evealed a
sequential development of antibody to parental and then to each strain of mutant
virus. Characterization of two of the mutant viruses showed that they were antigenically
stable, virulent in cell culture and when inoculated into new sheep, elicited
antibodies which cross reacted with the parental virus from which they were
derived. This continuous mutation of Visna virus in persistently infected sheep
may be a mechanism for the production of chronic disease.