WNV is considered to be endemic in many countries of Africa, Asia, southern Europe, and North America. Since 2001, serologic evidence of the spread of WNV into Latin America, the Caribbean, and South America has been reported. Epidemics appear in the human population at infrequent intervals in some of these countries, and there is evidence of viral transmission between Africa and Europe by migrating birds. Most outbreaks have occurred during the summer months and can continue until cold nights reduce mosquito vector activity, notably Culex spp.
