Farmers should avoid the temptation to retain calves persistently infected (PI) with bovine viral diarrhoea (BVD) as a means of acting as a vaccinator for the rest of the herd.

Speaking at a farm walk organised by Ballygawley Vets earlier this week, Mairead O’Grady from MSD Animal Health acknowledged that some farmers believe that there is a benefit to be gained from retaining a PI.

In theory, that animal will expose otherwise healthy animals to the virus, which will help these cattle develop a level of immunity against the disease.

What must be avoided is contact between a PI and cows or heifers in early to mid-pregnancy (30 to 120 days), especially animals that have had no recent contact with, or been vaccinated against, the disease. Where a susceptible cow is exposed to a PI during pregnancy, the virus is likely to be passed on to the unborn calf resulting in the birth of a PI animal.

“The problem is that a PI is not consistent as a vaccinator. They would need to get around all breeding cattle before these animals become pregnant. In practice, it just doesn’t work and the risks are too great, so cull any PIs,” advised O’Grady.

A PI within a batch of cattle will also shed virus, which will suppress the immune system in other animals leading to an increase in disease problems such as scours and pneumonia at housing, with associated veterinary costs.

When it comes to weaning this autumn, O’Grady advocated having pneumonia vaccinations completed in advance of housing, and removing cows in stages from cow and calf groups at grass. She also highlighted the danger of increased lungworm burdens in cattle this year after the relatively wet, but warm summer.

Farmers who haven’t dosed spring-born calves so far should discuss a strategy with their vet, she said.