A species of LENTIVIRUS, subgenus ovine-caprine lentiviruses (lentiviruses, ovine-caprine), that can cause chronic pneumonia (maedi), mastitis, arthritis, and encephalomyelitis (visna) in sheep. Maedi is a progressive pneumonia of sheep which is similar to but not the same as jaagsiekte (pulmonary adenomatosis, ovine). visna is a demyelinating leukoencephalomyelitis of sheep which is similar to but not the same as scrapie.