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 incidence 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.