Maedi and visna are contagious sheep diseases which were introduced into Iceland in 1933 by imported sheep
of Karakul breed. Maedi, a slowly progressing pneumonia, and the central nervous system disease visna were shown to be
transmissible in sheep and most likely caused by a virus. In 1957, visna virus was isolated in tissue culture from sheep
brain and maedi virus was isolated the following year from sheep lungs. Both viruses showed similar cytopathic effect in
tissue culture. Electron microscope studies of ultrathin sections from visna virus infected cells demonstrated spherical
particles, 70-100 nm in diameter, which were formed by budding from the cell membrane. Later studies showed identical
particles in maedi virus infected cultures. These, and several other comparative studies, strongly indicated that maedi and
visna were caused by strains of the same virus, later named maedi-visna virus (MVV). Comparative studies in tissue
culture suggested that MVV was related to RNA tumor viruses of animals, the oncornaviruses. This was later supported
by the finding that MVV is an RNA virus. A few months after reverse transcriptase was demonstrated in oncornaviruses,
the enzyme was also found in MVV virions. Thus, MVV was classified as a retrovirus together with the oncornaviruses.
However, MVV is not oncogenic in vivo or in vitro and was in 1975 placed in a subgroup of retroviruses named
lentiviruses, which cause cytopathic effect in vitro and slowly progressing inflammatory disease in animals, but are nononcogenic.
In the early 1980s, the causative agent of AIDS was found to be a non-oncogenic retrovirus and was classified as a
lentivirus. Thus, HIV became the first human lentivirus.
Keywords: Maedi, visna, maedi-visna virus, slow infections, oncornavirus, reverse transcriptase, retrovirus, lentivirus, HIV, AIDS
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