The emergence of antigenic variants is a rare event in long-term visna virus infection in vivo.

Six sheep persistently infected with visna virus were studied for 4 1/2 to 5 3/4 years until they became ill. Virus was isolated at intervals from peripheral blood leukocytes and cerebrospinal fluid (CSF), and at the time of sacrifice from various parts of the brain and the lungs. Both brain and lungs showed lesions typical of advanced visna/maedi. All the sheep formed antibodies in sera and CSF. Virus isolates from each sheep were tested in neutralization tests against sera and CSF collected from the same animal. In one sheep all isolates were found to be identical to the inoculated virus by this test. In each of the other sheep an antigenic variant emerged from 1 to 3 years after inoculation and remained in circulation even after the formation of autologous antibodies. In one case a variant was isolated from the lungs, whereas in all cases the virus isolated from the brain was identical to the inoculated virus. The results show that antigenic variants are rare in visna and do not seem to have a role in the pathogenesis of the disease.