Part 8 Basic Facts About Bvd
This section contains veterinary information on BVD. This has been produced by SAC, and is included in the online CPD ( www.gov.scot/bvd). There is also great deal of useful information including videos, presentations and podcasts on BVD on the Royal Veterinary College's website at: www.rvc.ac.uk/bvd.
BVDV is closely related to border disease virus ( BDV) of sheep and to classical swine fever virus ( CSFV). The three viruses are grouped together as pestiviruses. Pestiviruses do not infect humans.
BVDV can infect goats, sheep, South American camelids and pigs. There is serological evidence of infection in wild ruminants in Scotland and beyond. Furthermore, the pestivirus of sheep, BDV, can infect cattle and result in the generation of persistently infected cattle. In Scotland the close contact between sheep and cattle that occurs on many farms creates the opportunity for BVDV to infect sheep and BDV to infect cattle and while the frequency with which this occurs is unknown, it has been considered unlikely to be of significance in relation to national control. To mitigate this potential risk breeding cattle should not graze in the same field as sheep except in the extensive hill situation.
Persistent infection ( PI) with BVDV is the mechanism that allows this virus to spread and persist within a population of cattle. The PI animal is a potent source of infection releasing the virus in secretions from the respiratory, digestive and reproductive tracts. The generation of PI animals can only occur where infection of the dam occurred some time during the first trimester, crossed the placenta and infected the conceptus prior to the onset of the development of its immune system.
Transient infection can have a significant negative impact on the fertility of naive cattle, but transiently infected animals are much less infectious to others than PI cattle. In bulls infected transiently the virus can be released in the semen for a limited time. In all but a few exceptional cases this is only for a short period of time, but the semen of some bulls remains infectious while the bull itself is no longer infected.
Transient infection also has a prolonged impact on an animal's ability to resist the normal diseases of calfhood and it is this immunosuppressive effect that is considered to be of greatest financial significance to the cattle industry.
Vaccination on its own will not eradicate BVD from a herd or from the country. However, vaccination is and will continue to be an important tool in disease control for many, if not most, herds. BVDV vaccination is designed to protect dams in early pregnancy to avoid the production of PI cattle. Vaccination is also useful to protect susceptible cattle before they go through a mart. A decision on whether, what and when to vaccinate is one to be taken by the vet and keeper together. In considering whether to vaccinate you should consider the BVD infection risk factors for the herd involved, such as:
- Are there neighbouring holdings with cattle?
- Is there nose-to-nose contact with neighbouring cattle?
- Does the keeper buy-in cattle or is it a closed herd?
- Are bought-in cattle of known BVD status?
- Are cattle sent to shows?
- Are bulls brought in for breeding, and is their BVD status known?
The vaccines available are very effective, but care must be taken to ensure that the datasheet is followed precisely. This means storing the vaccine correctly and using the right doses, at the right time for that vaccine, including boosters.
Consideration should be given to scheduling check-test sampling and vaccination to ensure that vaccination does not lead to a failure in the check-test.
PI animals shed huge amounts of virus and present a significant challenge even to vaccinated animals. Inadequately vaccinated animals in contact with PI cattle are at significant risk.
Farmers can reduce the risk of buying in cattle infected with BVD if they recognise the BVD status of cattle being offered for sale. They need to know the BVD status of the seller's herd plus any tests done on the animal being sold. Buying pregnant cattle is always risky since the BVD status of the calf is unknown and can only be tested when it is born.
Summary of Key Points