Beef Cattle Virus Diarrhoea
BVD is a pestivirus, strains of which can infect cattle, sheep, deer and goats.New Zealand surveys have shown that the virus is widespread, approximately 60% of cattle and 80 to 90% of farms have been infected with the BVD virus.
Susceptible animals become infected by close contact with transiently or persistently infected (PI) carriers, which secrete the virus in tears, saliva, nasal secretions, milk, urine, faeces and semen.In cattle the manifestations of infection are complex and any combination of the following outcomes may be seen.Bovine Virus Diarrhoea: BVD follows the infection of young naive stock, resulting in diarrhoea, mouth ulcers, and weight loss or reduced growth rates. An outbreak can take several weeks to spread through a mob of calves and the farmer often wrongly suspects worm drench failure. Subclinical depression of growth rates, without obvious disease, can also occur. In addition to preventing achievement of target liveweights, the virus is also profoundly immunosuppressive and affected calves can succumb to secondary diseases such as bronchopneumonia.
Persistent Infection: Persistently infected (PI) animals are born PI and excrete the virus continuously for the duration of their lifetime. PI animals are either born to a naive cow who became infected during pregnancy, or are the progeny of a PI cow. They are often stunted and have poor survival rates but some appear clinically normal and healthy and are retained in the herd, producing a PI calf every year. They make up approximately 1% of the adult cattle population. PI breeding bulls usually have low fertility regardless of the immune status of the cows, thus the screening of all breeding bulls for BVD virus is advised.Infertility: Infection of the cow at mating or during pregnancy can result in
- Failure to conceive
- Death of the embryo, resulting in long returns to service
- Abortion of a fresh or mummified foetus.
- Abnormal Calves: This syndrome includes premature births, stillborn calves, stunted PI calves and the birth of weak or uncoordinated calves.
The virus can cause various developmental defects, many of which are easily recognisable eg. brain, eye and skeletal deformities.Mucosal Disease: Mucosal disease is an acute, severe illness of cattle, typically between 6 and 18 months of age. It results in severe mouth and gastrointestinal ulceration with profuse diarrhoea and eventual death, usually within 2 weeks. Mucosal disease only occurs in PI animals.
The ideal control programme would be to test and cull PI animals, maintain a closed herd, test and quarantine new arrivals and vaccinate all stock annually. However, vaccination alone can be used effectively. Vaccination possibilities include:
- Whole herd vaccination and continue with annual boosters.
- Progressive vaccination of younger animals initially. Annual boosters will then progressively lead to a fully vaccinated herd.Treatment is symptomatic. A definite diagnosis of BVD is often difficult to confirm – especially in older animals. A diagnosis and vaccination regime may be based on an extensive farm history as well as diagnostic tests.