West Nile Virus

Lyle R. Petersen, M.D., M.P.H.
Centers for Disease Control and Prevention

Definition/Key Clinical Features

• A virus in the Flaviviridae family transmitted by infectious mosquito bites
• Infected blood products or transplanted organs may transmit infection
• Historically confined to Africa, Asia, the Middle East, and Europe, but its geographic distribution has extended to the entire U.S., southern Canada, Mexico, Central America, the Caribbean, and Colombia
• Infection incidence peaks in late summer and early fall in temperate climates
• Year-round transmission may occur in tropical areas

• Sudden onset of fever
• Headache
• Nausea
• Vomiting
• Drowsiness
• Malaise
• Mental confusion
• Meningismus
• Skin rash
• Less common in severely ill patients
• Often arises upon convalescence in patients with West Nile fever

• Acute flaccid paralysis
• Hyperactive reflexes
• Parkinsonism
• Myoclonic jerking
• Coma

• Infection usually asymptomatic
• Excellent prognosis with West Nile fever or meningitis, although prolonged fatigue is common
• Although encephalitis or meningitis develops in fewer than 1% of infected persons, older persons are at increased risk, and immunocompromised persons (e.g., patients with hematologic malignancy, transplant recipients) are at extreme risk
• Clinical outcome of encephalitis is highly variable, but persistent neurologic deficits are common
• Mortality with encephalitis is 10%
• Acute flaccid paralysis mostly caused by anterior horn cell damage; may occur without meningitis or encephalitis; complete recovery is uncommon

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Differential Diagnosis

• Unexplained febrile illness
• Meningitis
• Encephalitis

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Best Tests

• Antibody-capture ELISA of IgM in serum or cerebrospinal fluid most sensitive, although often initially negative in patients presenting with West Nile fever
• Polymerase chain reaction testing of CSF reveals West Nile virus RNA in about 50% of patients
• PCR testing of CSF and sera may aid in diagnosis in severely immunocompromised patients, who may not develop West Nile virus antibodies

• MRI usually normal, but may show lesions in the basal ganglia and thalamus, white and gray matter, cerebellum, and spinal cord

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Best Therapy

• No proven antiviral agent is available
• Clinical trial information available at http://www.cdc.gov/ncidod/dvbid/westnile/clinicalTrials.htm
• Supportive treatment
  • Respiratory therapy
  • Treatment of any seizures
  • Prevention or treatment of bacterial superinfections
• Mosquito control and prevention of bites is the key to controlling outbreaks
• Universal screening of U.S. and Canadian blood supply

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Best References

Hayes EB, et al: Emerg Infect Dis 11:1167, 2005 [PMID 16102302]

Hayes EB, et al: Emerg Infect Dis 11:1174, 2005 [PMID 16102303]

Sejvar JJ, et al: Emerg Infect Dis 11:1021, 2005 [PMID 16022775]

Petersen LR, et al: JAMA 290:524, 2003 [PMID 12876096]

The authors have no commercial relationships with manufacturers of products or providers of services discussed in this module.