West Nile virus (WNV) is a vector-borne zoonotic disease belonging to the Japanese encephalitis serogroup of flaviviruses. These viruses are transferred by carriers, especially mosquitoes, and spread all over the world attacking vertebrates of several types, including birds and mammals. WNV is normally transmitted between birds by the Culex pipiens, a mosquito that tends to breed in foul standing water in drains and catch basins (Epstein, 2001; Pats et al., 2003). The disease generally appears at the end of the summer and the beginning of autumn, with incubation period of 3-15 days. West Nile fever (WNF) became recognized as a cause of fatal human meningitis or encephalitis in elderly patients during an outbreak in Israel in 1957. Encephalitis occurs also in horses, as well as causing mortality in certain domestic and wild birds (CDC website, 2004).
WNV is endemic in Africa, south-western Asia, eastern and southern Europe, and parts of the Mediterranean basin (Hubalek and Halouzka, 1999; Dohm et al., 2002). The virus was first detected in the western hemisphere in New York City in 1999 (Nash et al., 2001). The means by which WNV entered the Americas is unknown, but the genetic analysis indicated that the strain responsible for the outbreak was nearly identical to a strain that was circulating in Israel in 1998 (Lanciotti et al., 1999). Since 1999, it has moved rapidly westward and was detected in California in 2004 (Granwehr et al., 2004). Today, WNV exists also in Central and South America and in the Caribbean (Gibbs et al., 2006) and is therefore a vector-borne pathogen of global importance.
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