Intranasal vaccination of pigs against Aujeszky's disease.  4.  Comparison with one or two doses of an inactivated vaccine in pigs with moderate maternal antibody titres. 
Intranasal (IN) vaccination of pigs with low levels of maternally-derived antibody (MDA) has previously been shown to confer good protection against challenge with virulent Aujeszky's disease virus (ADV).  The objective of the present study was to determine the efficacy of IN vaccination with an attenuated ADV, in comparison with that of an inactivated vaccine given parenterally, in pigs with higher MDA titres at the time of vaccination.  In one experiment, vaccinations were done at 6 weeks of age, and in another experiment pigs were vaccinated at 4 and/or 9 weeks of age.  Two months after (the last) vaccination pigs were challenged intranasally with a virulent ADV.  Protection was evaluated on the basis of mortality, periods of growth arrest, fever and virus shedding after challenge.  The presence of MDA markedly depressed the serum-neutralizing antibody response after vaccination.  Sensitisation occurred after parenteral vaccination with an inactivated vaccine despite high MDA levels.  Although the intranasally-vaccinated pigs had lower levels of neutralizing antibody at the time of challenge, they were significantly better protected than pigs given 1 or 2 doses of the inactivated vaccine.  Comparing the present results with those of a previous study, it appears that the efficacy of parenteral as well as intranasal ADV vaccination decreases with increasing levels of MDA at the time of vaccination.