Also Known As
WNV
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This article waslast modified on June 29, 2018.
What is West Nile virus?

West Nile virus (WNV) is an infectious disease that 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. except Hawaii and Alaska.  According to the Centers for Disease Control and Prevention (CDC), WNV is the leading cause of encephalitis due to arboviruses in the country. Outbreaks of cases begin to appear in the summer and increase through the fall, and the medical community tracks its emergence and promotes awareness and means for its prevention.

WNV is a mosquito-borne virus and belongs to the flavivirus group that causes illness in many regions of the world. 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. It is estimated that about 1 in 200 mosquitoes harbors the virus.

Although WNV is usually not transmitted person-to-person, there have been cases of WNV being passed on to others through blood donations, organ transplants, and rarely 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 those infected with West Nile virus will have no symptoms. About 20% will experience mild to moderate 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 there is central nervous system involvement. 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 fatality rate of those with the more serious form of the disease is about 10% and is highest among the elderly. Individuals with compromised immune systems or underlying conditions such as diabetes, kidney disease, or high blood pressure also have an increased risk of severe disease.

  • Tests

    Diagnosis of West Nile virus (WNV) infection in a person is usually made through review of the person's symptoms and exposure. It is confirmed by testing the person's blood and/or cerebrospinal fluid.

    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 blood for WNV, screen living donors of tissue and organs, and to track the spread of WNV through a community and across the country. Testing includes:

    • 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:

    • Using mosquito repellent
    • Wearing long-sleeved clothing and pants when outdoors
    • Staying indoors at dawn and dusk when mosquitoes are most active
    • Eliminating standing water sources around the home that attract mosquitoes 


    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 currently is focused on supportive care and symptom management. In severe cases, hospitalization may be required.

    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 health 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 safety and efficacy records.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

(2013 June 14). West Nile Virus in the United States: Guidelines for Surveillance, Prevention, and Control. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/westnile/resources/pdfs/wnvGuidelines.pdf through http://www.cdc.gov. Accessed May 2014.

Cunha, B. (2013 December 17). West Nile Encephalitis. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/234009-overview through http://emedicine.medscape.com. Accessed May 2014.

Lindsey, N. et. al. (2013). West Nile Virus and Other Arboviral Diseases United States, 2012. Medscape Multispecialty from Morbidity and Mortality Weekly Report. 2013;62(25):513-517. [On-line information]. Available online at http://www.medscape.com/viewarticle/807540 through http://www.medscape.com. Accessed May 2014.

Delgado, J. and Hillyard, D. (Updated 2013 November). Arboviruses. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/Arboviruses.html?client_ID=LTD through http://www.arupconsult.com. Accessed May 2014.

Murray, K. et. al. (2013). West Nile Virus, Texas, USA, 2012. Medscape Multispecialty from Emerging Infectious Diseases. 2013;19(11):1836-1838. [On-line information]. Available online at http://www.medscape.com/viewarticle/819868 through http://www.medscape.com. Accessed May 2014.

Salinas, J and Steiner, M. (Updated 2012 January 18). West Nile Virus. Medscape Multispecialty [On-line information]. Available online at http://emedicine.medscape.com/article/312210-overview through http://emedicine.medscape.com. Accessed May 2014.

Pittman, M. et. al. (2013 June 24). Recognition and Management of West Nile Virus in the ED. Medscape Multispecialty [On-line information]. Available online at http://www.medscape.com/viewarticle/804206 through http://www.medscape.com. Accessed May 2014.

Sources Used in Previous Reviews

American Association of Blood Banks. "Risk of West Nile Virus (WNV) Infection by Blood Transfusion." Press Release: September 5, 2002.

Brown, David. "Virus Test May be Ready by Summer ’03: West Nile Risk May Make Screening of Blood a Widespread Necessity." The Washington Post, September 25, 2002: A2.

Centers for Disease Control and Prevention: West Nile Virus Basics. Available online at http://www.cdc.gov/ncidod/dvbid/westnile/index.htm through http://www.cdc.gov.

National Institutes of Health: West Nile Virus Information. Available online at http://www.nih.gov/news/westnile.htm through http://www.nih.gov.

National Institute of Allergy and Infectious Diseases. Fact Sheet: Research on West Nile Virus. Available online at http://www.niaid.nih.gov/factsheets/westnile.htm through http://www.niaid.nih.gov.

S Petersen LR, Roehrig JT, and Hughes JM. "Perspective: West Nile Virus Encephalitis." The New England Journal of Medicine, 347(16):1225-1226, October 17, 200. Abstract available online at http://content.nejm.org/cgi/content/abstract/NEJMp020128v1?eaf through http://content.nejm.org.

Weiss, Rick. "Organ Recipients Had Virus: West Nile's Presence Also Raises Blood Supply Concerns." The Washington Post, September 4, 2002: A1, A6.

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 1001-1002.

Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp 1626.

(2007 August 28). FDA Approves Second West Nile Screening Test for Donated Blood and Organs. FDA News [On-line press release]. Available online at http://www.fda.gov/bbs/topics/NEWS/2007/NEW01691.html through http://www.fda.gov. Accessed on 9/09/07.

(2007 March 2). FDA Approves First Fully Automated Test to Screen for west Nile virus in Blood and Tissue Donors. FDA News [On-line press release]. Available online at http://www.fda.gov/bbs/topics/NEWS/2007/NEW01578.html through http://www.fda.gov. Accessed on 9/09/07.

(2006 August 25). Recommendations for Protecting Laboratory, Field, and Clinical Workers from West Nile Virus Exposure. National Institute for Occupational Safety and Health [On-line information]. Available online at http://www.cdc.gov/niosh/topics/westnile/reclab.html through http://www.cdc.gov. Accessed on 9/09/07.

(2005 September 4). What You Need to Know about Mosquito Repellent. CDC West Nile Virus Fact Sheet [On-line information]. Available online at http://www.cdc.gov/ncidod/dvbid/westnile/mosquitorepellent.htm through http://www.cdc.gov. Accessed on 9/09/07.

(2005 September 27). West Nile Virus Fact Sheet. CDC West Nile Virus Fact Sheet [On-line information]. Available online at http://www.cdc.gov/ncidod/dvbid/westnile/wnv_factsheet.htm through http://www.cdc.gov. Accessed on 9/09/07.

(2007 August 1, Reviewed). Testing and Treating West Nile Virus in Humans. CDC Questions and Answers [On-line information]. Available online at http://www.cdc.gov/ncidod/dvbid/westnile/qa/testing_treating.htm through http://www.cdc.gov. Accessed on 9/09/07.

(2007 June 22). West Nile Virus Sequelae Can Be Long-Term. Medscape Reuters Health Information [On-line information]. Available online at http://www.medscape.com/viewarticle/558721 through http://www.medscape.com. Accessed on 9/09/07.

Murray, K. et. al. (2007 April 05). Depression after Infection with West Nile Virus. Medscape from Emerg Infect Dis. 2007;13(3):479-481. Available online at http://www.medscape.com/viewarticle/554199 through http://www.medscape.com. Accessed on 9/09/07.

(2007 May 1). West Nile virus. MayoClinic.com [On-line information]. Available online through http://www.mayoclinic.com. Accessed on 9/09/07.

Salinas, J. and Steiner, M. (Updated November 6). West Nile Virus. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/312210-overview through http://emedicine.medscape.com. Accessed November 2010.

Cunha, B. (Updated 2010 October 18). West Nile Encephalitis. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/234009-overview through http://emedicine.medscape.com. Accessed November 2010.

Lindsey, N. et. al. (2010 September 13). West Nile Virus Activity — United States, 2009. Medscape Today from Morbidity & Mortality Weekly Report. 2010;59(25):769-772. © 2010 Centers for Disease Control and Prevention (CDC). Available online at http://www.medscape.com/viewarticle/725089 through http://www.medscape.com. Accessed November 2010.

Mayo Clinic Staff (2010 June 26) West Nile virus. MayoClinic.com [On-line information]. Available online at http://www.mayoclinic.com/health/west-nile-virus/DS00438/METHOD=print through http://www.mayoclinic.com. Accessed November 2010.

Dugdale, D. (Updated 2010 September 15). West Nile virus. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/007186.htm. Accessed November 2010.

Neitzel, D. et. al. (2009 August 11). False-Positive Results with a Commercially Available West Nile Virus Immunoglobulin M Assay --- United States, 2008. Medscape Today from Morbidity & Mortality Weekly Report. 2009;58(17):458-460. © 2009 Centers for Disease Control and Prevention (CDC) [On-line information]. Available online at http://www.medscape.com/viewarticle/706830 through http://www.medscape.com. Accessed November 2010.

Hillyard, D. et. al. (Updated 2010 January). Arboviruses. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/Arboviruses.html?client_ID=LTD through http://www.arupconsult.com. Accessed November 2010.

(December 28, 2010) Centers for Disease Control and Prevention. West Nile Virus. Available online at http://www.cdc.gov/ncidod/dvbid/westnile/index.htm through http://www.cdc.gov. Accessed February 2010.