As Bovine respiratory disease (BRD) remains one of the most common and economically significant diseases affecting calves, even in well-managed herds, a structured investigation of both clinical and subclinical cases is essential.
The objectives are to ensure early detection and appropriate treatment of affected calves, identify the pathogens involved and the contributing risk factors, assess treatment outcomes, and implement corrective actions to improve herd respiratory health and reduce the risk of recurrence.
When the first signs of BRD are observed on a farm: what steps should be followed?
From clinical examination to the implementation of an action plan, timely investigation and intervention are essential to safeguard herd health and reduce the impact of BRD.
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When walking through a calf barn, the first signs of bovine respiratory disease (BRD) are often subtle. In many cases, calves may already have developed lung lesions before obvious respiratory symptoms become visible.
From simple observation at a distance to a complete veterinary examination, numerous indicators can alert farmers and veterinarians to the presence and spread of BRD within a herd.
Several visual and behavioral changes can be identified during routine observation:
Some of these changes can also be detected using precision livestock farming technologies, including accelerometers, feeding behavior monitoring systems and automated activity sensors.
A more detailed evaluation may reveal respiratory abnormalities such as:
Depending on their severity and combination, these clinical signs can be incorporated into standardized scoring systems such as the Wisconsin Calf Respiratory Scoring Chart or the California BRD Scoring System to support treatment decisions.
Any calf suspected of being sick should undergo a thorough clinical examination by a veterinarian, including assessment of:
Not every coughing or depressed calf is affected by BRD. Differential diagnosis should always be considered, including heat stress or other environmental stressors, aspiration pneumonia and other systemic diseases that may cause similar clinical signs.
Careful observation of calves at least twice daily is critical for the early detection of BRD. Several studies have shown that lung lesions may be present before overt clinical signs become apparent.
The earlier affected calves are identified and treated, the lower the bacterial burden is likely to be and the greater the probability of achieving clinical cure after a single treatment. Early intervention also helps limit disease spread and reduces the risk of long-term impacts on growth and productivity.
Although clinical examination is essential for detecting disease, it cannot reliably distinguish between viral and bacterial infections, nor can it identify the specific pathogens involved.
To accurately determine the causative agents and adapt treatment and prevention strategies, appropriate sampling and laboratory analyses are required.
Understanding which pathogens are involved in a BRD outbreak is essential for selecting the most appropriate treatment and implementing effective control measures. However, successful sampling on farm requires proper organization, appropriate restraint of animals, and strict hygiene procedures.
Respiratory viruses typically infect the upper respiratory tract before spreading through the trachea to the lower airways, where they may contribute to the development of pneumonia.
The main bacterial pathogens associated with BRD are often present in the upper respiratory tract of healthy calves. Under the influence of stressors, impaired immunity, or viral infections, these bacteria may proliferate and spread into the lungs.
Sampling aims to identify pathogens involved in the outbreak and assess lung inflammation through detection of bacteria (culture), bacterial and viral DNA or RNA (PCR), inflammatory cells or inflammatory biomarkers.
Whenever possible, samples should be collected from:
Recently treated or chronic cases are generally less informative because treatment may alter pathogen recovery and chronic lesions are often associated with secondary invaders.
Sampling methods typically relevant in the field are:
Increasing evidence suggests that deep nasopharyngeal samples can provide valuable information on pathogens circulating within a herd and may reflect pathogens involved in lower respiratory tract disease. However, results should always be interpreted alongside clinical findings and, when available, imaging or post-mortem observations. Pooled sampling may offer a cost-effective approach for PCR-based pathogen detection. For example, samples from 10 clinically affected calves can be combined into two pools of five swabs for laboratory analysis.
This method is often considered one of the most representative sampling techniques for investigating BRD, as it allows simultaneous assessment of lower airway inflammation and pathogen detection. When performed by trained personnel with appropriate restraint, BAL can be completed rapidly and safely under field conditions.
This method is more invasive and requires experienced operators.
Adequate restraint is required for BAL and TTW procedures to ensure collection of sufficient material while minimizing stress and discomfort to the calf.
Blood sampling may provide useful epidemiological information through serological testing. Serology can demonstrate previous exposure to specific pathogens, support herd-level investigations. However, serology generally reflects exposure rather than active infection and should not be used alone to identify pathogens responsible for current clinical disease.
Necropsy of freshly dead calves remains one of the most informative diagnostic tools during a BRD outbreak. Whenever possible, calves should have died within the previous 12 hours, be untreated or minimally treated, and be representative of the outbreak. The examination should assess distribution of lung lesions, extent and pattern of consolidation, presence of fibrinous pleuropneumonia, pulmonary abscesses, pleuritis or pleural effusion.
The following samples should be collected:
Appropriate sterile sampling materials should be used and samples should be labeled carefully, refrigerated immediately after collection, and shipped to the laboratory as quickly as possible.
As no single sampling method provides all the answers, a combined diagnostic approach is generally recommended. Deep nasopharyngeal swabs collected from several representative calves provide valuable herd-level information on circulating pathogens, while targeted BAL sampling can help characterize lower respiratory tract inflammation and support bacteriological culture, antimicrobial susceptibility testing, and treatment decisions.
Whenever available, lung necropsy findings should be integrated with ante-mortem sampling results, as post-mortem examination remains the reference method for linking pathogens to pulmonary lesions.
The most commonly identified pathogens in bovine respiratory disease (BRD) include Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, Mycoplasma bovis, BRSV, BPIV-3, BHV-1 (IBR) and BVDV. Mixed infections are frequent and often contribute to disease severity.
What analyses were performed? What were the results? What conclusion can we draw?
PCR is widely used in BRD investigations because it is:
However, PCR has important limitations: It detects genetic material from both live and dead organisms: a positive result does not necessarily indicate active disease. Quantification is not always available, although pathogen load can be extremely valuable for interpretation.
Bacterial culture remains useful because it confirms the presence of viable organisms and allows antimicrobial susceptibility testing. Its drawbacks are that it is slower and may be affected by previous antibiotic treatments.
Cytological examination helps assess the inflammatory response by identifying neutrophilic inflammation and intracellular bacteria, suggesting active bacterial infection.
Serology detects antibodies produced by the animal against a pathogen, rather than the pathogen itself. It is mainly used for herd-level investigations and to assess pathogen circulation within a group. It can help identify pathogens that may no longer be detectable by PCR at the time of sampling and is relatively easy to perform on a large number of animals. It indicates exposure, but not necessarily an active disease. The highest diagnostic value is obtained by testing an acute sample collected at the beginning of the outbreak and a convalescent sample collected 2 to 4 weeks later. A significant increase in antibody titre strongly suggests recent infection and active circulation of the pathogen within the herd.
PCR identifies the pathogen, cytology assesses inflammation, culture confirms viable bacteria, and serology documents exposure and circulation within the herd.
Detecting a pathogen does not automatically mean that it is responsible for the clinical signs or lesions observed. Laboratory results must always be interpreted in the light of the clinical, epidemiological and management context.
Before drawing conclusions, several key questions should be answered:
Laboratory tests identify pathogens; they do not diagnose disease on their own. The final diagnosis results from combining laboratory findings with clinical examination, lung ultrasonography, necropsy findings and herd epidemiology. Effective BRD investigation therefore requires a holistic interpretation rather than reliance on a single test result.
Pathogens trigger BRD, but inadequate passive immunity, poor ventilation, overcrowding, stress and management deficiencies often determine whether exposure results in clinical disease. A complete BRD investigation should therefore evaluate both infectious agents and underlying risk factors before defining an action plan.
Doyle et al.: Agreement Among 4 Sampling Methods to Identify Respiratory Pathogens in Dairy Calves with Acute BRD. J Vet Intern Med. 2017, 31(3):954-959.
Woolums A.: Methods of Antemortem Sampling for Identification of Microbial Agents in BRD. 2019, AABP Proceeding, vol 55, N°2.
Pardon B., Buczinski S.: Bovine Respiratory Disease Diagnosis. Vet Clin Food Anim. 2020, 36: 425-444.
Probo, M., Veronesi, M.C. : Clinical Scoring Systems in the Newborn Calf: An Overview. Animals 2022, 12, 3013.
Kamel, M.S.; Davidson, J.L.; Verma, M.S.: Strategies for Bovine Respiratory Disease (BRD) Diagnosis and Prognosis: A Comprehensive Overview. Animals 2024, 14, 627.
O’Donoghue, S.; Waters, S.M.; Morris, D.W.; Earley, B. A Comprehensive Review: Bovine Respiratory Disease, Current Insights into Epidemiology, Diagnostic Challenges, and Vaccination. Vet. Sci. 2025, 12, 778.