Immunization with S19Δ Conferred Protection in Water Buffaloes against Virulent Challenge with Strain S544.

Vaccination of cattle and buffaloes with strain 19 has been the mainstay for control of bovine brucellosis. However, vaccination with S19 suffers major drawbacks in terms of its safety and interference with serodiagnosis of clinical infection. S19∆, a perosamine synthetase B gene deletion mutant, overcomes the drawbacks of the S19 vaccine strain. The present study aimed to evaluate the potential of S19Δ vaccine candidate in the natural host, buffaloes. Safety of S19∆, for animals use, was assessed in guinea pigs. Protective efficacy of vaccine was assessed in buffaloes by immunizing with normal dose (4 × 10 colony forming units (CFU)/animal) and reduced dose (2 × 10 CFU/animal) of S19Δ and challenged with virulent strain of S544 on 300 days post immunization. Bacterial persistency of S19∆ was assessed in buffalo calves after 42 days of inoculation. Different serological, biochemical and pathological studies were performed to evaluate the S19∆ vaccine. The S19Δ immunized animals showed significantly low levels of anti-lipopolysaccharides (LPS) antibodies. All the immunized animals were protected against challenge infection with S544. Sera from the majority of S19Δ immunized buffalo calves showed moderate to weak agglutination to RBPT antigen and thereby, could apparently be differentiated from S19 vaccinated and clinically-infected animals. The S19Δ was more sensitive to buffalo serum complement mediated lysis than its parent strain, S19. Animals culled at 6-weeks-post vaccination showed no gross lesions in organs and there was comparatively lower burden of infection in the lymph nodes of S19Δ immunized animals. With attributes of higher safety, strong protective efficacy and potential of differentiating infected from vaccinated animals (DIVA), S19Δ would be a prospective alternate to conventional S19 vaccines for control of bovine brucellosis as proven in buffaloes.