Visna Virus - Viral Infection

Viral Infection

First described in 1954 by Bjorn Sigurdsson in Iceland, Maedi-Visna virus was the first lentivirus to be isolated and characterized, accomplished in 1957 by Sigurdsson. "Maedi", Icelandic for dyspnoea, and "visna", Icelandic for "wasting" or "shrinking" of the spinal cord, refer to endemic sheep herd conditions that were only found to be related after Sigurdsson's work.

Visna infection may progress to total paralysis leading to death via inanition; however, if helped to obtain water and food, animals may survive for long periods of time, sometimes greater than ten years. Viral replication is almost exclusively associated with macrophages in infected tissues, however replication is restricted in these cells—that is, the majority of cells containing viral RNA do not produce infectious virus.

The disease was introduced to Iceland following an import of Karakul sheep from Germany in 1933. The susceptibility to Maedi-Visna infection varies across sheep breeds, with coarse-wool breeds apparently more susceptible than fine-wool sheep. Attempts at vaccination against Maedi-Visna virus failed to induce immunity, occasionally causing increased viremia and more severe disease. Eradication programs have been established in countries worldwide.