The neuropathogenesis of visna virus infection in sheep

Visna virus is a lentivirus which causes chronic, progressive encephalitis in sheep. The virus is highly tropic for cells of the monocyte/macrophage lineage and completion of the viral replicative cycle occurs only in mature macrophages. Combined immunohistochemistry and in situ hybridization studies have demonstrated an association between the number of infected macrophages in the brain and the severity of the lesions. A neurovirulent visna virus strain, developed by multiple sequential intracerebral passages in sheep, had an enhanced ability to replicate in macrophages. Visna virus-infected animals produce several cytokines, one of which augments inflammation by inducing expression of MHC class II antigens on macrophages.