West Nile Virus Infection Symptoms and Risk Factors

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West Nile virus was first observed in the U.S. during the summer of 1999 and is believed to be permanently established (endemic) in the U.S. at this time. A member of the flavivirus family, West Nile virus is related to the St. Louis encephalitis virus that is also found in the U.S. West Nile virus is commonly found in Africa, the Middle East, and in the western parts of Asia. It infects mosquitoes, birds, horses, humans, and some other mammals. In 2012, West Nile virus infections reached epidemic levels in Texas, and were reported in many other states.

Humans normally acquire the viral infection through a mosquito bite. The early fall, from late August to early September, is the most common time for infection to occur in the U.S. West Nile virus has the potential to cause a very serious illness, although 80% of people infected will not develop any symptoms at all. The others (about 20% of infected people) most commonly develop a mild illness, sometimes termed West Nile fever, that is characterized by:

West Nile fever develops two to 15 days following the bite of an infected mosquito and persists for a few days to a few weeks.