Effects of age at vaccination on efficacy of Brucella abortus strain RB51 to protect cattle against brucellosis.

ABSTRACT

OBJECTIVE: To establish that female calves vaccinated with Brucella abortus strain RB51 at 3, 5, and 7 months of age are protected against infection and abortion when challenged exposed during their first pregnancy. ANIMALS Polled Hereford heifer calves obtained from a brucellosis-free herd. PROCEDURE Calves were inoculated SC at 3, 5, or 7 months of age with strain RB51 (n = 26), strain 19 (n = 16), or sterile saline solution (n = 15). Calves were bred at 16 to 17 months of age and challenged exposed during the first pregnancy with virulent B abortus strain 2308. RESULTS: After vaccination, none of the heifers given strain RB51 developed serum antibodies that reacted in the standard tube agglutination test, but reacted in a dot-blot assay, using RB51 antigen. B abortus was cultured from biopsy specimens of superficial cervical lymph nodes in the RB51 and S19 vaccinates at 10 weeks, but not at 12 weeks after vaccination. All 4 heifers that had been vaccinated with RB51 at 3 months of age were protected against infection and abortion when challenged exposed. Vaccination at 5 and 7 months of age gave equivalent protection. Heifers given strain 19 were 95% protected and controls (given saline solution) had a high influence of infection and abortion. CONCLUSIONS: Strain RB51 is protective at doses comparable to those of strain 19 in calves 3 to 10 months of age. CLINICAL RELEVANCE Immunogenicity and failure to induce antibodies that interfere with the serologic diagnosis of field infections of B abortus make strain RB51 an effective vaccine.