West Nile Virus

West Nile encephalitis is a viral infection of the brain transmitted by Culex mosquitoes, which breed in stagnant water and bite most often after dusk. Mosquitoes become infected after feeding on infected birds. Other mammals, including horses, can become infected with West Nile virus, but their role in the cycle of infection is uncertain. Recent evidence also indicates that the virus can be transmitted by transplanted organs and by blood transfusion. West Nile virus is found in Africa, Europe, West Asia, the Middle East, and North America. In countries in the temperate zone, such as the United States and Canada, the disease occurs chiefly in late summer and early fall. Further south, the disease may occur year-round.

The incubation period ranges from 3 to 15 days. Most infections are mild or asymptomatic. In severe cases, the virus infects the central nervous system, leading to fever, headache, neck stiffness, confusion, lethargy, and sometimes coma and death. The virus may also cause a polio-like illness. There is no vaccine or treatment for West Nile virus infections. Mosquito protection measures are strongly advised, especially after dusk, in areas where West Nile virus has been reported.

For detailed, up-to-date maps of the distribution of West Nile virus infections in the United States, go to the U.S. Geological Survey or the Centers for Disease Control. For information about the spread of West Nile in Canada, go to Health Canada.

From the Centers for Disease Control (CDC)

Background: Virus History and Distribution