Visna-maedi virus (also known as Visna virus, Maedi-visna virus and Ovine lentivirus) from the genus Lentivirus and subfamily Orthoretrovirinae, is a 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 (OPP), particularly in the United States, or Montana sheep disease. White blood cells of the monocyte/macrophage lineage are the main target of the virus.
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 mæði 'dyspnoea') and visna (Icelandic visna '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 given assistance in eating and drinking, infected 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 have failed to induce immunity, occasionally causing increased viremia and more severe disease. Eradication programs have been established in countries worldwide.
Associated diseases and Clinical Signs Visna – Maedi is a chronic viral disease prevalent in adult sheep. The disease is rarely found in certain species of goat. Maedi Visna virus is also referred to as ovine progressive pneumonia (OPP). This disease corresponds to two clinical entities caused by the same virus. Maedi is a form that results in a chronic progressive pneumonia. Visna refers to the neurological form of the disease and predominantly causes meningoencephalitis in adult sheep. This disease has inflicted many economic losses worldwide due to the long incubation period and the high mortality rate of sheep and goats. MV virus can infect sheep of any age but clinical symptoms rarely occur in sheep less than two years old. The onset of the diseases is gradual resulting in relentless loss of weight in addition to breathing problems. Cough, abortion, rapid breathing, depression, chronic mastitis and arthritis are also additional symptoms observed. These symptoms appear mostly in animals over the age of three and therefore might spread to other flocks before clinical diagnosis can be achieved. Animals showing the above symptoms might die within six months of infection. This causal lentivirus can be found in monocytes, lymphocytes and macrophages of infected sheep in the presence of humoral and cell mediated immune response and can also be detected by conducting several serological tests. Transmission of the disease occurs most commonly via the oral route caused by ingestion of colostrum or milk that contains the virus or inhalation of infected aerosol droplets. Due to variation of the strains of MVV, some of the association clinical symptoms may be more pre-dominant in a flock relative to others along with differences in genetic susceptibility patterns.
Viral replication
Entry Visna Maedi virus (VMV) belongs to the small ruminant lentivirus group (SRLV). In general, SRLVs enter the cell through the interaction of their glycosylated envelope protein with a cellular receptor on the cell's plasma membrane facilitating fusion of the viral and cellular membrane. However, the specific cellular receptor that VMV binds is not entirely certain. A few studies have proposed MHC class II, CD4 and CXCR4 proteins as possible receptors however, none of these proteins have been established as the main receptor. Another study suggests that C-type lectins part of the mannose receptor (MR) family play a role as an alternative SRLV receptor. The mannose receptor is a 180-kDa transmembrane protein with eight tandem C-type lectin carbohydrate recognition domains (CRD) of which CRD4 and CRD5 are essential in recognizing mannose, fucose and N-acetyl glucosamine residues. Studies suggest that VMV gains entrance to the cell via mannosylated residues on its envelope proteins. MR is involved in recognizing the surface of pathogens and is involved in phago- and endocytosis and mediating antigen processing and presentation in a variety of cells including monocyte/macrophages and endothelial cells.
Replication Visna Maedi virus is a retrovirus meaning its genome consists of a (+)RNA that undergoes reverse transcription and then is integrated into the host's genome after infection. This integration is what leads to VMV's lifelong persistent infection. VMV has a long incubation period. During the initial outbreak among sheep in Iceland, there was no sign of clinical disease until six years after the importation of the Karakul sheep, which brought the virus from Germany to Iceland. Susceptibility to infection also increases with a higher level. VMV infects cells of the monocyte lineage, but only replicates at high levels when the monocytes are more mature/differentiated. Infected differentiated monocytes, also known as macrophages, will continuously present VMV antigens inducing T-lymphocytes to produce cytokines that in turn induce the differentiation of monocytes.
Viral transmission
Horizontal transmission Horizontal transmission plays an important role among livestock due to their often close quarters, especially during winter stabling. Free virus or virus infected cells are generally transferred in through inhalation of respiratory secretions. Additionally, fecal-oral transmission often occurs through contamination of drinking water. Sexual transmission has also been shown to be possible. No link has yet been made between transmission and other excretory products such as saliva and urine.
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