BACKGROUND AND DISTRIBUTION A West Nile (WN) virus was first isolated in 1937 from the peripheral blood of a woman in the West Nile province of Uganda in Central Africa. Since then, WN viruses have been reported from North Africa (Egypt, Israel), East, Central, and South Africa, Asia (India, Pakistan), Borneo, Europe (Cyprus, France, Romania) and, most recently, the northeastern USA. Tests for antibody to WN suggest it has also been present in Thailand, the Philippines, Malaysia, Turkey, and Albania.West Nile viruses are members of the virus family Flaviviridae and are closely related to Japanese encephalitis viruses from the Old World and St. Louis encephalitis (SLE) viruses from the New World. In addition, WN cross-reacts in a variety of serological tests, including the plaque reduction neutralization test, with Murray Valley encephalitis (MVE), Usutu, Kunjin, Kokobera, Stratford, and Alfuy viruses. It was this cross reactivity of the New York City (NYC) WN with SLE serologic reagents that initially confused this virus with SLE. Additional tests that used direct examination of the gene sequence of the NYC virus identified it as a WN-like virus, not SLE.West Nile is represented by at least 2 distinct antigenic groups. One, the African-Middle Eastern group, contains WN isolates from the Congo, Egypt, Israel, Uganda, South Africa, Pakistan, France, and Eastern Europe. The second antigenic group contains WN isolates from India and South Africa.HISTORY The earliest reported epidemics caused by WN were in Israel. The first involved more than 500 clinical cases in 1950. Additional epidemics were reported from Israel in 1951, 1952, 1953, and 1957. The 1950 and 1957 epidemics were reported from a site 40 miles north of Tel Aviv, while the 1951 epidemic occurred at a site 15 miles southeast of Tel Aviv. In 1952, 1953, and 1954, cases were reported from both areas, indicting the ability of this virus to be extremely focal in its epidemic tran...
