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West Nile virus
Other Resources UpToDate PubMed

West Nile virus

Contributors: Edith Lederman MD, Noah Craft MD, PhD
Other Resources UpToDate PubMed


West Nile virus (WNV) is caused by infection with a flavivirus. The vector of transmission is the Culex, Aedes, and Mansonia mosquitoes as well as some ticks with birds as intermediate hosts.

The virus is endemic in several temperate climates around the world including North America, Europe, Africa, and Australia. Most cases in these regions occur during the late summer and fall.

Most cases are asymptomatic. Patients usually present with mild illness of fever, myalgias, and an associated macular and papular rash on the upper body. Occasionally, an associated hepatitis or encephalitis can be life threatening. The incubation period is typically 3-14 days. Fever and severe frontal headache, backache, and anorexia may precede the central nervous system (CNS) signs and symptoms of encephalitis (confusion, neck stiffness, cranial nerve palsies, and generalized weakness), for which the mortality rate is 40%. Mild illnesses usually resolve in less than 1 week, but prolonged fatigue is common. Symptoms of encephalitis may persist for weeks or may be permanent.


A92.30 – West Nile virus infection, unspecified

57311007 – West Nile virus

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Differential Diagnosis & Pitfalls

Other causes of Viral exanthem or rash and fever (with or without encephalitis):
  • Exanthematous drug eruption
  • Drug-induced hypersensitivity syndrome (DRESS)
  • In Mononucleosis, pharyngitis and adenopathy are more significant.
  • Dengue fever
  • Japanese spotted fever
  • Mediterranean spotted fever
  • Queensland tick typhus
  • Endemic typhus
  • Epidemic typhus
  • Rubella
  • Measles
  • Ehrlichiosis
  • Urticaria
  • Mycoplasma exanthem
  • Enteric fever
  • Bacterial infections such as streptococcal (Scarlet fever)
Note: Acute seroconversion of Human immunodeficiency virus primary infection can present with mononucleosis-like symptoms and exanthematous rash. Consider this diagnosis in those at risk.

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Last Updated:10/12/2017
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West Nile virus
A medical illustration showing key findings of West Nile virus (Mild) : Diarrhea, Fatigue, Fever, Headache, Vomiting, Rash, Arthralgia, Myalgia
Clinical image of West Nile virus - imageId=2220835. Click to open in gallery.  caption: 'Numerous tiny, erythematous papules, each with a surrounding blanched halo, on the leg.<br/><br/>Image source: Centers for Disease Control and Prevention (CDC). This image is in the public domain and thus free of any copyright restrictions.'
Numerous tiny, erythematous papules, each with a surrounding blanched halo, on the leg.
Copyright © 2024 VisualDx®. All rights reserved.