Maedi-visna is a multi-systemic disease of sheep caused by a non-oncogenic, exogenous retrovirus of the Lentiviridae subfamily. It is mainly transmitted through the ingestion of milk from infected sheep, although disease can be spread within flocks through direct contact and transmission can occur also in utero. Maedi usually begins insidiously and leads to a classical interstitial pneumonia. On postmortem examination, the lungs may be 2-4 times heavier than normal, due to decreased elasticity and fibrosis. Regional lymph nodes are enlarged with formation of lymphoid follicles with active germinal centers. Visna - the neurological form of the disease, is characterized by chronic and active meningoencephalomyelitis and chorioiditis with massive infiltrations of mononuclear cells around the blood vessels, microglial nodules formation and astrogliosis. Virus isolation is very specific but of variable sensitivity. The cytopathic effects are characteristic, consisting of the appearance of stellate cells and syncytia. The diagnostic methods currently used are based on the detection of either antibodies or the viral genome. The three most commonly tests used, are the PCR and RT-PCR, agar gel immunodiffusion test (AGID) and the enzyme-linked immunosorbent assay (ELISA). Western blot analysis and radio-immunoprecipitation are also performed, but only in specialized laboratories. No commercial vaccines are available to control infection, and to date, monitoring programs have proved to be the only effective tool in controlling this disease.