Visna virus (also known as Visna-maedi virus, Maedi-visna virus and Ovine lentivirus) from the genus Lentivirus 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.