Development of combination vaccine for prophylactic immunization of domestic ruminants against Rift Valley Fever and pulpy kidney.

Abstract

Rift Valley Fever (RVF) and pulpy kidney (PK) are two severe veterinary diseases of domestic ruminants that have a negative impact on the sheep industry. Control and prevention of these diseases depend on strict adherence to vaccination programs. However, epidemics of RVF occur at irregular intervals, leading to periodic vaccination programs that are implemented following emergency warnings of extensive heavy rainfalls and floods. The present vaccination strategy has been consistently reported to result in low vaccination of RVF coverage across Africa and the Arabian peninsula. This approach delay the vaccination of livestock and result in a significant number of animals not developing protective immunity before the onset of infection. These challenges highlight the urgent need for an alternative effective vaccination strategy for livestock in endemic areas to improve the RVF vaccination coverage. In contrast, the epsilon toxoid has been widely used to immunise livestock against PK. The current research aimed to develop a safe and immunogenic RVF/PK combination vaccine as a strategy to improve vaccination coverage against RVF in endemic regions. The first objective of the study was to evaluate safety and efficacy of the inactivated RVF Smithburn (SB) vaccine as one of the oldest products widely used for immunisation of ruminants against RVF. The SB vaccine has a residual pathogenic effect, which limits its use to non-pregnant animals. The live-attenuated RVFV SB strain was utilized for developing an inactivated vaccine against RVF. The BEI-inactivated RVFV SB vaccine candidate was proven to be safe and elicited an immune response in merino sheep. Consequently, the inactivated RVFV SB was used for formulation of a combination vaccine product with epsilon toxoid at predetermined concentrations. The RVF/PK combination vaccine was evaluated for safety and immunogenicity in a clinical trial with OBP-commercial inactivated RVFV and alumformulated PK vaccines serving as positive controls. The results demonstrated that the RVF/PK combination vaccine was safe for use in sheep, and no clinical signs against RVF or PK infections were observed following primary and secondary injections. The RVF/PK combination vaccine elicited neutralizing antibodies at levels comparable to those of commercial monovalent products for RVF and PK. These antibody levels remained high throughout the duration of the study. These findings suggest that the RVF/PK combination vaccine may be a possible alternative strategy for improving vaccination coverage against RVF.