It should be pointed out that only two mutants were examined by inoculation into sheep,
but if mutants generally evolve this antigenic relationship to parental virus, such mutations
would function to the advantage of the evolving mutant since antibodies to the parental
virus cause little neutralization of mutants. This pattern of one way cross reactions would
favour a progressive antigenic drift away from the parental strain.
Although the mechanisms of derivation of mutant Visna viruses with respect to the role
of antibody is open to speculation, the mutants appear to differ from the parental strain
only in antigenicity as detected by neutralization. Their unique antigenicity was stable
after multiple subcultures and plaque purifications and their efficiency of replication and