Why "natural vaccination" won't work
Author: Carolyn Gates
Given the costs of vaccinating breeding stock against BVD, many farmers and veterinarians have asked why we can’t just leave persistently infected (PI) heifers in the mob to naturally infect their herd mates prior to the mating period. After all, animals that develop immunity to BVD after recovering from natural infection are believed to stay protected against the virus for life whereas animals that are vaccinated must be re-boostered every year to maintain immunity. So “natural vaccination” would seem to make economic sense, right?
Unfortunately, new research findings show that there are several reasons why this strategy won’t work for New Zealand cattle herds.
Starting in September 2017, we ran a field study with 75 beef herds across New Zealand that were not currently vaccinating against BVD to measure the level of immunity in heifers prior to the start of the 2017/2018 mating period. Participating veterinarians randomly chose 15 heifers that had just delivered their first in calf in 2017 and collected blood samples from each to measure their level of antibodies against BVD. We specifically targeted this age group because they are the youngest in the breeding mob and therefore the least likely to have been previously exposed to BVD. We also assumed that if at lease one PI calf was born into the mob, the heifers would have all been at risk of getting naturally infected with BVD through contact with those calves on pasture.
After the first round of sampling, there were 32 / 75 herds (42.7%) that showed evidence of active BVD infection based on results from pooled antibody ELISA, which is our first line screening test for determining the BVD status of beef herds. Only 12 of these herds (37.5%) had heifer mobs that were completely immune to BVD. This suggests that contact between infected and susceptible animals on pasture is not high enough to ensure that all animals are “naturally vaccinated” before the mating period. These findings also tell us that we won't be able to break the BVD cycle in infected herds without taking active measures like vaccinating breeeding stock and culling any identified PIs.
We are just in the process of re-sampling the negative heifers from these herds to see how many became infected with BVD during the mating period when they were at greatest risk of complications such as poor fertility, abortion, and creating new PI calves. This will help us estimate the costs of BVD infections in beef herds and therefore the potential benefits to you from implmenting BVD management plans. Stay tuned for more updates!