Cattle remain antibody positive for years after exposure to BVD virus so to determine if there is evidence of recent exposure to BVD virus in a herd it is the young stock that should be tested.
· Five from each separate group aged 9-18 months old should be blood sampled for the BVD antibody check test
To be considered as a group the cattle must have been mixing for at least two months. If there has been no mixing between groups or fields of cattle then only testing a sample of animals from one group will not necessarily reflect the situation in another group. If the cattle have been together for less than two months then ten young stock from each group should be tested.
Maternally derived antibodies from colostrum can interfere in the interpretation of results in calves up to nine months old. If calves are sold before this age then testing should be done on six to nine month old calves instead but the older they are the better. It is usually possible to determine if antibodies detected in this age group are likely to be due to maternally derived antibody from colostrum or from natural exposure from virus but it is not always clear cut. Antibody positive animals may need to be retested to demonstrate that the antibodies are from colostrum i.e. they have declined and that they are not due to virus exposure.
The alternative option, if calves are being sold before nine months old, is to test all calves that are born into the herd for virus (using blood or else ear tissue collected using specially designed ear tags).
If calves have been vaccinated with a respiratory vaccine containing a BVD component then sampling should be left until at least three months after completion of the vaccination course.
The use of BVD vaccine does not interfere with the ability to detect PIs in a herd.