Most West Nile infections never announce themselves
About 80% of West Nile virus infections cause few or no symptoms. Roughly 20% bring fever, headache, vomiting, or rash; under 1% invade the nervous system as encephalitis or meningitis, with about a 10% death risk among those neuroinvasive cases. Birds amplify the virus; mosquitoes carry it to people.
Symptoms, when they appear, usually begin three to fourteen days after a bite. West Nile fever—the febrile syndrome in about one in five infections—is often described as a mild three-to-six-day flu-like illness with fever, headache, fatigue, joint pain, and sometimes gastrointestinal upset; fewer than one-third develop a rash. Neuroinvasive disease (under 1%) includes meningitis, encephalitis, meningoencephalitis, or a poliomyelitis-like flaccid paralysis. Encephalitis is the most common neuroinvasive form; 30–50% of those patients show marked muscle weakness.
Female mosquitoes are the main vector. In the United States, Culex pipiens, C. tarsalis, and C. quinquefasciatus dominate by region; in Europe, C. pipiens leads. Preferred bird hosts include corvids and thrushes such as the American crow and robin. Birds that build high viremia can reinfect mosquitoes; mammals and many bird species act as dead-end hosts. The virus was found in Uganda in 1937 and reached North America in 1999; U.S. cases cluster in August and September. Rare human routes include transfusion, transplant, and mother-to-child exposure; U.S. blood banks have screened donors since 2003.
No human vaccine exists; a horse vaccine does. Prevention means fewer standing-water breeding sites and bite avoidance with repellents, screens, and nets. There is no specific antiviral treatment—pain relief is supportive. Age over sixty, infancy, immunosuppression, organ transplant history, hypertension, and diabetes raise severe-disease risk. A CCR5 gene mutation that partially protects against HIV appears enriched among severe West Nile patients (about 4–4.5% versus 1% in the general population). Bird surveillance can flag outbreaks before human spikes.
Source: West Nile fever
