The efficacy of an inactivated IBR vaccine in the prevention of intra-uterine infection and its use in a disease-control programme.

The effects of administration of an established, inactivated IBR vaccine were studied in 30 cows from two herds (one seropositive and one seronegative). All acquired immunity which, after subsequent intratracheal infection with IBR virus, prevented development of symptoms in the cows and protected their foetuses against viral infect-ion in utero. The calves were all healthy and were born at normal term. Ten non-vaccinated cows from the seronegative herd responded to the experimental infection with mild respiratory disease and abortion of four out of 10 foetuses. Organs from the aborted foetuses were found to have IBR virus. In a breeding herd, without clinical signs of disease but with 40 % of cows tested as seropositive, a 2-year disease-control programme was initiated. A total of 234 newborn calves were examined and it was shown that immunization of their darns with an inactivated vaccine conferred full in utero protection against IBR-virus infection. When such calves are reared in isolation they can be used as the nucleus for a seronegative breeding herd.