Indication of transmission of BVDV in the absence of persistently infected (PI) animals.

In a closed dairy herd all animals were tested serologically for BVD antibodies twice a year during a 6-year period. Seroconversions were detected every year. At the start of the 6-year monitoring period blood samples from all animals were examined by virus isolation. No persistently infected animals were identified. Entire-herd culturing for BVDV was repeated at the end of the third year. Samples from all newborn female calves were examined for BVDV at approximately 2 months of age and older. During the entire monitoring period BVDV was isolated in one newborn calf twice with an interval of 3 weeks. The mother had seroconverted during pregnancy. Five congenitally infected non-PI calves were identified, the mothers of which had seroconverted during late pregnancy; repeated sampling proved the calves to remain seropositive in a seronegative age cohort. Although direct and indirect introduction of BVDV from outside the herd can never be excluded it seems highly unlikely in this closed herd. The findings indicate that transmission of BVDV can take place over a long period of time in the absence of PI animals. This observation may have serious consequences for control programmes.