Virulence of temperature-sensitive mutants of foot-and-mouth disease virus.

A number of temperature-sensitive mutants isolated from two strains of foot-and-mouth disease virus were examined for their virulence in suckling mice. The majority of the mutants were found to be less virulent than the parent virus strains, ranging from slight to total attenuation, but two mutants retained parental levels of virulence. There was no correlation between mutant cut-off temperatures and virulence, or the revertant content of mutant preparations and virulence. It was not always possible to regain parental levels of virulence by isolating phenotypic revertants or wild-type recombinants from genetic crosses, but recombinants were significantly more virulent than either of the two input viruses in a genetic cross.