Epidemiology of Undifferentiated Ruminant Respiratory Disease
Amelia R. Woolums
RESPIRATORY DISEASE IN DAIRY CALVES. Respiratory disease in dairy calves has been described to occur both in epizootics (outbreaks) and in an endemic (enzootic) pattern (“enzootic calf pneumonia”).
At the farm level, the occurrence of producer-identified calf respiratory disease is skewed, with few or no calves being identified with respiratory disease on many farms and many calves being identified with respiratory disease on a few farms. The median (and range of) incidence for producer-identified respiratory disease in the first 90 days of life was 3% to 22% (0% to 52%).39,40 Case fatality rates for respiratory disease in dairy calves have been reported to range from 2.6% to 9.4%.40,41 Respiratory disease accounts for a significant proportion of the total mortality (proportionate mortality) in calves raised on dairy farms, with proportionate mortality rates of 24% to 52% reported.41-43 Respiratory disease in dairy calves can be identified as early as the first week of life.44,45 In reports describing dairy calf respiratory disease in the first 90 to 120 days of life, the median age at which calves were first identified by producers to have respiratory disease was between 27 and 37 days,45,46 and the age at which the highest percentage of calves are treated ranged from 35 to 70 days41,47,48 ( 5 to 10 weeks). In an assessment of calves in three Canadian dairies, weekly transthoroacic ultrasound in the first weeks of life indicated that 57% of calves were found to have lung consolidation on at least one examination.7 In a cross-sectional study of 39 dairies in which calves were examined by transthoracic ultrasonography once in the winter and once in the summer, the median calf-level prevalence of respiratory disease as defined by lung consolidation was 8% in winter and 15% in summer.49The agreement between producers and veterinarians in identification of dairy calf respiratory disease was evaluated in two studies.
Van Donkersgoed and colleagues46 found that the risk of respiratory disease in Saskatchewan dairy calves was 39% as diagnosed by the farmer and 29% as diagnosed by the veterinarian. Virtala and colleagues42 found that the risk of respiratory disease was 11% in New York dairy calves when diagnosed by producers and 26% when diagnosed by a veterinarian. In both sudies the agreement between veterinarian and producer diagnosis was poor at the level of the individual calf but moderate at the level of the herd. For calves subjected to necropsy, and using necropsy as the gold standard, the antemortem diagnosis of respiratory disease by producers was determined to have a sensitivity of 56% and a specificity of 100%.Respiratory disease in dairy calves has a negative impact on their subsequent growth and productivity. In one study, calves treated for respiratory disease in the first 90 days of life were 2.5 times more likely to die before weaning than calves that were not treated.50 Multiple investigators have shown that calfhood respiratory disease decreases calf growth.51-53 Dairy heifers treated for respiratory disease weighed 10.6 kg less at 6 months of age than untreated calves.51 Calves with signs of respiratory disease in the first 3 to 4 months of life are more likely to leave the herd before calving, calve later, and produce less milk in their first lactation compared to calves without respiratory disease.
RESPIRATORY DISEASE IN PREWEANING BEEF CALVES. Respiratory disease was reported to be the leading cause of preweaning death for beef calves 3 weeks of age or older on U.S. cow-calf operations.54 Sixteen percent of deaths in beef calves born alive are attributed to BRD.55 Two reports described the pattern of respiratory disease occurring over several years in beef calves still nursing their dams in a single large herd.56,57 In 110,412 calves over 20 years, the annual incidence of preweaning calf BRD in the herd varied from 3% to 24%, with an average annual incidence of 11%.
Annual mortality among calves with preweaning respiratory disease varied from 7% to 17%, with an annual average of 13%. In some years the incidence of preweaning BRD was higher than the incidence of respiratory disease after calves were weaned and transferred to the feedlot. Calves identified with preweaning respiratory disease weighed 7.7 kg less at birth than calves not identified with preweaning respiratory disease. A mail survey of cow-calf producers in Quebec found that 16% of respondents with fewer than 40 cows recognized calves with BRD, as compared to 36% of respondents with 40 cows or more.58 In a mail survey of producers in six U.S. states, 21% of operations reported observing one or more calves with preweaning respiratory disease. Of these, 89% treated one or more calves, and 46% reported one or more calf deaths attributed 59to preweaning respiratory disease.59
RESPIRATORY DISEASE IN FEEDLOT AND STOCKER CATTLE. In U.S. and Canadian cattle production systems, weaned beef calves are typically transported from the farm where they are born (cow-calf operation) to feedlots where they are fed and managed until they have reached the appropriate weight for harvest. Calves may enter feedlots by direct delivery from their cow-calf operation of origin or through purchase at local or regional auction markets by an “order buyer” acquiring calves for feedlots. Alternatively, calves may not enter a feedlot directly but instead may be sent for temporary management to a “backgrounding” (also known as “conditioning”) operation, where they may receive vaccinations or other treatments such as anthelmintics and growth implants, with dehorning and/or castration if necessary. At the backgrounding operation calves may be reassorted based on their size, breed type, and/or gender into new groups before being sent to feedlots. Similarly, after sale, weaned calves are sometimes transported to “stocker” operations, where they are housed or “stocked” on pasture for weeks to months with supplemental feed given as needed, along with vaccinations and other treatments, before being transported to a feedlot at a later date.
Undifferentiated bovine respiratory disease is the most common cause of morbidity and mortality in feedlot and stocker cattle. The disease is also known as “shipping fever” because it commonly occurs after cattle have been transported. The pattern of BRD in North American feedlot cattle has been well characterized.60,61 In a study by Alexander and colleagues,61 data from 17,696 cattle in a single feedlot were evaluated, and factors related to respiratory disease morbidity were also evaluated. Respiratory disease was the cause of treatment for 50% of the cattle treated, with 58% of BRD cases developing in the first 40 days on feed. In a study by Ribble and colleagues,60 data were collected on all 58,885 spring-born calves entering a single western Canadian feedlot over a 4-year period. Across the 4 years of the study, total (crude) mortality was fairly consistent, ranging from 2.4% to 4.8%, whereas the mortality caused specifically by BRD varied more than tenfold (0.25% to 2.73%). Across years, proportionate mortality caused by BRD ranged from 10% to 57%. Epidemic curves constructed for each of the 4 years showed that peak mortality occurred approximately 16 days after arrival at the feedlot and that at least 50% of BRD mortalities occurred within 3 weeks of arrival. Similar results were found in the Alexander study, where 58% of BRD cases were found to develop in the first 40 days on feed. The consistent onset of fatal disease in cattle within days of their arrival at the feedlot indicates that the disease process is often underway in affected animals before feedlot entry. Ribble and colleagues demonstrated that when the incidence of fatal BRD was high (greater than 2%), the disease clustered within truckload groups of calves and also, in 1 year, within pens.
Another study evaluated patterns of BRD in the first 100 days after feedlot entry in lots (cohorts) of cattle entering 10 U.S. feedlots over 8 years.62 For 1.2 million cattle in 7553 lots, the cumulative within-lot BRD morbidity ranged from 0.3% to 84.3%, with a median of 4.2% and a mean of 8.8%.
This study demonstrated that the pattern of BRD occurrence in the first 100 days on feed can vary greatly among lots of cattle. In some lots 80% of BRD cases occurred within the first 10 days on feed, whereas in other lots 80% of BRD cases had not occurred until the cattle had been on feed for more than 90 days. In some lots the proportion of cattle treated for BRD increased rapidly over relatively few days, and in others the proportion treated increased slowly over several weeks. In some lots a rapid increase in the proportion of cattle treated for BRD occurred relatively late, between 60 and 100 days on feed. The reasons for the different patterns of BRD occurrence that were observed in different lots of cattle were not determined, but they are likely related to a combination of factors related to the cattle and their environment.Feedlot BRD has a significant negative impact on productivity of cattle. Numerous reports have demonstrated that BRD can decrease average daily gain, final weight at slaughter, and carcass quality, with these impacts decreasing producer profit.63-67 Feedlot BRD has been shown to decrease average daily gain during the entire feeding period by 0.06 to 0.08 kg.65,66 In one study, cattle treated once, twice, or three times for BRD yielded $41, $58, or $292 less, respectively, than cattle not treated for BRD.63 Another study evaluating data from more than 212,000 cattle entering Midwestern U.S. feedlots over a 5-year period similarly showed that increasing number of BRD treatments led to decreasing financial return; net returns for fall-delivered cattle treated three or more times were $72 less than cattle not treated.68 Although the negative impacts of BRD are well established, affected cattle sometimes exhibit compensatory growth in the later phase of feeding period, so that cattle with BRD do not always perform more poorly than unaffected cattle when the entire feeding period is considered.67 Therefore the exact impact of BRD on the performance and profit returned by a group of feedlot cattle will vary substantially depending on numerous variables, including the specific risk factors the cattle have encountered; the management of health and nutrition in the feedlot; the cost of feed, medications, and labor; and the value of the cattle at the time they are purchased and marketed.
The epidemiology of BRD in stocker cattle is similar to that described for feedlot cattle, although morbidity and mortality rates may be higher because cattle defined to be at “high risk” for BRD make up a large proportion of many stocker cattle groups. In high-risk cattle kept on native Texas range over 3 years, BRD morbidity ranged from 31% to 49%. Net returns for steers and bulls treated for BRD were decreased from 10% to 21%, and heifers treated twice or more for BRD had 19% lower conception rates at first breeding than unaffected heifers.69 Many reports indicate that BRD is the leading cause of disease in stocker cattle.69-71 And although Pinchak and colleagues reported death rates of less than 1% in stocker cattle on grass, anecdotal reports indicate that BRD mortality rates in high-risk stocker cattle can be alarmingly high, sometimes approaching 10% to 20%. In trials testing health practices designed to improve health outcomes for high-risk stocker cattle over a 28- to 84-day receiving period, BRD morbidity rates ranged from 62% to 72% and BRD mortality ranged from 0% to 13%.72,73
RESPIRATORY DISEASE IN ADULT DAIRY OR BEEF CATTLE. Although anecdotal reports indicate that adult beef or dairy cows can experience outbreaks of respiratory disease with high morbidity and an important impact on milk production in dairy herds, the epidemiology of respiratory disease in adult cows has not been well characterized. Surveys of U.S. producers indicated that respiratory disease is the cause of 3.4% of the deaths that occur in adult beef cattle54 and 11.3% of deaths that occur in adult dairy cattle.74 The lack of data regarding respiratory disease in adult cows is likely due to the fact that other diseases that affect reproductive performance and milk yield are of greater economic significance in this class of cattle. However, the few published descriptions of respiratory disease outbreaks confirm that the problem can on occasion be of major importance to individual herds.
RESPIRATORY DISEASE IN SHEEP AND GOATS. Little information is available regarding the epidemiology of respiratory disease in sheep and goats in North America. A survey of U.S. producers indicated that 7% of death in adult sheep is due to respiratory disease, whereas 12% of lamb deaths are due to respiratory disease.75 Feedlots reported that shipping fever pneumonia accounted for 13% of feedlot lamb deaths, and other respiratory disorders accounted for 29% of lamb deaths.35