The influence of maternal immunity on the transmission of pseudorabies virus and on the effectiveness of vaccination.

The purpose of this study was to investigate whether maternal immunity could prevent transmission of pseudorabies virus (PRV) among pigs, and whether it reduced the effectiveness of a single or double vaccination with regard to the transmission of PRV. In five experiments, the transmission of PRV, expressed as the reproduction ratio R, was compared in groups of pigs with maternal immunity and in groups of pigs without maternal immunity. Transmission of PRV among unvaccinated pigs with maternal immunity (R = 0.2) was significantly lower than among pigs without maternal immunity (R = 6.3). Furthermore, maternal immunity in young pigs prevented transmission of PRV, as R was significantly below one. In once-vaccinated groups, PRV spread extensively among pigs with maternal immunity (R = 23), but did not spread extensively among pigs without maternal immunity (R = 0.6). In twice-vaccinated groups, transmission of PRV among pigs with maternal immunity (R = 0.6) did not differ significantly from the transmission of PRV among pigs without maternal immunity (R = 0.3). Thus, a single vaccination of pigs with PRV strain 783 at 10 weeks of age, when they still possessed maternal immunity, seemed not sufficient to prevent transmission of PRV. Virus transmission could be reduced, however, if maternally immune pigs were vaccinated twice at 10 and 14 weeks of age.