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aedi-Visna is a viral disease of sheep caused by a lentivirus. It is characterized by progressive interstitial pneumonia or meningoencephalitis. Sometimes arthritis, ill-thrift and indurative mastitis are observed. The disease most often progresses slowly, but irreversibly. The disease occurs in all major sheep producing countries with the exception of Australia, New Zealand, and Finland. The international movement of sheep has facilitated the spread of the disease. Sheep and goats are the only species known to be susceptible (1). All breeds of sheep appear to be susceptible to infection, but there may be differences in breed susceptibility in seroprevalence in flocks with more than one breed of sheep (2, 3). The prevalence of infection varies between farms, and countries. Rates of seropositivity increase with age, and flock seroprevalence is influenced by the average age of the flock 1 . Flock seroprevalence also has been positively associated with the use of foster ewes, allowing lambs older than 1 day to have contact with other lambing ewes, flock size, close contact during confinement for lambing stocking density on pasture, and the length of time that the flock has been in existence (4, 5). Rates of seropositivity are much higher in flocks that also are infected with pulmonary adenomatosis than those which are not. The disease is spread by the respiratory route, ingestion of infected milk and in utero infection. The relative importance of these routes appears to vary with the flock and its management, but lateral transmission is important in all. Economic importance of the disease rests with losses associated with decreased longevity and mortality with clinical disease, decreased value of cull animals and possible effects of sub-clinical infection on productivity (1). As no treatment or vaccine is available at the present time for the disease, the eradication programs are based on early detection of the disease accompanied by elimination of the animals carrying the virus. This involves detection and culling of seropositive animals where lateral transmission is the dominant mode of transmission in the flock. All sheep on the farm are serologically tested annually or twice a year, and seropositive animals and their progeny of less than 1 year of age are culled and kept separately from the seronegative ones. Flock status with respect to the presence or absence of infection and the determination of the infection status of an individual sheep currently relies on serological testing. Agar gel immunodiffusion (AGID) tests and Enzymelinked immunosorbent assay (ELISA) tests are used in most countries. Indirect ELISA and competitive ELISA tests may have better sensitivity, depending on the antigen used (6, 7). The value of serological testing with current methodologies rests primarily with the establishment of the infection status of the flock. A negative test in an individual sheep M

Serological study of Maedi-Visna virus among sheep flocks in Kerman province of Iran