BVD control success and pitfalls
Bovine virus diarrhea (BVD) has been first described as an inconspicuous transmissible diarrhea of cattle in 1946. The infection is caused by a small RNA virus (BVDV) with a relatively high mutation rate. Two genotypes exist, BVDV-1 and BVDV-2. Today BVD is considered to be the economically most important disease of cattle. The acute postnatal infection is usually not recognized by the farmer since clinical signs are mild. However, the transient immunosuppression opens the way for secondary infections of the respiratory or enteric tract. As a result herd health declines and considerable damage occurs. In very rare cases highly virulent variants of BVDV, usually BVDV-2, cause acute postnatal infections with high mortality. Outbreaks of these virulent variants are usually self-limiting.
Major damage is done by the interference of BVDV with fertility. At any time during pregnancy the virus is able to cross the placenta of susceptible pregnant cattle, thus producing a wide range of damage: Infections during early pregnancy may result in the birth of persistently infected (PI) calves. Later on infections may result in different malformations, abortions and stillbirths. Infections during the last trimester of pregnancy result in the birth of normal calves with precolostral antibodies against BVDV. 90 percent of PI animals die within their first two years of life. The rest may survive for years. At any time PI animals shed large amounts of virus, thus serving as reservoirs ensuring virus persistence in the bovine population. In summary BVD is a very stealthy disease that often escapes the attention of farmers and veterinarians.
It took a long time until it was recognized that removal of PI animals is the central element of an efficient BVD control. Removal of PI cattle must be accompanied by strict biosecurity measure. There are two basic concepts of BVD control:
Systematic control (usually state driven):
a) Initial serological screening and classification of the entire population in non-infected and infected herds in order to identify infected herds (Bulk milk or young stock). PI animals are identified and removed. The status of the herds will be monitored continuously.
b) Voluntary control:
This approach is suitable in countries where no systematic BVD control program is in place. Participating farmers first check their herd status either by testing bulk milk serology or young stock. When the results indicate that there is an active BVD infection in the herd a search for PI animals is started. PI cattle must be removed and the herd has to be monitored regularly. In order to protect the herd from re-introduction of BVDV it is strongly recommended to vaccinate the herd regularly.