The Maedi-Visna virus (MVV) and Caprine Arthritis-Encephalitis virus (CAEV) belong to the genus ‘Lentivirus’ within the family ‘Retroviridae’. The terms Maedi and Visna are derived from Icelandic (Maedi: dyspnoea; Visna: fatigue) and describe the two main forms of the Maedi-Visna (MV) disease caused by the virus. Caprine Arthritis-Encephalitis (CAE) is a disease of goats caused by the CAEV virus. Depending on their age, affected animals show encephalitis, arthritis and mastitis. MVV and CAEV are characterized by long incubation periods and a chronic-progressive course of disease. CAEV of goats and MVV of sheep are closely related. The two viruses have a common group antigen.