Selection of a Variant of Western Encephalitis Virus of Low Pathogenicity for Study as a Live Virus Vaccine *

Abstract

Summary Western encephalitis virus strains obtained from naturally infected birds by inoculation into chick embryos, young chicks and hamster kidney tissue culture were subcultured by various methods in an effort to develop variants of low pathogenicity. Prolonged passage of the virus in chick embryos and chicks did not reduce the neuro-pathogenicity of the virus for mice and hamsters. A variant of low pathogenicity was obtained by cloning on chick embryo cells. Clones were at first selected on the basis of pathogenicity of the progeny of the cloned virus for young adult mice inoculated intracerebrally. Later clonal selections were done on the basis of the relative average survival time (in days) (AST) in infant mice inoculated intracerebrally and intraperitoneally, looking for variants having a long AST or the same AST for both routes of inoculation. The clone 15 variant was selected for study as a live virus vaccine because the neurotropic character of the virus was either markedly reduced or eliminated and animals vaccinated with the virus were immune to subsequent intracerebral challenge with the natural virus as obtained from wild birds.