Bovine Viral Diarrhoea (BVD)
Managing disease within a herd can be difficult, and identifying BVD, a repeated cause of reproductive and respiratory problems is essential as it can have a significant economic impact on a farm’s profitability. It is prevalent in many countries and problems related to the disease are believed to be increasing.
The Bovine Viral Diarrhoea Virus (BVDV) is the cause of bovine viral diarrhoea in cattle and can be transmitted throughout a herd in a number of ways, including congenital infection of an unborn foetus. Those congenitally infected with BVD, who do survive birth itself, will harbour the virus their entire lives. The virus may also cause a pregnant cow to abort the foetus, to suffer a stillbirth or reabsorb the foetus.
An early diagnosis of this disease is paramount, but in adults, the signs can be variable and difficult to detect. Acute infections manifest in fever, loss of appetite, lethargy, eye and nasal discharge, lesions, diarrhoea and a reduction in milk production.
The most common birth defect in calves is cerebellar hypoplasia and the signs of this include failure to nurse, stumbling, tremors, wide stance and lack of voluntary coordination of muscle movement known as ataxia. In extreme cases, death occurs. If those calves infected with BVD at the foetal stage are not lost through abortion, their development may be impaired, and they are classed as persistently infected (PI) animals.
Other infections that can affect infected cattle, whilst transient in nature, can cause a decrease in the production of milk and affect the general health of the animal. These other infections also include pneumonia, diarrhoea and reproductive diseases, all of which can lead to the death of the animal in extreme cases.
Once an infected animal has been identified it should be culled as treatment of bovine viral diarrhoea is limited to supportive therapy only. PI animals do not respond to vaccine and should also be culled.
Research is being undertaken to consider the possibility of breeding animals that are likely to be less susceptible to BVD but there is no conclusive evidence at this time that any one breed has an advantage.
One strategy is to make a BVD infected animal less infectious by increasing the number of antibodies it has. Research has shown that animals carrying a high number of antibodies shed less of the virus and for a shorter period of time, which helps to minimise transmission of the disease. Housing calves in individual pens rather than together will have a positive impact on the spread and risk of infection itself and is a useful prevention method, as is reducing stocking density.
Ensuring high-value colostrum is available to newborn calves, and that a strategic vaccination programme is in place, may also reduce the numbers of susceptible animals within a herd. The core aim of any control programme is to prevent foetal level infection and should be combined with the control of other infections which can lead to losses from reproductive complications.
As with controlling any virus on a farm, there needs to be a strict policy of culling infected animals, use of vaccination where appropriate, and a robust biosecurity strategy that all staff follow to manage any BVD spread.
There are two categories of vaccine available for BVD treatment. Killed virus (KV) vaccines are expensive and require more than one dose during the immunisation process. However, these are more easily handled than modified live virus (MLF) vaccines, which are susceptible to deactivation when subjected to changes in temperature or with contact to certain chemicals. Liaison with your veterinary surgeon is paramount to ensure your treatment regime has the best possible chance of success.