Virulence of temperature-sensitive mutants of Sindbis virus in neonatal mice

The virulence in neonatal mice of temperature-sensitive (ts) mutants of Sindbis virus was determined by measurements of mean survival time and 50% lethal dose after intracerebral injection. For 11 ts mutants, mean survival time was determined by the ribonucleic acid (RNA) phenotype, RNA+ mutants killing the mice sooner than RNA- mutants for the same titer of virus injected. Mortality caused by seven ts mutants was, with one exception, correlated with the proportion of revertants recovered after death. A82, a presumed double mutant showing low reversion, showed no detectable lethality. The pathogenicity of this mutant could be detected by inhibition of weight gain, which was proportional to the titer of virus injected. A low-level persistence, independent of the titer injected, occurred up to 7 days after injection. This was followed by complete clearance. It is concluded that the virulence of Sindbis virus may be considerably altered by mutation, and that this is related to events occurring at the cellular level.