Also Known As
WNV
This article was last reviewed on
This article waslast modified on June 4, 2019.
What is West Nile virus?

West Nile virus (WNV) is a virus spread most often by the bites of mosquitoes. The infection goes unnoticed in most people, but in a few cases it can cause severe illness.

The infectious disease was first discovered in Uganda, Africa in 1937. It then spread slowly through the Middle East, Eastern Europe, and West Asia. The first U.S. cases of WNV were reported in New York in 1999. Since then, the viral infection has moved steadily west and south and is now found in Canada and every state in the U.S.  

According to the Centers for Disease Control and Prevention (CDC), WNV is the leading cause of brain inflammation (encephalitis) caused by viruses spread by insects (arboviruses) in the continental U.S. Outbreaks of cases begin to appear in the summer and increase through the fall. The medical community tracks cases as they grow in number while promoting awareness and tips for preventing infections.

WNV belongs to a group of viruses called flaviviruses. Other flaviviruses are responsible for conditions such as dengue fever and yellow fever. These infections can affect both humans and a variety of animals. WNV tends to be seasonal in the United States, but in tropical areas of the world, flavivirus infections may occur year-round.

Birds are the primary WNV hosts and carriers, and mosquito bites are the most common route of transmission. When a mosquito bites an infected bird, the mosquito becomes infected and can then transmit the virus to other animals or humans that it bites.

Although WNV is usually not spread from person to person, rarely there have been cases of WNV being passed on to others through blood transfusions, organ transplants, and from a mother to her baby during pregnancy, delivery, or through breast milk while nursing. Blood centers in the United States now routinely screen donated units of blood for the presence of WNV.

For the most current numbers of confirmed human cases of WNV in the U.S. and the number of deaths attributed to it as the cause, visit the CDC's West Nile virus web site.

Accordion Title
About West Nile Virus
  • Signs and Symptoms

    About 80% of people infected with West Nile virus will have no symptoms. About 20% will experience mild to moderate non-specific flu-like signs and symptoms that develop between 3 and 14 days after a bite from an infected mosquito. These may include:

    • Fever
    • Nausea, vomiting
    • Headaches
    • Body aches
    • Skin rashes on the chest, stomach and back
    • Swollen lymph nodes


    These symptoms may last for a few days or persist for several weeks but have no long-term health impact.

    Less than 1% (about 1 in 150) of those infected will develop more serious disease, in which the central nervous system is affected. This can lead to life-threatening encephalitis and/or meningitis. Signs and symptoms may include:

    • High fever
    • Headache
    • Extreme muscle weakness
    • Neck stiffness
    • Stupor, disorientation
    • Tremors, convulsions
    • Loss of vision
    • Coma
    • Numbness, paralysis (rarely)


    These may last several weeks and the effects on the central nervous system may be permanent. The death rate for people with the more serious form of the disease is about 10%. The risk of severe illness and death is highest among the elderly and individuals with compromised immune systems or underlying conditions such as diabetes, kidney disease, or high blood pressure.

  • Tests

    Diagnosis of West Nile virus (WNV) infection in a person is usually made by reviewing your signs and symptoms and your possible exposure to the virus. It is confirmed by testing your blood and/or cerebrospinal fluid (CSF).

    WNV testing is used to determine whether someone is currently or has recently been infected with WNV. It is also used to screen units of donated blood for WNV, screen living donors of tissue and organs, and to track the spread of WNV through a community and across the country. (For details, see the article on West Nile Virus Testing.)

    Blood and CSF tests include:

    • WNV antibody tests for two different classes, IgM and IgG
    • WNV nucleic acid amplification test (NAAT) to detect the genetic material of the virus (viral RNA)
  • Treatment and Prevention

    Prevention of West Nile virus (WNV) depends upon protecting against mosquito bites by:

    • Wearing long-sleeved shirts and long pants, preferably light-colored
    • Using EPA-registered insect repellents containing DEET, picaridin, oil of lemon eucalyptus (OLE), or IR3535m and only used as directed, including precautions for pregnant women and children
    • Treating shoes, socks, pants, and tents with permethrin, a repellant
    • Sleeping in rooms that are screened against insects or sleep under a mosquito net
    • Emptying water from mosquito breeding sites, such as buckets, pots, or old tires


    Communities also take preventive measures by monitoring the seasonal risks and movement of WNV and spraying for mosquitoes as warranted.

    Treatment of milder cases of West Nile virus infection focus on supportive care, such as plenty of fluids and rest, and symptom management, such as over-the-counter pain relievers to reduce fever. People with severe cases may need to be hospitalized.

    No vaccine or specific antiviral drug treatment is available for humans at this time. Research and clinical testing of potential WNV vaccines are ongoing and healthcare practitioners remain optimistic that a solution will be found. Vaccines for other flaviviruses, such as yellow fever, have been available for about 70 years and have well-established records for safety and effectiveness.

View Sources

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