West Nile Virus Information for Clinicians

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

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.

• 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:

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.