Maedi-visna virus and caprine arthritis-encephalitis virus: distinct species or quasispecies and its implications for laboratory diagnosis.

The lentiviruses responsible for causing maedi-visna or ovine progressive pneumonia in sheep and caprine arthritis-encephalitis in goats have long been considered distinct, albeit related, viral species. Evidence, primarily in the form of nucleic acid sequence data, suggests this distinction may not be as absolute as once thought. These lentiviruses might better be viewed in the context of viral quasispecies whose individual members exhibit varying host range and pathogenic capabilities. Implications for diagnostic testing and control of these diseases are discussed.