Pathogenicity of different strains of Rift Valley fever virus in Swiss albino mice.

Laboratory mice were inoculated with 3 different strains of Rift Valley fever virus (RVFV): namely, the prototype, the Nigerian, and the Lunyo variant strains of RVFV. Animals were inoculated with either infective mouse brain or serum by the i.c. or i.p. route and organs of inoculated animals examined for virus content and histopathological changes. Animals inoculated with either the classical or the Nigerian encephalitis only, to a ditropism involving mild lesions in the brain and liver or the typically severe RVFV hepatitis. The type of lesion shown was dependent on the source of inoculum and the route of inoculation. On the other hand, animals inoculated by the i.c. route with the infective brain material of the Lunyo variant virus showed a mild encephalitis, while the use of the i.p. route or infective blood source of the Lunyo virus resulted in the development of typical RVFV hepatitis. Typical intranuclear eosinophilic inclusion bodies were found in both the brain and liver of animals inoculated with the Lunyo variant virus. These inclusion bodies were found only in the livers of mice inoculated with the infective blood material of either the prototype or the Nigerian strain. There was no correlation between the virus titres in the different organs and the severity of histopathological lesions.