29 August 1997

VACCINATION BEST ANSWER TO ANY VIRUS DIARRHOEA

PRODUCERS suffering bovine virus diarrhoea infection in their herd can now prevent further outbreaks by vaccinating.

Wiltshire vet Andrew Norton, Marlborough, explains that widespread BVD infection was found throughout the UK in Vet Investigation Centre tests last autumn. "It was difficult to find herds that hadnt been exposed to the virus."

After infection, BVD remains within a herd as stock carry the virus. However, Mr Norton warns that immunity built up by animals after an outbreak can suddenly fail and clinical disease re-occurs.

In some cases, adult cattle have died. However, most outbreaks result in embryo loss, which may only be seen as poor fertility, stillbirth, abortion and high calf mortality, he says (see panel).

But often no signs are seen until animals are stressed at calving, then either a change in the virus or exposure to a new strain of virus causes mucosal disease, and a fatal scour.

"Until last year it was a frustrating and expensive disease, with no treatment available. The only option then was to blood test suspect calves to identify carriers and cull them. Now there is a vaccine – Bovidec – which is well tested."

Many producers will know if they have BVD within the herd after having abortion cases tested, otherwise a sample of cows can be tested for infection.

Each vaccine dose costs just over £10 a head. In the first year, two doses must be given three weeks apart with the second dose at least a week before serving. In all-year-round calving herds, stock may have to be batched and vaccination dates split. Vaccination with a single dose should be repeated annually.

When tests before vaccination show only low levels of antibody, it may be that older cows are infected which means first lactation heifers are at greatest risk of BVD as they have no immunity. In this situation only the heifers, which are at most risk, could be vaccinated, reducing cost, suggests Mr Norton. &#42

Immunity to BVD, built up by animals, can suddenly fail, with clinical disease then re-occurring, warns Wilts vet Andrew Norton.

BOVINE VIRAL DIARRHOEA

BVD can affect animals in different ways depending on their age at infection, warns Andrew Norton.

He explains that a pregnant animal can suffer early embryo loss. However, when the foetus is infected later, but before 120 days, when its immune response system develops, it will not recognise the virus as foreign and therefore will not kill it.

That calf then carries the virus; it may be stillborn, have brain damage, be weak at birth or just be prone to other diseases. When stressed, the infected animal may suffer mucosal disease, which is an acute fatal scour, he explains.

Some carrier animals show no infection, but can shed the virus and infect other animals. For example, a bull brought in to serve a group of heifers may show no infection but can infect all the heifers with devastating effects, warns Mr Norton.

When non-pregnant animals are infected, they may only suffer a short period of diarrhoea with little ill-effect, but in serious cases they may die, he says.