West Nile Virus first appeared in the United States in 1999 in the New York City area, and has spread rapidly across the United States. Last year saw the first confirmed deaths in Wisconsin from infections caused by West Nile Virus. West Nile virus is an arbovirus (arthropod-borne virus) transmitted to birds and mammals by the bite of an infected mosquito. The virus cycles in nature between birds and mosquitoes, but can infect humans and domestic animals. Horses are particularly sensitive to the virus.