IMMUNE-RESPONSES AND PROTECTION AGAINST INFECTION AND ABORTION IN CATTLE EXPERIMENTALLY VACCINATED WITH MUTANT STRAINS OF BRUCELLA-ABORTUS

Twenty-four 10-month-old Polled Hereford heifers were inoculated SC wi th live cells of one of the following strains of Brucella abortus: S19 DELTA31K (n = 4), S19DELTASOD (n = 4), RB51 (n = 4), and strain 19 (n = 6); controls (n = 6) were given saline solution. Heifers given the d eletion mutants S19DELTA31K and S19DELTASOD, and those given strain 19 developed antibody responses to B abortus and cutaneous reactions to brucellin. Heifers given strain RB51 did not develop antibodies that r eacted in the standard tube agglutination test, but sem reacted in tes ts, using an antibody dot-blot assay containing RB51 antigen. The S19D ELTA31K and S19DELTASOD strains of B abortus isolated from lymph node tissue after vaccination did not differ genetically from the master st ock strain. All heifers were bred naturally at 16 to 17 months of age, and were challenge-exposed intraconjunctivally with virulent B abortu s strain 2308 during the fifth month of pregnancy. All vaccinated heif ers were protected (ie, none aborted and none had B abortus isolated f rom their tissues after parturition). Calves born from vaccinated dams were free of B abortus. Antibody responses in heifers after challenge exposure were an indicator of immunity. All 5 control heifers (nonvac cinated) developed serum antibodies after challenge exposure; 3 aborte d, and 1 delivered a small, weak calf at 8.5 months of gestation. Thus , live mutant strains of B abortus can induce protective immunity when given at 10 months of age, and strain RB51 is a strong candidate for further testing.