|Group:||Group VI (ssRNA-RT)|
Visna virus (also known as visna-maedi virus, maedi-visna virus and ovine lentivirus) from the genus lentivirinae and subfamily Orthoretrovirinae, is a "prototype" retrovirus that causes encephalitis and chronic pneumonitis in sheep. It is known as visna when found in the brain, and maedi when infecting the lungs. Lifelong, persistent infections in sheep occur in the lungs, lymph nodes, spleen, joints, central nervous system, and mammary glands; The condition is sometimes known as "ovine progressive pneumonia", particularly in the United States, or "Montana sheep disease". White blood cells of the monocyte/macrophage lineage are the main target of visna virus.
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.
Model system for HIV infection
Though it does not produce severe immunodeficiency, visna shares many characteristics with human immunodeficiency virus, including the establishment of persistent infection with chronic active lymphoproliferation; however, visna virus does not infect T-lymphocytes. The relationship of visna and HIV as lentiviruses was first published in 1985 by visna researcher Janice E. Clements and colleagues in the HIV field. It has been postulated that the effects of maedi-visna infection in sheep are the "equivalent" of central nervous system disease and wasting syndrome found in human AIDS patients. Despite limited sequence homology with HIV, the genomic organization of visna is very similar, allowing visna infection to be used as an in vivo and in vitro model system for HIV infection.
Research using visna was important in the identification and characterization of HIV. Nucleotide sequence analysis demonstrated that the AIDS virus was a retrovirus related to visna and provided early clues as to the mechanism of HIV infection.
Many countries have some sort of national programme to prevent and control a situation where the virus spreads.
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