Ovine progressive pneumonia and maedi-visna are chronic diseases of sheep caused by lentiviruses (family Retroviridae) that are structurally and antigenically similar. Progressive pneumonia virus and maedi (meaning “dyspnea”) virus induce chronic progressive pneumonias that present with similar clinical signs. Visna (meaning “wasting”) is the term used in many parts of the world to refer to the neurologic form of the disease in sheep, resulting initially in unilateral pelvic paresis, progressing to paralysis. A closely related lentivirus-induced disease in goats, caprine arthritis and encephalitis (CAE, see Caprine Arthritis and Encephalitis), affects the nervous system and joints. Reported seroprevalence for lentiviral infection in sheep varies widely, ranging from 49% in the western USA to 9% in the north Atlantic region. This variation has been reported in other countries as well and may result from varied climatic conditions (arid vs more lush climates) and management (range conditions vs close confinement).