West Nile Virus

West Nile virus (WNV) is a mosquito-borne flavivirus. WNV primarily causes disease in birds, humans, and horses and is transmitted by many different species of mosquitoes. Since 1999, more than 27,600 U.S. horses have been confirmed with WNV neuro-invasive disease with an estimated average case-fatality rate of 30–40%. During 2002 alone, over 15,000 horses were affected in the U.S. WNV is now considered endemic with yearly activity in the U.S. (with an average 300 cases per year), Canada, Mexico and the Caribbean.