We know that stresses associated with weaning, transport, comingling, castration, dehorning and feedlot processing increase plasma cortisol concentrations and challenge the immune system. We also know that calves arriving in feedyards often have been exposed to respiratory pathogens during the marketing and transport process, and that the combined effects of stress, suppressed immunity and disease exposure can reduce the benefits of vaccinations.

And yet, feedyards typically vaccinate cattle, including high-risk calves, with a modified-live viral (MLV) vaccine shortly after arrival

Texas A&M animal scientist John Richeson has conducted research on stress-induced immunosuppression in cattle, and particularly on vaccine responses. He recently outlined the subject and built a case for delaying MLV vaccination during the Academy of Veterinary Consultants conference in Denver. In illustrating the stress and disease exposure feeder calves often experience, he describes a typical scenario.

·         Tuesday:  Calves are separated from their dams and transported to a market.

·         Wednesday: Calves are handled and sold through the market, and likely exposed to BRD viruses.

·         Thursday: Calves are comingled on a truck, transported to an order-buyer facility and comingled again.

·         Thursday overnight: Calves are transported to a feedyard, disrupting their circadian rhythms.

·         Friday: Calves are unloaded at the feedyard.

·         Saturday: Calves are processed at the feedyard, including administration of a MLV vaccine.

Given those experiences, he says, it should come as no surprise when those calves suffer a BRD outbreak about 14 days after arrival.

Richeson says research has shown acute stress can actually help boost the immune response, but chronic stress suppresses immunity. Also, while a MLV vaccine administered to stressed calves can provide an enhanced antibody response, it also can contribute to chronic stress. Several studies comparing MLV vaccination upon arrival versus delayed for 14 days have shown reduced BRD cases in the delayed-vaccination calves, while some similar studies have found no difference.

Richeson stresses the difference between vaccine efficacy and vaccine efficiency. A strong response in terms of antibody titers suggests vaccine efficacy. Vaccine efficiency refers to the actual results in terms of reduced morbidity and mortality and improved performance. So, a vaccine could be efficacious, providing a good antibody response, but not be efficient due to external factors such as chronic stress. MLV vaccine antigens, although attenuated, could contribute to that chronic stress in new arrivals.

Delaying administration of the MLV vaccine has nothing to do with the vaccine response, Richeson says. It relates instead to removing an additional challenge during a period of stress-induced immunosuppression, while providing later protection against BRD viruses.

A recent feedyard trial, results of which were published in the peer-reviewed journal Bovine Practitioner, shows that withholding MLV viral vaccinations until 30 days after arrival could reduce BRD re-pulls in high-risk cattle without affecting the percentage of initial pulls.

Watch for the January issue of Bovine Veterinarian for an in-depth article on the research into delayed vaccinations for high-risk calves.