This outbreak is the first recognized introduction of WN virus into North America and the western hemisphere. It had previously been documented in at least 17 countries, with a geographic distribution from Africa to Eurasia. Multiple species of mosquitoes can serve as vectors for the virus. Canada has both the appropriate mosquito vectors and vertebrate reservoir hosts.
Symptoms can be inapparent to mild.
Three to twelve days following the bite of an infected mosquito, symptoms will range from a slight fever and headache to rapid onset of severe headache, high fever, stiff neck, muscle weakness, and disorientation. Illness is more common and more severe in adults than in children. Only two cases occurred in children, one age five years and one age 15. These findings are not unexpected, and are similar to rates found in a comparable survey following a 1996 outbreak of West Nile encephalitis in Bucharest, Romania. A red-tailed hawk was found in early February in Westchester County, N.Y. Testing in Connecticut has shown that it was infected with WN virus.