Visna-maedi virus

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Maedi-visna is a slow virus infection of sheep leading to a progressing lymphoproliferative disease which is invariably fatal. It affects multiple organs, but primarily the lungs where it causes interstitial pneumonia (media). Infection of the central nervous system was commonly observed in Icelandic sheep (visa), infection of mammary glands (hard udder) in sheep in Europe and the USA, and infection of the joints in sheep in the USA. The name o vine progressive pneumonia (OPP) is commonly used in the USA and o vine antivirus (Lvov) infection is also a name used for media-visa. A related infection of goats, caprice arthritis-encephalitis (CASE), is common in Europe and the USA. The natural transmission of media-visa is mostly by the respiratory route, but also to newborn lambs by colostrum and milk. Intrauterine transmission seems to be rare and venereal transmission is not well documented. Macrophages are the major target cells of media-visa virus (MVP), but viral replication is greatly restricted in the animal host, apparently due to a post transcriptional block. The low-grade viral production in infected tissues can explain the slow course of the disease in sheep. The lesions in media-visa consist of infiltrates of lymphocytes, plasma cells, and macrophages, and are detectable shortly after experimental transmission. Several studies indicate that the lesions are immune mediated and that toxicity T-lymphocytes may be important effect or cells. The persistence of the MVP infection is explained by a reservoir of latent infected blood and bone marrow monocytes, which migrate into the target organs and mature into macrophages with pro viral DNA transcription, but limited replication of virus. The MVP particles are morphological similar to those of other retroviruses and the mode of replication follows the same general pattern. The genome organization and gene regulation resembles that of other retroviruses. In addition to gag, pol and env, MVP has three auxiliary genes (tat, rev and via), which seem to have similar functions as in other retroviruses, with a possible exception of the tat gene. A determination of the 9200 nucleotide sequence of the MVP genome shows a close relationship to CASE virus, but limited sequence homology with other retroviruses, and only in certain conserved domains of the reverse transcription and possibly in the surface protein. MVP infection in sheep and HIV-1 infection in humans have a number of features in common such as a long clinical period following transmission, and a slow development of multi organ disease with fatal outcome. A brief early acute phase, which is terminated by the immune response, is also an interesting common feature. Like HIV-1, MVP is macrophages tropic and the early stages of the HIV-1 infection which affect the central nervous system and the lungs are in many ways comparable to media-visa. In contrast to HIV-1, MVP does not infect T-lymphocytes and does not cause T-cell depletion and immunodeficiency. This is responsible for the difference in the late stages of the HIV-1 and MVP infections and the final clinical outcome. Despite limited sequence homology, certain proteins of MVP and HIV-1 show structural and functional similarities. Studies of MVP may therefore help in the search for new drugs against retroviruses, including HIV-1.

This chapter presents a classification and description of the Media/Visna virus. This disease belongs to the Retrovirus family.. The Media/Visna virus is also known as Zwoegerziekte virus, o vine progressive pneumonia virus, and the Graffias–Reinet form of Jaagsiekte. Media is the Icelandic word for labored breathing. It is a progressive pneumonia of adult sheep, ending fatally. Visna, or wasting, occurs less frequently. It is seen as progressive hind limb paralysis following the development of lesions in the nervous system. The hosts to Media/Visna virus are sheep and goats. Although there are no reports of Media/Visna from Australia and New Zealand, seismological evidence is lacking. Agar gel immunodeficient and enzyme-linked immunodeficient assay are some of the diagnostic techniques that are routinely used for the seismological demonstration of infection from the disease. The complement fixation test is not reliable with field sera, but it is used routinely in some laboratories for experimental work. Fluorescent antibody is another technique for the diagnosis of Media/Visna virus.

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