West Nile Virus Information for Clinicians

Fact sheet from the CDC
This page has been automatically translated from English. MSDH has not reviewed this translation and is not responsible for any inaccuracies.

Clinical Features

Mild Infection

Most WNV infections are mild and often clinically unapparent.

  • Approximately 20% of those infected develop a mild illness (West Nile fever).
  • The incubation period is thought to range from 3 to 14 days.
  • Symptoms generally last 3 to 6 days.

Reports from earlier outbreaks describe the mild form of WNV infection as a febrile illness of sudden onset often accompanied by

  • malaise
  • headache
  • nausea
  • rash
  • vomiting
  • anorexia
  • myalgia
  • lymphadenopathy
  • eye pain

The full clinical spectrum of West Nile fever has not been determined in the United States.

Severe Infection

Approximately 1 in 150 infections will result in severe neurological disease.

  • The most significant risk factor for developing severe neurological disease is advanced age.
  • Encephalitis is more commonly reported than meningitis.

In recent outbreaks, symptoms occurring among patients hospitalized with severe disease include:

  • fever
  • gastrointestinal symptoms
  • weakness
  • change in mental status
  • A minority of patients with severe disease developed a maculopapular or morbilliform rash involving the neck, trunk, arms, or legs.
  • Several patients experienced severe muscle weakness and flaccid paralysis.
  • Neurological presentations included:
  • ataxia and extrapyramidal signs
  • optic neuritis
  • cranial nerve abnormalities
  • polyradiculitis
  • myelitis
  • seizures

Although not observed in recent outbreaks, myocarditis, pancreatitis, and fulminant hepatitis have been described.

Clinical Suspicion

Diagnosis of WNV infection is based on a high index of clinical suspicion and obtaining specific laboratory tests.

  • WNV, or other arboviral diseases such as St. Louis encephalitis, should be strongly considered in adults >50 years who develop unexplained encephalitis or meningitis in summer or early fall.
  • The local presence of WNV enzootic activity or other human cases should further raise suspicion.
  • Obtaining a recent travel history is also important.

Note: Severe neurological disease due to WNV infection has occurred in patients of all ages. Year-round transmission is possible in some areas. Therefore, WNV should be considered in all persons with unexplained encephalitis and meningitis.

Last reviewed on Sep 6, 2023 request edits
Mississippi State Department of Health 570 East Woodrow Wilson Dr Jackson, MS 39216 866‑HLTHY4U Contact and information

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