West Nile virus (WNV), a member of the family Flaviviridae (genus Flavivirus), is a mosquito-borne virus first isolated in 1937 in the West Nile district of Uganda. The disease in humans is characterized by a dengue-like illness with fever, and a more severe form is characterized by central nervous system involvement, including encephalitis, meningitis, and myelitis. WN encephalitis was first reported in the Western Hemisphere in the summer of 1999, there was an outbreak in New York City. Epidemic WNV strains in North America are severely pathogenic, however, attenuated WNV strains were found in Texas and Mexico in 2003. The principal vectors of WNV transmission in North America are Culex. pipiens, Cx. Quinquefasciatus, Cx. restuans, Cx salinarius and Cx talsalis. The number of WN fever case has exceeded 27,000 since 1999 in the United States and 4,600 since 2002 in Canada. The first imported case of West Nile fever in Japan was confirmed in September, 2005. The patient had returned to Japan from the United States and developed symptoms the next day. There is currently no WN vaccine for use in humans. An inactivated WNV vaccine for use in horses has been available since 2001. A DNA vaccine, a chimeric live attenuated vaccine, and a recombinant vaccine have also been licensed for use in horses.