Bovine respiratory disease complex

What it is

Bovine respiratory disease (BRD) is not a single disease but a multifactorial syndrome: host, environmental and management factors combine with several infectious agents to produce bronchopneumonia. Respiratory viruses (bovine respiratory syncytial virus, bovine herpesvirus 1, parainfluenza 3, bovine viral diarrhoea virus and bovine coronavirus) and the stress of transport, weaning or commingling damage the respiratory epithelium and mucociliary clearance. Bacteria that normally live in the nasopharynx then descend to the lung: Mannheimia haemolytica, the most important, Pasteurella multocida, Histophilus somni and Mycoplasmopsis (formerly Mycoplasma) bovis.

M. haemolytica produces a leukotoxin that destroys leukocytes and causes severe fibrinous pleuropneumonia. The classic term "shipping fever" refers to the onset of disease after transport and arrival at a feedlot.

Who it affects

• Rearing calves, especially of dairy breeds, before and around weaning (so-called enzootic calf pneumonia).
• Beef calves newly arrived at feedlots, with the highest risk in the first weeks after arrival.
• Risk factors include transport, passage through auction markets, commingling of cattle from different sources, abrupt weaning, castration and dehorning, poor transfer of passive immunity, poorly ventilated housing and sudden temperature changes.

Anatomy involved

• Lungs: lesions concentrate in the cranioventral parts, that is the cranial lobes, the middle lobe and the ventral part of the caudal lobes; the right cranial lobe is among the most affected. The marked lobulation of the bovine lung, with thick interlobular septa and almost no collateral ventilation, means that obstruction of a bronchiole leaves its lobule airless and gives fibrinous pneumonia its marbled appearance, with septa thickened by fibrin and oedema. Cattle also have a small lung capacity for their body mass, which reduces their respiratory reserve.
• Trachea: the tracheal bronchus, which arises from the right wall of the trachea before its bifurcation, ventilates the right cranial lobe and receives inhaled agents early. Bovine herpesvirus 1 causes necrotic tracheitis that damages the mucociliary apparatus. The trachea is also the route for sampling the lower airways by tracheal wash.

Clinical signs

Fever, depression, reduced feed intake, nasal and ocular discharge, cough and increased respiratory rate. Severe cases show dyspnoea, with open-mouth breathing and an expiratory grunt. Auscultation reveals abnormal sounds over the cranioventral thorax and, in pleuropneumonia, sometimes pleural friction rubs. M. bovis can also cause otitis media and arthritis in calves. Many animals with lung lesions never show obvious signs, and some are only detected at slaughter.

Diagnosis

• Clinical examination and scoring systems: in dairy calves, scores that add rectal temperature, cough, nasal and ocular discharge and ear position; in feedlots, visual detection of sick animals by pen riders, supported by rectal temperature.
• Lung ultrasonography: shows lung consolidation on the farm, especially in the cranioventral intercostal spaces, and detects animals with pneumonia that show no signs.
• Aetiological tests: culture with susceptibility testing and PCR on bronchoalveolar lavage, tracheal wash or deep nasopharyngeal swab samples, taken before treatment from several animals in the group.
• Necropsy of dead animals, to identify the agents and lesions in the group.

Treatment and prevention

Treatment relies on early antimicrobials, chosen according to local susceptibility, previous response on the farm, suspicion of M. bovis and the authorisation of each product; the animal must be reassessed after treatment. Non-steroidal anti-inflammatory drugs can be used case by case to reduce fever and discomfort. Drug use in food animals is subject to national regulations and to meat and milk withdrawal periods. In high-risk groups arriving at the feedlot, metaphylaxis (treatment of the whole group) is used, a practice that accounts for a large share of antimicrobial use.

Prevention depends above all on management: adequate colostrum feeding, good ventilation without draughts, avoiding commingling of cattle from different sources, reducing the stress of transport and weaning, and vaccination and early weaning before shipment (preconditioning). Vaccines against the viruses and bacteria of the complex are widely used, but the evidence that vaccination on arrival at the feedlot reduces disease is scarce.

Key studies

• ENOVAT guideline (Scahill et al., 2026): European guideline developed with the GRADE method from systematic reviews of randomised trials. No antimicrobial class showed a clinically meaningful advantage in the need for re-treatment. It suggests penicillin G, oxytetracycline or florfenicol as first choice, keeps macrolides as second line and recommends against fluoroquinolones except in exceptional, justified circumstances. It recommends against adding a non-steroidal anti-inflammatory drug solely to avoid re-treatment. Some panel members declared past links with industry.
• Scahill et al. (2026), systematic review and meta-analysis of 17 randomised trials with 4909 animals. Adding a non-steroidal anti-inflammatory drug to the antimicrobial did not reduce the need for re-treatment (RR 0.94; 95% CI 0.84–1.05; moderate certainty). The comparison of the anti-inflammatory alone against the antimicrobial rested on two trials and its certainty is very low. The outcome measured does not capture effects on pain or fever. It contradicts the widespread belief that the anti-inflammatory improves cure.
• Timsit et al. (2016), systematic review and Bayesian meta-analysis of 7 feedlot studies comparing disease detected in the pens with lung lesions at slaughter. Clinical detection had a sensitivity of only 0.27 and a specificity of 0.92: most animals with lesions go undetected. Heterogeneity between studies was large.
• Buczinski et al. (2026), systematic review and meta-analysis of lung ultrasonography in calves (26 studies), corrected for the imperfect reference test. With a consolidation threshold of 1 cm or more, sensitivity was 68.4% and specificity 91.2%; with 3 cm or more, 58.2% and 95.8%. The included studies were few and heterogeneous.
• Buczinski et al. (2021), systematic review and meta-analysis of 27 articles in dairy heifers. Heifers with calfhood BRD had 2.85 times higher odds of dying and 2.30 times higher odds of leaving the herd before first calving, gained 0.067 kg/day less and produced 121.2 kg less milk in their first lactation. Two of the authors worked for a pharmaceutical company.
• O'Connor et al. (2019), systematic review and network meta-analysis of bacterial and viral vaccines given on arrival at the feedlot (53 studies; 14 in the analysed network). It found little compelling evidence that these vaccines reduce the incidence of BRD, contrary to a very widespread practice.