Visna virus could be recovered from peripheral blood leukocytes of sheep for years after intracerebral inoculation. Viruses recovered from sheep prior to and several months after development of antibody were antigenically identical to the parental strain used for inoculation. Subsequently, mutant viruses which were not neutralized by the animals' sera were obtained. Longitudinal studies of leukocyte viruses collected from two infected sheep showed that more than one strain of virus could co-exist in the animal. Virus neutralization tests using sequentially collected sera and the viruses recovered from leukocytes revealed 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.


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