Bovine viral diarrhea virus infection in a dairy herd with high prevalence of persistently infected calves.

A dairy herd including approximately 50 milking cows and 40 heifers and calves was investigated. This herd was detected with high prevalence of calves persistently infected (PI) with bovine viral diarrhea virus (BVDV). Nine PI animals including a milking cow and 8 newborn calves were detected in the herd within 4 months. Prevalence of PI animals in this herd was estimated 7.0% which was very high compared to that estimated in previous reports. All newborn PI calves were strongly suspected to have a single origin of infection as estimated from the homology of the virus genes. The cause of high prevalence could not be clarified. Removal of PI animals and continuous examination of newborn calves were important for the elimination of BVDV from the herd.