Learn how to tackle bovine viral diarrhoea as part of Farmers Weekly’s disease check series.
The obvious clinical sign of BVD in cattle is mucosal disease which shows itself as ulceration of the mucosal surfaces such as the mouth, throat and intestines.But this represents a small part of a much bigger problem, says Den Leonard of Lambert Leonard and May. “Underneath, the herd is likely to be experiencing reduced fertility, increased pregnancy losses and immuno-suppression.”
DairyCheck figures show 57% of tested herds have been exposed to BVD, with 67% of beef herds testing positive through BeefCheck.With one abortion costing about £500, increased abortion rates from BVD are costly, says Paddy Gordon of Shepton Vet Group.
Immune suppression will make animals more susceptible to diseases such as pneumonia and neospora and may worsen the effect of infectious diseases such as TB, says Mr Leonard.
How is it spread?
BVD virus is spread via nasal secretions and faeces, so close penning of animals can increase transmission, explains Mr Leonard. “Bought-in animals are also a common route of infection.”BVD infection can also be spread vertically from cow to calf. “When the cow is infected during pregnancy the virus is able to pass to the foetus. The calf’s developing immune system can mistakenly think the virus is part of itself so it doesn’t mount an immune response.
“The calf is then born carrying and shedding the virus for the rest of its life and is termed a persistently infected (PI) animal.”A PI animal will always produce a PI calf and on test they show as antibody negative, but virus positive.
Testing and control
“When a herd is experiencing endemic infection or has a risk of infection, vaccination is essential,” says Mr Leonard. “And vaccinating is so cheap, it makes no sense not to.”
Testing regimes will vary depending on individual cases, but may involve testing for BVD antibodies to establish exposure to the disease, followed by a different test for antigen identifying PI animals shedding the virus when BVD is identified.
The only way to stop the cycle of infection is to cull all PIs and vaccinate, says Robert Anderson, Merlin Vet Group. . “PIs offer a serious threat to cow health and profitability.”
Worryingly, DairyCheck Plus figures showed almost 10% of herds tested had at least one PI animal in the milking herd.
Not many dairy herds have the luxury of being BVD free but, in such a case, farmers should bulk milk test for the disease every month, says Mr Leonard.
Screening in beef herds should involve testing a proportion of nine month old, weaned calves for antibodies. When results are positive, indicating exposure to virus, then identifying PIs with further tests is important, adds Mr Anderson.
And even when vaccinating, it is important to continue testing a proportion of nine month old calves for antibodies every year.
For biosecurity all bought in breeding animals should be tested for virus and vaccinated before entering the herd, he says.
Buying pregnant heifers is particularly risky as the foetus may already be infected so the cow will give birth to a PI – something known as Trojan cow syndrome.
For more on our disease check series click here.