Bovine respiratory disease (BRD) is an important cause of morbidity and mortality in both preweaned and weaned dairy calves. The syndrome is initiated by stress, but calves are more susceptible with partial or complete failure of passive transfer.

A wide variety of bacterial and viral pathogens play a role, including bacteria considered normal flora (Pasteurella multocida, Mannheimia haemolytica, Bibersteinia trehalosi and Histophilus somni). Mycoplasma bovis and Trueperella pyogenes may also be contributing factors.

Viral pathogens may be immunosuppressive (bovine viral diarrhea virus) or may cause direct damage to the respiratory tract (bovine herpes virus-1, parainfluenza virus type 3, bovine respiratory syncytial virus and bovine coronavirus).

Stress, immunosuppression and viral damage allow for bacterial pathogens to overwhelm the lower airway defenses, causing airway consolidation and clinical pneumonia.

A 2011 study showed that dairy heifers with multiple cases of BRD before first calving have two times greater odds of not completing their first lactation compared to counterparts that weren’t diagnosed with BRD.


That study found no association between the incidence of diarrhea or navel infections and the odds of completing first lactation.

In addition, lung lesions in dairy calves have been associated with decreased average daily gain (ADG). Research has shown that preweaned ADG has a significant effect on first-lactation milk yield. Thus, BRD has an important effect on future heifer productivity.

The ultimate challenge of BRD is early detection, as significant pulmonary lesions may exist without outward signs (i.e., subclinical BRD). Standardized clinical scoring systems have been developed to increase the sensitivity of detecting BRD.

The Calf Health Scoring App was developed at the University of Wisconsin – Madison School of Veterinary Medicine to apply twice-weekly health screening in preweaned calves.

The score includes systemic health parameters, such as appetite, attitude, umbilical and joint assessments, fecal consistency and rectal temperature, and specific respiratory parameters, such as cough, nasal and ocular discharge, and ear position.

A thoracic ultrasound score (0 to 5) is also included on the app. A thoracic ultrasound is highly accurate in detecting lung consolidation, with a score of 3 or higher (meaning at least one lung lobe is consolidated) indicative of pneumonia.

Thoracic ultrasound is an objective tool that, when combined with clinical scoring, allows for differentiation between upper respiratory disease, clinical and subclinical BRD.

A systematic thoracic ultrasound can be performed with a transrectal probe and 70 percent isopropyl alcohol as a transducing agent. Clipping is not needed and, once the technique is learned, an ultrasound diagnosis can be achieved in less than a minute. Veterinarians with access to a portable ultrasound can provide this service.

On the right side, the cranial aspect of the cranial lung lobe is the most common location for pneumonia in dairy calves. On the left side, the caudal aspect of the cranial lobe is also a common site of pneumonia.

Specific findings on ultrasound that indicate a poor prognosis include caudal lung lobe consolidation, lung abscessation and lung necrosis. Following intratracheal inoculation with P. multocida, clinical signs (increased rectal temperature and increased respiratory rate) correlated well with postmortem changes and ultrasound scores.

Agreement in diagnosing consolidation (pneumonia) when performing thoracic ultrasound is considered good among novice and experienced operators. Incorporating thoracic ultrasound during routine calf scoring provides an insight into the herd prevalence of BRD and helps identify problems and determine a timeline before an outbreak with severe consequences occurs.

Along with clinical scoring and thoracic ultrasound, calves that are determined to have subclinical or clinical pneumonia should have samples submitted for pathogen isolation. Results will assist your veterinarian in determining optimal treatment and vaccination protocols.

A practical option in the field is a nasopharyngeal swab for aerobic culture and PCR (for detection of M. bovis and viruses). A previous study showed that M. haemolytica and M. bovis isolated from the nasopharynx were highly representative of isolates from the lung.

In addition, a recent study in Quebec showed that nasopharyngeal carriage of M. bovis was associated with increased odds of clinical signs, lung consolidation and decreased average daily gain.

Prevention of BRD in dairy calves includes provision of adequate colostrum. In addition, ventilation and housing must be managed to balance the benefits of increased starter intake and social development with group housing and the disadvantage of the increased risk of respiratory disease in mechanically ventilated barns.

Within naturally ventilated housing, increased nesting scores and decreased airborne bacterial concentrations in calf microenvironments decrease the prevalence of respiratory disease.

Quality, consistency and quantity of milk and milk replacer feedings also have a significant effect on the innate immune system as well as the rate of starter intake. Therefore, high-quality nutrition is an important defense against BRD.

The combination of a standardized scoring system along with effective treatment and vaccination protocols based on pathogen isolation will reduce postweaning pneumonia, chronic pneumonia and otitis media.

Although early vaccination may not increase a calf’s antibody level, it may slow the rate of decline of maternally derived antibodies, initiate cell-mediated immunity and provide local immunity (intranasal administration).

Recently, a study in dairy heifer calves was unable to find significant differences in BRD prevalence following intramuscular vaccination of 2- and 5-week-old calves.

However, it has been shown that calves with high initial antibody levels against bovine respiratory syncytial virus and bovine herpes virus-1 had lower odds of BRD compared to calves with lower antibody levels against both viral pathogens. These findings highlight the importance of adequate colostrum and indicate that populations of calves with a higher prevalence of failure of passive transfer may respond differently to vaccination.

Your veterinarian can help you establish the optimal timeline for implementing clinical and ultrasound scoring in preweaned calves to reduce calf loss and decrease the prevalence of BRD in weaned heifers.

With the Calf Health Scoring App, data can easily be transferred directly to your veterinarian in an Excel format. This data can be monitored for trends.

Pathogen data from nasopharyngeal swabs can help your veterinarian optimize treatment and vaccination protocols. Adequate colostrum delivery and ventilation are also paramount to ensuring calf health.

Proactive clinical and ultrasound screening in the preweaning period can help to reduce the prevalence of BRD, improve calf health and promote heifer success on dairy farms.  PD

Sarah Raabis is a large-animal medicine resident at the University of Wisconsin – Madison. Email Sarah Raabis