It may not be necessary to blood sample every animal in the herd when looking for PIs. As a PI cow will always produce a PI calf then if the calf is not a PI its dam will not be a PI. Therefore if there are good records of which calf belongs to which cow then the initial testing can be restricted to young stock up to calving age, plus the bulls and any cows that do not have an offspring that can be tested (e.g. if they were barren or aborted or if their calf died).
Calves can be blood tested from one month of age or they can be tested at any age using ear tissue collected with the specially designed ear tags. An animal cannot become a PI after it is born so the result is valid for its life.
If any virus (also referred to as antigen) positive animals are identified they should be retested for BVD antigen after at least 21 days to confirm if they are PI animals in case they were transiently infected with virus. The dam of any PI should also be tested for BVD antigen/virus. Only about 7% of PIs are born from PI dams – in the vast majority of cases their dams have been naïve to virus and have been exposed to it for the first time in the first third of pregnancy.
Samples can be pooled for testing for virus by PCR.
After the PIs have been removed the next stage is to test all calves born for antigen/virus for 12 months after the last PI animal/s was removed. If no further PIs are identified a check test can be done when the young stock reach nine months old to confirm if the herd is free of active infection.