EFFECTS OF AGE AT VACCINATION ON EFFICACY OF BRUCELLA-ABORTUS STRAIN RB51 TO PROTECT CATTLE AGAINST BRUCELLOSIS

Objective-To establish that female calves vaccinated with Brucella abo rtus strain RB51 at 3, 5, and 7 months of age are protected against in fection and abortion when challenged exposed during their first pregna ncy. Animals-Polled Hereford heifer calves obtained from a brucellosis -free herd. Procedure-Calves were inoculated SC at 3, 5, or 7 months o f 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 cha llenged exposed during the first pregnancy with virulent B abortus str ain 2308. Results-After vaccination, none of the heifers given strain RB51 developed serum antibodies that reacted in the standard tube aggl utination test, but reacted in a dot-blot assay, using RB51 antigen. B abortus was cultured from biopsy specimens of superficial cervical ly mph nodes in the RB51 and S19 vaccinates at 10 weeks, hut not at 12 we eks after vaccination. All 4 heifers that had been vaccinated with RB5 1 at 3 months of age were protected against infection and abortion whe n challenged exposed. Vaccination at 5 and 7 months of age gave equiva lent protection. Heifers given strain 19 were 95% protected and contro ls (given saline solution) had a high incidence of infection and abort ion. Conclusions-Strain RB51 is protective at doses comparable to thos e of strain 19 in calves 3 to 10 months of age. Clinical Relevance-lmm unogenicity and failure to induce antibodies that interfere with the s erologic diagnosis of field infections of B abortus make strain RB51 a n effective vaccine.