West Nile Virus (WNV) is a mosquito-borne illness that was first detected in the U.S. at New York City in 1999. Since then, the disease has spread across the United States. In 2003, WNV activity occurred in 46 states and caused illness in over 9,800 people.
The majority of people who are infected with the virus, about 80 in 100, have few or no symptoms. About 20 in 100 will develop West Nile fever, described as a mild flu-like illness, which lasts 3-6 days. Symptoms may include fever, headache, nausea, body aches, mild skin rash, and swollen lymph nodes. One person in 150 may develop brain inflammation (encephalitis), which can lead to muscle weakness, partial paralysis, confusion, and death. Of the few people that develop encephalitis, a small proportion die but, overall, this is estimated to occur in less than 1 out of 1000 infections.
West Nile Virus has been detected in over 40 other species of vertebrates, including alligators and horses, making it one of the most widely infectious diseases ever recorded. Most species will acquire symptoms similar to those in humans: widespread, low grade infections with some mortality.
WNV is transmitted to people and animals through infected mosquitoes. The virus is most prevalent from May to October when mosquitoes are most abundant. An infected mosquito may bite any animal, but not all animals will become infected. The time between the mosquito bite and the onset of the illness, ranges from 3-15 days in humans.
The disease is most destructive among bird populations. Infection has been reported in more than 200 bird species, and had been detected in dead birds from over 138 species since the outbreak began in 1999. The most severe illnesses are seen in the American Crow, where almost all infected birds are killed within 5 days of contracting the virus.