Demyelination in mice resulting from infection with a mutant of Semliki Forest virus

Twelve of 34 weanling mice (35%) developed lesions in the brain and spinal cord following i.p. infection with 10(2) p.f.u. of a mutant of Semliki Forest virus (SFV). Six of 12 mice examined 13 days post infection (p.i.) showed meningo-encephalomyelitis with focal spongiform lesions in the grey and white matter. The spongiform lesions were characterised by necrosis of putative oligodendrocytes, myelinic vacuolation and mononuclear cell infiltration. Only one of six mice examined 21 days p.i. and one of six mice examined 28 days p.i. showed lesions which comprised reactive and dystrophic changes in the white matter. Spongiform lesions and pycnotic nuclei were not seen at these times. Viral nucleocapsids were seen in the early stages of the disease in putative necrotic oligodendrocytes. Mature virus particles were not seen. This was in contrast to mice infected with virulent wild-type SFV when lesions were more severe and were accompanied by large numbers of immature and mature virus particles. It is suggested that the demyelination in mice infected with mutant SFV results primarily from selective destruction of oligodendrocytes by the mutant virus.