Editor’s note: We’ll have a series of articles summarizing highlights from the recent Bovine Respiratory Disease Symposium and Academy of Veterinary Consultants Conference on this site over the rest of this month. Check back often.
Assessing the risk of cattle needing treatment for bovine respiratory disease (BRD) requires consideration of far more than exposure to patahogens.
During the recent BRD symposium in Denver, Mississippi State University professor David Smith, DVM, PhD, noted that pathogens represent a “causal agent” in respiratory disease. However, while a causal agent is “necessary” for disease, it might not be “sufficient” to cause disease on its own.
Mannheimia haemolytica for example, is one of the most common bacterial pathogens involved in BRD, but it often is present in bovine respiratory tracts in healthy animals, without any indication of disease. Disease occurs when exposure to the pathogen occurs along with other factors such as dust, stress, cold or concurrent infection with respiratory viral agents such as bovine viral diarrhea virus (BVDV). At some point – and that point varies depending on the animal’s genetics – those factors add up to become sufficient to cause disease.
When a set of calves are weaned on a truck, marketed through a sale barn, comingled at an order-buyer facility, shipped to a feedyard, processed and probably comingled again, they are at high risk for BRD. No single factor causes the outbreak of disease; it results from the additive effects of stress, fatigue, dehydration and exposure to pathogens. Complicating the issue further, those pathogens include a range of viruses such as BRSV, BHV1, BVDV and bovine coronavirus, and bacteria including Mannheimia haemolytica, Pasteurella multocida, Histophilus somni and Mycoplasma bovis, either alone or in combination.
Smith notes that the bovine lungs are inherently susceptible to infection, and have low capacity relative to the animal’s size. Even a small loss of lung capacity reduces oxygen transfer to vital organs and limits performance.
It is no coincidence that BRD tends to spike in calves at around 90 to 120 days of age, Smith says. Even if a calf receives adequate colostrum at birth, serum concentrations of maternal antibodies drops by one-half every 16 to 20 days after colostrum ingestion. By 96 to 120 days of age, the calf retains less than 2% of the antibodies it absorbed from colostrum. Most high-risk calves, he adds, have little to no immunity when marketed at weaning.
BRD commonly affects pre-weaned calves and recent arrivals in feedyards and stocker operations. Late-day BRD cases in heavy feedyard cattle, while less common, results in substantial economic losses due to the lost production costs invested in the animal at that stage.
Smith is involved with a group of veterinarians – Veterinary Advancement of Systems Thinking (VAST) – who explore how the systems approach can apply to animal health and production strategies. Dr Smith outlined the “systems-thinking” concept in more depth in a presentation during the subsequent Academy of Veterinary Consultants (AVC) conference.
The systems approach fits well with the BRD challenge, Smith says, by exploring causal relationships and decisions across the production chain. For example, he returns to the ongoing acceptance of high-risk cattle in the marketing system. Buyers count on mass treatment to mitigate BRD risk and compensatory gains to make up for treatment costs. A systems approach would consider all the factors involved, such as cow nutrition, cow vaccinations and calf vaccinations back at the ranch, and build in better economic signals to reward practices that minimize risk to the buyer.
Smith echoed a common theme from the conference, discussing the three pillars of animal disease: the animal, the pathogen and the environment. All three play a role in outbreaks, but perhaps too much of the industry’s focus has been on the pathogens. A systems approach dedicates equal attention to the animal – its nutrition, immune system and stress levels – and the environment in which it is raised, marketed and finished. The pathogens will always be present, but management practices beginning at the cow-calf level and throughout an animal’s life could leave them relatively impotent.
For more summaries from the BRD Symposium and Academy of Veterinary Consultants Conference, see these articles from BovineVetOnline: