It is well known that bovine respiratory disease (BRD) can be a costly and time-consuming blight on any farming enterprise. Several pathogens and risk factors inter-play in a BRD outbreak, making treatment and prevention challenging. Pathogens implicated include viruses such as bovine herpesvirus 1 (BHV1) (the causative agent of infectious bovine rhinotracheitis (IBR; Figure 1)), bovine respiratory syncytial virus (RSV) and parainfluenza 3 virus (PI3) with bovine viral diarrhoea disease (BVD) contributing to immune suppression, and bacteria (eg Mycoplasma bovis, Pasteurella multocida, Mannheimia haemolytica).
Although respiratory disease is most often identified as a cause of morbidity and mortality in post-weaning calves, pre-weaning heifers can also be affected.
This article aims to outline the main pillars of a respiratory disease outbreak investigation, focusing on a holistic and herd health-based approach.
Data analysis and history taking prior to the farm visit
Typically, veterinary advice is sought in the case of a higher than expected incidence of morbidity and mortality due to respiratory disease or perceived vaccine failure. More insidious production losses (such as poor growth rates) associated with respiratory disease often go unnoticed. A lot of information may be gleaned prior to a farm visit from good farm records, such as calf morbidity and mortality data, including age of calves affected and risk period, seasonal disease patterns, housing structure, maximum number of calves in any one housing structure at any one time, calf nutrition pre- and post-weaning, vaccinations and treatment protocols.
Be aware that disease recording can be very poor and available data for analysis prior to a farm visit can be limited. Be conscious that any information collected at a farm visit can be subject to recall bias and may be inaccurate. This is often an area for improvement.
Examining individual calves
Initial investigations should centre around clinical examination of individual calves. All animals should be examined where fewer than 20 animals are affected. In larger group sizes a representative sample of 50 calves can be sampled.
There are several available scoring systems to screen for respiratory disease in calves, the most common one being the Wisconsin calf health system (McGuirk and Peek, 2014). Animals are scored on a clinical scale based on nasal and ocular discharge (Figure 2), ear position, appetite, cough, demeanour and naval and joint appearance.
An animal with a score of four or more is defined as clinically affected with BRD. Most clinical parameters can be established from outside the pens with minimal fuss and time spent on each calf. Often (in practical terms) clinical decision making regarding treatment is made on pyrexic rectal temperature (higher than 39.5°C or 103.1°F).
The scoring tool can also be used to examine the ability of the farm staff to detect respiratory disease. Ideally 85 percent or more of affected animals should be detectable by trained farm staff, but in reality, this target is seldom achieved without further education and knowledge. Early detection is key to treatment success and all too often chronic low-level respiratory disease becomes an acceptable part of calf rearing operations due to poor detection and treatment failures.
While Wisconsin respiratory scoring can provide a useful “snapshot” of the prevalence of BRD on the day of an investigation visit, more regular, routine scoring of calves can have many advantages including: containing outbreaks within pens, providing an early opportunity for treatment intervention and determining cure rates to treatment.
Colostrum management practices need to be thoroughly reviewed in any BRD outbreak The three Qs of colostrum management are that calves should receive high quality colostrum in sufficient quantity as quickly as possible.
Calves should be fed 10 to 15 percent of their bodyweight of high-quality colostrum (more than 50g/L of IgG) within their first 6 to 12 hours of life. Since the abomasal capacity of a new-born calf is 2.5 to 3 litres, it may be necessary to split the feed between two separate feeds. Be aware that highly contaminated colostrum (especially colostrum contaminated with coliforms) can interfere with IgG absorption, so hygienic colostrum harvest and storage is important.
Housing evaluation and risk factor assessment
Poorly ventilated calf housing can result in aerosolisation of respiratory disease pathogens. Mixing of different age groups of calves can result in transfer of pathogens, particularly from older to younger less immunologically competent animals. Mixing of calves from different sources and high stocking densities may also be problematic. Winter temperatures (particularly in draughty housing) can hamper growth rates, since feed rations are rarely tailored to account for this.
Basic biosecurity is important in any calf rearing facility including gloves, disinfectant and clean boots and clothing. Sick calves should ideally be contained, or if detected early enough, isolated in a separate airspace.
Further diagnostic testing
Once clinical parameters have been established, use the youngest age group of affected calves for further diagnostic testing. Animals with acute signs, representative of the group’s clinical picture, should be selected since sampling chronically affected animals will not yield useful results.
The aims of diagnostic testing should be decided beforehand. Be aware that therapeutic decisions often need to be made before results are available, for sick animals, but laboratory results may inform preventative decision making (such as on vaccination or housing) in the future. It is also important to discuss the cost of diagnostic testing before beginning.
Serum for failure of passive transfer testing
Total proteins of more than 5.5g/L in calf serum is indicative of adequate passive transfer. This is an indirect measure of serum IgG and is a simple and cheap option. Alternatively, a Brix refractometer can be used calf side to assess failure of passive transfer (Brix higher than 8.5 percent is indicative of adequate passive transfer).
Two serum samples can be collected at a 14- to 21-day interval to gauge antibody titre increases to respiratory disease pathogens. This can be a costly option and single sample serological testing is often employed in practice, although this option does not provide as clear a diagnosis. BVD check testing may also be prudent.
If the aim of the investigation is simply to improve early detection and initiate a more effective treatment protocol, nasal swabs can be taken from six untreated calves with clinical disease. From each calf, two deep nasal swabs are taken for aerobic and anaerobic bacteria. One of the swabs is submitted for bacterial culture and the second is submitted for Mycoplasma bovis culture.
Bronchoalveolar fluid collection from pre-weaned calves in a herd with respiratory disease can be an efficient way to characterise the type and severity of respiratory inflammation.
Fresh mortalities or freshly euthanised calves can provide useful diagnostic information and an opportunity to examine all organs. Sometimes the problem is not respiratory disease at all!
Vaccination and treatment protocols
Investigation of calf respiratory disease outbreaks should also include a review of any vaccination protocol. At-risk calves can be vaccinated any time from seven days of age, depending on maternally derived immunity and whether respiratory disease is pre- or post-weaning. Intranasal and subcutaneous vaccinations are available. Sensitiser and booster vaccinations should be given strictly at the required time interval depending on the product used to ensure efficacy, as compliance is often poor.
Treatment for BRD often centres around antimicrobial therapy. The decision to treat entire groups of calves with antibiotics should not be taken lightly, particularly in the current climate of prudent antimicrobial use and compliance. Non-steroidal anti-inflammatory treatments are also often useful ancillary therapy.
A holistic approach to calf respiratory disease investigations takes time and attention to detail to ensure that nothing is missed. Client expectations should be managed and costs of diagnostic testing discussed. Basic epidemiological principles like morbidity and mortality incidences, age of onset and risk factors should not be overlooked.