Evaluation of a Formalin-Inactivated Rift Valley Fever Vaccine in Sheep

Abstract

SUMMARY A formalin-inactivated Rift Valley fever ( rvf ) vaccine prepared in cell culture for human use was immunogenic in sheep. Vaccine was administered as a single dose of diluted (1:5) or undiluted vaccine with or without an adjuvant. Serum-neutralizing antibodies induced by rvf vaccine persisted for at least 7 months. Seven of 11 vaccinated sheep with prechallenge plaque-reduction neutralization ( prn 80 ) antibody titers of ≤ 10 were protected against challenge exposure with 10 6 plaque-forming units of Zagazig 501 strain of rvf virus. Challenge exposure induced abortion in 2 of 2 pregnant sheep. Five sheep with prn 80 titers ≥ 1:20 were protected from detectable viremia after challenge exposure. Additionally, 5 of 6 lambs (3 months old) were protected (by maternal antibodies) against challenge exposure. Challenge control sheep developed clinical disease and detectable viremia after exposure. Virus was isolated from saliva of 1 challenge control sheep and virus was transmitted by contact exposure to 1 of 4 seronegative contact-control sheep. Immunization of sheep with formalin-inactivated rvf vaccine induced a priming effect against rvf viral antigens. Challenge exposure with rvf virus resulted in significantly higher neutralizing titers in vaccinated sheep than in nonvaccinated sheep.