Ability of a mutagenized virus variant to protect young lambs from Rift Valley fever.

A live attenuated vaccine virus variant of Rift Valley fever (RVF) virus was developed by passaging a human isolate in tissue culture under the influence of the mutagen 5-fluorouracil. This virus variant (MV P12) has been assessed in this study as to its suitability as a vaccine, by testing its pathogenicity in young lambs and measuring its ability to induce a protective immune response. Even high doses of the vaccine virus failed to induce any of the clinical or histopathologic changes associated with classical RVF virus infection. Although the vaccine induced mild pyrexia when given in high doses, viremia was not induced. Neutralizing antibody and a protective immune response was elicited with even low doses of vaccine virus. These data, along with data of other workers on the lack of abortigenicity of this virus variant, indicate that the MV P12 variant of RVF virus is an excellent candidate for a safe and effective vaccine against RVF.