Four mutants of Semliki Forest virus (SFV) strain L10 showing altered virulence in weanling mice were selected following mutagenesis with -nitro--methyl-′-nitrosoguanidine. Intraperitoneal (i.p.) injection of wild-type (wt) virus resulted in the death of all mice given more than 30 p.f.u. Protection of mice surviving injection with lower doses of virus was not obtained. A proportion of mice infected with doses from 10 to 10 p.f.u. of mutant virus survived infection. Such surviving mice were protected against an otherwise lethal challenge with wt virus. Three mutants which were non-lethal at doses of 10 p.f.u. were studied further. The mutants M4 and M103 established a transient viraemia which was quickly cleared; unlike the wild-type, the virus did not enter the brain or spinal cord. The mutant M136 produced a viraemia similar to the wild-type, but reached a lower titre in the brain; it was not recovered from the spinal cord. Severe necrotic and inflammatory lesions, associated with large numbers of virus particles, were present in the brain and spinal cord (CNS) of mice 5 days p.i. with wt virus. Mice infected with M136 showed degenerative lesions in the grey and white matter of the CNS at 13 days p.i. Virus particles could not be detected at this time either by electron microscopy or infectivity assays.


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