Summary: Ovine lentivirus (OvLV) strains vary in cytopathogenicity, but the correlation, if any, between viral cytophenotypes and in vivo pathogenicity is poorly understood. To examine this issue and to evaluate changes in OvLV strains following passage in vivo, biological and genetic characteristics of OvLV isolates following in vivo passage were compared with those of the parent strain used for inoculation of two sets of isogeneic twin lambs. Plaquepurified OvLV strains with slow/low (84/28) and rapid/high (85/34) cell culture phenotypes were used for inoculation of the lambs. The phenotypes of the parent OvLV strains were compared with virus isolates from the four lambs by assaying virus replication and cytopathogenicity in goat synovial membrane cells. Viral population genetic differences in the env region were compared by polymerase chain reaction (PCR) amplification and denaturing gradient gel electrophoresis (DGGE) of the fragments. Virus isolates recovered from rapid/high virus-infected sheep were more lytic and developed syncytia earlier than viruses reisolated from sheep inoculated with the slow/low strain. Isolates from lambs infected with 84/28 were more cytopathic in all assays than was their parent strain. Isolates from animals infected with 85/34 were more lytic and syncytiogenic than the parent strain, but responded similarly in replication assays. Although there were no consistent phenotypic differences between virus isolates recovered from sets of twins with markedly different degrees of lymphoid interstitial pneumonia (LIP), the DGGE band patterns of PCR amplified env fragments of the virus isolates from the twin lamb set with severe LIP, but not the set with slight LIP, were distinctly different from those of the parental viruses.
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