Human visceral leishmaniasis (VL) and canine visceral leishmaniasis (CVL) are the
most important emerging diseases with high prevalence in Latin American countries
and are mainly caused by Leishmania (L.) chagasi (Syn=L. infantum). CVL has a great
impact on Brazilian public health because domestic dogs are the most important VL
peri-domicile reservoirs in both urban and peri-urban areas. Our findings highlight
the complexity of cellular immunological events related to the natural infection from
dogs by L. chagasi, additionally correlating major peripheral blood phenotypic markers
with clinical status and tissues parasite density. Our main results demonstrated that
lower frequency of circulating B cells and monocytes are important markers of severe
CVL, whereas increased levels of CD8+ lymphocytes appear to be the major phenotypic
feature of asymptomatic disease. Determination of the isotypes patterns during CVL
demonstrated that asymptomatic dogs and those with low parasitism are associated with
an increase of IgG1, while the symptomatic dogs and those with high parasitism are
associated with an increase of IgG, IgG2, IgM, IgA and IgE immunoglobulins. Pioneer
findings obtained by our group showed a correlation between clinical status of CVL
with degree of tissue parasite density. This data demonstrated that asymptomatic dogs
presented low parasitism while symptomatic dogs are associated with high parasite
load in various tissues such as skin, bone marrow and spleen. We have also investigated
the association between tissue parasitism and CVL clinical forms. Regardless of clinical
status, skin and spleen are the major sites of high parasite density during ongoing
CVL. Furthermore, we demonstrated that bone marrow and spleen parasite density are
the most reliable parasitological markers to decode the clinical status of CVL. In
this article, we have reviewed some aspects of the histopathological and immunological
events occurring in natural and experimental L. chagasi/L. infantum infection, pointing
out the main L. chagasi-parasitized tissue. We have discussed the importance of the
association between parasite density, immunological/histopathological aspects and
clinical status of the CVL, their current applications, challenges for the future
and potential opportunities in CVL research.