The virus is passed from cow to foetus and, if this occurs in the first third of pregnancy, a persistently infected (PI) calf will be born. These will then go on to shed virus throughout their lives,
so infecting herdmates and the next generation of calves.
“The timing of when a vaccine is given is critical to the development of an immune response, which will protect the unborn foetus,” explains vet Jon Reader from Synergy Farm Health, an XLVets practice. “Working out when you want her to calve and then calculating back shows when either a primary course of BVD vaccine or a booster should be given, but get this wrong and you will end up with what is known as an immunity gap”.
In the case of heifers, they MUST be fully protected before being served for the first time, and cows must have had immunity boosted before they are pregnant again. However, far too often heifers are adequately protected following an initial course of vaccinations, but given their age and date of service, do not receive further protection until they enter the adult herd.
If heifers calve just after the herd booster, then there is a very real possibility that they will go for two years without a booster and be severely exposed when they become pregnant as a first lactation heifer. This is termed the immunity gap; a period when the animal is not protected from the virus at all. Obviously, this period should ideally be as short as possible.
With this in mind, Jon Reader has developed an online tool called Mind the Gap. Farmers simply input their breeding protocols for maiden heifers, as well as the timings of their annual herd booster. The app will then highlight times when animals may be at risk if immunity has waned, without sufficient boosting of immunity by vaccination.