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Clinician Information

Clinical Features
Clinical Suspicion
Diagnosis and Reporting
Diagnostic Testing
Reporting Suspected West Nile virus Infection
Laboratory Findings

Clinical Features

Mild Infection

Most West Nile virus infections are mild and often clinically unapparent.

  • Approximately 20 percent 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 West Nile virus infection as a febrile illness of sudden onset often accompanied by

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

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
  • weakness
  • gastrointestinal symptoms
  • 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
  1. ataxia and extrapyramidal signs
  2. cranial and nerve abnormalities
  3. myelitis
  4. optic neuritis
  5. polyradiculitis
  6. seizures
  • Although not observed in recent outbreaks, myocarditis, pancreatitis, and fulminant hepatitis have been described.

Clinical Suspicion

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

  • West Nile virus, 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 West Nile virus enzootic activity or other human cases should further raise suspicion.
  • Obtaining a recent travel history is also important.

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


Diagnosis and Reporting

Procedures for submitting diagnostic samples and reporting persons with suspected West Nile virus infection vary among states and jurisdictions. Links to state and local websites are available at www.cdc.gov/ncidod/dvbid/westnile/city_states.htm.

Procedures for submitting specimens to the Illinois Department of Public Health Laboratory and requisition forms can be found at www.idph.state.il.us/envhealth/wnvguidelines.htm. Go to Health Care Provider information and then click on West Nile Virus Testing Guidelines.


Diagnostic Testing

West Nile virus testing for patients with encephalitis or meningitis can be obtained through local or state health departments.

  • The most efficient diagnostic method is detection of IgM antibody to West Nile virus in serum or cerebral spinal fluid (CSF) collected within 8 days of illness onset using the IgM antibody capture enzyme-linked immunosorbent assay (MAC-ELISA). [The CDC surveillance case definitions that accompany "Epidemic/Epizootic West Nile Virus in the United States: Revised Guidelines for Surveillance, Prevention and Control" suggest that IgM antibody should be present by the eighth day after onset of symptoms and may persist for 12 months or longer.]
  • Since IgM antibody does not cross the blood-brain barrier, IgM antibody in CSF strongly suggests central nervous system infection.
  • Patients who have been recently vaccinated against or recently infected with related flaviviruses (e.g., yellow fever, Japanese encephalitis, dengue) may have positive West Nile virus MAC-ELISA results.

Reporting Suspected West Nile virus Infection

Refer to local and state health department reporting requirements: www.cdc.gov/ncidod/dvbid/westnile/city_states.htm

  • West Nile virus encephalitis is on the list of designated nationally notifiable arboviral encephalitides.
  • Aseptic meningitis is reportable in some jurisdictions.

In Illinois, cases of arboviral disease, such as West Nile virus, are required to be reported to the local health department. Contact information for local health departments can be found at www.idph.state.il.us/local/home.htm.

The timely identification of persons with acute West Nile virus or other arboviral infection may have significant public health implications and will likely augment the public health response to reduce the risk of additional human infections.


Laboratory Findings

Among patients in recent outbreaks

  • Total leukocyte counts in peripheral blood were mostly normal or elevated, with lymphocytopenia and anemia also occurring.
  • Hyponatremia was sometimes present, particularly among patients with encephalitis.
  • Examination of the cerebrospinal fluid (CSF) showed pleocytosis, usually with a predominance of lymphocytes.
  • Protein was universally elevated.
  • Glucose was normal.
  • Computed tomographic scans of the brain mostly did not show evidence of acute disease, but in about one-third of patients, magnetic resonance imaging showed enhancement of the leptomeninges, the periventricular areas, or both.


Treatment is supportive, often involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections for patients with severe disease.

Ribavirin in high doses and interferon alpha-2b were found to have some activity against West Nile virus in vitro, but no controlled studies have been completed on the use of these or other medications, including steroids, antiseizure drugs, or osmotic agents, in the management of West Nile virus encephalitis.

For additional clinical information, please refer to Petersen LR and Marfin AA, "West Nile Virus: A Primer for the Clinician [Review]," Annals of Internal Medicine (August 6) 2002: 137:173-9.

For clinical and laboratory case definitions, see "Epidemic/Epizootic West Nile Virus in the United States: Revised Guidelines for Surveillance, Prevention, and Control, 2001,"at www.cdc.gov/ncidod/dvbid/westnile/surv&control.htm


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Illinois Department of Public Health
535 West Jefferson Street
Springfield, Illinois 62761
Phone 217-782-4977
Fax 217-782-3987
TTY 800-547-0466
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