Consumer edition active

West Nile Virus Infection

ByStefania Carmona, MD, University of Alabama at Birmingham
Reviewed/Revised Modified Aug 2025
v48470682
VIEW PROFESSIONAL VERSION
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, Central and South America, and Caribbean Islands, as well as in Africa, the Middle East, southern Europe, Russia and eastern Europe, 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.

  • 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.

  • 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.

West Nile virus is present in many species of birds. Mosquitoes become infected when they bite infected birds. Bites from infected mosquitoes then spread West Nile virus to people and other animals. 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 or delivery.

Symptoms of West Nile Virus Infection

Most (4 out of 5) people with West Nile virus infection have no symptoms. About 1 in 5 develop fever along with 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 adults and people with certain chronic medical conditions, such as diabetes or high blood pressure, are more likely to develop severe illness. About 10% of 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.

Diagnosis of West Nile Virus Infection

  • 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. (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.

Treatment of West Nile Virus Infection

  • 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 Acetaminophen to relieve fever and pain

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

Prevention of West Nile Virus Infection

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

Avoiding mosquito bites is key to preventing West Nile virus infection.

More Information

The following English-language resource may be useful. Please note that The Manual is not responsible for the content of this resource.

  1. Centers for Disease Control and Prevention: West Nile virus

Drugs Mentioned In This Article

quizzes_lightbulb_red
Test your KnowledgeTake a Quiz!
iOS ANDROID
iOS ANDROID
Download the free Merck Manual App iOS ANDROID