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" (OPP), particularly in the
United States, or "Montana sheep disease".
White blood cells of the
monocyte/
macrophage lineage are the main target of visna virus.