West Nile Fever (WNF) is a viral disease of birds and is transmitted to humans by vector mosquito bites. Humans, horses and other vertebrates are dead-end hosts. After incubation period of five and 15 days symptoms from mild flu-like illness up to severe encephalitis can occur (similar to those of Rift Valley fever with headache, weakness, fever and sometimes nausea and vomiting). In addition, skin rashes are common. Mortality is relatively high, perhaps up to 5% of those diagnosed, although this may be lower because most people infected show no symptoms; there may be polio-like presentation in some cases.
West Nile fever occurs in Egypt, Israel, India, France, Africa and the northern Mediterranean area.
In late summer and early fall of 1999, infections were recognized for the first time in the North America. The cause remains uncertain but is likely to be importation of animals, perhaps through the illegal bird trade. Since its original introduction into the New York City area, WNV has caused disease in humans, horses and a wide variety of birds and other vertebrates, spreading into the eastern two thirds of the USA and also into Canada and the Caribbean Basin. The apparent ability of WNV to be disseminated by infected birds and to persist from year to year indicates that it will continue to be a public health problem for the foreseeable future.