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.