West Nile fever - Definition

West Nile fever (West Nile encephalitis) a febrile illness caused by West Nile virus, that is transmitted from birds to the common Culex mosquito and then to people. The virus is closely related to other flaviviruses including those responsible for St. Louis encephalitis, Japanese encephalitis and Murray Valley encephalitis.

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, pain in the abdomen, a rash, and swollen glands (lymphadenopathy). These features are usually but not always mild. More severe cases usually involve neuroinvasion and are characterized as West Nile meningitis or West Nile encephalitis. Fatal cases tend to involve infants and small children under age 5, the aged, and people with an impaired immune system.