Respiratory disease problems represent a major area of concern for all phases of cattle
production. All types and all ages of cattle are susceptible to respiratory problems,
and in some production settings, respiratory disease is the single most important
cause of livestock morbidity and mortality. In recent national surveys, respiratory
disease is reported to account for 24.5% of preweaned dairy heifer calf deaths, and
it is the leading cause of death in weaned heifer calves, accounting for 44.8% of
calf death losses [54]. In adult dairy cows, respiratory disease is less important
than mastitis, lameness, metabolic diseases, and reproductive disorders as a cause
of morbidity, but it still affects 2.5% of adult dairy cattle on a yearly basis, and
9.6% of dairy cow deaths are attributed to respiratory disease [54]. In preweaned
beef calves over 3 weeks of age, respiratory problems represent 21% of health problems,
occurring in approximately 0.8% of all calves [52]. Respiratory disease accounts for
16.3% of total beef calf death loss and 6.0% of total breeding cattle death loss on
cow–calf operations [52]. Shipping fever was recently reported to occur in 14.4% of
feedlot cattle, and this respiratory problem was more than 4 times more prevalent
than the next leading cause of morbidity, which was acute interstitial pneumonia [56].
Annual death loss estimates due to respiratory disease for all cattle and calves in
the United States exceed 1.2 million animals, with an estimated total economic loss
greater than $478 million [53].
These morbidity and mortality estimates underscore the tremendous importance of respiratory
disease to cattle producers. Considerable effort over many years has been focused
on improving our understanding of this problem. Despite improvements in our understanding
of pathogenesis, characteristics of causative agents, vaccine technology, and means
of prevention and treatment, it seems that respiratory disease remains one of the
foremost cattle health concerns.
The challenge that the authors were presented with in writing this article was to
consider the role that biosecurity could play in reducing the occurrence or effect
of respiratory disease. It seems that little research has specifically evaluated the
effects of biosecurity management practices on the occurrence of the problem in livestock
operations. Indeed, recognizing the multifactorial etiology of infectious respiratory
disease and the ubiquitous presence of the pathogens involved leads to the conclusion
that attempts to decrease disease prevalence must incorporate multiple management
steps, of which biosecurity practices are only a single component. Although biosecurity
practices have equal potential to decrease respiratory disease losses in all food
animal species, the authors focus this article primarily on bovine respiratory disease
complex. This article addresses major areas of respiratory pathogen control and provides
some suggestions for practical intervention.
Overview of bovine respiratory disease
Bovine respiratory disease is not a single entity, nor is it attributable to a single
cause [2]. One useful scheme for characterizing respiratory tract diseases in a practical
manner distinguishes three different categories of problems [40]. These include the
bovine respiratory disease complex (BRDC), epitomized by shipping fever pneumonia
and enzootic calf pneumonia; acute interstitial pneumonias; and metastatic pneumonia.
This scheme excludes many problems that involve only the upper respiratory tract,
although these problems may predispose to lower tract infections. The interstitial
pneumonias are most commonly attributed to toxicoses, and metastatic pneumonias are
secondary complications of disease in other organ systems that spread hematogenously
to the lung. Although these disease problems are frequently fatal for affected cattle,
they occur sporadically and are generally not considered to be contagious. The authors
focus their attention for this discussion on BRDC. This problem has an infectious
origin, and it is by far the most frequently occurring form of cattle respiratory
disease. Cattle of all ages and in a variety of circumstances can be affected by BRDC,
but the disease most commonly manifests in young dairy calves (enzootic calf pneumonia)
and in beef calves recently arrived at feedlots (shipping fever pneumonia).
Research over the past several decades has provided an increasingly clear picture
of how BRDC occurs and why it is so common. Unfortunately, this knowledge has not
led to a commensurate decrease in the morbidity and mortality associated with this
problem, primarily because animals are commonly managed in ways that predispose to
disease development.
Bovine respiratory disease complex refers to bacterial bronchopneumonia that may or
may not be complicated by previous or concurrent viral or Mycoplasma infection [2].
Numerous bacterial species can be isolated from the lungs of affected animals. In
feedlot cattle and adult cattle, Mannheimia (Pasteurella) haemolytica is considered
the most important pathogen, with lesser roles attributed to Pasteurella multocida
and Hemophilus somnus. In younger calves, these same pathogens play a role, but Mycoplasma
spp. are also considered to be important. Arcanobacterium pyogenes, Fusobacterium
spp., and Bacteroides spp. are frequently isolated from animals with chronic, abscessing
lung lesions but do not play a major role in acute bronchopneumonia. Less common bacterial
isolates, including Streptococcus spp., Staphylococcus spp., Pseudomonas aeruginosa,
and Chlamydia spp., are also occasionally identified in young calves.
All of the bacterial pathogens considered important in BRDC can be isolated from the
upper respiratory tract of healthy cattle and calves. These pathogens are considered
ubiquitous in cattle populations, not because they can be found in each animal, but
because they are readily identified in the nasopharynx of some animals in most populations.
In the absence of other predisposing causes of disease, it seems that the simple presence
of these bacterial agents is not of major significance. The disease complex is best
characterized as being multifactorial, only occurring when a combination of factors
involving the animal, environment, and infectious agents are present.
Viral pathogens are implicated in the development of BRDC, although the final pulmonary
pathology is primarily caused by bacterial pathogens [2]. The principal viruses involved
in BRDC include bovine herpesvirus 1 (infectious bovine rhinotracheitis), bovine parainfluenza
virus type 3, bovine respiratory syncytial virus, and bovine viral diarrhea virus.
Lesser roles are attributed to bovine coronavirus, adenovirus, rhinovirus, reovirus,
and enterovirus. These viral pathogens primarily infect the upper respiratory tract,
resulting in rhinitis, tracheitis, and bronchitis. Their ability to cause direct pulmonary
disease is generally limited except for bovine respiratory syncytial virus, which
can also cause severe lung damage as the primary agent. All of these viral pathogens
predispose the lung to bacterial infection and bronchopneumonia. The primary role
of these agents in BRDC is to promote bacterial challenge to the lungs by compromising
respiratory tract defense mechanisms.
The predisposing causes of BRDC act synergistically and are most commonly identified
in combination rather than as single causative problems. The list of predisposing
animal factors is long and includes animal age, decreased immune responsiveness due
to animal stress, lack of previous viral exposure or vaccination, inadequate passive
immunoglobulin transfer in young calves, nutritional deficiencies, and dehydration.
Environmental risk factors include high air humidity or dust content, rapidly changing
environmental temperatures, extreme heat or cold, and high concentrations of noxious
gases such as ammonia. Several risk factors may increase pathogen density or pathogen
exposure, although these risk factors probably act by other means as well. For example,
commingling cattle from multiple sources may increase exposure to antigenically heterogeneous
viral pathogens, while also increasing animal stress. Poor ventilation and high humidity
can increase pathogen density and survival time but also can increase noxious gas
concentrations and adversely affect pulmonary function. Animal crowding increases
airborne pathogen exposure but also induces animal stress and reduces immune responsiveness.
Prevention of bovine respiratory disease complex
When evaluating the rate of BRDC occurrence in cattle populations, it is clear that
efforts to prevent this disease have not been effective on an industry-wide basis,
although some individual producers have successfully used prevention strategies. The
two biggest areas of BRDC effect are in the form of enzootic calf pneumonia of dairy
calves and shipping fever pneumonia of feedlot cattle. Given our current understanding
of this disease problem, it is clear that the animal management systems employed for
these groups of animals (i.e., dairy calf–rearing systems and feedlot cattle–receiving
systems) have failed to rigorously apply knowledge of disease pathogenesis and prevention
into their processes. This situation may be changing currently, as the beef production
industry increasingly uses quality-assurance principles in production systems and
develops marketing procedures and animal-purchasing practices that reward improvements
in animal health [12], [35], [46]. Similarly, the dairy industry has begun to recognize
the economic benefit of improved calf health and increasingly uses specialized calf-rearing
systems [1], [44].
Because the purpose of this article is to examine the role of biosecurity management
in respiratory disease prevention, the authors do not attempt to provide a complete
review of BRDC preventive practices. Many of the important means of preventing BRDC
do not employ biosecurity but are targeted toward enhancing animal immune preparedness
and enhancing animal response to infectious challenge. Effective respiratory disease
preventive practices are those targeted at reducing identified risk factors for disease
development [1], [2], [35], [46]. These practices include management to improve animal
nutrition with special emphasis on micronutrient nutrition, practices that reduce
animal stress, reduced commingling of animals, improved animal transportation and
feedlot receiving practices, improved preconditioning and vaccination programs that
emphasize vaccination before shipment and during times of low calf stress, and improved
ventilation with reduced crowding.
It is important to consider the factors that drive the development and implementation
of disease prevention and biosecurity programs. The most apparent of these factors
is the effect on animal production and growth; however, all interventions have their
cost, and these costs must always be considered relative to the potential economic
returns. Unfortunately, information regarding the financial impact of herd biosecurity
programs is limited, and estimates based on clinical experience must often be applied.
Other issues, including herd pathogen status and its effect on livestock marketing,
food product quality assurance, drug residues, injection site lesions, antimicrobial
resistance, and animal welfare also contribute to the forces that drive the development
of biosecurity programs. Ultimately, a biosecurity program must be integrated into
the overall herd management. It must be developed using a team approach that addresses
the concerns of the producer, the economic effect on the production unit, the influence
on product quality, and public health concerns. The veterinarian is best suited to
effectively develop and implement such programs.
The multifactorial nature of BRDC and the ubiquitous presence of respiratory pathogens
are important concepts when considering the role that biosecurity can play in decreasing
the prevalence of disease. For infectious diseases in which point source pathogen
exposure, high susceptibility, and high virulence are prominent features of disease
transmission (e.g., anthrax, foot-and-mouth disease, rabies, and so forth), limiting
animal contact with the pathogen is a key feature of disease prevention and may even
provide the means of disease eradication. Alternatively, when the causative pathogens
are endemic in a population and individual susceptibility is dependent on numerous
interrelated factors, the management of animal resistance and risk factors may be
proportionally more important for disease prevention than biosecurity practices. It
appears that BRDC prevention requires a combination of management to enhance animal
resistance plus management to reduce exposure to the pathogens. The important point
is not to de-emphasize the value of reducing pathogen introduction, exposure, and
transmission (i.e., biosecurity) but to also stress the importance of other management
features that promote animal resistance. It is particularly important that preventive
management practices be coordinated and used in combination, because no single management
procedure will be successful without the complement of other practices. It is likely
that our inability to reduce the prevalence of respiratory disease in cattle is, in
part, attributable to our failure to integrate multiple aspects of respiratory disease
prevention practices, including biosecurity.
The fundamental concept of biosecurity is to decrease pathogen transmission between
animals. Transmission of respiratory pathogens occurs by close nose-to-nose contact,
environmental or fomite exposure, and airborne exposure. Increased contact between
shedding and susceptible individuals increases pathogen spread. Environmental exposure
through common areas and equipment that involve oral or nasal contact such as feed
bunks, water troughs, and salt blocks may be an even greater risk, however.
Total environmental pathogen load is extremely important in considering respiratory
pathogen transmission. Environmental contamination from animals in contact is the
primary source of most respiratory pathogens. Individual animal shedding is quite
variable and depends on the etiologic agent, the time course of the disease, the clinical
severity, and the immune response of the host. In general, clinically ill animals
shed greater numbers of pathogens than normal or asymptomatic animals; however, it
must be recognized that individuals periodically shed both viral and bacterial respiratory
pathogens without evidence of disease. Well-vaccinated animals may also periodically
shed pathogens and should not necessarily be considered completely safe from disease
transmission.
The persistence of the pathogen in the environment also contributes to pathogen exposure.
Environmental pathogen survival times depend on many factors, including organic material,
moisture, direct sunlight, and exposure to disinfectants. Environmental survival times
for most viral respiratory pathogens are probably on the order of minutes to several
hours [18], [47]. Survival times for bacterial pathogens may be longer depending on
the environmental conditions and the organism. Airborne transmission is dependent
on numerous factors, including ambient temperature, relative humidity, airborne particle
(dust) density, ventilation, prevailing wind, and structural or geographic obstructions
[47]. Airborne transmission of typical viral respiratory pathogens can occur over
distances as far as 4 meters and possibly further [29], [30]. Airborne transmission
of other viruses such as foot-and-mouth disease virus or pseudorabies virus has been
shown to occur over many miles, however [10], [11], [15], [45], [47]. Adding to the
complexity of pathogen transmission, it seems that the efficiency of transmission
is different between different strains of a given pathogen [30]. Understanding how
management practices can reduce either pathogen shedding or exposure is the key to
creating effective biosecurity programs.
Biosecurity and bovine respiratory disease complex
The term biosecurity is used for those management and hygiene practices that reduce
introduction, exposure, and transmission of infectious agents. Although biosecurity
may not provide the single most important component of respiratory disease prevention,
reducing pathogen exposure is a valuable part of any infectious disease management
system. Little information is available to specifically evaluate the effect of individual
biosecurity practices in prevention of BRDC, but there are some important respiratory
disease prevention practices that limit pathogen exposure and good reason to more
closely evaluate the role that biosecurity could play in the future. The authors emphasize
five areas of biosecurity management that should be more rigorously applied for the
reduction of respiratory disease prevalence in cattle, including (1) strategic vaccination,
(2) calf biosecurity, (3) housing ventilation, (4) commingling and animal contact,
and (5) bovine viral diarrhea virus control.
Strategic vaccination
Many improvements in vaccine technology have occurred over the past few decades, and
practitioners have an array of improved bovine respiratory pathogen vaccines at their
disposal [35]. Unfortunately, the current respiratory pathogen vaccines have not all
been scrutinized for efficacy to the most desirable degree, and many do not protect
against respiratory disease nearly as effectively as some veterinarians and producers
would like to believe. Although vaccines directed at specific conserved proteins,
such as toxoid vaccines, may completely prevent a particular disease, vaccines against
complex disease agents that have multiple antigenic strains are unlikely to be capable
of such levels of protection. Respiratory vaccines are better viewed as disease modifiers
than absolute preventive agents.
Vaccines are usually used as a means to decrease the likelihood or severity of disease
occurrence in the individual animal receiving the vaccination. Indeed, vaccine efficacy
may be evaluated in many ways, but the more rigorous evaluations involve the ability
of a vaccinated animal to withstand a challenge of disease or pathogen exposure [42].
Practitioners tend to view vaccination as one of the management factors that enhance
animal resistance to infection and thus augment the value of biosecurity management
by working to reduce susceptibility to infectious disease rather than decrease exposure
and transmission. For respiratory disease prevention, however, effective vaccination
can also serve as part of a biosecurity management system. In addition to preventing
disease, a vaccine's efficacy might also be considered for its ability to limit pathogen
shedding when infection does occur. Vaccine-induced immunity often results in decreased
magnitude and duration of pathogen shedding [6], [16], [59]. Because exposure is directly
related to pathogen concentration in the environment, it follows that vaccine-induced
reductions in shedding should decrease transmission within a susceptible population.
Proper vaccine use and a well-managed vaccination program can be viewed as part of
a complete biosecurity program. At a minimum, a good vaccination program should include
the following:
•
Proper storage and administration of the vaccine as indicated by the manufacturer's
labeled recommendations.
•
Vaccination of all susceptible animals, including both resident and incoming animals.
•
Application of the vaccine to systemically healthy, well-nourished, minimally stressed,
and immunocompetent cattle.
•
Strategic timing of vaccination so that it precedes contact with new animals long
enough to allow an appropriate immune response.
•
Revaccination as recommended for the particular vaccine product.
Calf biosecurity
Biosecurity management of calves is extremely important for development of healthy
animals. Many of the biosecurity recommendations for newborn calves focus on decreasing
the transmission of enteric pathogens; however, these same principles can be important
for minimizing respiratory disease problems. Several details of calf biosecurity management
deserve emphasis.
Environmental and housing factors significantly affect calf health and viability.
Differences in calf management for cow–calf herds versus dairies are related to the
relative risk of respiratory disease between these two production groups. Beef calves
are generally raised in open-range situations that effectively dilute the exposure
to respiratory pathogens. Although beef calves are continually exposed to pathogens
shed from adult cattle and other calves, the magnitude of pathogen exposure before
weaning is generally low, resulting in relatively little respiratory disease. In contrast
to many enteric pathogens, the environmental survival time of the respiratory pathogens
is limited [18], and accumulation of pathogens in the environment is not considered
a primary concern.
Dairy calf housing has a significant effect on the incidence of respiratory disease
in neonatal calves. Although the common viral respiratory pathogens can be transmitted
over distances up to 4 meters [29], [30], properly spaced calf hutches seem to effectively
limit aerosol transmission of respiratory pathogens. The short survival of these pathogens
in the environment limits the transmission between successive occupants of an individual
hutch. Disinfection procedures that are used for enteric diseases should be more than
sufficient to decrease respiratory pathogen transmission (see article by Barrington
et al. in this issue). In contrast, there is a high risk of respiratory disease transmission
in group-raised neonatal calves. Factors including the number of animals, relative
animal density, housing facilities, and ventilation conditions significantly contribute
to transmission in grouped calves and are discussed in subsequent sections of this
article.
Numerous management practices can decrease exposure and transmission of respiratory
pathogens to calves in dairy operations. Feeding pasteurized milk or milk replacer
is a useful biosecurity practice for minimizing the spread of enteric agents such
as Salmonella spp. or Mycobacterium avium subsp. paratuberculosis. These practices
are also effective at limiting ingestion of potential respiratory pathogens. Mycoplasma
spp. bacteria are commonly implicated in newborn calf disease, including enzootic
calf pneumonia [2], [37], [48], [49]. Although Mycoplasma spp. may spread by the airborne
route, it is also a common mastitis pathogen and can be shed from clinically or subclinically
infected cows [24], [37], [49]. Nasopharyngeal colonization occurs after oral ingestion
of contaminated milk, potentially resulting in clinical respiratory disease in calves
[37]. Mycoplasma spp. and other pathogens can also spread hematogenously after ingestion
by a susceptible calf [24], [37]. Similarly, other potential respiratory pathogens
such as Streptococcus spp., Staphylococcus spp., Salmonella spp., and Escherichia
coli can be recovered from milk and spread hematogenously to the lungs after oral
ingestion. Bovine viral diarrhea virus is shed in the milk of persistently infected
cattle. Ingestion of bovine viral diarrhea virus–contaminated milk can result in respiratory
and systemic infections, possible immune suppression, and respiratory disease.
Proper cleaning and disinfection of calf feeding equipment, including nursing bottles,
buckets, and mixing utensils, should be performed. Equipment should be cleaned with
a detergent and disinfected between uses. A common and economical disinfectant is
standard household bleach used at a 1:10 dilution. Bottles and equipment that are
potentially shared between multiple animals should be soaked for 15 to 20 minutes
in this solution. Although bleach will not completely kill all potential pathogens,
it is effective at significantly decreasing viable numbers and thus contributing to
decreased exposure and transmission between feedings.
Prompt removal of dairy calves from the maternity pen environment, where they are
exposed to numerous adult cow pathogens, can also decrease transmission of potential
respiratory pathogens. Newborn calves should not have direct contact with older calves
and adults. Calf hutch spacing should be evaluated, with a minimum of 4 feet of separation
between calves. Worker hygiene can minimize contamination of calf feed and the calf
environment. Appropriate vaccination of dams before colostral production can increase
passive transfer of effective antibodies, reducing the risk of exposure and potential
shedding after infection. It has been demonstrated that good colostral transfer to
beef calves was associated with decreased occurrence of disease episodes and improved
calf performance all the way through the growing and finishing period in feedlot animals
[36]. It is unlikely that the passive transfer of immunoglobulins per se is specifically
responsible for beneficial effects on the long-term health of animals, but profound
effects may result from management that improves newborn health and disease resistance.
This in turn provides for improved nutrition, growth, physiologic well-being, and
decreased total pathogen load.
Numerous calfhood husbandry procedures should be considered as standard biosecurity
protocols for all infectious diseases, including respiratory disease. Sick animals
should be identified and separated from healthy animals. A specific calf-isolation
area should be established, with consideration to animal comfort and ease of cleaning
and disinfection. Where practical, individual equipment should be used for each separate
calf. Specific care and treatment personnel should be identified, and animals with
suspected infectious diseases should be treated after handling healthy animals. Additional
personnel hygiene protocols include dedicated coveralls to be used in the sick pens,
the use of rubber overboots, and disinfectant footbaths. Personnel should be encouraged
to wash their hands before and after entering the sick pens and between caring for
animals with dissimilar disease conditions. In many cases, equipment and facilities
need to be made available to help establish such procedures.
Similar biosecurity management practices can be used in cow–calf herds. Although feeding
pasteurized milk or milk replacer is obviously not a practical management practice,
milk-borne exposure to pathogens can be minimized by proper attention to the adult
cows. Adult cattle must be appropriately vaccinated to provide optimal colostral immunity
to the calves and to decrease adult cow infections and shedding. Adult cow nutrition
should be optimized to improve colostrum quality. Adult cow nutrition can also have
a dramatic effect on calving ease and decrease the incidence of dystocia. Special
attention should be placed on high-risk calves, including calves delivered with manual
assistance, cesarean section, born in inclement weather, weak or premature calves,
and multiple births. Such calves often do not nurse colostrum in a timely fashion
or have impaired absorption of immunoglobulin. Cows should be evaluated for evidence
of clinical mastitis and treated or culled as appropriate. Decreased morbidity can
be observed by minimizing the time that beef cow–calf pairs spend in a designated
calving area, where pathogen loads tend to increase throughout the calving season.
Bovine viral diarrhea virus surveillance and eradication in cows and calves should
also be used (see discussion in a following section).
As can be seen from the preceding discussion, many of the management practices that
contribute to biosecurity of respiratory disease are standard quality-assurance practices
that are recommended for basic calf health.
Ventilation
Good ventilation is a critical aspect of animal management and can profoundly affect
respiratory health. Several discussions of ventilation and its effect on animal health
are present in the literature [2], [3], [4], [5], [13], [14], [19], [27], [31], [34],
[38], [39], [43], [51], [58]. Proper ventilation serves eight primary functions:
1.
It decreases the airborne pathogen concentration
2.
It eliminates noxious gases (ammonia, hydrogen sulfide, carbon dioxide, carbon monoxide,
and methane)
3.
It decreases airborne dust contamination
4.
It decreases airborne endotoxin levels
5.
It maintains optimum ambient temperature
6.
It maintains optimum environmental humidity levels
7.
It eliminates drafts
8.
It eliminates areas of stagnant air
With respect to biosecurity, one of the most important aspects of proper ventilation
is the reduction in the concentration of airborne pathogens. All of the important
viral and bacterial respiratory pathogens can spread aerogenously and can attain high
concentrations in poorly ventilated housing areas. Airborne pathogen concentration
is a function of many factors, including animal type, housing system, stocking rate,
bedding, humidity, dust particle density and size, and finally, elimination through
ventilation. Improved ventilation is one important means whereby airborne pathogen
concentration can be readily decreased within the given constraints of an operation;
however, pathogen removal is not a linear function, and practical and theoretical
limits are often observed [33]. Studies of building ventilation for humans demonstrate
potential reductions in airborne exposure of pathogens and disease incidence, although
improved ventilation beyond that which provides comfort may not be practical or provide
significant additional benefit [33]. As the airborne pathogen load rises, ventilation
provides progressively less protection against respiratory infections. It is important
to realize that stocking rate has a more dramatic effect on airborne pathogen density
than ventilation [33], [58]. For example, a two-fold increase in stocking rate requires
nearly a 10-fold increase in ventilation to maintain the same airborne pathogen density
[58]. Ventilation cannot overcome grossly inadequate housing, management, or hygiene
within a production unit.
Along with stocking density, there are other practical concerns that contribute to
airborne pathogen density and transmission. One of these is related to animal handling
and excitement. It is extremely important to handle grouped animals in a calm environment
with minimal animal activity and stress. Increased animal activity not only increases
dust exposure (which contains airborne pathogens) but also increases ventilatory rate,
ventilatory effort, and tidal volume, which in turn increases the amount of aerosolized
pathogen shed by infected animals and the amount of pathogen inhaled by susceptible
animals. The increased dust exposure will also adversely affect mucociliary clearance
and respiratory defense mechanisms.
Part of the effect of ventilation is to minimize airborne contaminants that can impair
respiratory function and defense mechanisms [34], [38], [39], [58]. Significant airborne
contaminants include ammonia, hydrogen sulfide, carbon dioxide, carbon monoxide, methane,
dust particles, and endotoxin. Ammonia and hydrogen sulfide are toxic gases and can
contribute to respiratory damage, decreased mucociliary clearance, decreased alveolar
macrophage activity, and overall compromise to respiratory defense mechanisms. Carbon
dioxide, carbon monoxide, and methane contribute primarily as asphyxiative gases and
generally do not contribute to significant impairment of the respiratory tract.
Dust particles also contribute to the impairment of respiratory defense mechanisms.
Dust particles can arise from both organic and inorganic sources. In general, particles
greater than 5 μm are filtered out by the nasal passages; most particles from 2 to
5 μm are removed by the mucociliary clearance of the trachea and bronchi, and particles
less than 2 μm can penetrate to the alveolar spaces [47], [58]. Organic and inorganic
dust particles can impair mucociliary clearance and overload alveolar macrophage phagocytic
clearance [58]. Organic dust particles are generally of more concern in confinement
and intensive housing situations. In animal housing environments, most of the organic
dust arises from fecal material, skin, and hair. Organic dust is significant in that
it often contains high endotoxin and pathogen levels [38], [39]. Inhaled endotoxin
can contribute to pulmonary compromise by initiating inflammatory reactions within
the alveoli and alveolar vascular endothelium.
Appropriate ventilation is also important in maintaining acceptable humidity and ambient
temperature levels within confinement or semi-open housing. Observed thermoneutral
ranges (the range of air temperature that sustains optimal performance) for a variety
of domestic livestock are available (Table 1
) [58]. In general, livestock can perform adequately within a fairly wide thermoneutral
range. Higher temperatures, especially when combined with high humidity, tend to be
more problematic than low temperatures [34]. Depending on the given climate and temperature
ranges of a geographic region, housing ventilation will need to be designed to provide
either heating or cooling or both. Cold temperatures and perhaps temperature fluctuations
can decrease mucociliary clearance and predispose animals to respiratory disease [17].
Often, wide temperature fluctuations are more detrimental to animal health because
they do not allow suitable adaptation over time.
Table 1
Estimated thermoneutral values (°C) for several livestock species and age groups
Livestock group
Minimum acclimated temperature
Minimum ideal temperature
Maximum ideal temperature
Maximum acclimated temperature
Newborn calf
10
10
25
37
1-month-old calf
0
0
25
30
Veal calf
−15
−5
22
30
Beef cows
−17
−10
20
27
Dairy cow
−25
0
22
27
Ewe
−10
−5
25
37
Newborn lamb
20
20
32
37
Growing lamb
−12
0
22
37
From Wathes CM, Jones CD, Webster AJ. Ventilation, air hygiene and animal health.
Vet Rec 1983;113:554–9; with permission.
There is minimal information on how ambient temperature directly relates to airborne
pathogen biosecurity. Increased ambient temperature results in increased respiration
and may increase pathogen shedding from infected animals. The direct effects of ambient
temperature on pathogen survival are relatively unknown. Some studies suggest that
the concentration of airborne particles is increased at low temperatures, and airborne
bacterial concentrations were higher in winter than in summer [47].
There is slightly more information concerning the effects of relative humidity on
pathogen survival and thus, airborne biosecurity [47], [58]. In general, viruses with
a hydrophobic lipid outer shell (i.e., enveloped viruses) survive better in lower
humidity, and lipid-free viruses (i.e., foot-and-mouth disease virus) are more stable
in moist air [18], [47]. The four primary viral respiratory pathogens in cattle (bovine
herpsevirus 1, bovine parainfluenza virus type 3, bovine respiratory syncytial virus,
and bovine viral diarrhea virus) are all enveloped viruses and would be considered
more stable in dry air, although the authors are unaware of specific studies documenting
this conclusion. Gram-negative bacteria have outer phospholipid membranes and are
also expected to be more stable in dry air [47]. Mycoplasmaare reported to be sensitive
to relative humidity between 40% and 70% [58]. Extrapolation of these limited data
suggests that typical airborne pathogens associated with respiratory disease in domestic
animals survive better in cool, dry air such as is observed in the late fall, winter,
and early spring months. Although this correlates with clinical observations concerning
the relative seasonal incidence of respiratory disease, a direct association has not
been established. In beef cattle, seasonal increases in respiratory disease also correlate
with seasonal management practices associated with movement of cattle to feedlots
and increased animal density. It is likely that climate and management factors act
together to dramatically increase pathogen exposure and transmission in feedlots.
Alternatively, the high humidity that can be observed with dairy confinement housing
in cold weather probably contributes to increased respiratory disease because of the
higher pathogen density associated with increased aerosolized particle concentrations.
Ventilation systems should be constructed to provide even airflow throughout the structure
without areas of air stagnation or drafts. Pockets of air stagnation have higher levels
of airborne contaminants and contribute to the exposure and transmission of respiratory
pathogens. Air stagnation can often be remedied by appropriate use of inexpensive
fans. Correcting draft conditions can be more problematic and often requires complete
evaluation of the housing structure for air leaks and evaluation of the ventilation
system, especially air intake vents.
Guidelines for housing of livestock have been reported, including recommendations
for ventilation (Table 2
) [2], [5], [13], [27], [34], [51], [58]. Appropriate ventilation should flow from
younger to older animals to minimize spread of pathogens to the more susceptible animals.
The total air volume should be completely changed 4 times per hour in winter, and
it should be changed up to 30 times per hour in summer [5], [51]. The ventilation
system should provide constant rather than intermittent airflow. In the winter, the
goal of ventilation is to minimize airborne pathogen density, remove excess moisture
from animal respiration, and maintain adequate ambient temperature (10–13°C, 50–55°F).
Although higher ventilation rates improve air quality, they are inefficient because
they require excessive heating costs. Supplemental heating may be necessary as the
outside temperature falls or stocking density decreases. At optimal stocking densities,
livestock generally produce enough animal heat to maintain adequate ambient temperature
in confined housing when outside temperatures remain above −8°C [51]. Winter ventilation
is a compromise between the removal of airborne contaminants and the maintenance of
ambient temperature. The primary goal of summer ventilation is to minimize ambient
temperature and relative humidity. This requires high ventilation flow rates, which
also enhance air quality. The goal is to maintain an ambient housing temperature to
no more than 2°C above the outside temperature [5], [51].
Table 2
Recommended space requirements for calves
Confinement housing
Age of calf (wk)
<6
6–12
12–16
Air volume (m3/calf)
6
10
15
Open housing
Age of calf (mo)
0–5
5–8
9–12
13–15
16–24
Sheltered area (ft2/calf)
21
25
28
32
40
Outside open area (ft2/calf)
30
35
40
45
50
Total area (ft2/calf)
51
60
68
77
90
Confinement housing From Klingborg DJ. Preventing calf pneumonia. Compend Contin Educ
Pract Vet 1986;8:F112–14; with permission.
Fundamental recommendations for ventilation systems in confinement and open-sheltered
housing
Confinement housing
◦
Minimum of four air changes per hour (winter)
◦
Total exhaust capacity for up to 30 air changes per hour (summer)
◦
Continuous (not intermittent) ventilation
◦
Single-speed fans, not variable-speed fans, should be used
◦
Fans must be able to sustain 1/8-inch static pressure
◦
One must allow for two to four different ventilation rates using multiple fans
◦
Enough inlet slot area should be provided to allow minimal inlet velocity of 100 fpm
(winter) and 800 fpm (summer)
◦
Thermostats should be used to control ventilation fans
◦
Thermostats should be located at eye level near the center of the barn
◦
The ventilation rate should be altered by stepping up the number of fans used for
each level
◦
Wall fans should be mounted near the ceiling but collect air using ducts from within
38 cm (15 in) of the floor
◦
The fresh air intake should be located near the ceiling but at least 4 feet from any
exhaust fan
◦
Adjustable eave slot inlets should be used to distribute incoming air uniformly
◦
A system for supplemental heat in the winter should be provided
Open-shelter housing
◦
Ventilation occurs through both open sides and the roof (ridge ventilation)
◦
Fully closed ends should be no more than 30 feet wide
◦
End widths greater than 30 feet require inlet ventilation
◦
Widths of 60 to 70 feet result in pockets of air stagnation
◦
The building should be oriented with the long axis perpendicular to the prevailing
wind
◦
Open sides should face away from the prevailing wind
◦
Ventilation fans should be directed out of the downwind side of the building
◦
The building should be located upwind of other structures that might block air flow
◦
One should avoid placing shelter within 75 feet of other existing shelters or other
obstructions
Data from Refs. [2], [5], [13], [34], [51].
Relative humidity levels should be maintained between 50% and 80%, and ammonia levels
should not exceed 10 ppm [2], [34], [47], [51], [58]. Maximum recommended stocking
densities should not be exceeded (see Table 2) [26]. Separate age groups of cattle
should be maintained in separate barns or be separated by barrier walls. Calf hutches
for individual dairy calves provide an ideal means of managing relative calf isolation
and limiting airborne transmission if they are properly positioned and spaced. Recommendations
for calf hutches include one calf per hutch with a minimum separation of 4 feet between
hutches. Hutches should be placed at least 10 feet from older-cattle enclosures and
50 feet from livestock building exhaust fans.
Commingling and animal contact
Many cattle management systems provide numerous opportunities for exchange of respiratory
pathogens from animal to animal. Assembling groups of beef calves for a feedlot often
involves mixing calves from different origins, congregation of animals at sale barns
or other holding pens, and movement in congested cattle transports. These activities
are well known to increase the rate of respiratory disease occurrence by stressing
the animals and providing circumstances that decrease disease resistance. These same
animal contact and crowding circumstances can dramatically increase exposure to pathogens,
often including pathogens to which the animal has not developed prior immunity.
In a recent national survey, more than 50% of dairy producers housed sick animals
in a manner that allowed direct nose-to-nose contact with healthy herdmates [54].
Many dairy producers expand their herds by purchasing animals from other sources,
but less than 25% of them provide any quarantine time for the incoming animals. For
producers who introduced 15% or more of their total animal inventory during an expansion,
16.6% reported an increase in occurrence of respiratory disease during the year [55].
During the early phases of respiratory disease, the shedding rates of pathogens via
respiratory secretions increases dramatically. Commingling, crowding, and the animal
stresses that are involved in animal movement can precipitate respiratory problems.
These same factors can increase spread of pathogens to other animals with close contact.
Although quarantine may not be effective against diseases with chronic carrier states
such as Johne's disease, it can substantially decrease the risk of spreading respiratory
pathogens. Furthermore, the duration of respiratory pathogen shedding has also been
well characterized. In general, nasal shedding of viral respiratory pathogens is significantly
reduced by 14 days after infection but may persist longer in individual animals, which
suggests that quarantine for approximately 14 to 21 days should significantly reduce
the exposure and transmission of these pathogens within an operation. Practical suggestions
for limiting pathogen spread by contact include quarantine of incoming livestock,
maintenance of hospital areas that do not allow contact with healthy animals, prevention
of animal contact between different age groups of cattle, minimizing the time animals
spend in market channels, and limiting the introduction of new animals to assembled
herds or pens of cattle.
The concepts of pathogen transmission within grouped housing can be effectively applied
to weaned dairy calves. Calves receive relatively low pathogen exposure while in calf
hutches. On weaning and grouping in calf pens, the risk of exposure increases dramatically.
It is important to appreciate that the risk of exposure rises with the number of calves
housed together. For example, if one estimates that 5% of calves born in a herd with
bovine viral diarrhea virus are persistently infected, then the probability of bovine
viral diarrhea virus exposure in a group of 10 calves is approximately 0.4 (1–0.9510).
If the stocking rate increases to 30 calves, the probability nearly doubles to 0.78
(1–0.9530). Limiting the number of calves per pen to less than seven is associated
with decreased respiratory disease mortality [28] (also see article by Smith in this
issue).
It must be emphasized that one animal can expose an entire pen of animals by simple
close contact, airborne transmission, or environmental transmission at common housing
areas such as feed bunks and water troughs. By dividing animals into smaller groups,
the number of animals exposed is lowered significantly. Using the same example of
bovine viral diarrhea virus exposure, if one splits the 30 calves into three separate
pens, the probability of having all 30 calves exposed to bovine viral diarrhea virus
falls from 0.78 to the comparatively negligible level of 0.064 [(1–0.9510)3]. Simple
segregation of animals is not sufficient unless physical barriers for fence line contact,
separation of food and water troughs, segregation of likely fomites, and blocking
airborne spread are used. These same principles can be applied to any group-housing
situation. Such management and housing decisions must be made based on a balance between
the risk and cost of disease versus the availability and cost of facilities and labor.
Minimizing the role of bovine viral diarrhea virus in bovine respiratory disease
It was noted previously that the common bovine respiratory pathogens are considered
to be ubiquitous in cattle populations in the United States and most other countries.
Although this does not suggest that every animal harbors each pathogen, these agents
can be found routinely in the nasopharynx of healthy and diseased animals within most
herds. In contrast, some European countries have successfully eradicated some viral
respiratory pathogens such as bovine herpesvirus 1 and bovine viral diarrhea virus
(BVDV) from cattle populations. Under the currently prevailing practices within the
United States and many other countries, the authors do not suggest testing or identification
of most respiratory pathogens as a viable means to identify carriers or to exclude
the animals from introduction into a herd. The exception to this is BVDV. Although
BVDV is not considered a primary pneumopathogen, it is considered to have an important
role in respiratory disease of cattle [2], [41]. The immunosuppressive effects of
the virus and the close association of BVDV infection and respiratory disease occurrence
in some epidemiologic studies suggest that the virus plays a role by promoting secondary
bacterial lung infection.
Although BVDV vaccines have been improved over the past several years, vaccination
alone rarely eliminates BVDV from an infected herd. An effective BVDV biosecurity
program must include the identification and removal of persistently infected animals,
BVDV screening of incoming animals and their calves, and a comprehensive vaccination
program [7], [8], [25], [57]. Persistently infected cattle do not mount an effective
immune response against the virus and are capable of shedding large amounts of the
virus into the environment through multiple routes. Persistently infected animals
have been implicated as the primary means by which BVDV infection is maintained in
assembled dairy herds, and they are also considered a significant threat for transmission
in cow–calf and feedlot operations [20], [21], [22], [23], [32], [50], [60]. With
the development of new tests over the past several years, our ability to accurately
and expediently identify persistently infected animals has dramatically improved [9],
[25]. The serum immunoperoxidase monolayer assay (IPMA) and antigen capture ELISA
tests and the immunohistochemistry test of skin biopsy material have appropriate sensitivity
and specificity for detecting persistently infected cattle.
The authors do not know of significant published research that evaluates the effect
of test-and-cull strategies for BVDV on the occurrence of BRDC; however, elimination
of persistently infected animals from herds can have significant positive effects
in decreasing other BVDV manifestations such as reproductive failure. Implementing
test-and-cull procedures for persistently infected animals may prove to be a powerful
means of decreasing BRDC prevalence.
In general, all cattle introduced into a herd should be tested for BVDV before purchase
or entry. Acute BVDV infections of pregnant cattle can result in animals that are
BVDV negative at the time of testing while the fetus is persistently infected. It
is critical that all calves from newly introduced pregnant animals also be tested
immediately after birth. To establish a BVDV-negative herd, it is generally more effective
and economical to test calves as they are born rather than screen adult populations.
A negative result for a calf indicates that not only the calf but also all of the
calf's maternal ancestors are not persistently infected. A single positive test on
a calf does not differentiate between acute and persistent infection. A confirmatory
test may be performed in 4 weeks, or the animal may be assumed to be persistently
infected and euthanized or sold for slaughter. The dams of all persistently infected
calves should be traced and tested as well to determine their status. In most cases,
these animals will test negative, indicating fetal exposure due to acute infection
during gestation. Bulls should also be tested because they can contribute to animal
exposure within a herd.
Summary
Although biosecurity practices play a role in minimizing respiratory disease in cattle,
they must be used in combination with other management strategies that address the
many other risk factors. Because the pathogens involved in bovine respiratory disease
are enzootic in the general cattle population, biosecurity practices aimed at the
complete elimination of exposure are currently impractical. Several animal husbandry
and production management practices can be used to minimize pathogen shedding, exposure,
and transmission within a given population, however. Various combinations of these
control measures can be applied to individual farms to help decrease the morbidity
and mortality attributed to respiratory disease.