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West Nile Virus Infection


Thomas M. Yuill

, PhD, University of Wisconsin-Madison

Last full review/revision May 2020| Content last modified May 2020
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West Nile virus infection is a viral disease spread primarily from mosquitoes to people. Most people have mild or no symptoms, but some people develop a severe infection that affects the central nervous system.

  • West Nile virus infection is present in the contiguous United States, southern Canada, Mexico, South America, and Caribbean Islands, as well as in Africa, the Middle East, southern Europe, the former Soviet Union, India, and Indonesia. A closely related virus, Kunjin virus, is present in Australia.

  • Symptoms, if any, vary in severity but may include fever, headache, body aches, joint pain, vomiting, diarrhea, or rash.

  • Rarely, people develop a serious, sometimes fatal, infection affecting the central nervous system.

  • To diagnose West Nile virus infection, doctors do a spinal tap or blood tests to check for the antibodies to the virus.

  • There are no vaccines to prevent West Nile virus infection in humans, but people can reduce the risk of transmission by taking steps to prevent mosquito bites.

  • Most people with West Nile virus infection do not need treatment, but those who develop infections in their brain or spinal cord require close monitoring and supportive treatment, such a mechanical ventilation.

West Nile virus is present in many species of birds. Mosquitoes become infected when they bite infected birds. Infected mosquitoes then spread West Nile virus to people and other animals by biting them. In a very small number of cases, West Nile virus has been spread through blood transfusions and organ transplantations and from a mother to baby during pregnancy, delivery, or breastfeeding.


Most (4 out of 5) people 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 people develop severe central nervous system involvement with encephalitis (a brain infection), meningitis (infection of the tissues that line the brain and spinal cord), or weakness and paralysis. Symptoms of central nervous system infection include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, seizures, muscle weakness, vision loss, numbness, and paralysis. Older people and people with certain chronic medical conditions, such as diabetes or high blood pressure, are more likely to develop severe illness. About 1 in 10 people with severe central nervous system involvement die.

Most people with typical symptoms recover completely, but fatigue and weakness can last for weeks or months. People whose central nervous system was affected often have long-lasting adverse effects on their brain and nerves.


  • Blood tests

  • A spinal tap

West Nile virus infection is suspected in people who have typical symptoms (especially those suggesting meningitis or a brain infection).

To diagnose West Nile virus, doctors may do blood tests for West Nile virus-specific antibodies. (Antibodies are proteins produced by the immune system to help defend the body against a particular attack.)

If people have symptoms of meningitis, a spinal tap (lumbar puncture) is done to obtain a sample of cerebrospinal fluid for analysis. (Cerebrospinal fluid is the fluid that flows through the tissues that cover the brain and spinal cord.) A sample of cerebrospinal fluid is sent to a laboratory to be tested for West Nile virus-specific antibodies.

Doctors may use reverse transcriptase-polymerase chain reaction (RT-PCR) to make many copies of the virus's genetic material. This technique enables doctors to rapidly and accurately identify the virus.


There is no vaccine to prevent West Nile virus infection in people. A vaccine is available for equine animals.

Avoiding mosquito bites is key to preventing West Nile virus infection. To prevent mosquito bites, people should take the following precautions:

  • Wear long-sleeved shirts and long pants.

  • Stay in places that have air conditioning or that use window and door screens to keep mosquitoes out.

  • Sleep under a mosquito net in places that are not adequately screened or air-conditioned.

  • Use strong insect repellents—those with ingredients such as DEET (diethyltoluamide) or other approved active ingredients—on exposed skin surfaces.

  • Treat clothing and gear with permethrin insecticide (do not apply it directly to the skin).

For children, the following precautions are recommended:

  • Do not use insect repellent on infants under 2 months old.

  • Do not use products containing oil of lemon eucalyptus (para-menthane-diol) on children under 3 years old.

  • For older children, adults should spray repellent on their own hands and then apply it to the children's skin.

  • Dress children in clothing that covers their arms and legs, or cover the crib, stroller, or baby carrier with mosquito netting.

  • Do not apply insect repellent to the hands, eyes, mouth, or any cut or irritated skin.


  • Supportive care

There is no specific treatment for West Nile virus infection. Treatment of West Nile virus infection is supportive. It includes the following:

  • Rest

  • Fluids to prevent dehydration

  • Acetaminophen to relieve fever and pain

Healthcare professionals closely monitor people whose central nervous system is affected and provide mechanical ventilation, if needed.

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