Lesions may be found in gray and white matter. The earliest changes seen in the CNS are degenerative and seem to be the result of viral replication in glial cells, especially astrocytes, followed by viral-induced demyelination.

The role of T cells in acute demyelination is unknown. The degree of myelin destruction has been correlated with the amount of viral antigen in the tissues. Whether progressive demyelination in chronic disease is immune-mediated is also unknown.