Virus du Nil occidental

West Nile virus is a member of the flavivirus genus within the Japanese encephalitis antigenic complex of the family Flaviviridae ^[2]. It was first isolated in a woman in the West Nile district of Uganda in 1937 and later identified in birds in the Nile delta ^[2]. The virus is maintained in nature through a mosquito-bird-mosquito transmission cycle, with birds serving as reservoir hosts ^[2].

Human infection is frequently asymptomatic but can range to rare encephalitis, paralysis, and death ^[1][2]. Among neuroinvasive disease cases in the United States, common acute symptoms include fever, nausea or vomiting, fatigue, headache, altered mental status, and focal weakness, with pooled mortality near 9% and frequent ICU admission ^[3]. West Nile fever develops in about 25% of infections with prolonged symptoms, while neuroinvasive disease occurs in under 1% but carries roughly 10% fatality and considerable long-term morbidity such as persistent weakness after paralysis ^[3][4].

WNV epidemics were originally localized to Europe, Africa, the Middle East, and parts of Asia, with human infections reported worldwide for over 50 years ^[1][2]. The 1999 introduction via New York produced a dramatic outbreak that spread across the continental United States and subsequently throughout the Americas ^[2][4]. The largest outbreaks occurred in Greece, Israel, Romania, Russia, and the USA, often along major bird migratory routes ^[2]. Since 1999 the contiguous US has reported 16,196 neuroinvasive cases and 1,549 deaths, with incidence highest in the Midwest from mid-July to early September ^[4].

Human infection most often follows bites from infected mosquitoes, which acquire the virus by feeding on viremic birds and later inject it via salivary glands during blood meals ^[2]. The virus is maintained in a mosquito-bird-mosquito cycle, with Culex mosquitoes as principal vectors and vertical transmission sustaining mosquito populations ^[2]. Less common routes include contact with infected animal tissues, organ transplant, blood transfusion, breast milk, and one reported transplacental case ^[2].

During outbreaks, emphasis shifts to aggressive adult mosquito control to lower abundances of infected biting mosquitoes ^[4].

1. 1 Rossi SL et al. West Nile virus. Clin Lab Med. 2010 Mar. PMID: 20513541. doi: 10.1016/j.cll.2009.10.006. PubMed: https://pubmed.ncbi.nlm.nih.gov/20513541/
2. 2 World Health Organization. West Nile virus [Internet]. cited 23 Oct 2026. Available from: https://www.who.int/news-room/fact-sheets/detail/west-nile-virus
3. 3 Roberts JA et al. Clinical, Prognostic, and Longitudinal Functional and Neuropsychological Features of West Nile Virus Neuroinvasive Disease in the United States: A Systematic Review and Meta-Analysis. Ann Neurol. 2025 Jul. PMID: 40008684. doi: 10.1002/ana.27220. PubMed: https://pubmed.ncbi.nlm.nih.gov/40008684/
4. 4 Petersen LR et al. West Nile virus: review of the literature. JAMA. 2013 Jul 17. PMID: 23860989. doi: 10.1001/jama.2013.8042. PubMed: https://pubmed.ncbi.nlm.nih.gov/23860989/