Age-dependent and strain-related differences of virulence of Semliki Forest virus in mice.

Factors that influence the virulence of Semliki Forest virus for mice have been studied. In vivo experiments showed that maximum brain infectivities following i.p. inoculation of adult mice with a virus strain of low virulence were less than those found in mice showing clinical signs in virulent infections. The virulent strains generally caused death before antibody could be detected in brain tissue; the less virulent strain caused an infection that was neuroinvasive, but infectivity increased less rapidly in the brain and allowed antibody to intervene before clinical signs were apparent. IgG3 antibody was first detected in brain tissue coincident with the beginning of the decline of brain infectivities in avirulent infections; other sub-classes of IgG antibody were not detected until later. In vitro experiments showed differences in abilities of SFV strains to replicate in tissue culture and adult mouse brain tissue, and differences in susceptibility to infection of brain tissue from mice of various ages. These in vitro results could provide a basis for differences of virulence.