VACCINATION OF NEWBORN PIGS IN THE PRESENCE OF LOW-LEVELS OF PSEUDORABIES COLOSTRAL ANTIBODIES

To test the hypothesis that newborn pigs with pseudorabies virus (PRV) colostral antibodies might be actively immunized with a PRV glycoprotein gIII-deleted vaccine (Omnimark-PRV), 23 piglets were obtained from four sows that had been immunized 4 weeks and 2 weeks before farrowing with this vaccine. Thirteen piglets were immunized with Omnimark-PRV when they were less than 3 days old and ten piglets served as non-vaccinated controls. Piglets were weaned at 28 days of age and challenged with virulent PRV(Shope) when they were 49 days old, at which time the vaccinated and control pigs were seronegative for PRV virus neutralizing (VN) and gIII antibodies, and all control pigs and ten vaccinees were seronegative for PRV antibodies by the latex agglutination test (LAT). Two vaccinees were LAT(+) and one was LAT(+/-). Central nervous system signs and/or respiratory disease signs were observed in six of ten control pigs with the death of one control, while two of 13 vaccinees showed only very mild and transient clinical disease signs and there were no fatalities. Non-vaccinees lost weight until postchallenge day (PCD) 6 and did not regain prechallenge weight until PCD 8. All vaccinees gained weight after challenge and at PCD 11 had mean weight gains nearly twice that of the controls. On PCD 11, the geometric mean titre for VN antibodies of non-vaccinees was 9.3, while that of vaccinees was 49.0, indicating that the vaccinated group had been immunologically primed.