Host and Environmental Risk Factors for Undifferentiated Ruminant Respiratory Disease*
Amelia R. Woolums
In general, risk factors for respiratory disease in all ruminants are related to (1) factors that impair the host physiologic and immunologic responses that defend against respiratory infection, and (2) characteristics of the environment that overwhelm the ability of these physiologic and immunologic responses to prevent respiratory infection.
More specifically, host risk factors are related to (1) mechanical respiratory defense by the anatomic features of the upper respiratory tract and the mucociliary elevator; (2) innate immune defense provided by soluble factors such as lysozyme and defensins and by cells such as granulocytes, macrophages, and natural killer cells; and (3) acquired immune defense provided by antibodies on mucosal surfaces and in serum and by cells such as T-helper lymphocytes and T-cytotoxic lymphocytes. Characteristics of the host that can impair these mechanisms include other concurrent disease, nutritional deficiency, genetics, pronounced or chronic stress related to events such as mixing animals from various sources, crowding, and/or long-distance transport. In young animals, failure of transfer of passive immunity is a cause of transient immunodeficiency that may contribute to respiratory disease.Environmental factors can also contribute to the development of respiratory disease in ruminants. High concentrations of airborne particulates, infectious agents, or noxious gases impair the function of host respiratory defense mechanisms or enhance the transmission of infectious respiratory pathogens. Inadequate housing can lead to cold or heat stress, which may increase nutritional requirements beyond what is available and lead to increased disease susceptibility. Crowding or mixing animals of different sources and/or age-groups can increase opportunities for transmission of infectious respiratory pathogens.
Longdistance transport is well known to predispose animals to respiratory disease; this is due to close contact with other animals, which enhances transmission of infectious agents, as well as to inhalation of particulates and noxious gases in truck exhaust. Mixing animals from multiple sources, such as occurs when animals are sold through livestock auctions, increases risk for respiratory disease by increasing opportunities for exposure to respiratory and other pathogens; psychological stress associated with the establishment of new social hierarchies may also contribute to immune suppression and increased rates of disease.When respiratory disease occurs in any group of ruminants, a systematic evaluation of host and environmental factors that may increase risk of respiratory disease should be undertaken. The purpose of this evaluation is to identify modifications in management and husbandry that should help limit ongoing respiratory disease and should decrease risk of future respiratory disease. Although this approach is logical, there is unfortunately little research that confirms that management modifications decrease respiratory disease in ruminants; it is to be hoped that future work will address this knowledge gap. Specific risk factors relevant to various classes of ruminants are discussed below.
DAIRY CALVES. In young calves, adequate transfer of passively acquired antibody and possibly also maternal cells from colostrum are very important for defense against respiratory and other infections. Nutritional deficiency, stress associated with weaning, mixing of calves from different sources, castration and/or dehorning, concurrent disease, and temperature extremes may all affect host defense mechanisms. Environmental risk facrtors are related to (1) air quality, including humidity and the concentration of bacteria, other particulates, and noxious gases in air; (2) ambient temperature extremes; and (3) opportunities for exposure of calves to respiratory pathogens through new introductions, mixing animals from different locations or age-groups on the same farm, or contact with older animals on the same farm.
When dairy calves are affected by either epizootic respiratory disease (outbreaks) or enzootic calf pneumonia, careful evaluation of management practices that influence both host and environmental risk factors may help prevent future disease.Specific risk factors for respiratory morbidity and mortalitiy in dairy calves have been evaluated in many studies.39 42,44,46,4','6 '8 It is noteworthy that factors significantly associated with respiratory morbidity and/or mortality in some studies are not always associated with disease in other studies; this is likely because of differences in the populations studied or in methods of collection or analysis of data. Factors identified as signficant in more than one study may be relatively more important. Knowledge of factors associated with dairy calf respiratory disease can help veterinarians focus efforts to modify management to limit or prevent respiratory disease in herds where calf respiratory disease is a problem. Calf-level and farm-level factors that have been found to be significantly associated with dairy calf respiratory disease (morbidity or mortality) are presented in Box 31.5.
PREWEANING BEEF CALVES. As compared to dairy calf respiratory disease, less is known about factors that increase risk for respiratory disease in preweaning beef calves. Calf-level factors associated with preweaning respiratory disease over several years in a large U.S. herd included the year of the calf’s birth, the location of the calf’s group on the operation, whether or not the calf experienced dystocia requiring major assistance, the calf’s sex (with male calves being at greater risk), and the age of the calf’s dam (with calves born to heifers being at greater risk).57
■ BOX 31.5
Factors Associated With Respiratory Disease in Dairy Calves
Risk Factors Associated With Increased Respiratory Disease Morbidity in Dairy Calves Calf-Level Factors
Having diarrhea39
Serum total IgG ≤1200 mg/dL9 or serum total protein supervised calving76
Serum total IgG >800 mg/dL46
Receiving antibiotic preventatively40
Receiving supplemental antibody product to prevent diarrhea8
Farm-Level Factors
Decreased total bacterial counts in calf pen air91
Solid barriers between individual calf pens91
Bedding deep enough for calves to nest91 Antibiotics in feed or milk replacer40
IgG, Immunoglobulin G.
A survey of cow-calf producers in Quebec58 found that farms with fewer than 40 cows were less likely to report a problem with preweaning calf pneumonia than farms with 40 or more cows (16% versus 36% of producers recognizing nursing calf pneumonia). The rate of preweaning calf pneumonia was associated with calving season length, with a longer calving season being associated with more cases of calf pneumonia. Another study in Quebec found that the incidence of preweaning calf respiratory disease in the first month of life was decreased if the herd vaccinated cows for respiratory viruses, with a 13% incidence in herds not vaccinating cows versus a 6% incidence
70 in herds vaccinating cows.
More recent research has evaluated herd-level risk factors for preweaning calf respiratory disease in U.S. cow-calf herds. An analysis of U.S. Department of Agriculture National Animal Health Monitoring System (NAHMS) data collected by producer survey found that introduction of steers from outside sources, herd breed makeup, number of visits to the herd by outsiders, administration of antibiotics in feed to prevent calf respiratory disease, and the herd being the primary source of income for the producer were factors associated with preweaning calf respiratory disease treatment rate.80 A mail survey of cow-calf producers in three eastern U.S. states and three Plains states found that the occurrence of preweaning calf respiratory disease was significantly associated with larger herd size, respiratory disease in cows, and diarrhea in calves.59 Calving season length was associated with calf respiratory disease in Plains but not eastern herds. Cumulative treatment incidence was associated with winter calving, bringing calves into the herd from outside sources, giving calves supplemental feed, and use of a heat synchronization program. A case-control study of herd-level risk factors for respiratory disease in preweaning beef calves in three U.S.
states found that herd size, use of an estrus synchronization program, and use of intensive grazing were associated with treatment of at least 5% of calves for respiratory disease.81 Taken together, the available evidence suggests that factors that increase opportunities for introduction of respiratory pathogens new to the herd and factors that increase opportunities for close contact between calves, and among cows and calves, are important risk factors for respiratory disease in preweaning beef calves.FEEDLOT AND STOCKER CATTLE. Risk factors for BRD in feedlot and stocker cattle include factors related to events on the cow-calf operation where cattle originate, events that occur while cattle are in transit, and events that occur at arrival. On the cow-calf operation of origin, factors such as poor nutrition and uncontrolled parasitism are suspected to affect later BRD risk in the feedlot or stocker operation, but data confirming this are currently sparse. Cattle breed has been related to BRD risk.82 Fetal programming, which refers to epigenetic effects driven by maternal experiences while the cow is pregnant but that affect the health of the calf long after it is born, is an active area of research; this may lead to clearer information about the relationship between events on the farm of origin and later BRD in stocker and feedlot cattle.
The mere fact that feedlot and stocker cattle are subjected to transport from the farm of origin increases their risk for BRD. Other factors related to increased rates of respiratory disease include sale through auction markets and prolonged time in market channels (sales barns, transport vehicles). Excessive dehydration or “shrink” from transport has been shown to account for significant morbidity and mortality in the feedlot. Movement through multiple auctions greatly increases the risk of respiratory disease. Several studies have confirmed that mixing cattle from multiple sources prior to or at feedlot entry is an important risk factor for feedlot respiratory disease.61,83,84 Shipment for longer distances prior to feedlot entry has been associated with increased rates of respiratory disease in some studies83,85 but not others.86 Changes in weather have been associated with respiratory disease; Alexander and colleagues61 found that a wide range between the high and low environmental temperatures in the first 2 weeks after feedlot entry was associated with increased respiratory disease.
Cernicchiaro and colleagues87 found that several weather-related variables such as maximum wind speed, mean wind chill, and temperature range were significantly associated with BRD risk, although the effect of weather was dependent on various characteristics of the cattle such as their average weight, risk category, and month of arrival and the number of cattle in the group. Heavier weight at entry has been shown to decrease feedlot BRD risk.82,83 Castration at arrival has been shown in numerous studies to increase morbidity and mortality in feedlot and stocker cattle; it is preferable for male calves to be castrated or banded early in life to prevent the need for castration postweaning. Placement of water troughs so that cattle in adjoining pens shared them markedly increased risk for BRD in one study; the odds of BRD occurring was more than four times greated for cattle that shared water with cattle in adjacent pens compared to cattle that did not.88 Metaphylactic antibiotic administration at arrival has been shown repeatedly to decrease risk for subsequent respiratory disease (see more discussion of metaphylaxis in the Metaphylactic Antimicrobial Therapy section later).Calves that arrive in the feedlot with moderate concentrations of serum antibodies against respiratory viruses have sometimes been shown to have a decreased incidence of BRD, indicating that natural exposure or vaccination prior to arrival can be protective. Starting calves on diets containing 75% concentrate or greater or feeding corn silage as a major dietary component during the first month is associated with higher mortality and morbidity rates, possibly because of inhibition of certain immune functions by systemic acidosis. Feeding of nonprotein nitrogen, such as urea, is associated with increased rates of BRD.
Smith and Gill89 have presented a helpful summary of risk classification of cattle in stocker and backgrounding operations. Stocker and backgrounding cattle can be grouped into one of three categories: (1) high risk, exposed: cattle with little or no history of vaccination, adaptation to hay or concentrate diet, and other preventative health management and with a history of mixing with cattle from multiple sources; (2) high risk, not exposed: as for high risk, exposed, but with no history of mixing with cattle from multiple sources; and (3) low risk: cattle that have been vaccinated, weaned, and turned out for a period before shipment. By nature most stocker and backgrounding cattle are in categories 1 or 2 and are thus at high risk for respiratory disease due to a variety of factors, including inadequate immunity, stress of recent weaning, and recent transport. Cattle may require dehorning and/or castration when entering the facility, which will further affect their health and productivity. Respiratory disease in high-risk, not exposed calves often peaks at 2 weeks post entry because these calves are not mixed with calves from other sources before they reach the stocker and backgrounding operation.
ADULT DAIRY AND BEEF CATTLE. Studies confirming risk factors for respiratory disease of adult dairy and beef cattle in modern North American production systems are lacking, but it is assumed that factors are similar to those described previously for calves and for cattle entering stocker operations and feedlots.
SHEEP AND GOATS. Many of the risk factors that have been found to predispose sheep and goats to undifferentiated respiratory disease are similar to those affecting cattle. Respiratory disease is more common in younger animals, after shipping or storms, and in crowded conditions. In addition, cold stress is an important risk factor for respiratory disase in young lambs and recently shorn adult sheep and goats. In contrast, heat stress may predispose to respiratory disease in unshorn sheep that do not have access to shade. Respiratory disease is also associated with semiconfinement or total confinement of sheep in poorly ventilated barns.