Estimation of herd incidence of infection with bovine virus diarrhoea virus (BVDV) in herds previously without animals persistently infected with BVDV.

On a previous occasion, all animals in 9 herds had been bled for bovine virus diarrhoea virus (BVDV) and antibodies. No animals persistently infected (PI) with BVDV were detected. Three years later 10 animals in each herd were tested again. By this time 60 out of 90 previously seronegative animals had seroconverted. Seroconversions had occurred in 8 of the 9 herds corresponding to a incidence risk of herd infection of 0.52 per year. The incidence risk of seroconversions in individual animals was 0.31. Examination of young stock for antibodies and determination of antibody titer in bulk milk were good indicators for ongoing infections in the herds. The number of herd infections seemed to be higher than could be explained from purchase of PI animals.