West Nile Virus

The West Nile virus (WNV) belongs to the genus Flavivirus (family Flaviviridae) and was previously classified as a group B arbovirus. These disease-causing pathogens are spread to humans by insects, usually mosquitoes. Other flaviviruses include the Yellow fever virus, Japanese encephalitis virus, dengue virus, and the Saint Louis encephalitis virus (see sections on flaviviruses in Chapters 19 and 23). The West Nile virus was isolated first in Uganda in 1937 and then in other parts of the Eastern Hemisphere (Africa, Asia, the Middle East, and Europe). Since 1999, when it emerged in the Western Hemisphere (for the first time in the New York City area), it has been classified as an emerging infectious disease in the United States ( http://cdc.gov/ncidod/dvbid/westnile/surv&control05Maps.htm). The WNV infection is considered to be zoonotic by nature because the first step in the transmission of WNV usually takes place when a mosquito bites an infected bird, and the virus primarily cycles between mosquitoes and birds, which are highly susceptible to infection; more than 40 species of mosquitoes are capable of transmitting the virus. However, infected mosquitoes can also transmit WNV to humans and other incidental hosts [other mammals, birds, and even reptiles (e.g., alligators)].