Date of this Version
West Nile virus (WNV) was introduced into the United States (US), specifically in New York City (NYC), in 1999; this translocation represented a major shift out of its normal geographical distribution of Africa, the Middle East, Europe and the western parts of Asia (Center for Disease Control 1999a). The route or method of entry into the US is still unknown. WNV is in the genus Flavivirus, the family Flaviviridae and is closely related to some other viruses in this family, such as Japanese encephalitis virus in Southeast Asia, Murray Valley encephalitis virus in Australia and St. Louis encephalitis (SLE) virus in North and South America. The principal vertebrate hosts for these viruses are wild birds, but few cases of clinical disease or mortality of wild birds were reported previously from natural infection with these viruses, although significant morbidity and mortality occurred in humans and domestic animals (Monath 1988). Natural maintenance of these arboviruses (arthropod-borne viruses) involves their transmission from infected mosquitoes to susceptible birds. A variety of wild birds may become infected, however some species are incompetent hosts for the viruses and do not regularly infect mosquitoes. On the other hand, infections in reservoir competent wild bird species produce high amounts of the virus in their blood (viremia) for the duration of several days and subsequently infect the mosquitoes that feed upon them, completing the transmission cycle. These competent bird species frequently maintain and amplify the particular virus.