INFLUENCE OF PASSIVE-IMMUNITY ON PIG IMMUNIZATION WITH DELETED AUJESZKYS-DISEASE VACCINES MEASURED BY THE AMOUNT OF WILD VIRUS EXCRETION AFTER CHALLENGE

Four attenuated glycoprotein I deleted Aujeszky's disease virus (ADV) vaccines were compared on the basis of their ability to induce immunity in the presence of passive antibodies. The relative severity of clinical disease and amount of viral excretion following experimental challenge with virulent ADV were determined among groups of eight pigs that were unvaccinated or vaccinated with one of the four ADV vaccines. Vaccinated pigs received two vaccine doses, the first administered when passively acquired serum antibodies were still detectable at 10 weeks of age, and the second four weeks later. The experiment was divided into two trials, with vaccinated and unvaccinated control groups in each trial. Challenge with virulent ADV took place at 18 weeks of age for the first lot and 19 weeks of age for the second. Differences among the vaccines were observed with regard to clinical protection and viral excretion. Virulent virus was excreted by most of the vaccinated pigs from two to seven days after challenge. In the case of two of the vaccines, no virus excretion was detected in several of the pigs. It was confirmed that mean serum neutralizing titers at challenge are inversely associated with amount of viral excretion post-challenge. Difficulties in the standardization of vaccine trials with passive antibodies were discussed.