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West Nile virus (WNV) is a mosquito-borne zoonotic arbovirus that can cause a fatal neurological disease in humans.

WNV was once only commonly found in Africa and the Middle East but it has now spread globally, with the first case in the Western Hemisphere being identified in New York City in 1999. Over the next 5 years, the virus spread across the continental United States, north into Canada, and southward into the Caribbean Islands and Latin America. WNV also spread to Europe, beyond the Mediterranean Basin, and a new strain of the virus was identified in Italy in 2012. WNV is now considered to be an endemic pathogen in Africa, Asia, Australia, the Middle East, Europe and in the United States, which in 2012 has experienced one of its worst epidemics.

Human infection is most often the result of bites from infected mosquitoes. The infection is asymptomatic in approximately 80% of cases. 20% of cases will develop Wes Nile fever (headache, tiredness, body aches, nausea, skin rash), with a small percentage developing a severe form of the disease that can cause neurological symptoms and death.

Treatment often includes hospitalization, IV fluids, and respiratory support. There is no vaccine for WNV. 

Source: World Health Organization