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West Nile Virus


Thomas M. Yuill

, PhD, University of Wisconsin-Madison

Last full review/revision Mar 2020| Content last modified Mar 2020
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West Nile virus is a flavivirus that is now the primary cause of arbovirus encephalitis in the US. Most patients have mild or no symptoms. About 1 out of 150 patients develop a severe infection involving the central nervous system. Diagnosis is by serologic testing. Treatment is supportive with close monitoring for severe infection.

West Nile virus was first introduced into the US in 1999 in New York City. It is now present in all 48 contiguous states (not in Alaska), southern Canada, Mexico, South America and Caribbean Islands. West Nile virus also is widely distributed in Africa, Middle East, southern Europe, the former Soviet Union, India, and Indonesia.

West Nile virus is present in many species of birds. Many infected birds are asymptomatic but others, especially crows and jays, become sick and die and thus possibly are an indicator of disease in an area. Horses infected by West Nile virus may become ill and die. Occurrence of equine cases are a good indication of West Nile virus transmission in a locality. An equine vaccine is available. The virus is transmitted among birds and to humans mainly by the culex mosquito but also may be transmitted by blood transfusion, organ transplantation, or occasionally transplacentally to a fetus.

Symptoms and Signs of West Nile Virus

Most (4 out of 5) patients with West Nile virus infection have no symptoms. About 1 in 5 develop fever along with other symptoms such as headache, body aches, joint pain, vomiting, diarrhea, or rash. About 1 in 150 patients develop severe central nervous system involvement with encephalitis, meningitis, or flaccid paralysis. Symptoms of central nervous system infection include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. Severe illness can affect any age, but those > 60 years or with certain chronic medical conditions (eg, diabetes, hypertension) are at greater risk. About 1 in 10 people with severe central nervous system involvement die.

Acute flaccid paralysis may occur that is usually clinically and pathologically identical to that in poliomyelitis, with damage of anterior horn cells. West Nile virus acute flaccid paralysis can occur without fever or apparent viral prodrome and often presents as isolated limb paresis or paralysis and may progress to respiratory paralysis requiring mechanical ventilation.

Most people with typical fever and myalgia recover completely, but fatigue and weakness can last for weeks or months. Patients who recover from West Nile virus encephalitis or acute flaccid paralysis often have residual neurologic deficits.

Diagnosis of West Nile Virus

  • Serologic testing

West Nile virus infection is diagnosed by finding West Nile virus-specific IgM antibodies in the serum or cerebrospinal fluid (CSF). These antibodies are usually detectable 3 to 8 days after onset of illness and persist for 30 to 90 days, but longer persistence has been documented. False-positive results may result from cross-reactive antibodies due to infection with other flaviviruses, recent immunization with flavivirus vaccines (yellow fever or Japanese encephalitis), or from nonspecific reactivity.

Viral cultures and tests to detect viral RNA (eg, reverse transcriptase-polymerase chain reaction [RT-PCR]) can be done on serum or CSF and used to confirm infection.

Treatment of West Nile Virus

  • Supportive care

Supportive care for severe West Nile virus illness includes

  • Close monitoring of patients with encephalitis for the development of elevated intracranial pressure and seizures

  • Close monitoring of patients with encephalitis or acute flaccid paralysis for inability to protect their airway

  • Mechanical ventilation, if needed

Acute respiratory failure may develop rapidly, and prolonged ventilatory support may be required.

Prevention of West Nile Virus

  • Community-level mosquito control programs

  • Personal protective measures to avoid mosquito bites

No West Nile virus vaccines are licensed for use in humans.

Personal protective measures to decrease exposure to infected mosquitoes, including using diethyltoluamide (DEET), mosquito netting, and protective attire.

Blood and some organ donors are screened for West Nile virus by nucleic acid–based tests. Healthcare professionals should remain vigilant for the possible transmission of West Nile virus through blood transfusion or organ transplantation.

There is no evidence that humans acquire West Nile virus infection from handling dead or infected birds, but the CDC still recommends wearing gloves when handling dead birds (or any animal).

Key Points about West Nile Virus

  • West Nile virus is spread by among birds and transmitted to humans by the bite of an infected mosquito.

  • Most patients have mild or no symptoms, but some develop a severe infection involving the central nervous system.

  • Diagnosis is by serologic testing for West Nile virus–specific IgM.

  • Patients who develop severe infection should be closely monitored for elevated intracranial pressure, inability to protect their airway, and respiratory failure requiring mechanical ventilation.

More Information about West Nile Virus

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