Comparison of the efficacy of Brucella abortus strain RB51 and Brucella melitensis Rev. 1 live vaccines against experimental infection with Brucella melitensis in pregnant ewes

To test the efficacy of rough Brucella strain vaccines in sheep, a vaccine recently developed in cattle (Brucella abortus strain RB51) was assessed in comparison with the conventional Rev. 1 vaccine. Forty-five ewes from twel ve to fourteen months of age, from brucellosis-free flocks, were allotted t o three groups of fifteen ewes each. Group one was vaccinated by the conjun ctival route with 1.73 x 10(8) colony forming units (CFU) of Rev. I vaccine . Group two was vaccinated subcutaneously with 11 x 10(9) CFU of RB51 vacci ne and group three was considered as a control. All sheep were challenged a t two to three months of gestation with 5 x 10(7) CFU of virulent B. melite nsis H38. Vaccination with RB51 vaccine did not result in the production of any antibodies against the 0-side chain of lipopolysaccharide, as measured by conventional serological tests (Rose Bengal plate test and complement f ixation test). Protection of sheep against abortion and excretion of virule nt Brucella strain in vaginal fluid, aborted foetuses and/or non viable lam bs at parturition and abortion was significantly lower than that afforded b y Rev. 1 vaccine. The difference compared to the control group was not sign ificant. Data from this study suggest that the RB51 vaccine used for cattle vaccinat ion does not provide effective protection of sheep against abortion induced by B. melitensis.