Definition of West Nile fever

West Nile fever: A febrile disease caused by the West Nile virus that is transmitted from birds to the common Culex mosquito and then to people. The virus is named after the area it was first found in Uganda.

West Nile fever occurs in parts of Africa and Asia and, infrequently, in Southern Europe and the Middle East. The West Nile virus had never been seen in birds or people in the Western Hemisphere prior to an outbreak in the summer of 1999 in New York City.

Signs and symptoms include the sudden onset of drowsiness, headache and nausea due to encephalitis, pain in the abdomen, a rash, and swollen glands (lymphadenopathy). These features are usually but not always mild. Fatal cases tend to involve infants and small children under age 5, the aged, and people with an impaired immune system.

The outbreak in New York, which was entirely unexpected, reflects the reality of global travel today that can transport exotic viruses to new areas. If a mosquito-borne virus enters a new area, the entire mosquito population may be susceptible, putting everyone living there at risk.

West Nile fever is also known as West Nile encephalitis. The virus is closely related to other flaviviruses including those responsible for St. Louis encephalitis, Japanese encephalitis and Murray Valley encephalitis.