While a great deal of study has taken place on bovine respiratory disease (BRD) in feedlots, BRD also sometimes affects young calves, and less is known about the disease complex at that production stage. During the recent American Association of Bovine Practitioners conference, veterinarians provided some insight into pre-weaning incidences of BRD or “summer pneumonia” as it sometimes is called.

Risk factors for BRD in calvesRuss Daly, DVM, MS, DACVPM, from South Dakota State University (SDSU), said pre-weaning BRD outbreaks are not predictable, and when outbreaks occur, within-herd incidence can often be high. And, he says, outbreaks occur even in herds where calves are well-vaccinated at branding or turnout. Fortunately, he says, most affected calves respond well to treatment and death loss can be kept to low levels with diligent monitoring and timely treatment.

Daly says the typical reaction to outbreaks among producers and their veterinarians is to fall into firefighting mode, just trying to gain control of the disease. While timely treatment is important, he also advises taking a systematic approach toward mitigating the disease — at the kitchen table rather than at the chute. He encourages veterinarians to sit down with their clients to compile information on the outbreak to gain understanding of how it occurred, resolve it and prevent future problems.

He suggests gathering information on the animals affected including their age at the time of infection, where they were located on the ranch and, when possible, other information such as identification of sick calves’ dams, age of the dams and dystocia scores. Also discuss time events such as herd-management dates, group movements, introduction of other animals to the herd and weather events around the time of the onset of clinical signs. The veterinarian potentially can use this information to determine how the disease established and spread within the herd

BRD, of course, can involve several different pathogens including bacteria and viruses, and Daly encourages veterinarians to employ diagnostic testing in post-mortem examinations for any calves that die from BRD.

Daly said that veterinarians investigating these cases will also frequently perform ante-mortem testing. SDSU testing in herds with outbreaks of BRD in calves has found a variety of bacterial pathogens including Mannheimia haemolytica, Pasteurella multocida,Histophilus somni and Mycoplasma bovis. Tests also detected viruses associated with BRD including bovine respiratory syncytial virus, infectious bovine rhinotracheitis and coronavirus, which are frequently isolated from these cases. There is much to learn about properly interpreting results from ante-mortem samples, Daly says.

David R. Smith, DVM, PhD, DACVPM (Epi), from Mississippi State University, says risk factors and host-pathogen interactions for BRD in young calves are not well understood and difficult to study in the field, particularly as factors such as age, weather, dust and other pathogens can influence whether or not a calf becomes sick. For example, an agent might be necessary to cause disease, Smith says, but it might not be sufficient to cause disease, making it difficult to consistently observe the causal relationship.

“We suspect that the pathogens associated with BRD are common in beef herds,” Smith says, “but clinical cases of BRD are relatively rare, until some stressors in the system result in an outbreak.” Studies of BRD outbreaks in calves indicate the most common timeframe is around 120 days of age. Smith points out that antibodies from colostrum decline over time and are essentially gone by about 90 days of age. Calves often experience a gap in immunity before active immunity builds to adequate levels. There is some scattered incidence of BRD in much younger calves, probably related to a failure in their passive immune system such as from lack of quality colostrum soon after birth. However, BRD outbreaks in older calves may be due to loss of herd immunity after the majority of calves have lost their maternal immunity.

Some studies have shown higher incidence of BRD in calves from first-calf females compared with those from older cows, but the results are inconclusive, Smith says. In some herds steer calves seem to be more likely to contract BRD than heifers.

Amelia Woolums, DVM, MVSc, PhD, DACVIM, DACVM, from the University of Georgia, said that although BRD incidence in calves fluctuates widely between and within herds, it is the leading cause of calf deaths in calves 3 weeks of age and older, accounting for up to 16 percent of the total. Twenty years of data from over 110,000 cattle at the U.S. Meat Animal Research Center (MARC) show annual incidence ranging from 3 to 24 percent, with a mean incidence of 11 percent.

Based on the MARC data and several other datasets, Woolums lists these risk factors that appear to be associated with pre-weaning BRD:

• Year of birth — Incidence varies widely from one year to the next.

• Location of a group of calves on an operation

• Dystocia

• Sex of the calf — higher incidence in males

• Damage — higher incidence in calves born to heifers

• Operations that imported weaned steers

• Herds that had visitors

• Calves fed antibiotics to prevent BRD (Woolums speculates these operations might have initiated prevention measures after seeing a problem develop.)

• Larger herds (Incidence generally increases with larger herd sizes.)

• Occurrence of diarrhea in calves

• Occurrence of BRD in cows

• Longer calving seasons

• Herds that introduce new calves

• Creep feeding

• Heat synchronization programs

(Like creep feeding, these events congregate calves and increase contact.)

Consistent risk factors across the data sets include introduction of new cattle and management activities that confine or concentrate calves during the risk period.

Woolums adds that a survey of 61 veterinarians in six states asked respondents to select from a list of potential risk factors. The most common selections, in order of frequency, include:

• Weather

• Inadequate colostrum

• Introducing new cattle

• Failure to vaccinate calves with respiratory vaccines

• Failure to vaccinate cows with respiratory vaccines

• Calf diarrhea

• Nutritional deficiencies

• Presence of a calf persistently infected with BVDV

More research will be necessary to confirm whether the risk factors identified to date can be modified to limit pre-weaning calf BRD on operations where the disease is a problem. While veterinarians can work with clients to avoid or minimize some of these risk factors, some others are unavoidable or consistent with best management practices within an operation. In those cases, veterinarians and producers can benefit by understanding where and when those risks occur, to enable increased surveillance of calves exposed to these risk factors, and to allow timely intervention to prevent or control the disease.