Further evaluation of a mutagen-attenuated Rift Valley fever vaccine in sheep

A previous study demonstrated that a mutagen-attenuated Rift Valley fever virus (RVFV) vaccine, RVF MP-12, was immunogenic and non-abortogenic when ewes, 90 110 days pregnant, were inoculated with 5 × 10⁵ plaque-forming units (p.f.u.) of the virus strain. The ewes delivered live, healthy lambs that had no neutralizing antibody to RVFV until after they had ingested colostrum. To assess further the safety and protective capability of this candidate vaccine, six pregnant ewes were inoculated with 5 × 10³ p.f.u. of RVF MP-12 and challenged with 5 × 10⁵ p.f.u. of virulent ZH-501 strain of RVFV 30 days later. No viraemia was detected after vaccination or challenge and all six ewes delivered live, healthy lambs. Those lambs tested before their nursing did not have neutralizing antibody to RVFV but quickly acquired antibody titres of 1:320 to ≥ 1:10 240 after ingesting colostrum. To test the safety of the RVF MP-12 immunogen in neonates, lambs ≤ 7 days old, born to unvaccinated ewes, were inoculated with 5 × 10⁵ p.f.u. of RVF MP-12. With the exception of brief pyrexia in 18 of 26 lambs, and a transient low-titred viraemia in 16 of 26 lambs after inoculation, no untoward effects were observed. Serum-neutralizing antibody to RVFV was detected 5-7 days after inoculation. Lambs vaccinated with either 5 × 10⁵ or 5 × 10³ p.f.u. of RVF MP-12 were protected against virulent RVFV challenge at 14 days postvaccination.