The duration of antibodies against bovine virus diarrhoea virus in bulk milk.

Fifty-eight dairy herds, suspected to be recently infected with bovine virus diarrhoea virus (BVDV) due to a rise in BVDV antibodies in bulk milk, were followed over a two-year period. In 34 (59%) of these 58 herds (Group 1), pooled milk samples from heifers or pooled blood samples from calves were negative for BVDV antibodies. In this group as many as 53 and 76% of the herds again had antibody-negative bulk milk one and two years after the positive sample, respectively. Of the remaining herds, 5 and 17% had negative samples after one and 2 years respectively. Possible explanations for the limited duration of antibodies against BVDV in bulk milk are discussed. In 65% of the herds in Group 1, animals had been purchased and introduced into the herd, or the herds had been exposed to other forms of contact representing a risk of infection with BVDV. In the remaining 35% of the herds in this group, no explanation for the rise in BVDV antibodies in the bulk milk could be found. In this study the introduction of seropositive animals into the herd, and infection inducing seroconversion in one or more animals as the only result, seem to be the most probable explanations for the rise in antibody levels observed in Group 1.