For dairy herds, in addition to a check test of young stock (ten aged 9-18 months old), the bulk tank milk should be tested for BVD antibody. If it is positive for antibodies the next stage is to test milk from the first lactation heifers. Milk should be pooled from these animals and tested for antibodies.
A bulk milk sample can be tested for BVD virus by PCR test. This test is very sensitive and will detect the presence of a PI where there are up to 400 cows contributing to the bulk tank. The dry cows and any cows not contributing to the bulk tank (e.g. where milk is being withheld due to mastitis treatment) would need to be blood sampled individually. Alternatively the bulk milk virus PCR test could be repeated in a couple of months.
If the bulk milk sample is positive for virus by PCR testing the next stage is to identify the PIs. This can be done in steps as the PIs are more likely to be amongst the younger animals in the herd and in the lowest yielders. Therefore blood sampling for virus can initially be done on the first and second lactation cows and the lowest 10% by yield. If a PI animal is not identified in the younger cows and lowest yielders then individual sampling of older cows for virus is required.
If a virus positive animal is identified it should be retested after a minimum of three weeks for BVD antigen to determine if it is persistently infected. If a PI is identified it should be removed from the herd. Its dam should be tested, if still in the herd, and the bulk tank milk BVD virus PCR test repeated after a minimum of three weeks to determine if all PIs have been removed. The calves and immature stock would also need to be tested for virus.