Maedi-visna

Slow Virus Diseases

“…Neither visna nor maedi, the pulmonary analog of visna, had ever been observed in Karakul sheep, either in Iceland or in the flocks currently maintained in Germany (179). There is little reason to doubt that Icelandic sheep differ considerably genet ically from other European breeds, since Icelandic sheep were brought into that country by settlers more than 1000 years ago, and have lived in relative isolation since (149). Numerous early attempts to transmit visna infection to a variety of laboratory and domestic animals, including non-Icelandic sheep, have been unsuc cessful.…”

“…In all naturally occurring cases of visna virus infection, eSF pleocytosis is noted, and an elevation in eSF gamma globulin may also be present (149). The specificity of the increased gamma globulin has not been described, and it is known that visna virus persists in the presence of high titered neutralizing antibody.…”

“…This is consistent with the claim that the immune response in visna is deficient as compared with that in other viral infections (157). In this disorder, neutralizing antibodies are usually observed well in advance of clinical disease, and it has been proposed that an antigen-antibody reaction occurs on the surface of infected eNS cells, causing cell destruction and secondary demyelination (79,149). Visna virus has the property of causing cell fu sion and subsequent cell disintegration in vitro, even when virus does not replicate (2 1, 85).…”

“…Both the SSPE (95) and the visna-affected (149) host produce antibody to their respective viruses, often in greatly exaggerated quantities. The PML victim may likewise produce antibody to the papovavirus infecting him, but usually not at increased levels (139).…”

“…Cellular pathology with visna occurs in the CNS with the appearance of menin geal and subependymal infi ltrates. Myelin degeneration occurs secondary to exten sive microglial infiltrations which have a tendency to necrose and cavitate (149).…”