Metaphylaxis works. Numerous controlled and blinded trials have shown that in high-risk calves arriving at feedlots or stocker operations, mass treatment with an antibiotic significantly reduces BRD sick pulls and mortality. However, as pressure mounts to reduce the use of antibiotics in food animals, it becomes increasingly important to understand how metaphylaxis works, and how changes in the practice might affect results.
During the recent AABP Conference, Brian Vander Ley, DVM, PhD, an epidemiologist at the Great Plains Veterinary Education Center, outlined his hypothesis, looking at the timing of treatment and the post-metaphylaxis interval (PMI). Vander Ley described the “SIR Model” of epidemic disease, such as BRD, with SIR representing susceptible, infected, and resolved or recovered individuals in a population. Progress of the epidemic depends on the number of effective contacts, duration of the infection and infectiveness of the pathogen, modified by the proportion of susceptible and resolved hosts.
Vander Ley used a fire analogy, saying the number of contacts between hosts acts like fuel for the fire. Infectivity of the pathogen is analogous to the fuel being wet or dry, and duration corresponds with using either a match or a blowtorch to start the fire.
Calves classified as “high-risk” often ship from their home ranch at weaning, with unknown vaccination histories, minimal health-related management and high levels of stress, meaning a high percentage are susceptible to disease. Co-mingling at sale facilities and again at their destination results in a high number of contacts, or exposure to transmissible pathogens.
Metaphylaxis upon arrival, Vander Ley says, moves a high percentage of calves from the susceptible category to resolved, while also reducing transmissibility of BRD pathogens and duration of infections. This creates a sort of grace period during which the outbreak stalls. Then, during the PMI, the calves acclimate to their new environment, stress levels decline, feed intake increases and immune suppression drops off, in part due to immune response to existing pathogens among those calves that moved from susceptible to resolved.
Essentially, Vander Ley says, metaphylaxis interrupts disease progression while allowing development of immunity from exposure. This concept, Vander Ley says, could help explain why prophylactic antibiotic treatment, at the ranch prior to shipping, does not match the efficacy of metaphylaxis upon arrival, following exposure during the marketing process.
Vander Ley says successful metaphylaxis, in the context of his hypothesis, depends on these key assumptions:
- The bacterial pathogens involved are susceptible to the drug.
- Calves ae exposed and infected with the pathogen at the time of metaphylaxis, allowing them to develop immunity during the PMI.
- Introducing exposed but untreated animals to the group could “add fuel to the fire” by shifting the balance between infected and resolved animals.
For more on metaphylaxis strategies, see these articles from BovineVetOnline: