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SUMMARYA group of 20 Icelandic sheep were infected intracerebrally with visna virus strain 1514, and 209 virus isolates were obtained from the blood, cerebrospinal fluid, and central nervous system (CNS) over a period of 7 years, during which eight animals developed clinical signs of visna necessitating sacrifice. (i) Using type-specific antisera, it was found that 12 (16%) of 76 isolates tested escaped neutralization. These 12 variant viruses were distributed randomly among animals and over time, and did not replace the infecting strain even though all sheep developed homotypic antibody within 3 months of infection. The one exception was sheep no. 1557 (an animal without clinical visna), where the last six isolates were variants. (ii) A total of 35 blood and CNS isolates from seven of these sheep (including five with clinical visna) were tested against serial samples of their own sera. Autologous antisera neutralized all isolates tested with the exception of isolates from sheep 1557. None of the isolates obtained at sacrifice from the five sheep with clinical visna escaped neutralization with autologous antisera. These data suggest that although variant viruses are encountered at considerable frequency during long-term infection of Icelandic sheep, the variants usually do not replace the infecting strain. Antigenic drift does not appear to be essential for virus persistence or for the development of clinically evident CNS lesions.

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