescent erythrocytes
from the circulating blood does not cause an increase in size of the spleen; however,
splenomegaly can be observed when large numbers of defective erythrocytes must be
removed, as in cases of severe acute hemolytic anemia. Nonsinusoidal spleens lack
the fenestrated endothelium and perisinusoidal macrophages of canine sinusoids that
allow for slow processing of red blood cells to determine which are to be returned
to the circulation, pitted, or phagocytized. Instead, the macrophages of the red pulp
perform these functions, and phagocytized cells remain in the red pulp vascular spaces.
The location of the primary sites of pitting in nonsinusoidal spleens is unclear,
but it is likely that most erythrophagocytosis takes place in the red pulp vascular
spaces. The cat's spleen is deficient in pitting, and removal of Heinz bodies is slow;
however, some erythrophagocytosis does occur in the marginal sinus.
Figure 13-44
Phagocytosis of Foreign Material by Macrophages of the Splenic Marginal Zone (Calf
Injected Intravenously with Micronized Carbon Particles).
Carbon particles (black pigment) are present in macrophages of the marginal zone.
Macrophages phagocytize blood-borne foreign material, bacteria, viruses, and senescent
and/or damaged erythrocytes (as in immune-mediated anemias and infections with hemotropic
parasites). Eosin counterstain.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
The macrophages of the sinusoids, marginal sinus, and red pulp vascular spaces are
of bone marrow origin. From the bone marrow these cells circulate in the blood as
monocytes and migrate into the spleen. Some macrophages are replenished by local proliferation.
For example, after phagocytizing large amounts of material from the blood, the macrophages
of the PAMS migrate through the wall of the cuff into the adjacent red pulp, denuding
the PAMS of macrophages. After 24 hours, local residual macrophages have proliferated
to repopulate the PAMS. The fixed macrophages elsewhere in the body, namely, those
in connective tissue, lymph nodes (sinus histiocytes), liver (Kupffer cells), lung
(pulmonary intravascular macrophages and pulmonary alveolar macrophages), and brain
(resident and perivascular microglial cells), are also derived from bone marrow (see
Chapters 5, 8, 9, and 14Chapter 5Chapter 8Chapter 9Chapter 14).
Red Pulp Vascular Spaces
Storage or Defense Spleens.
Spleens are also classified as either storage or defense spleens, based on whether
or not they can store significant volumes of blood. The ability to store blood in
the spleen depends on the fibromuscular composition of the splenic capsule and trabeculae.
Splenic capsules and trabeculae with a low percentage of smooth muscle and elastic
fibers cannot expand and contract and are designated as defense spleens. These are
found in rabbits and human beings. The spleens of other domestic animal species have
storage and defense functions but are classified as storage spleens because the extensive
smooth muscle of the capsule and trabeculae allows the spleen to expand and contract.
The spleens of ruminants and pigs are intermediate in their amount of smooth muscle
and thus have limited storage capacity. Equine, canine, and feline spleens all have
considerable storage and contractile capacity because of their muscular capsule, increased
numbers of trabeculae, and the relatively small amount of splenic parenchyma devoted
to white pulp. The storage capacity in dogs and horses is remarkable: It has been
claimed that the canine spleen can store one-third of the dog's erythrocytes while
the animal sleeps and the equine spleen holds one-half of the animal's circulating
red cell mass (which is considered advantageous because it reduces the viscosity of
the circulating blood). Storage spleens expand and contract quickly under the influence
of the autonomic nervous system, via sympathetic and vagal fibers in the trabeculae
and reticular walls of the red pulp vascular spaces and other circulatory disruptions,
such as hypovolemic and/or cardiogenic shock. Thus storage spleens may be either grossly
enlarged and congested or small with a wrinkled surface and a dry parenchyma depending
on whether the spleen is congested from stored blood or shrunken from contraction
(see Uniform Splenomegaly and Small Spleens).
Hematopoietic Tissue.
In the developing fetus the liver is the primary site of hematopoiesis, with the spleen
making a minor contribution. Shortly before or after birth, hematopoiesis ceases in
the liver and spleen, and the bone marrow becomes the primary hematopoietic organ.
Under certain conditions, such as severe demand due to prolonged anemia, splenic hematopoiesis
can be reactivated; this outcome is called extramedullary hematopoiesis (EMH). Studies
have indicated that splenic EMH in dogs and cats most commonly occurs with degenerative
or inflammatory conditions (e.g., hematomas, thrombosis) and may occur without concomitant
hematologic disease (see Uniform Splenomegaly with a Firm Consistency). It is also
found in splenic nodular hyperplasia (see Splenic Nodules with a Firm Consistency).
In some species, such as the mouse, EMH is a normal function of the adult spleen and
not necessarily a response to disease or hypoxic challenge. The splenic red pulp also
contains large numbers of monocytes, which function as a reserve for generating tissue
macrophages in response to ongoing tissue inflammation in the body.
White Pulp.
White pulp consists of PALS, each with a splenic lymphoid follicle surrounded by a
marginal zone. Normally these foci of white pulp are so small that they may not be
visible on gross examination of a cross section of the spleen. However, if nodules
are enlarged either by lymphoid hyperplasia, amyloid deposits, or a neoplastic process
(e.g., lymphoma), they can become grossly visible on the cut surface, initially as
0.5- to 1.0-mm white circular foci scattered through the red pulp. In animals with
storage spleens, the distention of the red pulp from stored blood separates the foci
of white pulp (PALS and lymphoid follicles), making white pulp appear sparser. Splenic
white pulp is organized around central arteries in the form of PALS, which are populated
primarily by T lymphocytes (see Figure 13-38, Figure 13-39, Figure 13-40). Primary
splenic follicles are located eccentrically in PALS and are primarily composed of
B lymphocytes. When exposed to antigen, the splenic lymphoid follicles develop germinal
centers (see Lymphoid/Lymphatic System, Lymph Nodes, Function). Macrophages in the
white pulp follicles remove apoptotic B lymphocytes not selected for expansion because
of low binding affinity for antigen. Failure of these macrophages to phagocytize has
been experimentally correlated with decreased production of growth factors like TGF-β
and increased production of inflammatory cytokines that predispose the animal to autoimmune
conditions.
The marginal zone surrounds the marginal sinus at the interface of the white and red
pulp and consists of macrophages, DCs, and T and B lymphocytes. The blood supply of
the marginal sinus is from the radial branches of the central artery, and it serves
as the portal of entry into the spleen for recirculating B and T lymphocytes. From
here, T lymphocytes migrate to the PALS and B lymphocytes to the germinal centers.
Macrophages in the marginal zone capture blood-borne antigens, process them, and present
them to the lymphocytes. B lymphocytes that recognize antigens corresponding to their
receptors are activated, enter the follicle, and proliferate.
Macrophages in the marginal zone are phenotypically distinct from those in the red
pulp. The red pulp macrophages function primarily to filter the blood by phagocytizing
particles and by removing senescent or infected erythrocytes and pathogenic bacteria
and fungi. Marginal zone macrophages are divided into two types based on their location
and the type of cell surface receptors they possess. The first group is positioned
toward the periphery of the marginal zone, whereas the second group, the marginal
metallophilic macrophages (so called for their silver staining positivity), is at
the inner margin of the marginal zone closer to the splenic follicle and PALS. It
has been difficult to generate mammalian models that eliminate one of the two classes
of marginal zone macrophages, so the degree to which one group specializes in a particular
function is not clear. Some marginal zone macrophages actively phagocytize particulate
matter or bacteria (e.g., septicemias caused by Streptococcus pneumoniae, Listeria
monocytogenes, Campylobacter jejuni, or Bacillus anthracis) in the blood (see Fig.
13-40). They also play a similar role in limiting the spread of viral infections.
Other marginal zone macrophages phagocytize and process antigens. Thus macrophages
of the marginal zone serve to bridge the innate and adaptive immune responses by secreting
inflammatory cytokines to activate other immune cells and providing receptor-based
activation of marginal zone lymphocytes. Studies have shown that a loss of marginal
zone macrophages coincides with decreased antigen trapping by resident B lymphocytes
of the marginal zone and consequently a decrease in the early IgM response to antigens.
Dysfunction/Responses to Injury
The responses of the spleen to injury (Box 13-8
) include acute inflammation, hyperplasia of the monocyte-macrophage system, hyperplasia
of lymphoid tissues, atrophy of lymphoid tissues, storage of blood or contraction
to expel reserve blood, and neoplasia. These responses are also best considered on
the basis of the two main components of the spleen, the red and white pulp, and the
anatomic systems associated with each.
Box 13-8
Responses of the Spleen to Injury
Red Pulp
Inflammation
Acute inflammation with fibrin and necrosis
Abscesses, microabscesses
Granulomatous inflammation (diffuse, multifocal, focal)
Monocyte-Macrophage System
Hyperplasia
Infectious agents
Bacteremia/septicemia
Facultative intracellular pathogens (e.g., Mycobacterium bovis)
Fungi
Chronic hemolytic disease
Chronic splenic congestion
Red Pulp Vascular Spaces
Congestion
Blood storage or expulsion
White Pulp
Lymphoid hyperplasia (PALS, splenic lymphoid follicle)
Macrophage hyperplasia (marginal zone)
Atrophy (PALS, splenic lymphoid follicle; see Box 13-5)
Neoplasia
Primary
Lymphoma
Sarcoma (e.g., hemangiosarcoma, histiocytic sarcoma, leiomyosarcoma)
Metastatic
PALS, Periarteriolar lymphoid sheath.
Red Pulp
Monocyte-Macrophage System.
The distribution and function of macrophages in the spleen is described earlier in
the section on Structure and Function. These interactions are complex, and their relationships
to both innate and adaptive immunity are areas of intense study (see also Chapter
5). To facilitate filtering, all of the blood in the body passes through the spleen
at least once a day, and 5% of the cardiac output goes to the spleen. In dogs, blood
flow and transit time depend on whether the spleen is contracted or distended; blood
flow is slower in the distended spleen. The extent to which macrophages of the monocyte-macrophage
system phagocytize particles depends to a large degree on the sequence in which they
receive blood. In most species, macrophages of the marginal sinus are the first to
receive blood, and consequently phagocytized particles and bacteria tend to be more
concentrated here initially. However, there are differences among domestic animal
species; the cat, for instance, has a comparatively small marginal sinus, and thus
the PAMS play a larger role in phagocytosis.
The spleen is able to mount a strong response to blood-borne pathogens, which has
been demonstrated in several studies. The blood of immunized rabbits injected intravenously
with pneumococci cleared 98% of those bacteria within 15 minutes and 100% within an
hour. The blood of dogs injected with 1 billion pneumococci per pound of body weight
into the splenic artery was cleared of all bacteria in 65 minutes. After splenectomy,
blood-borne organisms multiply rapidly and may disseminate widely in the body to cause
an overwhelming postsplenectomy infection. Studies have also shown that the phagocytic
function of the spleen is critical in the control of plasmodium (causative agent of
malaria) in human beings and babesiosis in cattle. If the number of pathogenic bacteria
in the circulation exceeds the capacity of the splenic macrophages, as in cases of
severe septicemia, it may result in acute splenic congestion (see Uniform Splenomegaly
with a Bloody Consistency). This may be followed by inflammation with areas of necrosis,
fibrin deposition, and infiltration by neutrophils in bacteremias of pyogenic bacteria.
The marginal zone can be the initial site of response to blood-borne antigens and
bacteria delivered by the radial branches of the central arteries to the marginal
sinus. Similar to the response of the red pulp vascular spaces, the marginal zone
can become congested and with time (only hours with highly pathogenic organisms) may
contain aggregates of neutrophils and macrophages. Histologically, the congestion
and inflammation form a complete or partial concentric ring around the circumference
of the splenic nodule (see Anthrax).
Hyperplasia of the red pulp macrophages is also seen in chronic hemolytic diseases,
because there is a prolonged need for phagocytosis of erythrocytes. Similarly, chronic
splenic congestion, usually the result of portal or splenic vein hypertension, can
lead to proliferation of the macrophages present on the walls of the red pulp vascular
spaces and results in thickening of the reticular walls between the red pulp vascular
spaces. Macrophages in the red pulp also proliferate in response to fungi and facultative
intracellular pathogens (e.g., Mycobacterium bovis) arriving hematogenously to the
spleen. The number of red pulp macrophages may be augmented by monocytes recruited
from the blood to form granulomatous inflammation, which may be diffuse or multifocal/focal
(e.g., blastomycosis and tuberculosis, respectively).
Red Pulp Vascular Spaces.
The main response to injury of the red pulp vascular spaces is congestion (see Uniform
Splenomegaly with a Bloody Consistency), as well as the storage of blood or contraction
to expel reserve blood.
White Pulp.
The responses to injury within the white pulp are most pronounced in the splenic lymphoid
follicles. Lymphoid follicular hyperplasia is a response to antigenic stimuli and
results in the formation of secondary follicles; marked hyperplasia may be grossly
evident. Hyperplasia of splenic lymphoid follicles follows a similar sequence of events
and morphologic changes as seen in other secondary lymphoid organs and is discussed
in more detail in Lymphoid/Lymphatic System, Lymph Nodes, Dysfunction/Responses to
Injury. Similarly, atrophy of splenic lymphoid follicles has similar causes as lymphoid
atrophy in other lymphoid organs (see Box 13-5). Briefly, atrophy occurs in response
to lack of antigenic stimulation (e.g., from regression after antigenic stimulation
has ceased), from the effects of toxins, antineoplastic chemotherapeutic agents, microorganisms,
radiation, malnutrition, wasting/cachectic diseases, or aging, or when the bone marrow
and thymus fail to supply adequate numbers of B and T lymphocytes, respectively. The
follicles are depleted of lymphocytes, and with time, germinal centers and follicles
disappear. The amount of the total lymphoid tissue is reduced, and the spleen may
be smaller.
The response to injury of the monocyte-macrophage system in the marginal sinus and
marginal zone is also phagocytosis and proliferation.
Capsule and Trabeculae.
Lesions in the capsule and trabeculae are uncommon and include splenic capsulitis
secondary to peritonitis, and complete or partial rupture of the splenic capsule,
usually due to trauma.
Portals of Entry/Pathways of Spread
The two main portals of entry to the spleen for infectious agents are hematogenous
spread and direct penetration. The splenic capsule is thick, and thus direct penetration
is less common. Inflammation from an adjacent peritonitis is unlikely to penetrate
the capsule into the splenic parenchyma. Cattle with traumatic reticulitis may have
foreign objects migrate into the ventral extremity of the spleen, causing a splenic
abscess. Splenic abscesses also develop secondary to perforation of the gastric wall
in horses, due to foreign body penetration, gastric ulcers, or gastric inflammation.
Portals of entry used by microorganisms and other agents and substances to access
the lymphoid/lymphatic system are summarized in Box 13-6.
Defense Mechanisms/Barrier Systems
Defense mechanisms used by the spleen to protect itself against microorganisms and
other agents are the innate and adaptive immune responses, discussed in Chapters 3,
4, and 5Chapter 3Chapter 4Chapter 5. Other defense mechanisms are structural in nature
to protect against external trauma and include the thick fibrous capsule of the spleen.
Lymph Nodes
Structure
Lymph nodes are soft, pale tan, round, oval or reniform organs with a complex three-dimensional
structure. On gross examination of a cross section of lymph nodes, two main areas
are visible: an outer rim of cortex and an inner medulla (Fig. 13-45
). To understand the pathologic response of the lymph node, it is important to consider
its anatomic components and their relationship with antigen processing (Fig. 13-46
):
•
Stroma—Capsule, trabeculae, and reticulum
•
Cortex—“Superficial” or “outer” cortex (lymphoid follicles, B lymphocytes)
•
Paracortex—“Deep” or “inner” cortex (T lymphocytes)
•
Medulla—Medullary sinuses and medullary cords
•
Blood vessels—Arteries, arterioles, high endothelial venules (HEVs), efferent veins
•
Lymphatic vessels—Lymphatic afferent and efferent vessels; lymphatic sinuses (subcapsular,
trabecular, and medullary)
•
Monocyte-macrophage system—Sinus histiocytes
Figure 13-45
Structure of a Lymph Node.
1 and 2, Lymph node architecture consists of an outer cortex composed of lymphoid
follicles (B lymphocytes), inner/deep paracortex (T lymphocytes), and medulla (medullary
cords and sinus). Lower right, Histologic section. H&E stain. 3, Gross photograph
of a lymph node: The cortex (both outer and inner) is pale tan-pink, and the medulla
is dark red.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania;
Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee; and Dr.
J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
Figure 13-46
Cellular Zones of a Lymph Node.
1, Antigen arrives in the afferent lymphatic vessels, empties into the subcapsular
sinus, and drains into the trabecular and medullary sinuses. 2, As antigens travel
through the sinuses, they are captured and processed by macrophages and dendritic
cells (DCs), or antigen-bearing DCs in blood can enter through high endothelial venules
(HEVs). B lymphocytes encounter DCs charged with antigen, are activated, and migrate
to a primary follicle to initiate germinal center formation, creating secondary follicles.
3 and lower right image, lymphoid follicles. Germinal centers have a distinct polarity
(superficial or light zone and a deep dark zone), and the mantle cell rim partially
encircles the germinal center and is wider over the light pole of the follicle.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania;
Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee; and Dr.
J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
Stroma.
The lymph node is enclosed by a fibrous capsule penetrated by multiple afferent lymphatic
vessels, which empty into the subcapsular sinus (see also Figs. 13-45 and 13-46).
At the hilus, efferent lymphatic vessels and veins exit, and arteries enter the node.
Fibrous trabeculae extend from the capsule into the parenchyma to provide support
to the node and to house vessels and nerves. The lymph node is also supported by a
meshwork of fibroblastic reticular cells and fibers. Besides providing structural
support, this reticulum helps form a substratum for the migration of lymphocytes and
antigen-presenting cells to the follicles and facilitates the interaction with B and
T lymphocytes.
Cortex.
The outer/superficial cortex contains the lymphoid follicles (also referred to as
lymphoid nodules) (see Figs. 13-45 and 13-46). The follicles are designated as primary
if they consist mainly of small lymphocytes: Mature naïve B lymphocytes expressing
receptors for specific antigens exit the bone marrow and circulate through the bloodstream,
lymphatic vessels, and secondary lymphoid tissues. On their arrival at lymph nodes,
B lymphocytes exit through HEVs in the paracortex and home to a primary follicle (which
also contains follicular DCs in addition to the resting B lymphocytes). Lymphoid follicles
with germinal centers are designated as secondary follicles: B lymphocytes that recognize
the antigen for which they are expressing receptors are activated and proliferate
to form the secondary lymphoid follicles characterized by prominent germinal centers.
Germinal centers are areas with a specialized microenvironment that support the proliferation
and further development of B lymphocytes to increase their antigen and functional
capacity (see Lymphoid/Lymphatic System, Lymph Nodes, Function). The mantle cell zone
surrounds the germinal center and consists of small inactive mature naïve B lymphocytes
and a smaller population of T lymphocytes (approximately 10%).
Paracortex.
The diffuse lymphoid tissue of the paracortex (also referred to as the deep or inner
cortex) consists mainly of T lymphocytes, as well as macrophages and DCs (see Figs.
13-45 and 13-46). This region contains the HEVs through which B and T lymphocytes
migrate from the blood into the lymphoid follicles and paracortex, respectively. T
and B lymphocytes may also enter the lymph node via the lymphatic vessels.
Medulla.
The medulla is composed of medullary cords and medullary sinuses (see Figs. 13-45
and 13-46). The medullary cords contain macrophages, lymphocytes, and plasma cells.
In a stimulated node the cords become filled with antibody-secreting plasma cells.
The medullary sinuses are lined by fibroblastic reticular cells and contain macrophages
(“sinus histiocytes”), which cling to reticular fibers crossing the lumen of the sinus.
These macrophages phagocytize foreign material, cellular debris, and bacteria from
the incoming lymph.
Vasculature: Blood Vessels, Lymphatic Vessels, and Lymphatic Sinuses.
The blood vessels of the lymph node include arteries, arterioles, veins, and postcapillary
venules (HEVs) lined by specialized cuboidal endothelium (see Figs. 13-45 and 13-46).
Approximately 90% to 95% of lymphocytes enter lymph nodes through the HEVs, which
also play an important role in lymph fluid balance. The lymphatic vasculature consists
of afferent lymphatic vessels, which pierce the capsule and drain into the subcapsular
sinus. Lymph continues to drain through the trabecular sinuses to the medullary sinuses
and finally exits at the hilus via efferent lymphatic vessels.
All lymph nodes receive afferent lymphatic vessels from specific areas of the body.
The term lymphocenter is often used in veterinary anatomy to describe a lymph node
or a group of lymph nodes that is consistently present at the same location and drains
from the same region in all species. For example, the popliteal lymph node, caudal
to the stifle, drains the distal hind limb. The tracheobronchial nodes (bronchial
lymphocenter), located at the tracheal bifurcation, collect lymph from the lungs and
send it to the mediastinal nodes or directly to the thoracic duct. Because lymph from
a single afferent lymphatic vessel drains into a discrete region of a lymph node,
only these regions of the node may be affected by the contents of a single draining
lymph vessel (e.g., antigen, infectious organisms, or metastatic neoplasms [Fig. 13-47
]).
Figure 13-47
Subcapsular Sinuses with Metastatic Carcinoma, Lymph Node, Dog.
Subcapsular sinuses are sites for embolization, lodgment, invasion, and growth of
neoplastic emboli (arrows), most commonly carcinomas. Emboli initially lodge in that
portion of the lymph node drained by the branch of the afferent lymphatic vessel draining
the site of the primary carcinoma. H&E stain.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
The lymph node of the pig has a different structure. The afferent lymphatic vessels
enter at the hilus instead of around the periphery of the node and empty lymph into
the center of the node. The lymph drains to the “subcapsular” sinus (the equivalent
of the medullary sinuses of other domestic animals) and then into several efferent
lymphatic vessels, which pierce the outer capsule. This reversal of flow is the result
of an inverted nodal architecture, with the cortex in the middle of the node surrounded
by the medulla at the periphery. Thus a pig lymph node that is draining an area of
hemorrhage will have blood accumulate in the periphery (subcapsular) instead of in
the center of the node (which may be grossly visible).
Function
The functions of the lymph node are (1) to filter lymph of particulate matter and
microorganisms, (2) to facilitate the surveillance and processing of incoming antigens
via interactions with B and T lymphocytes, and (3) to produce B lymphocytes and plasma
cells. Material arriving in the lymph can be subdivided into free particles and larger
molecules, small molecules and free antigens, and antigen within DCs. It is helpful
to consider the paths taken by particles, molecules, antigens, and cells arriving
at a lymph node. The following account describes the journey of an antigen as it enters
a lymph node to trigger an immune response.
Antigen in the lymph arriving in the afferent lymphatic vessels empties into the subcapsular
sinus. Hydrostatic pressure here is low, and reticular fibers crossing the sinus impede
flow, and thus particles tend to settle, which facilitates phagocytosis by the sinus
macrophages. Lymph then flows down the trabecular sinuses that line the outer surface
of fibrous trabeculae, to the medullary sinus, and eventually exits via efferent vessels.
As antigens within the lymph travel through the sinuses, they are captured and processed
by macrophages and DCs. Alternatively, DCs charged with antigen can migrate within
blood vessels to the node and enter the paracortex via the HEVs. Circulating B lymphocytes
also enter across the HEVs, and if they encounter antigen-bearing DCs, there is a
local reaction involving the appropriate T helper lymphocytes, B lymphocytes, and
DCs. This results in the migration of the activated B lymphocytes to a primary follicle,
where they initiate formation of a germinal center.
Germinal centers, upon migration of antigen-activated B lymphocytes, develop a characteristic
architecture. Distinct polarity composed of a superficial or light zone and a deep
dark zone is present in cases of antigenic stimulation. The light zone, orientated
at the source of antigen, consists mainly of small lymphocytes, called centrocytes,
which have moderate amounts of pale eosinophilic cytoplasm. The cells of the dark
zone, called centroblasts, are large, densely packed lymphocytes with scant cytoplasm,
giving this area a darker appearance on H&E staining. The centroblasts undergo somatic
mutations of the variable regions of the immunoglobulin gene, followed by isotype
class switching (from IgM to IgG or IgA). During this process most centroblasts undergo
apoptosis, and cell fragments are phagocytized by macrophages, which are then termed
tingible (stainable) body macrophages. The cells that have survived the affinity maturation
process are now called centrocytes and along with T lymphocytes and follicular DCs,
populate the germinal center light zone. These post–germinal center B lymphocytes
leave the follicle as plasma cell precursors (immunoblasts or plasmablasts) and migrate
from the cortex to the medullary cords, where they mature and excrete antibody into
the efferent lymph. Some of these cells may colonize the region surrounding the mantle
cell zone to form a marginal zone. Marginal zones are apparent only in situations
of prolonged and intense immune stimulation and serve as a reservoir of memory cells.
The elliptical mantle cell cuff is wider over the light pole of the follicle, though
in instances of strong antigenic stimulation, the cuffs can completely encircle the
germinal center.
Dysfunction/Responses to Injury
Responses to injury are listed in Box 13-9
, and the responses are discussed on the basis of the following systems: sinus histiocytes
of the monocyte-macrophage system, cortex, paracortex, and medulla (medullary sinuses
and medullary cords).
Box 13-9
Responses of the Lymph Node to Injury
Hyperplasia
Sinus histiocytosis (monocyte-macrophage system)
Follicular hyperplasia (B lymphocytes)
Paracortical hyperplasia (T lymphocytes)
Atrophy
Lymphoid atrophy (see Box 13-5)
Inflammation
Acute or chronic lymphadenitis
Neoplasia
Primary (lymphoma)
Metastatic
Generally, enlarged lymph nodes can be distributed in several different patterns in
the body. First, all lymph nodes throughout the body (systemic or generalized) may
be enlarged (lymphadenopathy or lymphadenomegaly). This pattern is usually attributed
to systemic infectious, inflammatory, or neoplastic processes. If a single lymph node
or regional chain of nodes is enlarged, then the area drained by that node should
be checked for lesions (e.g., evaluate the oral cavity if the mandibular lymph nodes
are enlarged). Thus it is important to know the area drained by specific lymph nodes.
Mesenteric lymph nodes are normally larger because of follicular hyperplasia and sinus
histiocytosis, because these nodes continuously receive and respond to barrages of
antigens and bacteria from the intestinal tract.
Sinus Histiocytes (Monocyte-Macrophage System).
Sinus histiocytes (macrophages) are part of the monocyte-macrophage system and the
first line of defense against infectious and noninfectious agents in the incoming
lymph. In response to these draining agents, there is hyperplasia of the macrophages
(“sinus histiocytosis”), most notable in the medullary sinuses (Fig. 13-48
). Leukocytes, often monocytes, may harbor intracellular pathogens (e.g., Mycobacterium
spp., cell-associated viruses such as parvovirus), arrive in the blood or lymph, infect
the lymph node, and then are disseminated throughout the lymphoid tissues of the body
via the efferent lymph and circulating blood.
Figure 13-48
Medullary Sinus Histiocytosis and Medullary Cord Plasmacytosis, Lymph Node, Dog.
1, The medullary sinuses are filled with histiocytes (macrophages) in response to
drainage of infectious and noninfectious agents in the incoming lymph. 2, The medullary
cords are filled with plasma cells and fewer lymphocytes. Plasma cell precursors are
formed in the germinal centers, mature into plasma cells, and migrate to the medullary
cords. The presence of large numbers of plasma cells in the medullary cords indicates
ongoing production of antibody due to an antigenic stimulus.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Cortex (Lymphoid Follicles).
Follicular hyperplasia of the cortex is discussed in the section Lymphoid/Lymphatic
System, Lymph Node, Function. An antigenically stimulated lymph node that is undergoing
follicular hyperplasia is enlarged and has a taut capsule, and the cut surface may
bulge. Histologically, the follicles contain active germinal centers with antigenic
polarity (light and dark zones) (Figs. 13-49
and 13-50
; also see Fig. 13-46). Depending on the duration and continued exposure to the antigen,
there may also be concomitant paracortical hyperplasia and medullary cord plasmacytosis.
Less florid follicular reactions will have smaller separated germinal centers, whereas
nodes receiving persistent high levels of antigen stimulation may have coalescing
germinal centers (termed “atypical benign follicular hyperplasia”). In such cases
of chronic strong antigenemia, the highly reactive nodes may also exhibit colonization
of lymphocytes into perinodal fat, and germinal centers may contain irregular lakes
of eosinophilic material, known as follicular hyalinosis. As the immune response declines,
there is follicular lymphoid depletion and the concentration of lymphocytes in the
germinal centers is reduced, allowing the underlying follicular stroma (including
DCs and macrophages) to become visible. With ongoing lymphocyte depletion, the mantle
cell zones are thinned, less populated, and discontinuous. Eventually, residual mantle
cells collapse into the follicular stroma, forming clusters of small dark cells within
the bed of DCs and macrophages, referred to as fading follicles.
Figure 13-49
Benign Follicular Hyperplasia, Lymph Node, Dog.
Antigenic stimulation results in a secondary follicle with germinal center formation
(G). The centroblasts of the germinal center undergo somatic mutations and isotype
class switching, a process during which most centroblasts undergo apoptosis and cell
fragments are phagocytized by tingible body macrophages. The cells that have survived
the affinity maturation process (centrocytes) leave the germinal center as plasma
cell precursors. Some of these cells migrate to the medullary cords, where they mature
and excrete antibody into the efferent lymph, whereas others colonize the region surrounding
the mantle cell zone to form a marginal zone. In instances of strong antigenic stimulation,
the mantle cell cuffs can completely encircle the germinal center (arrows). H&E stain.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Figure 13-50
Benign Follicular Hyperplasia, Chronic Demodicosis, Prescapular Lymph Node, Dog.
There is diffuse hyperplasia of the lymphoid follicles (F) with prominent and often
coalescing germinal centers. H&E stain.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Paracortex.
Paracortical atrophy may result from a variety of causes, including deficiency in
lymphocyte production in the bone marrow, reduced differential selection of lymphocytes
in the thymus, or destruction of lymphocytes in the lymph node by viruses, radiation,
and toxins directly on the lymphocytes in the lymph node (see Box 13-5). Examination
of H&E-stained sections allows evaluation of follicular activity in the cortex and
the concentration of plasma cells in the medullary cords, which serve as a reasonable
estimate of B lymphocyte activity for comparison.
Paracortical hyperplasia may have a nodular or diffuse appearance depending on which
and how many afferent lymphatic vessels are draining antigen. This reaction may precede
or be concurrent with the germinal center reaction of follicular hyperplasia. Proliferation
of T lymphocytes has been reported in the paracortex (and PALS of the spleen) in malignant
catarrhal fever (MCF) in cattle and in pigs with porcine reproductive and respiratory
syndrome. PCV2 can cause a diffuse proliferation of macrophages within the paracortex.
Medulla (Medullary Sinuses and Cords).
Responses to injury by the medullary sinuses are dilation of the sinuses and proliferation
of histiocytes (“sinus histiocytosis”). Sinus macrophages proliferate in response
to a wide variety of particulate matter in the lymph, including bacteria and erythrocytes
(erythrophagocytosis) draining from a hemorrhagic area (see Lymphoid/Lymphatic System,
Disorders of Domestic Animals: Lymph Nodes, Pigmentation of Lymph Nodes). Dilation
of the sinuses due to edema occurs with many underlying conditions, including chronic
cardiac failure or drainage from an acutely inflamed area. As the inflammation progresses,
the sinuses become filled with neutrophils, macrophages, and occasionally fibrin,
in addition to the hyperplastic resident sinus histiocytes (see Fig. 13-48). Depending
on the intensity of the inflammation, the adjacent parenchyma may become affected
(see Lymphoid/Lymphatic System, Disorders of Domestic Animals: Lymph Nodes, Enlarged
Lymph Nodes [Lymphadenomegaly], Acute Lymphadenitis).
As pointed out in the section on Lymph Nodes, Function, after activation and proliferation
of B lymphocytes in the follicle, the immunoblasts formed there move to and mature
in the medullary cords, which as a result are distended with plasma cells that secrete
antibody into the efferent lymphatic vessels (“medullary plasmacytosis”). The concentration
of medullary plasma cells correlates with the activity of the germinal centers. As
the immune response subsides, the number of plasma cells decreases and the medullary
cords return to their resting state populated by few lymphocytes and scattered plasma
cells.
Portals of Entry/Pathways of Spread
The two main portals of entry to the lymph node for infectious agents and antigens
are afferent lymphatic vessels (lymphatic spread) and blood vessels (hematogenous
spread). Portals of entry used by microorganisms and other agents and substances to
access the lymphoid/lymphatic system are summarized in Box 13-6. Infectious microorganisms,
either free within the lymph or within lymphocytes or monocytes, are transported to
regional lymph nodes through lymphatic vessels. Agents may escape removal by phagocytosis
in one lymph node and be transported via efferent lymphatic vessels to the next lymph
node in the chain and cause an inflammatory or immunologic response there. This process
can continue serially down a lymph node chain, and if the agent is not removed, it
may eventually be transported via the lymphatic vessels to either the cervical or
thoracic ducts and then disseminated throughout the body.
Although most pathogens are transported to lymph nodes via afferent lymphatic vessels,
bacteria can be transported to lymph nodes hematogenously (free or within leukocytes
such as monocytes) in septicemias and bacteremias. Direct penetration of a lymph node
is uncommon, because it is protected by a thick fibrous capsule. Occasionally, inflammatory
cells or neoplasms can extend directly into nodal parenchyma from adjacent tissues.
Defense Mechanisms/Barrier Systems
Defense mechanisms used by the lymphatic system to protect itself against microorganisms
and other agents are the innate and adaptive immune responses, discussed in Chapters
3, 4, and 5Chapter 3Chapter 4Chapter 5. Other defense mechanisms are structural in
nature to protect against external trauma and include the thick fibrous capsules of
lymph nodes.
Hemal Nodes
Structure and Function
Hemal nodes are small, dark red to brown nodules found most commonly in ruminants,
mainly sheep, and have also been reported in horses, primates, and some canids. Their
architecture resembles that of a lymph node with lymph follicles and sinuses, except
that in the hemal node, sinuses are filled with blood (E-Fig. 13-8). Because erythrophagocytosis
can be present, it is presumed that hemal nodes can filter blood and remove senescent
erythrocytes, but as their blood supply is small, their functional importance is not
clear.
E-Figure 13-8
Hemal Node, Ruminant.
Hemal nodes resemble lymph nodes except that the sinuses are filled with blood. H&E
stain.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Mucosa-Associated Lymphoid Tissue
Structure and Function
MALT is the initial site for mucosal immunity and is crucial in the protection of
mucosal barriers. MALT is composed of both diffuse lymphoid tissues and aggregated
lymphoid (also known as lymphatic) nodules, which can be subcategorized based on their
anatomic location: (1) bronchus-associated lymphoid tissue (BALT), which is often
at the bifurcation of the bronchi and bronchioles; (2) tonsils (pharyngeal and palatine)
form a ring of lymphoid tissue at the oropharynx; (3) nasal-, larynx-, and auditory
tube–associated lymphoid tissues (NALT, LALT, and ATALT, respectively) within the
nasopharyngeal area; (4) gut-associated lymphoid tissue (GALT), which includes Peyer's
patches and diffuse lymphoid tissue in the gut wall; (5) conjunctiva-associated lymphoid
tissue (CALT); (6) other lymphoid nodules (e.g., genitourinary tract) (Fig. 13-51
).
Figure 13-51
Lymphoid Follicular Hyperplasia, Mucosa-Associated Lymphoid Tissue.
A, The mucosa contains multifocal, slightly raised, soft white nodules (arrows). B,
Prominent lymphoid follicles (arrows) have formed within the submucosa. H& E stain.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Diffuse lymphoid tissue consists of lymphocytes and DCs within the lamina propria
of the mucosa of the alimentary, respiratory, and genitourinary tracts. These cells
intercept and process antigens, which then travel to regional lymph nodes to initiate
the immune response, leading ultimately to the secretion of IgA, IgG, and IgM.
Solitary lymphoid nodules are localized concentrations of lymphocytes (mainly B lymphocytes)
in the mucosa and consist of defined but unencapsulated clusters of small lymphocytes
(primary lymphoid nodule). They are usually not grossly visible in the resting or
antigenically unstimulated state, but upon antigenic stimulation, they proliferate
and form germinal centers and surrounding mantle cell zones (secondary lymphoid nodules).
Aggregated lymphoid nodules consist of groups of lymph nodules, the most notable of
which are the tonsils and Peyer's patches. The aggregated lymphoid follicles of the
Peyer's patches are most obvious in the ileum. The latter are covered by a specialized
epithelium, the follicle-associated epithelium (FAE). The FAE is the interface between
the Peyer's patches and the luminal microenvironment and consists of enterocytes and
interdigitated M cells. M cells transport (via endocytosis, phagocytosis, pinocytosis,
and micropinocytosis) antigens, particles, bacteria, and viruses from the intestinal
lumen to the underlying area rich in DCs , which deliver the material to the lymphoid
tissue of the Peyer's patches. M cells also express IgA receptors, which allows for
the capture and transport of bacteria entrapped by IgA. The proportion of enterocytes
and M cells within the FAE is modulated by the luminal bacterial composition. For
instance, M cells increase in animals transferred from pathogen-free housing to the
normal environment. M cells may also be exploited as a portal for entry by some microbes
(see Lymphoid/Lymphatic System, Portals of Entry/Pathways of Spread). Table 13-5
lists the interactions of the MALT with different microorganisms.
Table 13-5
Function of Mucosa-Associated Lymphoid Tissue (MALT) in Viral and Bacterial Diseases
in Livestock
Function
Species
Microorganism
TONSILS
Portal of entry
Sheep and goats
Chlamydia psittaci
Initial site of infection
Cattle
BVDV
Early site of infection
Cattle
BHV-1
Site of replication
Pig
Porcine circovirus type 2PMWS infection
Carriers (reservoirs)
Horses
Streptococcus equi subsp. zooepidemicus
Cattle
Mannheimia haemolytica
Sheep
Salmonella spp.
Pasteurella haemolytica
Scrapie agent (PrPSc)
Pigs
Mycoplasma spp.
Streptococcus suis
Salmonella spp.
Yersinia pseudotuberculosis
GALT (PEYER'S PATCHES)
Portal of entry
Cattle
Brucella abortus
Mycobacterium avium ssp. paratuberculosis
Sheep and goats
Yersinia tuberculosis
BVDV, Bovine viral diarrhea virus; BHV-1, bovine herpesvirus 1; PMWS, postweaning
multisystemic wasting syndrome.
Data from Liebler-Tenorio EM, Pabst R: Vet Res 37:257-280, 2006.
Dysfunction/Responses to Injury
The responses of MALT to injury are similar to those of other lymphoid tissues: hyperplasia,
atrophy, and inflammation (Box 13-10
).
Box 13-10
Responses of Mucosa-Associated Lymphoid Tissue to Injury
Hyperplasia
Lymphoid hyperplasia with germinal center formation due to antigenic stimulation
Atrophy
Lymphoid atrophy (see Box 13-5)
Inflammation
Granulomatous (Johne's disease; see Diseases of Ruminants)
Hyperplasia.
Hyperplasia of lymphoid nodules is a response to antigenic stimulation and consists
of activation of germinal centers with subsequent production of plasma cells (see
Fig. 13-51, B). Lymphoid nodule hyperplasia is often present in chronic disease conditions,
such as BALT hyperplasia in chronic Dictyocaulus spp. (horses, cattle, sheep, and
goats) or Metastrongylus spp. (pigs) associated bronchitis or bronchiolitis. Mycoplasma
spp. pneumonias of sheep and pigs display marked BALT hyperplasia that can encircle
bronchioles and bronchi (“cuffing pneumonia”).
Hyperplastic lymphoid nodules can be so enlarged that they become grossly visible
as discrete white plaques or nodules (see Fig. 13-51, A). They can be seen in the
conjunctiva of the eyelids and the third eyelid in chronic conjunctivitis, the pharyngeal
mucosa in chronic pharyngitis, the gastric mucosa in chronic gastritis, and the urinary
bladder in chronic cystitis (follicular cystitis). The normal fetus has no detectable
BALT, though it may be present in fetuses aborted due to infectious disease.
Atrophy.
Atrophy of the diffuse lymphoid tissue and lymphoid nodules has the same causes as
atrophy affecting other lymphoid tissues (see Box 13-5) and includes lack of antigenic
stimulation, cachexia, malnutrition, aging, viral infections, or failure to be repopulated
by B lymphocytes from the bone marrow or T lymphocytes from the thymus. Lymphocytolysis
of germinal center lymphocytes of Peyer's patches is a characteristic lesion in BVDV
infection in ruminants and canine and feline parvovirus infections (“punched-out Peyer's
patches) (see Chapters 4 and 7).
Portals of Entry/Pathways of Spread
The main portals of entry to MALT for infectious agents are hematogenous spread and
through migrating macrophages, DCs , and M cells. Pathogenic bacteria such as Escherichia
coli, Yersinia pestis, Mycobacterium avium ssp. paratuberculosis (MAP), L. monocytogenes,
Salmonella spp., and Shigella flexneri can invade the host from the lumen of the intestine
through dendritic or M cells. Some viruses (e.g., reovirus) may be transported by
M cells. The scrapie prion protein (PrPSc) may also accumulate in Peyer's patches.
Many viruses, such as bovine coronavirus, BVDV, rinderpest virus, malignant catarrhal
fever virus, feline panleukopenia virus, and canine parvovirus, cause lymphocyte depletion
within the MALT. Portals of entry used by microorganisms and other agents and substances
to access the lymphoid system are summarized in Box 13-6.
Defense Mechanisms/Barrier Systems
Defense mechanisms used by MALT to protect itself against microorganisms and other
agents are the innate and adaptive immune responses, discussed in Chapters 3, 4, and
5Chapter 3Chapter 4Chapter 5.
Disorders of Domestic Animals: Thymus
Congenital Disorders
Congenital disorders of the thymus are discussed in detail in Chapter 5. Summaries
of the gross and microscopic morphologic changes are described in the sections on
Disorders of Horses and Disorders of Dogs.
Thymic cysts can be found within the developing and mature thymus and in thymic remnants
in the cranial mediastinum. Thymic cysts are often lined by ciliated epithelium and
represent developmental remnants of branchial arch epithelium and are usually of no
significance.
Inflammatory and Degenerative Disorders
Thymitis is an uncommon lesion and may be seen in PCV2 infection (see Disorders of
Pigs and also Chapter 4), enzootic bovine abortion (see Chapter 18), and salmon poisoning
disease of dogs (see Chapter 7). Infectious agents more commonly cause thymic atrophy.
Variable degrees of acquired immunodeficiency can be also be caused by toxins, chemotherapeutic
agents and radiation, malnutrition, aging, and neoplasia. Of infectious agents, viruses
most commonly infect and injure lymphoid tissues and include the following: EHV-1
in aborted foals (Fig. 13-52
), classic swine fever virus, BVDV, canine distemper virus, canine and feline parvovirus,
and FIV; severe thymic lymphoid depletion is an early lesion in FIV-infected kittens.
Figure 13-52
Equine Herpesvirus 1, Spleen, Aborted Foal.
Most of the splenic follicle is occupied by nuclear debris, the result of lymphocytolysis.
note: lymphocytolysis may be caused by other infectious and noninfectious agents (e.g.,
chemotherapeutic drugs). H&E stain.
(Courtesy College of Veterinary Medicine, University of Illinois.)
Environmental toxins, such as halogenated aromatic hydrocarbons (e.g., polychlorinated
biphenyls and dibenzodioxins), lead, and mercury have a suppressive effect on the
immune system. Halogenated aromatic hydrocarbons cause dysfunction of DCs through
several mechanisms that lead to atrophy of the primary and secondary lymphoid organs.
Heavy metals, such as lead, mercury and nickel, are immunosuppressive and generally
affect the levels of B and T lymphocytes, NK cells, and inflammatory cytokines. Other
metals, such as selenium, zinc, and vanadium, may be immunostimulatory at low doses.
The immunotoxic mechanisms may differ and include chelation of molecules and effects
on protein synthesis, cell membrane integrity, and nucleic acid replication. The toxic
effects of mycotoxins such as fumonisins B1 and B2 (secondary fungal metabolites produced
by members of the genus Fusarium) and aflatoxin (produced by Aspergillus flavus) include
lymphocytolysis in the thymic cortex.
Chemotherapeutic drugs inhibit the cell cycle through various mechanisms, and thus
all dividing cells, including lymphocytes, bone marrow cells, and enterocytes, are
sensitive to their effects. As such, bone marrow suppression, immunosuppression, and
gastrointestinal disturbances are common side effects of anticancer drugs. Purine
analogues (e.g., azathioprine) compete with purines in the synthesis of nucleic acids,
whereas alkylating agents like cyclophosphamide cross-link DNA and inhibit the replication
and activation of lymphocytes. Cyclosporin A specifically inhibits the T lymphocyte
signaling pathway by interfering with the transcription of the IL-2 gene. Methotrexate,
a folic acid antagonist, blocks the synthesis of thymidine and purine nucleotides.
The immunosuppressive effects of some of these agents is desirable for the treatment
of immune-mediated disease (e.g., immune-mediated hemolytic anemia) or to prevent
allograft rejection after transplantation. Corticosteroids may be given at an immunosuppressive
dose, though the degree of suppression is highly variable among species. Local or
palliative treatment of cancer may include radiotherapy (ionizing radiation) to target
and damage the DNA of the neoplastic cells. Although some immunosuppression may be
noted, particularly if bone marrow or lymphoid tissue is within the therapeutically
irradiated field, mounting evidence suggests that radiotherapy can induce a cascade
of proimmunogenic effects that engage the innate and adaptive immune systems to contribute
to the destruction of tumor cells.
Malnutrition and cachexia, which may occur with cancer, lead to secondary immunosuppression
through several complex metabolic and neurohormonal aberrations. Thymic function may
be impaired in young malnourished animals, resulting in a decrease in circulating
T lymphocytes and subsequent depletion of T lymphocyte regions of secondary lymphoid
organs. Lymphoid atrophy may result from physiologic and emotional stress, which can
cause the release of catecholamines and glucocorticoids.
Aging
As part of the general effects of aging in cells (see Chapter 1), all lymphoid organs
decrease in size (atrophy) with advancing age. In the case of the thymus this reduction
in size occurs normally after sexual maturity and is more appropriately termed thymic
involution. The term involution should be reserved for normal physiologic processes
in which an organ either returns to normal size after a period of enlargement (e.g.,
postpartum uterus) or regresses to a more primitive state (e.g., thymic involution).
Neoplasia
Because the thymus has both lymphoid and epithelial components, neoplasms may arise
from either component. Thymic lymphoma arises from the T lymphocytes in the thymus
(and very rarely B lymphocytes). It is most often seen in young cats and cattle and
less frequently in dogs (Fig. 13-53
) (see Hematopoietic Neoplasia). Thymomas arise from the epithelial component and
are usually benign neoplasms that occupy the cranial mediastinum of older animals.
Histologically, these neoplasms consist of clustered or individualized neoplastic
epithelial cells, often outnumbered by nonneoplastic small lymphocytes (“lymphocyte-rich
thymoma”). Thymomas are common in goats and often contain large cystic structures.
Immune-mediated diseases, including myasthenia gravis and immune-mediated polymyositis,
occur with thymomas in dogs, and also rarely in cats. Myasthenia gravis is caused
by autoantibodies directed toward the acetylcholine receptors, which lead to destruction
of postsynaptic membranes and reduction of acetylcholine receptors at neuromuscular
junction. Megaesophagus and aspiration pneumonia are common sequelae to this condition.
Figure 13-53
Thymic Lymphoma, Cat.
The large pale tan mass (M) fills the cranial mediastinum and caudally displaces the
lungs. H, Heart. (Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University
of Pennsylvania.)
Miscellaneous Disorders
Thymic Hyperplasia.
Asymptomatic hyperplasia may occur in juvenile animals in association with immunizations
and results in symmetrical increase in the size of the thymus. Autoimmune lymphoid
hyperplasia of the thymus has germinal center formation and occurs with myasthenia
gravis.
Thymic Hematomas.
See Disorders of Dogs.
Disorders of Domestic Animals: Spleen
Congenital Disorders
Asplenia or the failure of a spleen to develop in utero occurs rarely in animals,
and the effect on the animal's immune status is uncertain. (Splenic aplasia is present
in certain strains of mice, but because these are usually maintained under either
germ-free or specific pathogen–free [SPF] conditions, the effect of asplenia cannot
be evaluated.) Congenital immunodeficiency diseases are described in detail in Chapter
5, and in the sections on Disorders of Horses and Disorders of Dogs.
Splenomegaly
Gross examination of the spleen involves deciding whether the spleen is enlarged (splenomegaly),
normal, or small (see E-Appendix 13-2). Diffuse enlargement of the spleen may be due
to congestion (termed bloody spleen) or other infiltrative disease (termed meaty spleen).
The cut surface of congested spleens will exude blood, whereas meaty spleens are more
firm and do not readily ooze blood. The diseases and disorders having splenomegaly
are discussed using the following categories, which list the common causes of uniform
splenomegaly (Table 13-6
):
•
Uniform splenomegaly with a bloody consistency (bloody spleen) (Fig. 13-54, A
)
Figure 13-54
Uniform Splenomegaly.
A, Congested bloody spleen. This condition occurs secondary to compromises in vascular
flow into and out of the spleen (e.g., torsion), from intravenous barbiturates (e.g.,
euthanasia or anesthesia), and from acute hyperemia due to septicemia. B, Meaty spleen.
This condition may be due to proliferation of macrophages in cases of chronic septicemias,
hemolytic diseases, diffuse granulomatous disease, or neoplasia (e.g., lymphoma).
(A courtesy College of Veterinary Medicine, University of Illinois. B courtesy Dr.
A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
•
Uniform splenomegaly with a firm consistency (meaty spleen) (see Fig. 13-54, B)
•
Splenic nodules with a bloody consistency
•
Splenic nodules with a firm consistency
Table 13-6
Common Causes of Uniform Splenomegaly in Domestic Animals
Species
Congested (Bloody) Spleen
Firm (Meaty) Spleen
Horse
Barbiturate euthanasia or anesthesia
Acute septicemia
Salmonellosis
Acute hemolytic disease
EIA
Chronic septicemia
Salmonellosis
Chronic hemolytic diseases
EIA
IMHA
Hematopoietic neoplasia
Lymphoma
Cattle, sheep, and goat
Septicemia
Anthrax
Salmonellosis
Acute hemolytic disease
Babesiosis
Chronic septicemia
Salmonellosis
Chronic hemolytic diseases
Babesia
Anaplasmosis
Trypanosomiasis
Hemotropic mycoplasmosis
Hematopoietic neoplasia
Lymphoma
Pig
Septicemia
Salmonellosis
Splenic torsion
Chronic septicemias
Salmonellosis
Erysipelas
Chronic hemolytic disease
Hemotropic mycoplasmosis
Hematopoietic neoplasia
Lymphoma
Dog and cat
Barbiturate euthanasia or anesthesia
Splenic torsion with GDV (dog)
Chronic hemolytic disease
IMHA
Chronic infectious disease
Histoplasmosis
Leishmaniasis
Hematopoietic neoplasia
Lymphoma
Mast cell neoplasia
Histiocytic sarcoma
Extramedullary hematopoiesis
Amyloidosis
EIA, Equine infectious anemia; GDV, gastric dilatation and volvulus; IMHA, immune-mediated
hemolytic anemia.
Uniform Splenomegaly with a Bloody Consistency—Bloody Spleen.
The common causes of a bloody spleen are (1) congestion (due to gastric volvulus with
splenic entrapment, splenic volvulus [all of which compress the splenic vein], and
barbiturate euthanasia, anesthesia, or sedation), (2) acute hyperemia (due to septicemia),
and (3) acute hemolytic anemia (due to an autoimmune disorder or an infection with
a hemotropic parasite).
Congestion
Splenic Torsion.
Torsion of the spleen occurs most commonly in pigs and dogs; in dogs this usually
involves both spleen and stomach and is seen more often in deep-chested breeds (see
Chapter 7). In contrast to ruminants, in which the spleen is firmly attached to the
rumen, the spleens of dogs and pigs are attached loosely to the stomach by the gastrosplenic
ligament. It is the twisting of the spleen around this ligament that results initially
in occlusion of the veins, causing splenic congestion, and later in occlusion of the
artery, causing splenic infarction. In dogs the spleen is uniformly and markedly enlarged
and may be blue-black from cyanosis. It is often folded back on itself (visceral surface
to visceral surface) in the shape of the letter “C.” Treatment for this condition
is most often splenectomy.
Barbiturate Euthanasia, Anesthesia, or Sedation.
Intravenous injection of barbiturates induces acute passive congestion in the spleen
due to relaxation of smooth muscle in the capsule and trabeculae. This phenomenon
is seen most dramatically at autopsy (syn: necropsy) in horses and dogs that have
been euthanized or anesthetized with barbiturates. Grossly, the spleen is extremely
enlarged (Fig. 13-55
), and the cut surface bulges and oozes copious blood. Because of the splenic distention,
the splenic capsule can be fragile and easily ruptured. Histologically, the red pulp
is distended by erythrocytes, and the lymphoid tissues of the white pulp are small
and widely separated (Fig. 13-56
). Electric stunning of pigs at slaughter may result in a large congested spleen;
the mechanism is unknown, but it should not be confused with a pathologically congested
spleen. Splenic congestion in acute cardiac failure is rarely seen in animals.
Figure 13-55
Splenic Congestion From Barbiturate Euthanasia, Horse.
The spleen is enlarged and congested from storage of blood.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Figure 13-56
Splenic Congestion From Barbiturate Euthanasia, Dog.
The red pulp vascular spaces are markedly distended by blood. One white pulp splenic
follicle is present in the lower right. H&E stain.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Acute Congestion/Hyperemia.
Acute septicemias may cause acute hyperemia and concurrent acute congestion of marginal
zones and splenic red pulp. Microbes are transported hematogenously to these sites,
where they are rapidly phagocytized by macrophages. Enormous numbers of intravenous
bacteria can be cleared by the spleen from the blood in 20 to 30 minutes, but when
this defensive mechanism is overwhelmed, the outcome is usually fatal. The response
of the spleen depends on the duration of the disease. In acutely fatal cases, such
as anthrax and fulminating salmonellosis, distention by blood may be the only gross
finding. If the animal survives longer, as in swine erysipelas and the less virulent
forms of salmonellosis, there may be sufficient time for neutrophils and macrophages
to accumulate in the marginal sinuses, marginal zones, and splenic red pulp vascular
spaces.
Anthrax.
B. anthracis, the causative agent of anthrax, is a Gram-positive, large, endospore-forming
bacillus, which grows in aerobic to facultative anaerobic environments. Anthrax is
primarily a disease of ruminants, especially cattle and sheep (see Chapters 4, 7,
9, and 10Chapter 4Chapter 7Chapter 9Chapter 10). Once the spores are ingested, they
replicate locally in the intestinal tract, spread to regional lymph nodes, and then
disseminate systemically through the bloodstream, resulting in septicemia. B. anthracis
produces exotoxins, which degrade endothelial cell membranes and enzyme systems.
Grossly, the spleen is uniformly enlarged and dark red to bluish-black and contains
abundant unclotted blood. In peracute cases the only histologic lesion may be marked
congestion of the marginal sinuses and the splenic red pulp vascular spaces. At low
magnification, congestion of the marginal sinus may appear as a circumferential red
ring around the splenic follicle, and there is marked lymphocytolysis of follicles
and PALS. Intravascular free bacilli are noted and may be seen in impression smears
of peripheral blood, presumably because death is so rapid from the anthrax toxin that
there is insufficient time for phagocytosis to take place. If the animal lives longer,
scattered neutrophils are present in the marginal sinuses and red pulp vascular spaces
(Fig. 13-57
). Anthrax cases are not normally autopsied because exposure to air causes the bacteria
to sporulate—anthrax spores are extremely resistant and readily contaminate the environment.
Figure 13-57
Anthrax, Spleen, Monkey.
(See Fig. 13-40 for schematic illustration of the marginal zone.)
A, Acute septicemias may cause acute congestion of the marginal zone (double-headed
line) and then of the red pulp vascular spaces (not shown). B, Higher magnification
of A with marginal zone (double-headed line) and central artery (C) of the follicle.
C, Higher magnification of B with small aggregates of neutrophils within the marginal
zone (arrows).
D, Higher magnification of B with accumulation anthrax bacilli within the marginal
zone (arrows). This form produces anthrax toxins, which cause severe tissue injury,
resulting in inflammation and cell death. All H&E stain.
(Courtesy Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.
Photographed from slides provided by Toxicology Battelle Columbus to the Wednesday
Slide Conference [2003-2004, Conference 13, Case 1], Armed Forces Institute of Pathology,
Department of Veterinary Pathology.)
Acute Hemolytic Anemias.
Hemolytic diseases, including acute babesiosis, hemolytic crises in equine infectious
anemia, and immune-mediated hemolytic anemia, can cause marked splenic congestion.
The splenic congestion is due to the process of removal (phagocytosis) and storage
of large numbers of sequestered parasitized and/or altered erythrocytes from the circulation.
Histologically, there is dilation of the red pulp vascular spaces with erythrocytes
and erythrophagocytes. With chronicity there is hyperplasia of the red pulp macrophages,
hemosiderosis, and reduced congestion because the number of sequestered diseased erythrocytes
is diminished.
Uniform Splenomegaly with a Firm Consistency—Meaty Spleen.
The three general categories of conditions leading to uniform splenomegaly with a
firm meaty consistency are (1) marked phagocytosis of cells, debris, or foreign agents/material;
(2) proliferation or infiltration of cells as occurs in diffuse lymphoid and histiocytic
hyperplasia, diffuse granulomatous disease (E-Table 13-2), EMH, and neoplasia; (3)
storage of materials in storage diseases or amyloidosis. It is important to recognize
that more than one of these processes can occur in the same patient (e.g., dogs with
immune-mediated hemolytic anemia may have both marked erythrophagocytosis and EMH).
The appearance of the cut surface of a meaty spleen depends on the underlying cause.
In diffuse marked lymphoid hyperplasia, large, disseminated, discrete, white, bulging
nodules are visible. Spleens with diffuse infiltrative neoplasms, such as lymphoma,
are pink–light purple on cut surface.
E-TABLE 13-2
Granulomatous Diseases of the Spleen
Disorders
Agents
Noninfectious
Foreign bodies/particles
Colloids (e.g., India ink)
Infectious
Bacteria: facultative pathogens
Mycobacteriosis
Tularemia
Yersiniosis
Brucellosis
Fungi
Blastomycosis
Histoplasmosis
Coccidioidosis
Sporotrichosis
Protozoa
Toxoplasmosis
Neosporosis
Modified from Nieman RS, Attilo O: Disorders of the spleen in major problem in pathology,
Philadelphia, 1999, WB Saunders.
Phagocytosis and Proliferation of Cells
Diffuse Lymphoid Hyperplasia.
Lymphoid hyperplasia has been described in detail in the section on Dysfunction/Responses
to Injury. In cases of prolonged antigenic stimulation the lymphoid follicles throughout
the splenic parenchyma can become enlarged and visible on gross examination (Fig.
13-58
), leading to diffuse splenomegaly. In contrast to B lymphocyte hyperplasia of the
lymphoid follicles, certain diseases (e.g., malignant catarrhal fever in cattle) may
lead to T lymphocyte hyperplasia of the PALS.
Figure 13-58
Lymphoid Hyperplasia, Cross Section of Spleen, Dog.
The hyperplastic white pulp follicles are grossly evident as 1 to 3 mm in diameter
pale gray-white foci. These structures are not visible in the normal spleen but become
enlarged and visible from marked lymphoid hyperplasia.
(Courtesy Dr. S. Wolpert, USDA/FSIS; and Noah's Arkive, College of Veterinary Medicine,
The University of Georgia.)
Diffuse Histiocytic Hyperplasia and Phagocytosis.
Splenomegaly from hyperplasia and increased phagocytosis of splenic macrophages is
a response to the need to engulf organisms in prolonged bacteremia or parasitemia
from hemotropic organisms. Whereas acute hemolytic anemias cause splenomegaly with
congestion (bloody spleen), with chronicity there is decreased sequestration of diseased
erythrocytes and hence less congestion. Therefore in cases of chronic hemolytic disease,
splenomegaly is attributed to diffuse proliferation of macrophages, phagocytosis,
and concurrent hyperplasia of the white pulp due to ongoing antigenic stimulation.
For example, equine infectious anemia has cyclical periods of viremia, with immune-mediated
damage to erythrocytes and platelets, and phagocytosis to remove altered erythrocytes
and platelets. These cycles result in proliferation of red pulp macrophages, hyperplasia
of hematopoietic cells (EMH) to replace those lost, and hyperplasia of lymphocytes
in the white pulp.
Diffuse Granulomatous Disease.
Chronic infectious diseases may cause a uniformly firm and enlarged spleen, mostly
due to macrophage hyperplasia and phagocytosis, diffuse lymphoid hyperplasia, or diffuse
granulomatous disease. Diffuse granulomatous diseases (see E-Table 13-2) occur in
(1) intracellular facultative bacteria that infect macrophages (e.g., Mycobacterium
spp., Brucella spp., and Francisella tularensis); (2) systemic mycoses (e.g., Blastomyces
dermatitidis, Histoplasma capsulatum) (see Lymphoid/Lymphatic System, Disorders of
Domestic Animals: Lymph Nodes, Enlarged Lymph Nodes [Lymphadenomegaly]) (Fig. 13-59,
A and B
), and (3) protozoal infections that infect macrophages (e.g., Leishmania spp.). Some
of these organisms may also produce nodular spleens with the formation of discrete
to coalescing granulomas (e.g., M. bovis) (see Splenic Nodules with a Firm Consistency).
Figure 13-59
Histoplasmosis, Spleen, Dog.
A, There is uniform splenomegaly with a firm consistency (meaty spleen). B, Cross
section of spleen. The red pulp has been almost completely replaced by diffuse granulomatous
inflammation.
(Courtesy Department of Veterinary Biosciences, The Ohio State University; and Noah's
Arkive, College of Veterinary Medicine, The University of Georgia.)
Extramedullary Hematopoiesis.
EMH is the development of blood cells in tissues outside the medullary cavity of the
bone (E-Fig. 13-9). The formation of single or multiple lineages of hematopoietic
cells is often observed in many tissues and commonly in the spleen. The ability of
blood cell precursors to home, proliferate, and mature in extramedullary sites relies
on the presence of HSCs and pathophysiologic changes in the microenvironment (i.e.,
extracellular matrix, stroma, and chemokines). In the spleen, HSCs have been found
within vessels and adjacent to endothelial cells to form a vascular niche; thus splenic
EMH occurs in the red pulp, both within the red pulp vascular spaces and sinusoids
(of the dog). The predilection for EMH to occur varies among species (for instance,
splenic EMH persists throughout adulthood in mice), and the underlying mechanisms
are not completely understood, but four major theories to explain the causes of EMH
are (1) severe bone marrow failure; (2) myelostimulation; (3) tissue inflammation,
injury, and repair; and (4) abnormal chemokine production.
E-Figure 13-9
Extramedullary Hematopoiesis.
A, Marked diffuse splenomegaly (meaty spleen) from a ferret. B, The splenic parenchyma
contains numerous erythroid and myeloid precursors and megakaryocytes. H&E stain.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Because splenic EMH is often observed in animals without obvious hematologic abnormalities,
tissue inflammation, injury, and repair is the most likely mechanism of EMH in this
organ. In dogs and cats EMH occurs most frequently with degenerative and inflammatory
disorders, such as lymphoid nodular hyperplasia, hematomas, thrombi, histiocytic hyperplasia,
inflammation (e.g., fungal splenitis), and neoplasia. EMH in multiple tissues may
be observed in chronic cardiovascular or respiratory conditions, chronic anemia, or
chronic suppurative diseases in which there is an excessive tissue demand for neutrophils
that exceeds the supply available from the marrow (e.g., canine pyometra).
Primary Neoplasms.
Primary neoplastic diseases of the spleen arise from cell populations that normally
exist in the spleen and include hematopoietic components, such as lymphocytes, mast
cells, and macrophages, and stromal cells, such as fibroblasts, smooth muscle, and
endothelium. The primary neoplasms that result in diffuse splenomegaly are the round
cell tumors, including lymphoma (Fig. 13-60
), leukemia, visceral mast cell tumor, and histiocytic sarcoma. It is important to
note that all of these types of neoplasms can produce nodular lesions instead of—or
along with—a diffusely enlarged spleen. The different types of lymphoma in domestic
animals are discussed in the section on Hematopoietic Neoplasia. Secondary neoplasms
of the spleen are due to metastatic spread and most often form nodules in the spleen,
not a uniform splenomegaly.
Figure 13-60
Lymphoma, Spleen and Liver.
A, Dog. There is diffuse splenomegaly and multiple tan splenic nodules. Mild hepatomegaly
with an irregular surface corresponds to the neoplastic infiltration into the portal
areas. B, Cow. The spleen is diffusely infiltrated by neoplastic lymphocytes, which
have completely obliterated all normal architecture (absence of the red and white
pulp). H&E stain.
(A courtesy College of Veterinary Medicine, University of Illinois. B courtesy Dr.
M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Storage of Material
Amyloid.
The accumulation of amyloid in the spleen may occur with primary (AL) or secondary
(AA) amyloidosis (see Chapters 1 and 5). Rarely, severe amyloid accumulation may cause
uniform splenomegaly (Fig. 13-61
), in which the spleen is firm, rubbery to waxy, and light brown to orange. Microscopically,
amyloid is usually in the splenic follicles, which if large enough, are grossly visible
as approximately 2-mm-diameter gray nodules. Amyloid deposition can also be seen within
the walls of splenic veins and arterioles. Plasma cells tumors within the spleen may
also be associated with amyloid (AL) deposits.
Figure 13-61
Splenic Amyloid, Dog.
The spleen is enlarged, pale tan, firm, and waxy in this advanced case of amyloidosis.
(Courtesy College of Veterinary Medicine, University of Illinois.)
Lysosomal Storage Diseases.
Storage diseases are a heterogeneous group of inherited defects in metabolism characterized
by accumulation of storage material within the cell (lysosomes). Genetic defects,
which result in the absence of an enzyme, the synthesis of a catalytically inactive
enzyme, the lack of activator proteins, or a defect in posttranslational processing,
can lead to a storage disease. Acquired storage diseases are caused by exogenous toxins,
most often plants that inhibit a particular lysosomal enzyme (e.g., swainsonine toxicity
due to indolizidine alkaloid found in Astragalus and Oxytropis plant spp.). Storage
diseases typically occur in animals less than 1 year of age. In general, these substrates
are lipids and/or carbohydrates that accumulate in the cells, the result of the lack
of normal processing within lysosomes. Major categories of stored materials include
mucopolysaccharides, sphingolipids, glycolipids, glycoproteins, glycogen, and oligosaccharides.
Macrophages are commonly affected by storage diseases, and thus accumulations of macrophages
within several organs, including splenic macrophages, Kupffer cells of the liver,
and macrophages in the brain are often observed.
Splenic Nodules with a Bloody Consistency.
The most common disorders of the spleen with bloody nodules are (1) hematomas, including
those induced by nodular hyperplasia or occurring with hemangiosarcoma, (2) incompletely
contracted areas of the spleen, (3) acute splenic infarcts, and (4) hemangiosarcomas.
The term nodule has been applied rather loosely here. In some of these conditions,
such as incompletely or irregularly contracted areas of the spleen, the elevated area
of the spleen is not as well defined as the term nodule would imply.
Hematomas.
Bleeding into the red pulp to form a hematoma is confined by the splenic capsule,
and produces a red to dark red, soft, bulging, usually solitary mass of varying size
(2 to 15 cm in diameter) (Fig. 13-62
). Resolution of a splenic hematoma progresses over days to weeks, through the stages
of coagulation and breakdown of the blood into a dark red-brown soft mass (Fig. 13-63,
A
), infiltration by macrophages that phagocytize erythrocytes and break down hemoglobin
to form hematoidin and hemosiderin (see Fig. 13-63, B), and repair leading to fibrosis.
On occasion the capsule (splenic capsule and visceral peritoneum) over the hematoma
can rupture, resulting in hemoperitoneum, hypovolemic shock, and death.
Figure 13-62
Hematoma, Spleen, Dog.
The ventral extremity of the spleen has a large hematoma on its visceral surface.
Note the two nodules on the dorsal extremity (arrows) of splenic nodular hyperplasia,
a common site for hematomas to arise.
(Courtesy College of Veterinary Medicine, University of Illinois.)
Figure 13-63
Subcapsular Hematoma, Spleen, Dog.
A, Note the separation of the splenic capsule from the underlying parenchyma by a
mass of blood. B, The hematoma is subjacent to the capsule. Hematoidin (yellow) or
hemosiderin (brown) pigments may be seen in or around these lesions as a result of
the breakdown of erythrocytes. H&E stain.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
The origin or cause of many hematomas is unknown. Some are due to trauma, and others
may also be induced by splenic nodular hyperplasia. It is postulated that as the splenic
follicles become hyperplastic they distort the adjacent marginal zone and marginal
sinus, which compromises their drainage into sinusoids and red pulp vascular spaces.
The result is an accumulation of pooled blood surrounding the hyperplastic nodule,
which leads to hematoma formation. Splenic hematomas can also occur secondary to the
rupture of hemangiosarcomas within the spleen.
Incompletely Contracted Areas of the Spleen.
Incompletely or irregularly contracted areas of the spleen are caused by failure of
the smooth muscle to contract in response to circulatory shock (hypovolemic, cardiogenic,
or septic) or sympathetic “fight-or-flight” response, resulting in a lack of splenic
evacuation of stored blood. Grossly, incompletely contracted areas are characterized
by multiple, variably sized and irregularly shaped, dark red to black, raised, soft,
blood-filled “nodules.” These areas are usually at the margins of the spleen, and
the intervening tissues are depressed and pink-red, corresponding to the contracted
portions of red pulp devoid of blood. Incompletely contracted areas may be confused
with acute splenic infarcts or hematomas on gross examination.
Acute Splenic Infarcts.
Splenic infarcts are wedge-shaped or triangular hemorrhagic lesions that occur primarily
at the margins of the spleen. In dogs, splenic infarcts most often occur with hypercoagulable
states (e.g., liver disease, renal disease, Cushing's disease), neoplasia, and cardiovascular
disease. Splenic vein thrombi may occur in association with traumatic reticulitis,
splenic abscesses, portal vein thrombosis, and arterial thrombosis in bovine theileriosis
in cattle. Valvular endocarditis may also lead to multiorgan infarcts, including the
spleen. Splenic infarcts are common in pigs with classical swine fever.
Acute splenic infarcts may not always be grossly visible in the early stages but develop
into discrete, dark red and blood-filled, bulging, wedge-shaped foci with the base
toward the splenic capsule (Fig. 13-64, A
). With chronicity the lesion becomes gray-white and contracted due to fibrosis (see
Fig. 13-64, B).
Figure 13-64
Acute and Chronic Splenic Infarcts, Spleen, Dogs.
A, Acute splenic infarct (asterisk). Acute infarcts are red-black wedge-shaped foci
filled with blood. B, Chronic splenic infarct. Chronic infarcts are wedge-shaped (asterisk),
pale gray-white, firm, and often contracted due to fibrosis.
(A courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.
B courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Hemangiosarcoma.
Hemangiosarcoma is a malignant neoplasm of endothelial cells and is a common primary
tumor of the spleen, especially in dogs. Benign splenic hemangiomas are extraordinarily
rare. Grossly, hemangiosarcomas may appear as single, multifocal, or coalescing dark
red-purple masses and cannot be easily differentiated from a hematoma (Fig. 13-65
). On cut surface they are bloody with varying amounts of soft red neoplastic tissue;
in more solid areas the neoplasm can be slightly more firm and white-tan. Metastatic
spread occurs early in the disease process. Seeding of the peritoneum results in numerous
discrete red-black masses throughout the omentum and serosa of abdominal organs, and
hematogenous spread to liver and lung are common. Hemangiosarcomas in dogs also occur
in the right atrium of the heart, retroperitoneal fat, and skin (dermal and/or subcutaneous)
and multiorgan hemangiosarcomas are described in horses, cats, and cattle. Because
hemangiosarcomas have often metastasized at the time of initial diagnosis, it may
be difficult (and futile) to determine the primary site. Histologically, hemangiosarcomas
are composed of plump neoplastic endothelial cells, which wrap around stroma to form
haphazardly arranged and poorly defined blood-filled vascular spaces (Fig. 13-66
).
Figure 13-65
Hemangiosarcoma, Spleen, Dog.
A, There are multiple neoplastic nodules on the dorsal extremity and a large nodule
on the ventral extremity of the spleen. B, The ventral mass has been incised to reveal
the cut surface of the hemangiosarcoma.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Figure 13-66
Hemangiosarcoma, Spleen, Dog.
Neoplastic endothelial cells form haphazardly organized blood-filled vascular channels.
Mitotic figure (arrow). H&E stain.
(Courtesy Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.)
Splenic Nodules with a Firm Consistency.
The most common disorders of the spleen with firm nodules are (1) lymphoid nodular
hyperplasia, (2) complex nodular hyperplasia, (3) primary neoplasms, (4) secondary
metastatic neoplasms, (5) granulomas, and (6) abscesses.
Lymphoid and Complex Nodular Hyperplasia.
See Lymphoid/Lymphatic System, Disorders of Dogs.
Primary Neoplasms.
The primary neoplastic diseases of the spleen that result in firm nodules include
lymphoma (multiple subtypes), histiocytic sarcoma, leiomyoma, leiomyosarcoma, fibrosarcoma,
myelolipomas, liposarcomas, myxosarcomas, undifferentiated pleomorphic sarcomas, solid
hemangiosarcomas, and rare reports of primary chondrosarcomas. These locally extensive
neoplasms may be solitary or multiple, raised above the capsular surface, but usually
confined by the capsular surface. The consistency and cut surface appearance varies
depending on the type of neoplasm; spindle cell tumors like leiomyosarcomas and fibrosarcomas
will be white and firm, liposarcomas and myelolipomas are soft and bulging, and myxomatous
neoplasms are gelatinous. It is important to remember that many round cell neoplasms,
such as lymphoma, mast cell tumors, plasma cell tumors, myeloid neoplasms, and histiocytic
sarcomas, can form nodules or diffuse splenic enlargement (or both).
Metastatic Neoplasms.
Neoplasms that metastasize to the spleen usually result in enlarged nodular spleens
(Fig. 13-67
) and include any number of sarcomas, carcinomas, or malignant round cell tumors.
Metastatic sarcomas can include fibrosarcomas, leiomyosarcomas, chondrosarcomas, and
osteosarcomas. Mammary, prostatic, pulmonary, anal sac gland and neuroendocrine carcinomas
may metastasize widely to abdominal viscera, including the spleen.
Figure 13-67
Metastatic Carcinoma, Spleen, Cow.
The firm, lobulated, white mass is an undifferentiated carcinoma, which has metastasized
to the spleen.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Granulomas and Abscesses.
Microorganisms that cause diffuse granulomatous splenitis and uniform splenomegaly
may also cause focal to multifocal nodular lesions (e.g., Mycobacterium spp., fungal
organisms) (see Diffuse Granulomatous Diseases of the Spleen and also Enlarged Lymph
Nodes). Although there are a large number of diseases and conditions commonly caused
by bacteremia (e.g., navel ill, joint ill, chronic respiratory infections, bacterial
endocarditis, chronic skin diseases, castration, tail docking, and ear trimming and/or
notching), these rarely result in visible splenic abscesses. Pyogranulomas and abscesses
in the spleen (multifocal chronic suppurative splenitis) that do develop after septicemia
and/or bacteremia are usually caused by pyogenic bacteria such as Streptococcus spp.,
Rhodococcus equi (Fig. 13-68
), Trueperella pyogenes (Fig. 13-69
), and Corynebacterium pseudotuberculosis. Cats with the wet or dry form of feline
infectious peritonitis virus may have nodular pyogranulomatous and lymphoplasmacytic
inflammatory foci throughout the spleen. Splenic abscesses due to direct penetration
by a migrating foreign body are reported in cattle (from the reticulum) and less commonly
in the horse (from the stomach). Perforating gastric ulcers in horses due to Gasterophilus
and Habronema spp. have also reportedly led to adjacent splenic abscesses. Granulomas
and abscesses bulge from the capsule and cut surfaces, and the exudate can vary in
amount, texture, and color depending on the inciting organism and the age of the lesion.
Figure 13-68
Multiple Subcapsular Splenic Abscesses, Rhodococcus equi, Spleen, Horse.
(Courtesy Dr. P. Carbonell, School of Veterinary Science, University of Melbourne.)
Figure 13-69
Chronic Multifocal Suppurative Splenitis (Splenic Abscesses), Trueperella pyogenes,
Spleen, Cow.
Multiple encapsulated yellow-white abscesses are present throughout the parenchyma
of the spleen as a result of a previous bacteremia.
(Courtesy Department of Veterinary Biosciences, The Ohio State University; and Noah's
Arkive, College of Veterinary Medicine, The University of Georgia.)
Small Spleens (Splenic Hypoplasia and Atrophy)
The most common diseases or conditions that have small spleens are (1) developmental
anomalies, (2) aging changes, (3) wasting and/or cachectic diseases, and (4) splenic
contraction.
Developmental Anomalies
Splenic Hypoplasia.
Primary immunodeficiency diseases can result in splenic hypoplasia, as well as small
thymuses and lymph nodes (which may be so small as to be grossly undetectable in some
diseases). These diseases affect young animals and involve defects in T and/or B lymphocytes
(Fig. 13-70
). Spleens are exceptionally small, firm, and pale red and lack lymphoid follicles
and PALS. These diseases and their pathologic findings are discussed in Chapter 5
and in the sections on Disorders of Horses and Disorders of Dogs.
Figure 13-70
Severe Combined Immunodeficiency Disease, Spleen, Arabian Foal.
There is notable absence of the white pulp (the large pale pink areas are splenic
trabeculae). H&E stain.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Congenital Accessory Spleens.
Accessory spleens can be either congenital or acquired (see Splenic Rupture). Congenital
accessory spleens are termed splenic choristomas, which are nodules of normal splenic
parenchyma in abnormal locations. These are usually small and may be located in the
gastrosplenic ligament, liver, or pancreas (see Fig. 13-74, B).
Splenic Fissures.
Fissures in the splenic capsule are elongated grooves whose axes run parallel to the
borders of the spleen. This developmental defect is seen most commonly in horses but
also occurs in other domestic animals and has no pathologic significance. The surface
of the fissure is smooth and covered by the normal splenic capsule.
Aging Changes.
As part of the general aging change of cells as the body ages, there is reduction
in the number of B lymphocytes produced by the bone marrow and decline of naïve T
lymphocytes due to age-related thymic involution. Consequently, there is lymphoid
atrophy in secondary lymphoid organs. The spleen is small, and its capsule may be
wrinkled. Microscopically, the white pulp is atrophied, and splenic follicles, if
present, lack germinal centers. Sinuses may also collapse from a reduced amount of
blood, possibly because of anemia, which makes the red pulp appear fibrous.
Wasting/Cachectic Diseases.
Any chronic disease, such as starvation, systemic neoplasia, and malabsorption syndrome,
may produce cachexia. Starvation has a marked effect on the thymus, which results
in atrophy of the T lymphocyte areas in the spleen and lymph nodes, which is in part
mediated by leptin. B lymphocyte development is also diminished, because B lymphocytes
require accessory signals from helper T lymphocytes to undergo somatic hypermutation
and immunoglobulin isotype switching.
Splenic Contraction.
Contraction of the spleen is a result of contraction of the smooth muscle in the capsule
and trabeculae of storage spleens. It can be induced by the activation of the sympathetic
“fight-or-flight” response and is seen in patients with heart failure or shock (cardiogenic,
hypovolemic, and septic shock) and also occurs in acute splenic rupture that has resulted
in massive hemorrhage (hemoabdomen/hemoperitoneum). The contracted spleen is small,
its surface is wrinkled, and the cut surface is dry.
Miscellaneous Disorders of the Spleen
Hemosiderosis.
Hemosiderin is a form of storage iron derived chiefly from the breakdown of erythrocytes,
which normally takes place in the splenic red pulp. Thus some splenic hemosiderosis
is to be expected, and the amount varies with the species (it is most extensive in
the horse). Excessive amounts of splenic hemosiderin are seen when erythropoiesis
is reduced (less demand for iron) or from the rapid destruction of erythrocytes in
hemolytic anemias (increased stores of iron), such as those caused by immune-mediated
hemolytic anemias or hemotropic parasites. Excess splenic hemosiderin may also occur
in conditions such as chronic heart failure or injections of iron dextran or as focal
accumulations at the sites of old hematomas, infarcts, or trauma-induced hemorrhages.
Hemosiderin is also present in siderofibrotic plaques.
Siderofibrotic Plaques.
Siderofibrotic plaques are also known as siderocalcific plaques and Gamna-Gandy bodies.
Grossly, they are gray-white to yellowish, firm, dry encrustations on the splenic
capsule. Usually they are most extensive along the margins of the spleen but can be
elsewhere on the capsule (Fig. 13-71
) and sometimes in the parenchyma. With H&E staining these plaques are a multicolored
mixture of yellow (hematoidin), golden brown (hemosiderin), purple-blue (hematoxylinophilic
calcium mineral), and pink (eosinophilic fibrous tissue) (Fig. 13-72
; E-Figs. 13-10 and 13-11). Siderofibrotic plaques are extremely common in aged dogs
and may represent sequelae to previous hemorrhages from trauma to the spleen.
Figure 13-71
Siderofibrotic Plaques, Spleen, Macroscopic View, Dog.
Siderofibrotic plaque along the margins (golden-brown area) of the spleen; a focal
nodule of hyperplasia is also present. Both are common lesions in older dogs.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Figure 13-72
Siderofibrotic Plaques, Spleen, Microscopic View, Dog.
A, The thick splenic capsule contains fibrosis connective tissue (pink), linear bands
of mineral (dark purple), small lakes of hematoidin pigment (yellow), and hemosiderin-laden
macrophages (brown). H&E stain. B, The plaque is composed of fibrous connective tissue,
hemosiderin (blue) and hematoidin (orange) pigments, and mineral. Prussian blue reaction.
(A courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.
B courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
E-Figure 13-10
Siderofibrotic Plaques and Nodular Hyperplasia/Hematoma, Spleen, Dog.
A, Multiple confluent raised yellow-white plaques are present on the capsular surface
of the body of the spleen. Note the nodule of hyperplasia/hematoma (incised). B, The
plaque consists of fibrous connective tissue of the capsule, mineral/hemosiderin (purple-blue),
hemosiderophages (brown), and large areas of hematoidin (orange-yellow) pigment. H&E
stain.
(A courtesy College of Veterinary Medicine, University of Illinois. B courtesy Dr.
M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
E-Figure 13-11
Siderofibrotic Plaques, Spleen, Macroscopic View, Dog.
Note the yellow-white plaques on the capsular surface and along the border of the
spleen. These plaques may be the result of healing of sites of previous trauma and
hemorrhage.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Splenic Rupture.
Splenic rupture is most commonly caused by trauma, such as from an automobile accident
or being kicked by other animals. Thinning of the capsule from splenomegaly can render
the spleen more susceptible to rupture, and this may occur at sites of infarcts, hematomas,
hemangiosarcomas, and lymphoma. In acute cases of splenic capsular rupture, the spleen
is contracted and dry and the surface wrinkled from the marked blood loss (Fig. 13-73
). In more severe cases the spleen may be broken into two or more pieces, and small
pieces of splenic parenchyma may be scattered throughout the omentum and peritoneum
(sometimes called splenosis) (Fig. 13-74, A
). Clotted blood, fibrin, and omentum may adhere to the surface at the rupture site.
If the rupture is not fatal, the spleen heals by fibrosis, and there may be a capsular
scar. Occasionally there are two or more separate pieces of spleen adjacent to each
other and sometimes joined by scar tissue in the gastrosplenic ligament. The functional
capabilities of the small accessory spleens are questionable, although erythrophagocytosis,
hemosiderosis, hyperplastic nodules, EMH, and neoplasia can be present in these nodules.
Figure 13-73
Acute Splenic Rupture, Spleen, Dog.
The spleen has been almost transected by recent trauma. Because of the loss of blood,
the spleen is contracted, the surface is wrinkled, and the exposed parenchymal surface
is dry.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Figure 13-74
Accessory Spleens and Splenic Choristoma, Dogs.
A, Accessory spleens. The spleen had been broken into several parts, and the rupture
sites have healed by fibrosis. These small pieces of spleen (also referred to as daughter
or progeny spleens) are found on the gastrosplenic ligament. B, Splenic choristomas.
These nodules of normal splenic parenchyma are usually small and may be located in
the gastrosplenic ligament, liver, or pancreas. Splenic choristoma (arrow).
(A courtesy Dr. H.B. Gelberg, College of Veterinary Medicine, Oregon State University.
B courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Accessory spleens due to traumatic rupture should be distinguished from peritoneal
seeding of hemangiosarcoma and the developmental anomaly splenic choristomas (see
Fig. 13-74, B), which are nodules of normal splenic parenchyma in abnormal locations
(such as liver and pancreas).
Chronic Splenic Infarcts.
In the early stage, splenic infarcts are hemorrhagic and may elevate the capsule (see
Splenic Nodules with a Bloody Consistency). However, as the lesions age and fibrous
connective tissue is laid down, they shrink and become contracted and often depressed
below the surface of the adjacent capsule.
Parasitic Cysts.
Occasionally, parasitic cystic nodules are present within the spleen. These cysts
are intermediate stages of Echinococcus granulosus and Cysticercus tenuicollis and
are seen most commonly in wild animal species.
Disorders of Domestic Animals: Lymph Nodes
Small Lymph Nodes
The diseases or conditions with small lymph nodes are (1) congenital disorders, (2)
lack of antigenic stimulation, (3) viral infections, (4) cachexia and malnutrition,
(5) aging, and (6) radiation.
Congenital Disorders.
Primary immunodeficiency diseases are described in detail in Chapter 5 and in the
sections on Disorders of Horses and Disorders of Dogs. Neonatal animals with primary
immunodeficiency diseases often have extremely small to undetectable lymph nodes.
In dogs and horses with severe combined immunodeficiency disease (SCID), lymphoid
tissues, including lymph nodes from affected animals, are often grossly difficult
to identify and characterized by an absence of lymphoid follicles. Congenital hereditary
lymphedema has been reported in certain breeds of cattle and dogs. Grossly, the most
severely affected animals have generalized subcutaneous edema (see Fig. 2-10) and
effusions. In severe cases the peripheral and mesenteric lymph nodes are hypoplastic
and characterized by an absence of follicles. Nodes draining an edematous area may
be grossly enlarged from marked sinus edema.
Lack of Antigenic Stimulation.
The size of the lymph node depends on the level of phagocytosis and antigenic stimulation;
lymph nodes that are not receiving antigenic stimuli (e.g., SPF animals) will be small
with low numbers of primary lymphoid follicles and few, if any, secondary follicles
or plasma cells in the medullary cords. Conversely, nodes receiving constant antigenic
material (such as those draining the oral cavity or intestines) are large with active
secondary lymphoid follicles. The number of follicles increases or decreases with
changes in the intensity of the antigenic stimuli, and the germinal centers go through
a cycle of activation, depletion, and rest, as described previously (see Lymph Nodes,
Function). As the antigenic response wanes, germinal centers become depleted of lymphocytes,
and lymphoid follicles become smaller.
Viral Infections.
Many viral infections of animals target lymphocytes and cause the destruction of lymphoid
tissue. Of infectious agents, viruses most commonly infect and injure lymphoid tissues
and include the following: EHV-1 in aborted foals, classic swine fever virus, BVDV,
canine distemper virus, and canine and feline parvovirus.
Although some viruses destroy lymphoid tissue, others can lead to lymph node hyperplasia
(e.g., follicular B lymphocyte hyperplasia in FIV and paracortical T lymphocyte hyperplasia
in malignant catarrhal fever virus) or cause neoplasia (e.g., FeLV, BLV, and Marek's
disease).
Cachexia and Malnutrition.
Malnutrition and cachexia, which occur with cancer, lead to secondary immunosuppression
through several complex metabolic and neurohormonal aberrations. Starvation has marked
effect on the thymus with resultant atrophy of the T lymphocyte areas in the spleen
and lymph nodes and may also affect B lymphocyte development. Lymphoid atrophy may
result from physiologic and emotional stress and the concurrent release of catecholamines
and glucocorticoids. Glucocorticoids reduce B and T lymphocytes via redistribution
of these cells and glucocorticoid-induced apoptosis. T lymphocytes are more sensitive
to glucocorticoid-induced apoptosis than are B lymphocytes.
Aging.
As part of the general aging change of cells as the body ages, there is reduction
in the number of lymphocytes produced by the bone marrow and regressed thymus, and
consequently a reduction in the B and T lymphocytes in secondary lymphoid organs,
resulting in lymphoid atrophy. Consequently, lymph nodes are small, with loss of B
and T lymphocytes and plasma cells in the cortical follicles, paracortex, and medullary
cords, respectively.
Radiation.
Local or palliative treatment of cancer may include radiotherapy (ionizing radiation)
to target and damage the DNA of the neoplastic cells. Although some immunosuppression
may be noted, particularly if bone marrow or lymphoid tissues are within the irradiated
field, mounting evidence suggests that radiotherapy can induce a cascade of proimmunogenic
effects that engage the innate and adaptive immune systems to contribute to the destruction
of tumor cells. Fibrosis of tissues within the irradiated field also occurs as mainly
a late effect of chronic radiation.
Enlarged Lymph Nodes (Lymphadenomegaly)
Conditions causing lymphadenomegaly include (1) lymphoid hyperplasia (follicular or
paracortical), (2) hyperplasia of the sinus histiocytes (monocyte-macrophage system),
(3) acute or chronic lymphadenitis, (4) lymphoma, and (5) metastatic neoplasia.
Lymphoid Hyperplasia and Hyperplasia of the Monocyte-Macrophage System.
Detailed descriptions of lymphoid follicular hyperplasia, paracortical hyperplasia,
and hyperplasia of sinus histiocytes are in the sections on Lymph Nodes, Function,
and Lymph Node, Dysfunction/Responses to Injury. Follicular lymphoid hyperplasia can
involve large numbers of lymph nodes, as in a systemic disease, or can be localized
to a regional lymph node draining an inflamed or antigenically stimulated (e.g., vaccine
injection) area.
Acute Lymphadenitis.
Lymph nodes draining sites of infection and inflammation may develop acute lymphadenitis
(e.g., retropharyngeal lymph nodes draining the nasal cavity with acute rhinitis,
tracheobronchial lymph nodes in animals with pneumonia (Fig. 13-75
), and mammary [supramammary] lymph nodes in animals with mastitis). Grossly, affected
lymph nodes in acute lymphadenitis are red and edematous, have taut capsules, and
may have necrotic areas (Fig. 13-76
). In some instances the afferent lymphatic vessels may also be inflamed (lymphangitis).
The material draining to the regional lymph node may be microorganisms (bacteria,
parasites, protozoa, and fungi), inflammatory mediators, or a sterile irritant. In
septicemic diseases, such as bovine anthrax, the lymph nodes are markedly congested
and the sinuses filled with blood. Examination of these lymph nodes should include
culturing for bacteria and the examination of smears and histologic sections for bacteria
and fungi. Pyogenic bacteria, such as Streptococcus equi ssp. equi in horses (Fig.
13-77
), Streptococcus porcinus in pig, and Trueperella pyogenes in cattle and sheep, cause
acute suppurative lymphadenitis (see Disorders of Horses and Disorders of Pigs).
Figure 13-75
Acute Lymphadenitis, Tracheobronchial Lymph Nodes, Pig.
The tracheobronchial lymph nodes are draining the cranial lung lobes, which are consolidated
due to severe pneumonia. The nodes are enlarged and reddened. This appearance is due
to the “reversed” anatomic arrangement in the pig lymph node; the blood-filled sinuses
are obvious at the surface.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Figure 13-76
Acute Lymphadenitis, Lymph Node, Dog.
Acute lymphadenitis usually occurs when a regional lymph node drains a site of inflammation
caused by microorganisms and subsequently becomes infected. The lymph node is firm
and enlarged with a tense capsule. The cut surface bulges and is wet with blood, edema,
and an inflammatory cell infiltrate.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Figure 13-77
Acute Suppurative Lymphadenitis, Equine Strangles (Streptococcus equi ssp. equi),
Dorsal View of Larynx, Left and Right Retropharyngeal Lymph Nodes, Horse.
The lymph nodes are grossly distended with suppurative inflammation (pus).
(Courtesy College of Veterinary Medicine, University of Illinois.)
Histologically, the subcapsular, trabecular, and medullary sinuses and the parenchyma
of the cortex and medulla have focal to coalescing foci of neutrophilic inflammation,
necrosis, and fibrin deposition (Fig. 13-78
). If inflammation in the lymph node continues for several days or longer, the lymph
node is further enlarged by follicular hyperplasia and plasmacytosis of the medullary
cords from the expected immune response.
Figure 13-78
Acute Lymphadenitis (Early), Lymph Node, Dog.
A, The sinuses and the parenchyma of the cortex and medulla have coalescing foci of
neutrophilic inflammation, necrosis, hemorrhage, and fibrin deposition. H&E stain.
B, The medullary sinus contains numerous macrophages (sinus histiocytosis) and fewer
neutrophils. This is the type of early response seen when a lymph node drains an inflamed
area. Medullary cords are filled with lymphocytes and plasma cells. H&E stain.
(A courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.
B courtesy Dr. H.B. Gelberg, College of Veterinary Medicine, Oregon State University.)
Chronic Lymphadenitis.
The types of chronic lymphadenitis include chronic suppurative lymphadenitis, diffuse
granulomatous inflammation, and discrete granulomas. In chronic suppurative inflammation,
abscesses range in size from small microabscesses to large abscesses that occupy and
obliterate the whole node. Recurrent bouts of chronic lymphadenitis (e.g., regional
lymph node draining chronic mastitis in cows) lead to fibrosis and lymphoid hyperplasia,
in addition to chronic abscesses. The classic example of chronic suppurative lymphadenitis
with encapsulated abscesses is caseous lymphadenitis, a disease of sheep and goats
caused by C. pseudotuberculosis (Figs. 13-79
and 13-80
) (also see Disorders of Ruminants). It is also the cause of ulcerative lymphangitis
in cattle and horses and pectoral abscesses in horses.
Figure 13-79
Caseous Lymphadenitis, Corynebacterium pseudotuberculosis, Lymph Node, Sheep.
The entire lymph node is replaced by an abscess. This is an early stage of caseous
lymphadenitis, before the pus has become inspissated and lamellated.
(Courtesy Dr. K. Read, College of Veterinary Medicine, Texas A&M University; and Noah's
Arkive, College of Veterinary Medicine, The University of Georgia.)
Figure 13-80
Chronic Caseous Lymphadenitis, Corynebacterium pseudotuberculosis, Lymph Node, Sheep.
Three encapsulated chronic abscesses contain yellow-white caseous pus.
(Courtesy Dr. W. Crowell, College of Veterinary Medicine, The University of Georgia;
and Noah's Arkive, College of Veterinary Medicine, The University of Georgia.)
Focal Granulomatous Lymphadenitis.
Classic examples of focal to multifocal granulomatous lymphadenitis are Mycobacterium
tuberculosis complex, which includes M. bovis among others. Members of M. avium complex
cause similar lesions and have been described in a number of species, including dogs,
cats, primates, pigs, cattle, sheep, horses, and human beings. Infection may begin
by inhalation of aerosol droplets containing the bacilli, which may spread via the
lymphatic vessels to regional lymph nodes, resulting in granulomatous lymphangitis
and lymphadenitis (Fig. 13-81
). Initially lesions in the lymphatic system are confined to the lymphatic vessels
(granulomatous lymphangitis) and regional lymph nodes (e.g., the tracheobronchial
lymph nodes in the case of pulmonary tuberculosis), but once disseminated in the lymph
or blood, lymph nodes throughout the body will have lesions. Well-organized granulomas
consist of a central mass of macrophages with phagocytized mycobacteria, surrounded
by epithelioid and foamy macrophages and occasional multinucleated giant cells (Langhans
type). These inflammatory nodules are surrounded by a layer of lymphocytes enclosed
in a fibrous capsule. Over time the center of the granuloma may undergo caseous necrosis
due to the high lipid and protein content of the dead macrophages (see Chapter 3).
In bovine Johne's disease the mesenteric lymph nodes draining the infected intestine
can have noncaseous granulomas (Fig. 13-82
).
Figure 13-81
Tuberculosis (Mycobacterium bovis), Lymph Node, Ox.
The normal architecture of the lymph node has been completely obliterated by multiple
yellow-brown caseating granulomas, typical of M. bovis lesions.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Figure 13-82
Johne's Disease (Mycobacterium avium ssp. paratuberculosis), Lymph Node, Ox.
Several noncaseating granulomas (areas of pallor) have replaced the normal lymphoid
tissue (blue). Note the Langhans giant cell (arrow). H&E stain.
(Courtesy College of Veterinary Medicine, University of Illinois.)
Diffuse Granulomatous Lymphadenitis.
Coalescing to diffuse granulomatous lymphadenitis is seen in disseminated fungal infections
such as blastomycosis, cryptococcosis (E-Fig. 13-12), and histoplasmosis (see Disorders
in Dogs). In feline cryptococcosis (most often Cryptococcus neoformans), the inflammatory
response may be mild due to the thick polysaccharide capsule, which has strong immunomodulatory
properties and promotes immune evasion and survival within the host. Therefore the
nodal enlargement is due mainly to a large mass of organisms (see E-Fig. 13-12). Pigs
with PCV2 infection may have a multifocal to diffuse infiltrate of macrophages and
multinucleated giant cells of varying severity (see Disorders of Pigs).
E-Figure 13-12
Cryptococcosis (Cryptococcus neoformans), Cats.
A, Right mandibular lymph node. The lymph node (asterisk) is grossly enlarged with
complete loss of architecture. B, The fungal organisms (Cryptococcus neoformans) have
thick nonstaining capsules (arrows) and a lightly basophilic nuclear structure. Variable
amounts of granulomatous inflammation may be seen in cases of Cryptococcus. H&E stain.
(A courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee;
B courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Secondary (Metastatic) Neoplasms.
Carcinomas typically metastasize via lymphatic vessels to the regional lymph node.
Other common metastatic neoplasms include mast cell tumor and malignant melanoma.
Although sarcomas most often metastasize hematogenously, some more aggressive sarcomas
(e.g., osteosarcoma) may spread to regional lymph nodes. Histologically, single cells
or clusters of neoplastic cells travel via the afferent lymphatic vessels and are
deposited in a sinus, usually the subcapsular sinus (see Fig. 13-47). Here the cells
proliferate and can ultimately occupy the whole lymph node, as well as drain to the
next lymph node in the chain.
Pigmentation of Lymph Nodes
Red discoloration is caused by (1) draining erythrocytes from hemorrhagic or acutely
inflamed areas, (2) acute lymphadenitis with hyperemia and/or hemorrhage, (3) acute
septicemias with endotoxin-induced vasculitis or disseminated intravascular coagulation,
and (4) dependent areas in postmortem hypostatic congestion. Blood in pig lymph nodes
is especially obvious due to the inverse anatomy (the equivalent of the medullary
sinuses are subcapsular and thus readily visible in the unsectioned node). Initially
erythrocytes fill trabecular and medullary sinuses and then rapidly undergo erythrophagocytosis
by proliferating sinus macrophages. Hemosiderin deposition occurs within 7 to 10 days
in these macrophages, imparting a brown discoloration of the node.
Black discoloration is often present in the tracheobronchial lymph nodes due to draining
of carbon pigment (pulmonary anthracosis, see Chapter 9). Black ink from skin tattoos
will drain to the regional lymph node. These pigments are usually noted within the
medullary sinus macrophages.
Brown discoloration may be due to melanin, parasitic hematin, or hemosiderin. Melanin
pigment is seen in animals with chronic dermatitis when melanocytes are damaged and
their pigment is released into the dermis and phagocytized by melanomacrophages (pigmentary
incontinence) and drained to the regional lymph node. The mandibular lymph nodes often
contain numerous melanomacrophages in animals with heavily pigmented oral mucosa,
presumably due to chronic low levels of inflammation. This must be distinguished from
metastatic malignant melanomas. Lymph nodes draining areas of congenital melanosis
may have melanin deposits.
Parasitic hematin pigment is produced by Fascioloides magna (cattle) and Fasciola
hepatica (sheep) in the liver and then transported via the lymphatic vessels to the
hepatic lymph nodes.
Hemosiderin, an erythrocyte breakdown product, may form in a hemorrhagic node or arrive
in hemosiderophages draining from congested, hemorrhagic, or inflamed areas. Drainage
of iron dextran from an intramuscular injection may also cause hemosiderin pigment
accumulation within the draining lymph node.
Green discoloration is rare and may be caused by green tattoo ink (often used in black
animals); ingestion of blue-green algae, which drain to mesenteric lymph nodes; massive
eosinophilic inflammation; and in mutant Corriedale sheep, which have a genetic defect
that results in a deficiency in the excretion of bilirubin and phylloerythrin by the
liver. The phylloerythrin or a metabolite stains all the tissues of the body a dark
green, except for the brain and spinal cord, which are protected by the blood-brain
barrier.
Miscellaneous discolorations of lymph nodes may be seen with intravenously injected
dyes (e.g., methylene blue or trypan blue) or subcutaneous drug injections. Lymph
nodes may be yellow in severely icteric patients. The pigmented strain of MAP (Johne's
disease) may impart an orange discoloration in the mesenteric lymph nodes of sheep.
Miscellaneous Lymph Node Disorders
Inclusion Bodies.
Many viruses produce inclusion bodies, and some of these occur in lymph nodes. These
viruses include EHV-1 in horses, bovine adenovirus, cytomegalic virus in inclusion
body rhinitis and PCV2, herpesvirus of pseudorabies in pigs, and rarely parvovirus
in dogs and cats.
Emphysema.
Emphysema in lymph nodes is a consequence of emphysema in their drainage fields and
is seen most frequently in tracheobronchial lymph nodes in bovine interstitial emphysema
and in porcine mesenteric lymph nodes in intestinal emphysema (see Chapter 7). The
appearance of the lymph node varies with the extent of the emphysema. In severe cases
the lymph node is light, puffy, and filled with discrete gas bubbles, and the cut
surface may be spongy. Histologically, the sinuses are distended with gas and lined
by macrophages and giant cells. This change has been considered a foreign body reaction
to the gas bubbles. Macrophages and giant cells are also seen in afferent lymphatic
vessels (granulomatous lymphangitis).
Vascular Transformation of Lymph Node Sinus (Nodal Angiomatosis).
Vascular transformation of the sinuses is a nonneoplastic reaction to blocked efferent
lymphatic vessels or veins. This pressure-induced lesion results in the formation
of anastomosing vascular channels and may be confused with a nodal vascular neoplasm.
These proliferative but noninvasive masses usually begin in the subcapsular sinuses
and may be followed by lymphoid atrophy, erythrophagocytosis/hemosiderosis, and fibrosis.
The blockage may be caused by malignant neoplasms of the tissues that the lymph node
drains (e.g., thyroid carcinoma with nodal angiomatosis of the mandibular lymph node).
Neoplasia
See section on Bone Marrow, Disorders of Domestic Animals, Hematopoietic Neoplasia
for a discussion of the WHO classification of hematopoietic neoplasia that predominantly
arise and proliferate within bone marrow. This section will cover neoplasms of lymphoid
tissue(s) arising outside of bone marrow.
Lymphoma.
The term lymphoma (also known as lymphosarcoma) encompasses a diverse group of malignancies
arising in lymphoid tissue(s) outside of bone marrow. Grossly, there may be diffuse
to nodular enlargement of one or more lymph nodes (Fig. 13-83
), and the cut surface is soft, white, and bulging with loss of normal corticomedullary
architecture. There is great variation in the clinical manifestations and cytopathologic
features of lymphoma, which underlie the importance of classification to better predict
the clinical behavior and outcome. An understanding of lymphocyte maturation is crucial,
because the WHO classification of lymphoma postulates a normal cell counterpart for
each type of lymphoma (when possible). In other words, lymphoma can arise at any stage
in the development/maturation of a lymphocyte—from precursor lymphocytes (B or T lymphoblasts)
to mature lymphoid B and T, lymphocytes and NK cells (Table 13-7
and Box 13-11
).
Figure 13-83
Lymphoma, Cranial Mediastinal Lymph Nodes, Cat.
The cranial mediastinal lymph nodes are grossly enlarged (asterisks), fill the cranial
thoracic cavity, and have caudally displaced the lungs and heart.
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Table 13-7
Common Lymphoma Diagnoses in Domestic Animals Using the WHO Classification System
Lymphoma Subtype
Pattern
Cell Size
Grade
Postulated Normal Cell Counterpart
Defining Histopathologic Features
DLBCL
Diffuse
Large
Mid to high
Germinal center or post–germinal center (activated) B lymphocyte
Multiple nucleoli (centroblastic variant) or single central nucleoli (immunoblastic
variant)
PTCL
Diffuse
Large
Mid to high
Activated mature T lymphocyte
Nuclei of variable size and shape; eosinophils may be present
Burkitt-like lymphoma
Diffuse
Intermediate
High
Either germinal center or post–germinal center (activated) B lymphocyte
Uniform nuclear size with multiple distinct small nucleoli; numerous tingible body
macrophages
T-LBL
Diffuse
Intermediate
High
Naïve T lymphocyte
Dispersed chromatin that obscures nuclear detail
TZL
Nodular
Small to intermediate
Indolent
Activated mature T lymphocyte; paracortical
Abundant clear cytoplasm, sharp shallow nuclear indentations; expansion of paracortex
with peripheralization of “fading” follicles
MZL
Nodular
Intermediate
Indolent
Post–germinal center marginal zone B lymphocyte
Prominent single central nucleoli; expansion of marginal zone with encircling of “fading”
follicles
TCRLBCL
Nodular/diffuse
Large
Indolent to low
Germinal center B lymphocyte
Large irregularly-shaped nuclei with 1-2 prominent nucleoli; numerous small reactive
T lymphocytes
EATCL
Diffuse
Small to intermediate
Low
Intestinal intraepithelial T lymphocytes
Hyperchromatic nuclei with dispersed chromatin
DLBCL, Diffuse large B cell lymphoma; EATCL, enteropathy-associated T cell lymphoma;
MZL, marginal zone lymphoma; PTCL, peripheral T cell lymphoma; TCRLBCL, T cell–rich
large B cell lymphoma; T-LBL, T cell lymphoblastic lymphoma; TZL, T zone lymphoma,
WHO, World Health Organization.
Box 13-11
Histologic Classification of Hematopoietic Tumors of the Lymphoid System in Domestic
Animals
B Lymphocyte Lymphoid Neoplasms
Precursor B Lymphocyte
B cell lymphoblastic leukemia/lymphoma
Mature B Lymphocyte
B cell chronic lymphocytic leukemia/lymphoma
B cell lymphocytic lymphoma intermediate type
Lymphoplasmacytic
Follicular lymphomas
Mantle cell lymphoma
Follicular center cell lymphoma (I, II, III)
Nodal marginal zone lymphoma
Splenic marginal zone lymphoma
Extranodal marginal zone lymphoma of MALT
Hairy cell leukemia
Plasmacytic tumors
Indolent plasmacytoma
Anaplastic plasmacytoma
Plasma cell myeloma
Large B cell lymphomas
T cell–rich large B cell lymphoma
Diffuse large B cell lymphoma
Thymic B cell lymphoma
Intravascular large B cell lymphoma
Burkitt-like lymphoma
T Lymphocyte and NK Cell Lymphoid Neoplasms
Precursor T Lymphocyte
T cell lymphoblastic leukemia/lymphoma
Mature T Cell/NK Lymphocyte
Large granular lymphoproliferative disorders
T cell chronic lymphocytic leukemia
T cell LGL lymphoma/leukemia
NK cell chronic lymphocytic leukemia
Cutaneous T cell lymphomas
Cutaneous epitheliotropic lymphoma
Cutaneous nonepitheliotropic lymphoma
Extranodal/Peripheral T cell lymphoma
Adult T cell–like lymphoma/leukemia
Angioimmunoblastic lymphoma
Angiotropic lymphoma
Intestinal T cell lymphoma
Anaplastic large cell lymphoma
MALT, Mucosa-associated lymphoid tissue; LGL, large granular lymphocyte; NK, natural
killer.
Modified from Valli VE, Jacobs RM, Parodi AL, et al: Histologic classification of
hematopoietic tumors of domestic animals. In World Health Organization international
histological classification of tumors in domestic animals, second series (vol 8),
Washington, DC, 2002, Armed Forces Institute of Pathology.
Pathologists use gross features, histomorphologic features, immunophenotype (B or
T lymphocyte), and clinical characteristics to classify lymphomas. The morphologic
features used in histopathologic classification are the following:
•
Histologic pattern—Nodular or diffuse.
•
Cell size—The nuclei of the neoplastic lymphocytes are compared to the diameter of
a red blood cell (RBC ≅ 5 µm). Small is less than 1.5 times the diameter of an RBC;
intermediate is 1.5 to 2.0 times the diameter of an RBC; large is more than 2.0 times
the diameter of an RBC.
•
Grade—Mitotic figures are counted in a single high-power (400×) field. Indolent is
0 to 1; low is 2 to 5; mid is 6 to 10; high is more than 10.
Although there are numerous subtypes of lymphoma recognized under the WHO system,
a detailed discussion of each subtype is outside the scope of this textbook. However,
a select number of subtypes are more commonly seen in domestic animals (see Table
13-7) and currently best described in the dog (see Disorders of Dogs, Neoplasms, Lymphomas).
The most common types in dogs are large cell lymphomas and include diffuse large B
cell lymphoma and peripheral T cell lymphoma. T cell–rich large B cell lymphoma is
thought to be a variant of diffuse large B cell lymphoma with a distinctive reactive
T lymphocyte infiltrate. Intermediate cell lymphomas include B or T lymphocyte lymphoblastic
lymphomas and Burkitt-like lymphoma (both high grade), marginal zone lymphoma, and
the intermediate cell variant of T zone lymphomas (both indolent and nodular). Small
cell lymphomas most commonly diagnosed in domestic animal species include enteropathy-associated
T cell lymphoma, commonly seen in the cat, T zone lymphoma (small cell variant), and
small cell lymphoma. Cutaneous lymphomas are most often of T lymphocyte origin and
may be epitheliotropic or nonepitheliotropic, and a distinct entity of inflamed T
cell lymphoma has been recently described in dogs (see Chapter 17).
Plasma Cell Neoplasia.
Plasma cell neoplasms are most easily categorized as myeloma or multiple myeloma,
which arises in the bone marrow, and extramedullary plasmacytoma, which as the name
implies involves sites other than bone.
Multiple Myeloma.
See Bone Marrow and Blood Cells, Disorders of Domestic Animals, Types of Hematopoietic
Neoplasia, Plasma Cell Neoplasia.
Extramedullary Plasmacytomas.
Extramedullary plasmacytomas are most commonly diagnosed in the skin of dogs (also
cats and horses), where they constitute 1.5% of all canine cutaneous tumors (see Chapter
17). The pinnae, lips, digits, and chin are the most commonly affected locations,
and most lesions are solitary, though multiple plasmacytomas are infrequently diagnosed.
Other tissues affected include the oral cavity, intestine (colorectal in particular),
liver, spleen, kidney, lung, and brain; of these, the oral cavity and intestine (colorectal)
are involved most often. In one study, extramedullary plasmacytomas represented 5%
of all canine oral tumors and 28% of all extramedullary plasmacytomas diagnosed. Most
cutaneous extramedullary plasmacytomas are benign, and complete excision is usually
curative; oral cavity and colorectal extramedullary plasmacytomas are likely to behave
in a similar manner. More aggressive forms may occur at any site.
As with multiple myeloma, the neoplastic cells composing the tumor may vary from well
differentiated to pleomorphic, often within the same tumor. The cells often have a
characteristic perinuclear Golgi clearing or “halo,” and the more pleomorphic cells
exhibit karyomegaly and binucleation (Fig. 13-84
). Extramedullary plasmacytomas may produce monoclonal immunoglobulins with resulting
monoclonal gammopathy. Amyloid deposition (which may mineralize) is also observed
in a proportion of cases. Differentiation from other round cell tumors may be aided
by immunohistochemistry (MUM1/IRF4 is particularly sensitive and specific for plasma
cell neoplasms).
Figure 13-84
Plasmacytoma (Extramedullary), Oral Cavity, Dog.
Note the moderately well-differentiated plasma cells arranged in small clusters separated
by a fibrovascular stroma. H&E stain.
(Courtesy College of Veterinary Medicine, University of Illinois.)
Histiocytic Disorders.
Histiocytic disorders are frequently diagnosed in dogs and occur less often in cats.
Briefly, histiocytes are categorized as macrophages and DCs, the latter of which are
subdivided into Langerhans cells (LCs), found in skin, gastrointestinal, respiratory,
and reproductive epithelia (mucosae), and interstitial DCs (iDC), located in perivascular
spaces of most organs. The term interdigitating DCs describes DCs (either resident
or migrating) found in T lymphocyte regions of lymph nodes (paracortex) and spleen
(PALS); interdigitating DCs consist of both LCs and iDCs. These lineages can be differentiated
using immunohistochemical stains. Histiocytic disorders that are diagnosed in veterinary
medicine at this time include the following: canine cutaneous histiocytoma, canine
LC histiocytosis, canine cutaneous and systemic histiocytosis, feline pulmonary LC
histiocytosis, feline progressive histiocytosis, dendritic cell leukemia in the dog,
and histiocytic sarcoma and hemophagocytic histiocytic sarcoma in both dogs and cats.
Lymph node involvement is seen in many of these conditions. Rare reports of regional
lymph node metastasis in cases of solitary canine cutaneous histiocytoma have been
published. Lymphatic invasion with subsequent regional nodal involvement may be seen
in dogs with LC histiocytosis, which is a poor prognostic indicator and likely reflects
systemic infiltration. The normal architecture of tracheobronchial lymph nodes is
often effaced in cats with pulmonary LC histiocytosis.
Canine reactive histiocytoses are not clonal neoplastic proliferations but likely
reflect an immune dysregulation consisting of activated dermal iDCs (and T lymphocytes).
They are categorized as cutaneous histiocytosis (CH), involving skin and draining
lymph nodes, and a more generalized systemic histiocytosis (SH), affecting skin and
other sites (e.g., lung, liver, bone marrow, spleen, lymph nodes, kidneys, and orbital
and nasal tissues).
Histiocytic Sarcoma Complex.
Histiocytic sarcomas (HSs) are neoplasms of iDCs and therefore can arise in almost
any tissue, frequently the spleen, lung, skin, meninges, lymph nodes, bone marrow,
and synovium. Secondary involvement of the liver is common as the disease progresses.
This neoplasm is most commonly diagnosed in dogs, and a lower incidence is seen in
cats. Localized histiocytic sarcoma may be a focal solitary lesion or multiple nodules
within a single organ. Disseminated histiocytic sarcoma describes lesions that involve
distant sites and has replaced the term malignant histiocytosis. Breed predispositions
to histiocytic sarcoma complex are seen in Bernese mountain dogs, Rottweilers, golden
retrievers, and flat-coated retrievers, though the disease can occur in any breed.
Histiocytic sarcoma complex is considered to have a rapid and highly aggressive course,
and the clinical signs depend on the particular organ(s) involved.
Grossly, affected organs may be uniformly enlarged and/or contain multiple coalescing
white-tan nodules. Tissue architecture is effaced by sheets of pleomorphic round to
spindle-shaped cells. There is marked cellular atypia with numerous karyomegalic and
multinucleated neoplastic cells (Fig. 13-85
).
Figure 13-85
Histiocytic Sarcoma, Spleen, Dog.
The neoplastic cells are markedly pleomorphic with karyomegalic cells, binucleation,
and numerous mitotic figures. H&E stain.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Hemophagocytic Histiocytic Sarcoma.
Hemophagocytic histiocytic sarcoma is seen in dogs and cats and is a neoplasm of macrophages
of the spleen and bone marrow. Clinically, dogs present with hemolytic regenerative
anemia and thrombocytopenia, thus mimicking Evans's syndrome, though they are Coombs
negative. This form of histiocytic sarcoma carries the worst prognosis of the histiocytic
sarcomas, which is likely in part related to the severe anemia and coagulopathy. It
is characterized by a non–mass forming infiltrate of histiocytes within the bone marrow
and splenic red pulp, causing diffuse splenomegaly. The neoplastic cells exhibit marked
erythrophagocytosis, but the severe cellular pleomorphism seen in the histiocytic
sarcoma complex may be lacking. The neoplastic cells are often intermixed with EMH
and plasma cells. Metastasis is frequently to the liver, where the cells concentrate
within the sinuses. Tumor emboli within the lung are often present.
Disorders of Domestic Animals: Mucosa-Associated Lymphoid Tissue
MALT is involved in a variety of ways with bacteria and viruses, and these are summarized
for large animals in Table 13-5. These interactions include being a portal of entry
for pathogens (e.g., Salmonella spp., Yersinia pestis, MAP, and L. monocytogenes);
a site of replication for viruses (e.g., BVDV); a site for hematogenous infection
(e.g., panleukopenia virus and parvovirus); and a site of gross or microscopic lesions
in some viral diseases. Bovine coronavirus, BVDV, rinderpest virus, malignant catarrhal
fever virus, feline panleukopenia virus, and canine parvovirus cause lymphocyte depletion
within the MALT.
Disorders of Horses
Severe Combined Immunodeficiency
Severe combined immunodeficiency disease of Arabian foals is an autosomal recessive
primary immunodeficiency disorder characterized by the lack of functional T and B
lymphocytes caused by a genetic mutation in the gene encoding for DNA-dependent protein
kinase catalytic subunit (DNA-PKcs). This enzyme is required for receptor gene rearrangements
involved in the maturation of lymphocytes, and the resulting loss of functional T
and B lymphocytes leads to a profound susceptibility to infectious diseases. Though
normal at birth, these foals develop diarrhea and pneumonia by approximately 10 days
of age, often due to adenovirus, Cryptosporidium parvum, and Pneumocystis carinii
infections. Affected foals often die before 5 months of age. Lymph nodes and thymus
are small and often grossly undetectable, and the spleen is small and firm due to
the absence of white pulp (see Fig. 13-70). The development of genetic tests to identify
carriers of the disorder has led to a decrease in the prevalence of severe combined
immunodeficiency disease. Recently, severe combined immunodeficiency disease was diagnosed
in a single Caspian filly, though the exact genetic defect was not determined. Congenital
immunodeficiency diseases are also discussed in detail in Chapter 5.
Strangles
Streptococcus equi ssp. equi, the etiologic agent of equine strangles, is inhaled
or ingested after direct contact with the discharge from infected horses or from a
contaminated environment. The bacteria attach to the tonsils, penetrate into deeper
tissues, enter the lymphatic vessels, drain to regional lymph nodes (mandibular, retropharyngeal,
and occasionally parotid and cervical lymph nodes), and cause large abscesses (see
Fig. 13-77). Retropharyngeal enlargement from abscesses may lead to compression of
the pharynx and subsequent respiratory stridor and dysphagia. Abscesses may rupture
and discharge pus through a sinus to the skin surface or spread medially into guttural
pouches, where residual pus dries and hardens to form chondroids (which serve as a
nidus for live bacteria to persist in carrier animals). In up to 20% of these cases,
ruptured abscess material may spread via blood or lymph to other organs (metastatic
abscess formation, bastard strangles), including lung, liver, kidney, synovia, mesenteric
and mediastinal lymph nodes, spleen, and occasionally brain. Purpura hemorrhagica,
a type III hypersensitivity reaction, may result in necrotizing vasculitis in some
horses with repeated natural exposure to S. equi ssp. equi or after vaccination in
horses that have had strangles.
Rhodococcus Equi Infection
The typical manifestation of R. equi infection is chronic suppurative bronchopneumonia
with abscesses (see Chapter 9). Approximately 50% of foals also develop intestinal
lesions characterized by pyogranulomatous ulcerative enterotyphlocolitis, often over
Peyer's patches, and pyogranulomatous lymphadenitis of mesenteric and colonic lymph
nodes (see Chapter 7; see Fig. 13-68). Large abdominal abscesses may be the only lesion
in the abdomen and presumably originate from an infected mesenteric lymph node. The
diffuse lymphatic tissue in the lamina propria may contain granulomatous inflammation
with the phagocytized bacteria. Mediastinal pyogranulomatous lymphadenitis may compress
the trachea, causing respiratory distress. R. equi lesions also can develop in the
liver, kidney, spleen, or nervous tissue.
Lymphoma
Lymphoma is the most common malignant neoplasm in horses and mostly affects adult
animals (mean age 10 to 11 years) with no apparent breed or sex predisposition. The
most frequent anatomic locations of equine lymphoma are multicentric, cutaneous, and
gastrointestinal tract.
Multicentric lymphoma, defined as involving at least two organs (excluding the regional
lymph nodes), is the most common manifestation, followed by skin and gastrointestinal
tract types. Solitary locations have been reported in the mediastinum, lymph nodes,
ocular/orbital region, brain, spinal cord, oral cavity, and spleen. Of the multicentric
lymphomas, the most frequently observed type is T cell–rich large B cell lymphoma
(TCRLBCL), reportedly in one study affecting 34% of the cases. Peripheral T cell lymphoma
(PTCL) was the second most common, followed by diffuse large B cell lymphoma (DLBCL).
The most common lymphoma type in the gastrointestinal tract is also T cell–rich large
B cell lymphoma, followed by enteropathy-associated T cell lymphoma. Cutaneous lymphomas
in horses account for up to 3% of all equine skin tumors. T cell–rich large B cell
lymphoma is again the most common lymphoma subtype in the skin, representing up to
84% of all cutaneous lymphomas, and most frequently presents clinically as multiple
skin masses. Cutaneous T cell lymphoma (CTCL) is the second most common form and arises
as smaller solitary nodules. Thoroughbreds may have a higher incidence of cutaneous
T cell lymphoma compared to other breeds. Overall, horses with cutaneous T cell–rich
large B cell lymphoma appear to have a longer survival time than horses with other
types of lymphoma of the skin. Progesterone receptor–positive lymphomas have also
been identified in horses, and there is one report of subcutaneous tumor regression
following removal of an ovarian granulosa-theca cell tumor. There may be an increased
frequency of lymphoma in horses diagnosed with equine herpesvirus 5 (EHV-5, gammaherpesvirus),
when compared to healthy horses, although the exact cause-effect role of this observation
in lymphomagenesis5 is not yet known.
Histologically, the hallmark features of T cell–rich large B cell lymphoma include
a majority of small (nuclei approximately the size of an RBC), reactive, mature T
lymphocytes admixed with a neoplastic population of large B lymphocytes whose nuclei
are two to three times the diameter of an equine RBC. These large atypical cells are
often binucleated and have prominent eosinophilic nucleoli (Fig. 13-86
). The large cells may be observed in mitosis or in necrosis as single cells with
retracted cytoplasm and pyknotic nuclei. T cell–rich large B cell lymphoma is often
accompanied by the presence of a dense fibrovascular network.
Figure 13-86
T Cell–Rich Large B Cell Lymphoma, Skin, Horse.
A, The majority of the cells are small T lymphocytes and are mixed with fewer neoplastic
large pleomorphic B lymphocytes. H&E stain. B, The small reactive T lymphocytes are
strongly CD3 positive. Immunohistochemistry with anti-CD3, hematoxylin counterstain.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Disorders of Ruminants (Cattle, Sheep, and Goats)
Johne's Disease
Johne's disease primarily affects domestic and wild ruminants (and rarely pigs and
horses) and is due to infection by MAP. The characteristic lesions include granulomatous
enteritis usually confined to the ileum, cecum, and proximal colon; lymphangitis;
and lymphadenitis of regional lymph nodes (see Fig. 13-82). The bacteria are ingested,
engulfed by the M cells overlying Peyer's patches, and then transported to macrophages
in the lamina propria and submucosa. Among cattle, sheep, goats, and wild ruminants,
there is wide variation in the severity, distribution of lesions, primary inflammatory
cell type (lymphocytes, epithelioid macrophages, multinucleated giant cells), and
numbers of bacteria within lesions (multibacillary or paucibacillary). Histologically,
the architecture of the ileocecal lymph nodes may be partially replaced by aggregates
of epithelioid macrophages and multinucleated giant cells, and the remaining nodal
tissue contains large secondary follicles with reactive germinal centers. Cattle tend
to have noncaseating granulomas, whereas sheep and goats may have granulomas with
necrotic caseous centers and mineralization. Variable numbers of acid-fast bacilli
are detected within epithelioid macrophages. The intestinal lesions of Johne's disease
are described in detail in Chapter 7.
Anthrax
Anthrax is caused by B. anthracis, a Gram-positive bacillus found in spore form in
soil. Cattle, sheep, and goats become infected when grazing on infected soil, and
infection causes fulminant septicemia. The spleen in infected animals is markedly
enlarged and congested (see Uniform Splenomegaly with a Bloody Consistency; see also
Chapter 4).
Bovine Viral Diarrhea
Bovine viral diarrhea is caused by BVDV, a pestivirus. Cattle are the natural host,
but other animals such as alpacas, deer, sheep, and goats are also affected. BVDV
preferentially infects cells of the immune system, including macrophages, DCs, and
lymphocytes. The associated lesions in lymphoid tissues are severe lymphoid depletion
in mesenteric lymph nodes and Peyer's patches, whose intestinal surface may be covered
by a fibrinonecrotic membrane. Histologically, there is marked lymphocytolysis and
necrosis of germinal centers in Peyer's patches and cortices of lymph nodes. There
is thymic atrophy because the thymus is markedly depleted of lymphocytes and may consist
of only collapsed stroma and few scattered lymphocytes. BVD is discussed in detail
in Chapters 4 and 7.
Splenic Abscesses
Splenic abscesses can be the result of bacteremia (see Fig. 13-69) or direct penetration
by a foreign body from the reticulum (see Spleen and also Portals of Entry/Pathways
of Spread).
Caseous Lymphadenitis
C. pseudotuberculosis is a Gram-positive intracellular bacterium that causes caseous
lymphadenitis, a chronic suppurative disease of sheep and goats. The bacterium may
enter through skin wounds (e.g., shearing cuts in sheep, tagging, tail docking, or
castration), drain to the regional lymph node, and then be disseminated in lymph and
circulating blood to external and internal lymph nodes, as well as other internal
organs, including lung. External abscesses are most often detected in the “jaw and
neck” region, specifically in the mandibular and parotid lymph nodes. On gross examination
the abscesses are encapsulated and filled with greenish semifluid pus due to an infiltrate
of eosinophils (see Fig. 13-79). Over time the abscesses lose the greenish hue, and
contents become inspissated to form the characteristic concentric laminations (see
Fig. 13-80); old abscesses may reach a diameter of 4 to 5 cm.
Bovine Lymphoma
Bovine lymphoma is broadly classified into enzootic and sporadic forms. The enzootic
form, called enzootic bovine leukosis (EBL), is caused by BLV, a retrovirus common
in cattle. There is a higher prevalence in dairy cattle compared to beef breeds. BLV
is transmitted horizontally (e.g., blood, milk/colostrum, saliva) or iatrogenically
(e.g., rectal sleeves, instruments/equipment). Following infection, BLV invades and
integrates into the genome of infected B lymphocytes, resulting in a polyclonal B
lymphocyte lymphocytosis in approximately 30% of cattle. In approximately 1% to 5%
of BLV-infected cattle, a single clone will emerge, leading to the development of
B lymphocyte leukemia/lymphoma. The average incubation period between infection and
development of lymphoma is 7 to 8 years, and this low conversion rate suggests that
the latency period may be longer than the life span of most animals (dairy cattle
seldom live to the 7- to 8-year peak incidence of lymphoma occurrence). Other contributing
variables, such as genetic background, coinfections, and environmental factors, may
also play a role in lymphomagenesis. The exact mechanism of BLV-induced tumorigenesis
is poorly understood. Recently BLV microRNAs (miRNAs) were identified in preleukemic
and malignant B lymphocytes, which showed repression of structural and regulatory
gene expression. These findings suggested that miRNAs may play a key role in tumor
onset and progression.
Grossly, multiple tissues may be affected in cattle that develop lymphoma, including
peripheral lymph nodes (cephalic, cervical, sublumbar) (Fig. 13-87
), abdominal lymph nodes, retrobulbar region, abomasum, liver, spleen, heart, urogenital
tract, bone marrow, vertebral canal (Fig. 13-88
), and spinal cord. One study indicates most of these high-grade lymphomas are diffuse
large cell lymphomas (66%), and approximately 20% are intermediate cell lymphomas
(Burkitt-like and lymphoblastic lymphomas).
Figure 13-87
Lymphoma, Bovine Lymph Node.
The normal architecture of lymph node has been replaced by white lobules of neoplastic
lymphocytes.
(Courtesy College of Veterinary Medicine, University of Illinois.)
Figure 13-88
Lymphoma (Asterisks), Vertebral Canal, Epidural Space, Cow.
S, spinal cord.
(Courtesy Dr. J.M. King, College of Veterinary Medicine, Cornell University.)
The sporadic form of bovine lymphoma is most often of T lymphocyte immunophenotype
and has three subcategories: cutaneous, calf, and thymic. There is no known viral
cause for the sporadic form, and each subcategory has a much smaller prevalence compared
to the enzootic form. Of the three sporadic forms, the cutaneous form seems to be
the most common and manifests itself as multiple skin nodules in 1- to 3-year-old
cattle. The calf form presents as generalized lymphadenopathy with weight loss, lethargy,
and weakness in calves less than 6 months old. The thymic form is reportedly more
common in beef cattle, 6 to 24 months of age.
Disorders of Pigs
Postweaning Multisystem Wasting Syndrome
PCV2, a small single-stranded DNA virus, is highly prevalent in the domestic pig population.
Several clinical syndromes are attributed to PCV2 infection and collectively termed
PCV-associated diseases (PCVADs). These include postweaning multisystemic wasting
syndrome (PMWS), porcine respiratory disease complex (PRDC), porcine dermatitis and
nephropathy syndrome, and enteric disease (see Chapters 4 and 9).
The major postmortem findings of postweaning multisystemic wasting syndrome are poor
body condition, enlarged lymph nodes, and interstitial pneumonia. The lesions of the
lymphoid system are commonly observed in the tonsil, spleen, Peyer's patches, and
lymph nodes. Some pigs have all lymphoid tissues affected, whereas others may have
only one or two affected lymph nodes. The characteristic microscopic lesions are lymphoid
depletion of both follicles and paracortex with replacement by histiocytes, mild to
severe granulomatous inflammation with multinucleated giant cells, and intrahistiocytic
sharply demarcated, spherical, basophilic cytoplasmic inclusion bodies. Necrosis of
prominent lymphoid follicles (necrotizing lymphadenitis) is occasionally observed,
and PCV2 can be detected within the necrotic regions. The loss of lymphocytes may
be due to reduced production in the bone marrow, decreased proliferation in the secondary
lymphoid organs, or necrosis of lymphocytes.
Porcine Reproductive and Respiratory Syndrome
Porcine reproductive and respiratory syndrome (PRRS) is caused by an arterivirus and
causes two overlapping clinical syndromes: reproductive failure and respiratory disease.
The virus is transmitted by contact with body fluids (saliva, mucus, serum, urine,
and mammary secretions and from contact with semen during coitus), but often it first
colonizes tonsils or upper respiratory tract. The virus has a predilection for lymphoid
tissues (spleen, thymus, tonsils, lymph nodes, Peyer's patches). Viral replication
takes place in macrophages of the lymphoid tissues and lungs, though porcine reproductive
and respiratory syndrome virus antigen is found in resident macrophages in many tissues
and may persist in tonsil and lung macrophages. The result of this infection is a
reduction in the phagocytic and functional capacity of macrophages of the monocyte-macrophage
system. As a consequence, there is reduction in resistance to common bacterial and
viral pathogens. Most porcine reproductive and respiratory syndrome–infected pigs
are coinfected with one or more pathogens, including Streptococcus suis and Salmonella
choleraesuis. Infection with Bordetella bronchiseptica and Mycoplasma hyopneumoniae
appear to increase the duration and severity of the interstitial pneumonia.
The major lesions are interstitial pneumonia and generalized lymphadenopathy, and
tracheobronchial and mediastinal lymph nodes are most commonly affected. Coinfections
often complicate the gross and histopathologic changes. Lymph nodes are enlarged,
pale tan, occasionally cystic, and firm; some strains of virus also cause nodal hemorrhage.
Microscopically, the lesions in the lymph nodes, tonsils, and spleens consist of varying
degrees of follicular and paracortical hyperplasia and lymphocyte depletion in follicular
germinal centers.
Porcine Jowl Abscess
Streptococcus porcinus causes jowl abscesses in pigs. The bacteria colonize the oral
cavity and spread to infect tonsils and regional lymph nodes. The mandibular lymph
nodes are the most often affected and have multiple, 1- to 10-cm abscesses; the retropharyngeal
and parotid lymph nodes may also be involved (Fig. 13-89
). This once-prevalent disease is now rare, presumably due to improvements in husbandry,
feeder design, and hygiene. It is occasionally isolated in pigs with bacteremia.
Figure 13-89
Jowl Abscess, Pig.
The mandibular lymph nodes are markedly enlarged from a suppurative lymphadenitis
caused by Streptococcus porcinus.
(Courtesy Dr. J.M. King, College of Veterinary Medicine, Cornell University.)
Lymphoma
Lymphoma is the most frequently reported cancer of pigs based on abattoir surveys.
Affected pigs are typically less than 1 year of age, and there is no reported breed
predisposition, although a hereditary basis is suspected in cases arising in inbred
herds. The two main forms of porcine lymphoma are thymic/mediastinal and multicentric;
the latter is more common. Spleen, liver, kidney, bone marrow, and lymph nodes are
affected in the multicentric form, with visceral lymph nodes reportedly more commonly
involved than peripheral nodes. A recent study of lymphoma in 17 pigs found the majority
to be multicentric, and subtypes included the following: B lymphoblastic leukemia/lymphoma,
follicular lymphoma, diffuse and intestinal large B cell lymphoma, and peripheral
T cell lymphoma. One case each of thymic B cell and T cell lymphomas were also described.
Disorders of Dogs
Severe Combined Immunodeficiency Disease
Several types of severe combined immunodeficiency diseases have been described in
dogs. A mutation in DNA-PKcs (similar to Arabian horses) with an autosomal recessive
mode of inheritance is seen in Jack Russell terriers. An X-linked form of severe combined
immunodeficiency disease is well described in basset hounds and is caused by mutations
in the common γ-chain (γc) subunit of the receptors for IL-2, IL-4, IL-7, IL-9, IL-15,
and IL-21. A similar disease is seen in Cardigan Welsh corgi puppies, though it is
an autosomal mode of inheritance in this breed. The mutation inhibits the signal transduction
pathways initiated by any of these cytokines, which are critical for the proliferation,
differentiation, survival, and function of B and T lymphocytes. Affected dogs have
normal numbers of circulating B lymphocytes that are unable to class switch to IgG
or IgA and reduced numbers of T lymphocytes, which are nonfunctional due to the inability
to express IL receptors. Affected puppies are remarkably susceptible to bacterial
and viral infections and rarely survive past 3 to 4 months of age. The thymus of these
dogs is small and consists of only small dysplastic lobules with a few of Hassall's
corpuscles. Tonsils, lymph nodes, and Peyer's patches are often grossly unidentifiable
due to the severe lymphocyte hypoplasia. Congenital immunodeficiency diseases are
also discussed in detail in Chapter 5.
Thymic Hematomas
Thymic hemorrhage and hematomas have been reported in dogs and are most often seen
in young animals. A variety of causes are described, including ingestion of anticoagulant
rodenticides (warfarin, dicumarol, diphacinone, and brodifacoum), dissecting aortic
aneurysms, trauma (e.g., automobile accident), and idiopathic/spontaneous. Histologically,
hemorrhage variably expands the thymic lobules and septa, and in severe cases the
lobular architecture is obscured by hemorrhage. In cases of anticoagulant rodenticide
toxicosis, the medulla appears to be the main site of hemorrhage.
Gastrosplenic Volvulus
See Uniform Splenomegaly with a Bloody Consistency (also see Figs. 7-72 and 7-73).
Splenic Hematomas, Incomplete Splenic Contraction, Acute Splenic Infarcts, and Hemangiosarcomas
See the section on Splenic Nodules with a Bloody Consistency for discussion on splenic
hematomas (including those induced by nodular hyperplasia or occurring with hemangiosarcoma),
incomplete splenic contraction, acute splenic infarcts, and hemangiosarcomas.
Siderofibrotic Plaques, Splenic Rupture, and Accessory Spleens
See the section on Miscellaneous Disorders of the Spleen for discussions on siderofibrotic
plaques, splenic rupture, and accessory spleens.
Lymphoid and Complex Splenic Nodular Hyperplasia
Splenic nodular hyperplasia is common in dogs and categorized based on their cellular
components as lymphoid nodular hyperplasia or complex nodular hyperplasia. Hematomas
may arise within nodules of hyperplasia (see Splenic Nodules with a Bloody Consistency).
Lymphoid (or simple) nodular hyperplasia consists of a focal well-demarcated mass
composed of discrete to coalescing aggregates of lymphocytes. The lymphocytes may
form follicular structures with germinal centers and/or consist of a mixture of lymphocytes
with mantle and marginal zone cell morphologic features. The intervening tissue is
often congested and may contain plasma cells, but stroma is not observed (Fig. 13-90
; E-Fig. 13-13).
Figure 13-90
Lymphoid (Simple) Splenic Nodular Hyperplasia
A, Nodular hyperplasia, spleen, dog. B, The well-demarcated nodule (lower right of
image) is composed of hyperplastic lymphoid follicles, and the intervening tissue
is congested. H&E stain.
(A courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.
B courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
E-Figure 13-13
Nodular Hyperplasia, Spleen, Dog.
A, A hemispheric 4-cm diameter nodule protrudes from the capsular surface. B, Cross
section of the nodular mass with mottled red and white areas composed of red blood
cells and hyperplastic follicles.
(A courtesy College of Veterinary Medicine, University of Illinois. B courtesy Dr.
M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Complex nodular hyperplasia is a focal mass that contains two proliferative components:
lymphoid and stroma (E-Fig. 13-14). The lymphoid component resembles lymphoid nodular
hyperplasia described above. There is proliferation of the intervening stromal tissues
with fibroplasia, smooth muscle hyperplasia, and histiocytic hyperplasia; EMH and
plasma cells may also be present.
E-Figure 13-14
Complex Splenic Nodular Hyperplasia.
The splenic nodule contains a lymphoid component of discrete to merging nodular aggregates
of lymphocytes, characteristic of simple lymphoid nodular hyperplasia. In addition,
there is proliferation of splenic stromal elements with fibroplasia, smooth muscle
hyperplasia, and histiocytic hyperplasia. H&E stain.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Splenic Fibrohistiocytic Nodules
It has recently come to light that the entity splenic fibrohistiocytic nodule (SFHN),
first described in 1998, is not a single condition, but in fact a complex group of
diseases. Our better understanding of the spectrum of diseases once described under
the term splenic fibrohistiocytic nodule is due to increasing knowledge of histiocytic
disorders and immunochemistry. The original definition of splenic fibrohistiocytic
nodule is a nodule characterized by a stromal population of histiocytoid and spindle
cells intermixed with lymphocytes. Grading was based on the lymphocyte percentage
of the population (e.g., > 70% lymphocytes = grade 1; < 40% lymphocytes = grade 3);
dogs with grade 1 splenic fibrohistiocytic nodule had a much better 1-year survival
rate, and dogs with grade 3 nodules may develop sarcomas (often malignant fibrous
histiocytoma, a now outdated term).
With our increasing knowledge of histiocytic disorders and additional immunohistochemical
stains, diseases that likely were encompassed by the term splenic fibrohistiocytic
nodule include the following: complex and lymphoid nodular hyperplasia (see earlier),
stromal sarcoma, histiocytic sarcoma, marginal zone hyperplasia, marginal zone lymphoma,
and diffuse large B cell lymphoma (see Lymphoid/Lymphatic System, Disorders of Domestic
Animals: Lymph Nodes, Neoplasia, Lymphoma).
Histoplasmosis
Histoplasma capsulatum can cause a disseminated fungal disease that is widely endemic,
particularly in areas with major river valleys and temperate or tropical climates
(e.g., midwestern and southern United States). Free-living organisms in the mycelial
phase produce macroconidia and microconidia that are inhaled and converted to the
yeast phase in the lung. Yeasts are phagocytized and harbored by macrophages of the
monocyte-macrophage system. In some dogs the disease is limited to the respiratory
tract and causes dyspnea and coughing. However, in most dogs, the disease is disseminated
throughout the body, predominantly affecting the liver, spleen, gastrointestinal tract,
bone marrow, skin, and eyes; primary gastrointestinal disease is also reported. The
clinical signs in cases of disseminated histoplasmosis include wasting, emaciation,
fever, respiratory distress, diarrhea with hematochezia or melena, and lameness.
The clinicopathologic changes of disseminated histoplasmosis may include neutrophilia,
monocytosis, nonregenerative anemia in chronic infections, changes in total serum
protein level, and liver enzyme level elevations with hepatic involvement. The anemia
is likely a result of chronic inflammation, Histoplasma infection of the bone marrow,
and/or intestinal blood loss in dogs with GI disease. Cytologic examination is useful
for the diagnosis of histoplasmosis (tracheal wash preparations, aspirates of bone
marrow and lymph nodes), where organisms are often visible in macrophages (E-Fig.
13-15).
E-Figure 13-15
Histoplasmosis, Feline Transtracheal Wash.
A macrophage is laden with small, oval, encapsulated yeast forms. Wright's stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
Grossly, there is hepatosplenomegaly, the intestines are thickened and corrugated,
and the lymph nodes are uniformly enlarged (Fig. 13-91
) with loss of normal architecture (somewhat similar to lymphoma, though the nodes
tend to be more firm in histoplasmosis). Histologically within the node, there is
a multifocal to coalescing infiltrate of epithelioid macrophages with intracytoplasmic,
small (2 to 4 µm in diameter) yeast organisms with spherical basophilic central bodies
surrounded by a clear halo (Fig. 13-92
).
Figure 13-91
Mesenteric Lymph Node, Diffuse Granulomatous Lymphadenitis, Histoplasmosis, Dog.
The lymph node is enlarged, the cut surface shows loss of architecture, and the tissue
bulges because of the diffuse granulomatous inflammation (see Fig. 13-92).
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Figure 13-92
Histoplasmosis, Lymph Node, Dog.
Diffuse granulomatous lymphadenitis. Macrophages contain the phagocytized Histoplasma
capsulatum organisms (arrows). H&E stain.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Leishmaniasis
Leishmaniasis is a disease of the monocyte-macrophage system caused by protozoa of
the genus Leishmania. It occurs in dogs and other animals and is endemic in parts
of the United States, Europe, Mediterranean, Middle East, Africa, and Central and
South America. The protozoa proliferate by binary fission in the gut of the sand fly
and become flagellated organisms, which are introduced into mammals by insect bites,
where they are phagocytized by macrophages and assume a nonflagellated form. Cutaneous
and/or visceral forms of the disease are observed. In the visceral form, dogs are
emaciated and have general enlargement of abdominal lymph nodes and hepatosplenomegaly
(E-Fig. 13-16). Histologically, the lymph node sinuses and splenic red pulp are filled
with macrophages that contain intracytoplasmic, round, 2-µm-diameter organisms with
a small kinetoplast. Though there is an initial stage of lymphoid hyperplasia in the
spleen and lymph node, subsequent lymphoid atrophy occurs with chronicity. The atrophy
is due to impairment of follicular DCs, B lymphocyte migration, and germinal center
formation. There may be lymphoid atrophy of the spleen and lymph nodes in severe chronic
infections.
E-Figure 13-16
Leishmaniasis, Leishmania spp., Canine Popliteal Lymph Node Aspirate.
A macrophage contains multiple amastigotes with oval nuclei and smaller bar-shaped
kinetoplasts (arrow). Wright's stain.
(Courtesy Dr. M.M. Fry, College of Veterinary Medicine, University of Tennessee.)
Canine Distemper
Canine distemper virus preferentially infects lymphoid, epithelial, and nervous cells
(see Chapter 14). Dogs are exposed through contact with oronasal secretions, and the
virus infects macrophages within the lymphoid tissue of the tonsil and respiratory
tract (including tracheobronchial lymph nodes) and later disseminates to the spleen,
lymph nodes, bone marrow, MALT, and hepatic Kupffer cells. The virus causes necrosis
of lymphocytes (especially CD4 T lymphocytes) and depression of lymphopoiesis in the
bone marrow, leading to severe immunosuppression. Dogs are therefore susceptible to
secondary infections, including Bordetella bronchiseptica, Toxoplasma gondii, Nocardia,
Salmonella spp., and generalized demodicosis.
Canine Parvovirus
Canine parvovirus type 2 (CPV-2) is a highly contagious disease of dogs spread through
the fecal-oral route or oronasal exposure to contaminated fomites. The virus has tropism
for rapidly dividing cells, and replication begins in the lymphoid tissues of the
oropharynx, thymus, and mesenteric lymph nodes and then is disseminated to the small
intestinal crypt epithelium. By infecting lymphoid tissues, canine parvovirus type
2 causes immunosuppression directly through lymphocytolysis and indirectly though
bone marrow depletion of lymphocyte precursors. There is marked lymphoid atrophy of
thymus and follicles of the spleen, lymph nodes, and MALT—particularly of Peyer's
patches to produce the classic gross lesion of depressed oval regions of the mucosa
(so-called punched-out Peyer's patches).
Neoplasms
Thymomas.
See Lymphoid/Lymphatic System, Disorders of Domestic Animals: Thymus, Neoplasia.
Lymphomas.
Lymphoma is the most common hematologic malignancy in the dog. Using the WHO classification
scheme, several lymphoma subtypes are identified in dogs and clinically range from
slow-growing indolent tumors to highly aggressive tumors. Of all domestic animal species,
lymphoma is the most extensively studied in dogs. The most common clinical presentation
in dogs is generalized lymphadenopathy, with or without clinical signs such as lethargy
and inappetence.
The majority of lymphomas in dogs are large cell mid- to high-grade lymphomas, and
up to half of all lymphoma cases are subtyped as diffuse large B cell lymphoma. Diffuse
large B cell lymphomas are further subdivided into centroblastic or immunoblastic
based on nucleolar morphologic features (see Table 13-7 and Box 13-11), although it
is unclear if this difference has any prognostic significance. Histologically, lymph
node architecture is most often completely effaced by sheets of large neoplastic cells,
which may invade through the capsule and colonize the perinodal tissue. These dogs
are often treated with chemotherapy and achieve remission. The overall median survival
time for dogs with diffuse large B lymphocyte lymphoma is approximately 7 months,
although this number varies based on the study and the grade of the tumor (as determined
by mitotic figures). Peripheral T cell lymphomas–not otherwise specified are the second
most common subtype in dogs. This category includes all T cell lymphomas that do not
fit into the other categories (e.g., T zone lymphoma, enteropathy-associated T cell
lymphoma, and hepatosplenic T cell lymphoma). Peripheral T cell lymphoma also effaces
nodal architecture, and when compared to diffuse large B cell lymphoma, there is more
variation in nuclear size and morphologic features. Dogs with this subtype tend to
have shorter survival times.
Intermediate cell size, high-grade lymphomas are less common in dogs, and the two
most frequently encountered subtypes are lymphoblastic lymphoma (LBL) and Burkitt-like
lymphoma (BLL). Lymphoblastic lymphoma may be of B or T lymphocyte origin, though
T cell lymphoblastic lymphoma is more common of the two. It is important to recognize
a common misuse of the term “lymphoblast” in lymphoblastic lymphoma—by definition
in lymphoblastic lymphoma, lymphoblasts are intermediate-sized cells with a distinct
dispersed chromatin pattern, and not the large lymphocytes seen in cases of diffuse
large B cell lymphoma or peripheral T cell lymphoma. T lymphocyte lymphoblastic lymphoma
is an aggressive disease that is often resistant to treatment. Burkitt-like lymphoma
is a high-grade lymphoma of B lymphocytes.
Numerous other subtypes of lymphoma have been reported in dogs, including several
forms of cutaneous lymphomas, most often of T lymphocyte origin and epitheliotropic
(see Chapter 17). Hepatosplenic T cell lymphoma, thought to be of γ/δ T lymphocyte
origin, affects the liver and spleen without significant nodal involvement. Hepatocytotropic
T cell lymphoma is a distinct form of lymphoma with tropism for the hepatic cords;
clusters or individual neoplastic lymphocytes invade the hepatic cords, without hepatocyte
degeneration. Intravascular lymphoma is a proliferation of large neoplastic lymphocytes
within blood vessels of many tissues, leading to progressive occlusion and subsequent
thromboses and infarcts. This neoplasm does not form an extravascular mass, and neoplastic
cells are not found in peripheral blood smears or bone marrow.
Indolent lymphomas constitute up to 29% of all canine lymphomas. Indolent lymphomas
in dogs, in descending order of frequency, include T zone lymphoma (TZL), marginal
zone lymphoma (MZL), mantle cell lymphoma (MCL), and follicular lymphoma (FL). Mantle
cell lymphoma and follicular lymphoma are less commonly diagnosed than T zone lymphoma
and marginal zone lymphoma; therefore the reader is referred to the Suggested Readings
to learn more on mantle cell lymphoma and follicular lymphoma.
T Zone Lymphoma.
T zone lymphoma is the most common indolent lymphoma in dogs (Fig. 13-93
). It presents as a solitary or multiple peripheral lymphadenomegaly (often mandibular
lymph nodes) in otherwise healthy-appearing dogs. The characteristic histopathologic
architecture is a nodular expansion of the paracortex by neoplastic cells, which push
atrophied “fading” cortical follicles against the thinned capsule and trabeculae.
This unique architectural feature is best highlighted with immunohistochemical stains
(often CD3 for T lymphocytes and CD79a, pax5, or CD20 for B lymphocytes). The neoplastic
cells are small to intermediate in size with pale eosinophilic cytoplasm and oval
nuclei with sharp shallow indentations. Mitotic figures are rare. Dogs with this lymphoma
subtype tend to be diagnosed with an advanced stage of the disease, likely because
they present clinically healthy, without loss of appetite or activity level. Even
so, dogs with T zone lymphoma have a relatively long survival time compared to other
lymphomas: reports on median survival time range from 13 to 33 months, and data suggest
that dogs who do not receive chemotherapy actually have longer median survival times.
Figure 13-93
T Zone Lymphoma, Lymph Node, Dog.
A, The characteristic histopathologic architecture is a nodular expansion of the paracortex
by neoplastic cells, which push atrophied “fading” cortical follicles against the
capsule (C) and trabeculae (interconnected pink bands). H&E stain. B, The neoplastic
cells are small to intermediate in size, and mitotic figures are rare. H&E stain.
C, The neoplastic cells are T lymphocytes. Immunohistochemistry anti-CD3, hematoxylin
counterstain. D, The remnants of the “fading” cortical follicles are composed of B
lymphocytes. Immunohistochemistry anti-pax5, hematoxylin counterstain.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
Marginal Zone Lymphoma.
Marginal zone lymphoma is an indolent B lymphocyte neoplasm derived from the cells
of the marginal zone of lymphoid follicles. Most marginal zone lymphomas (and mantle
cell lymphomas) are assumed to originate in the spleen with slow spread to lymph nodes
and often present as a mottled white-red smooth spherical splenic mass. Histopathologic
assessment of tissue architecture is needed for a diagnosis of marginal zone lymphoma
and is characterized by a distinct nodular pattern in which the lighter-staining neoplastic
marginal zone cells form a dense cuff around small foci of darkly stained mantle cells
(fading follicles). The neoplastic marginal zone lymphocytes are intermediate in size
and have a single prominent central nucleolus. Mitotic figures are often rare or absent
early on and increase with disease progression.
Differentiating between marginal zone lymphoma and marginal zone hyperplasia (which
refers to a proliferation of marginal zone cells and contains a mixture of small and
intermediate lymphocytes) is challenging because marginal zone lymphoma arises on
the background of marginal zone hyperplasia. Additionally, lymphoid and complex nodular
hyperplasia are common in the dog spleen (see Disorders of Dogs), and it is possible
that many cases of nodular hyperplasia contain areas of marginal zone lymphoma. Therefore
immunophenotyping and molecular clonality are ultimately required for a definitive
diagnosis of marginal zone lymphoma. The overall median survival time in dogs with
splenic marginal zone lymphoma after splenectomy is approximately 13 months (even
longer if it is diagnosed as an incidental finding).
Plasmacytomas.
See Disorders of Domestic Animals: Lymph Nodes, Neoplasia, Plasma Cell Neoplasia,
Extramedullary Plasmacytomas (see Fig. 13-84).
Disorders of Cats
Feline Panleukopenia (Parvovirus)
Feline panleukopenia, caused by the single-stranded DNA virus feline parvovirus (FPV),
is a highly contagious and often lethal disease of cats and other Felidae, as well
as other species (including raccoons, ring-tailed cats, foxes, and minks). FPV is
transmitted by the fecal-oral route through contact with infected body fluids, feces,
or fomites. Following intranasal or oral infection, the virus initially replicates
in the macrophages in the lamina propria of the oropharynx and regional lymph nodes,
followed by viremia, which distributes the virus throughout the body. Because FPV
requires rapidly multiplying cells in the S phase of division for its replication,
replication occurs in mitotically active tissues (lymphoid tissue, bone marrow, and
intestinal mucosa). By infecting lymphoid tissues, FPV causes immunosuppression directly
through lymphocytolysis and indirectly through depletion of lymphocyte precursors
in the bone marrow. Consequently there is marked lymphoid atrophy of thymus, spleen,
lymph node, and MALT (particularly Peyer's patches).
Mast Cell Tumors
See Bone Marrow and Blood Cells, Disorders of Domestic Animals, Types of Hematopoietic
Neoplasia, Myeloid Neoplasia, Mast Cell Neoplasia and see E-Fig. 13-6.
Lymphoma
Lymphoma is the most commonly diagnosed neoplasm in cats, and the incidence is reportedly
the highest for any species. Mediastinal or multicentric lymphomas are seen in young,
FeLV-infected cats (see Fig. 13-53). With the advent of FeLV vaccine and routine testing,
the prevalence of FeLV-associated lymphoma is decreased. Currently the alimentary
tract is the most commonly affected site, and typically occurs in cats greater than
10 years of age (Figs. 13-94
and 13-95
). Other miscellaneous sites commonly affected are brain, spinal cord, eye, kidney,
and nasopharynx.
Figure 13-94
Alimentary Lymphoma, Stomach, Cat.
The stomach mucosa is markedly thickened by the neoplastic cells (gray-white areas
in the right half of the image); focal ulcers are also noted (asterisks).
(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)
Figure 13-95
Intestinal Small Cell Lymphoma, Jejunum, Cat.
Enteropathy-associated T cell lymphoma. A, The neoplastic cells expand the lamina
propria of intestinal villi and submucosa, lifting the crypts from the muscularis
mucosa. B, Higher magnification of a villus. The neoplastic lymphocytes in the lamina
propria are small and often colonize in clusters within the epithelium. H&E stain.
(Courtesy Dr. A.C. Durham, School of Veterinary Medicine, University of Pennsylvania.)
The retrovirus FeLV has long been recognized as a cause of lymphoma in cats—the risk
for lymphoma is increased sixtyfold in infected cats. Before the advent of a vaccine
in 1985, approximately 70% of cats (mainly young animals) with lymphoma were FeLV
positive. FeLV infects T lymphocytes and can cause myelodysplastic syndrome, acute
myeloid leukemias (see Myeloid Neoplasia), and T lymphocyte leukemia/lymphoma. In
the latter the mediastinum (thymus, mediastinal, and sternal lymph nodes) is the site
most commonly involved, although a multicentric distribution also occurs. Routine
FeLV vaccination has led to a significant decrease in the prevalence of FeLV infection,
which has resulted in a decrease in the proportion of mediastinal lymphomas.
The risk for developing lymphoma in FIV-infected cats is fivefold to sixfold higher
than in uninfected cats. Cats that underwent kidney transplantation and thus received
immunosuppressive drug therapy had a similar risk for developing lymphoma. Both FIV-infected
and posttransplantation cats predominantly developed extranodal, high-grade, diffuse
large B lymphocyte lymphomas. This form is also the most common subtype in human immunodeficiency
virus and posttransplantation patients caused by the Epstein-Barr virus (EBV). Therefore
it is reasonable to question whether these two groups of immunosuppressed cats may
be more prone to infection by a gammaherpesvirus similar to EBV, leading to lymphoma.
Recently a novel feline gammaherpesvirus (FcaGHV1) was discovered in domestic cats
with a 16% prevalence in North America, and further studies to investigate its role
in lymphomagenesis are needed.
The overall incidence of feline lymphomas has increased, mainly due to an increase
in gastrointestinal lymphomas. Mucosal T cell lymphoma, also known as enteropathy-associated
T cell lymphoma (EATCL type II), is the most common and arises from diffuse MALT of
the small intestine. The neoplastic cells are small (nuclei are equal to the diameter
of a feline RBC), mitotic figures are infrequent (low grade), and mucosal and crypt
epitheliotropism is common (see Fig. 13-95). A diagnosis of this subtype of lymphoma
may be difficult (particularly in endoscopic biopsy samples), because this disease
often is multifocal and concurrent with or arises within lymphoplasmacytic inflammatory
bowel disease (IBD). The neoplastic lymphocytes are morphologically similar to the
inflammatory lymphocytes. Early small cell mucosal T cell lymphomas often require
additional diagnostic testing, namely, immunohistochemistry and molecular clonality
testing (PCR for antigen receptor rearrangement [PARR]) to confirm a clonal neoplasm.
Transmural T cell lymphomas also occur focally or multifocally in the small intestine
of cats (best classified as enteropathy-associated T cell lymphoma type I) and by
definition must extend into the submucosa and muscularis. Some tumors invade the serosa
and adjacent mesentery. T cell large granular lymphocyte (LGL) lymphoma is often diagnosed,
and the intestinal segments orad and aborad to the transmural mass may also have mucosal
lymphoma. Gastrointestinal B cell lymphomas are less prevalent in cats but occur in
the stomach, jejunum, and ileocecocolic region as transmural lesions. Most are diagnosed
as diffuse large B cell lymphomas.
Lymphomas in other sites also occur less frequently in cats. The upper respiratory
tract (nasal and/or nasopharyngeal region) is a relatively rare site for lymphoma.
However, lymphoma is the most common primary nasal tumor, and diffuse large B cell
lymphomas (of immunoblastic type) are the predominant subtype. Both cutaneous (cutaneous
T cell lymphoma) and subcutaneous lymphomas (usually large cell lymphomas) are rare.
Presumed solitary ocular lymphomas have also been reported.
T cell–rich large B cell lymphoma, also referred to as feline Hodgkin-like lymphoma
in some studies, is composed of a mixture of reactive small lymphocytes and large
neoplastic B lymphocytes, many of which may be binucleated and/or have prominent nucleoli
(thus resembling the Reed-Sternberg cells of human Hodgkin's lymphoma). This disease
is typically characterized by a distinctive clinical presentation of an indolent unilateral
neoplasm of the cervical lymph nodes, which spreads slowly to adjacent nodes within
the chain. However, a proportion of cases may go on to develop into a more aggressive
multicentric large to anaplastic B lymphocyte lymphoma that can affect peripheral
and central nodes and multiple organs.
Suggested Readings
Suggested Readings are available at www.expertconsult.com.