Safety and Immunogenicity of Rift Valley Fever MP-12 and a Novel arMP-12?NSm21/384 Recombinant Vaccine Candidate in Native Breed of Black Head Sheep (Ovis aries) from Tanzania

Objective: Rift Valley fever (RVF) is a mosquito-borne zoonotic viral disease that affects humans and ruminants in Africa and the Arabian Peninsula. Efforts to develop effective vaccines have had limited success. Therefore, the aim of this study was to evaluate the safety and immunogenicity of RVF MP-12, and arMP-12ΔNSm21/384 vaccine candidates in sheep. Methods: One group of 6 sheep was vaccinated intramuscularly (IM) each with one ml of 1×105 PFU/ml of the RVF MP-12 and 9 sheep were inoculated IM each with one ml of the arMP-12ΔNSm21/384 vaccine candidate, and two control sheep received one ml each of only Eagle's Minimum Essential Medium. Blood samples obtained on days 14 and 0 before vaccination and on days 3, 4, and 5 post vaccination (PV) were tested for RVFV in Vero cells and by RT PCR assay, and samples collected at interval PV through day 87 and on days 7, 14 and 21 following revaccination on day 87 PV to test for RVFV neutralizing antibody response by the plaque reduction neutralization test. Results: All animals, including the controls remained in good health during the PV period as supported by normal body temperature, and the absence of clinical manifestations throughout this study. A viremia was not detected in any of the animals. Six of 6 animals that received the RVF MP-12 and 8 of 9 animals that received the arMP-12ΔNSm21/384 had antibody titers that ranged from 1:10 on day 5 PV to as high as 1:40 to 1:160 through day 87 PV. The antibody titers for these 15 animals following revaccination on day 87 PV with 1×104 PFU/ml of the MP-12 vaccine increased rapidly and by day 21 PV the titers for most animals ranged from 1:160 to 1:640. Conclusion: Overall, these findings based on a limited number of sheep indicated that both the MP-12 and the arMP-12ΔNSm21/384 are promising vaccine candidates for the prevention of RVF in sheep in Africa.