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 immunit y in the presence of passive antibodies. The relative severity of clin ical disease and amount of viral excretion following experimental chal lenge with virulent ADV were determined among groups of eight pigs tha t were unvaccinated or vaccinated with one of the four ADV vaccines. V accinated 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 i nto two trials, with vaccinated and unvaccinated control groups in eac h 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 t he 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 vac cines, no virus excretion was detected in several of the pigs. It was confirmed that mean serum neutralizing titers at challenge are inverse ly associated with amount of viral excretion post-challenge. Difficult ies in the standardization of vaccine trials with passive antibodies w ere discussed.