Effect of vaccinal and passive immunity on experimental infection of pigs with porcine parvovirus.

One group of 5 pigs was vaccinated twice at 6 and 8 weeks of age with an inactivated porcine parvovirus (PPV) vaccine. A 2nd group of 5 pigs was passively immunized at 9 weeks of age, and a 3rd group of 5 pigs served as nonimmunized controls. The immunity of all pigs was challenged with live PPV at 10 weeks of age. Four vaccinated pigs with hemagglutinating-inhibiting (HI) antibody titer of 160 to 640 and 3 passively immunized pigs with HI titer of 80 to 160 were resistant to PPV infection. Neither infective virus nor viral antigen was detected in their tissues. However, virus replicated in tissues of 1 vaccinated and 2 passively immunized pigs with HI titers of 80, and viral antigen was detected in their lymphoid tissues. There was a 4-fold increase of HI titer in 1 passively immunized pig. Viremia and viral excretion were not detected in any of the vaccinated or passively immunized pigs. In contrast, PPV replicated in the tissues of nonimmunized pigs; both virus and viral antigen were detected in their tissues, and they produced HI antibodies to PPV. All nonimmunized pigs developed viremia and shed virus in oral secretions and feces. Our data show that a high level of humoral antibodies is required for protection from PPV infection.