West Nile Virus Testing

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Also known as: WNV
Formal name: West Nile Virus, IgM and IgG Antibodies and Nucleic Acid Amplification Tests

At a Glance

Why Get Tested?

To determine the cause of viral meningitis or encephalitis that occurs during the summer season; to detect the presence of West Nile Virus (WNV) and to track its spread in the community and across the United States; to screen for WNV in donated units of blood, tissues, or organs

When to Get Tested?

When someone has symptoms suggesting WNV such as headache, fever, stiff neck, and muscular weakness and a diagnosis of encephalitis and/or meningitis

Sample Required?

Cerebrospinal fluid collected from a spinal tap and/or a blood sample drawn from a vein in your arm

Test Preparation Needed?


The Test Sample

What is being tested?

West Nile virus (WNV) is an infection that is transmitted to humans primarily by mosquitoes that have bitten infected birds. The virus is not passed directly from person-to-person, but there have been rare cases of WNV being transmitted to others through blood donations, organ transplants, and from a mother to child through breast milk.

Tests for WNV are used in conjunction with a person's signs and symptoms and history of exposure to detect and/or confirm an acute WNV infection. The tests help determine if a person is infected by measuring the antibodies produced in response to a WNV infection or by detecting the virus's genetic material in blood or cerebrospinal fluid (CSF).

About 80% of people infected with WNV experience no symptoms. In the other 20%, it causes flu-like symptoms such as headache, fever, nausea, muscular weakness, and/or a skin rash on the back or chest. These symptoms usually resolve without treatment within a few days to a few weeks. Only about 1 in 150 people infected with WNV becomes seriously ill with an infection that affects the central nervous system. These people may experience severe symptoms such as confusion, convulsions, high fever, neck stiffness, headaches, or a coma. They may have encephalitis and/or meningitis and/or may experience muscular paralysis. This serious form of WNV is much more common in the elderly and in the immunocompromised. While most symptoms resolve within several weeks, some nerve damage and paralysis may linger or be permanent.

How is the sample collected for testing?

Cerebrospinal fluid is collected from a spinal tap and/or a blood sample is drawn from a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

Ask a Laboratory Scientist

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Due to a dramatic increase in the number of questions submitted to the volunteer laboratory scientists who respond to our users, we have had to limit the number of questions that can be submitted each day. Unfortunately, we have reached that limit today and are unable to accept your inquiry now. We understand that your questions are vital to your health and peace of mind, and recommend instead that you speak with your doctor or another healthcare professional. We apologize for this inconvenience.

This was not an easy step for us to take, as the volunteers on the response team are dedicated to the work they do and are often inspired by the help they can provide. We are actively seeking to expand our capability so that we can again accept and answer all user questions. We will accept and respond to the same limited number of questions tomorrow, but expect to resume the service, 24/7, as soon as possible.

Article Sources

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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

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.

(March 9, 2009) National Institute for Allergy and Infectious Diseases. West Nile Virus Diagnosis. Available online at http://www.niaid.nih.gov/topics/westNile/understanding/Pages/diagnosis.aspx through http://www.niaid.nih.gov. Accessed January 2011.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Tabers Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosbys Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Petersen, L. and Marfin, A. (2002 August 6). West Nile Virus: A Primer for the Clinician. Annals of Internal Medicine Volume (137) 3:173-179 [On-line journal article]. Available online at http://www.annals.org/cgi/content/full/137/3/173 through http://www.annals.org.

(2003 September 24, Modified). West Nile Virus (WNV) Infection: Information for Clinicians. CDC, Division of Vector-Borne Infectious Diseases, West Nile Virus [On-line Fact Sheet]. Available online at http://www.cdc.gov/ncidod/dvbid/westnile/resources/fact_sheet_clinician.htm through http://www.cdc.gov.

(2003 September 10). Testing and Treating West Nile Virus in Humans. CDC, Division of Vector-Borne Infectious Diseases, West Nile Virus, 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.

(2003 July 15). Update on West Nile Virus. CDC Telebriefing Transcript [On-line News Conference]. Available online at http://www.cdc.gov/od/oc/media/transcripts/t030715.htm through http://www.cdc.gov.

(2003). Epidemic/Epizootic West Nile Virus in the United States: Guidelines for Surveillance, Prevention, and Control. CDC [On-line Guidelines]. PDF available for download at http://www.cdc.gov/ncidod/dvbid/westnile/resources/wnv-guidelines-aug-2003.pdf through http://www.cdc.gov.

Hayes, E. et. al (2004 February 27). Interim Guidelines for the Evaluation of Infants Born to Mothers Infected With West Nile Virus During Pregnancy. CDC, Division of Vector-Borne Infectious Diseases, West Nile Virus [On-line Clinical Guidelines, Published in MMWR (53) 7]. Available online at http://www.cdc.gov/ncidod/dvbid/westnile/congenitalinterimguidelines.htm through http://www.cdc.gov.

Martin, D. et. al (2000 May). Standardization of Immunoglobulin M Capture Enzyme-Linked Immunosorbent Assays for Routine Diagnosis of Arboviral Infections. Journal of Clinical Microbiology (38) 5:1823-1826 [On-line journal]. Available online at http://jcm.asm.org/cgi/content/full/38/5/1823 through http://jcm.asm.org.

(2003 July 9). FDA Clears First Test for West Nile Virus. FDA News [On-line News Release]. Available online at http://www.fda.gov/bbs/topics/NEWS/2003/NEW00920.html through http://www.fda.gov.

(2003 May). Revised Recommendations for the Assessment of Donor Suitability and Blood and Blood Product Safety in Cases of Known or Suspected West Nile Virus Infection. FDA [On-line Guidance for Industry]. Available online at http://www.fda.gov/cber/gdlns/wnvguid.htm through http://www.fda.gov.

(2002 December) Arbovirus IFA Slides. PANBIO [On-line package insert].  PDF available for download at http://www.panbio.com.au/prodinfo/I-WNV01X%20-%2004DEC02-005.pdf through http://www.panbio.com.au.

(2003 July 17, Revised). West Nile Virus IgM Capture ELISA. PANBIO [On-line package insert]. PDF available for download at http://www.panbio.com.au/prodinfo/E-WNV01M.pdf through http://www.panbio.com.au.

Bren, L. (2003 January-February). West Nile Virus: Reducing the Risk. FDA Consumer Magazine [On-line article]. Available online at http://www.fda.gov/fdac/features/2003/103_virus.html through http://www.fda.gov.

(2004 February 23). US Startled by Extent of West Nile in Blood Donors. Reuters by Yahoo! Health [On-line article]. Available online at http://health.yahoo.com/search/healthnews?lb=p&p=id%3A54298 through http://health.yahoo.com.

(2003 December 11). West Nile Virus IgM Capture ELISA. FOCUS Technologies, Package Insert [On-line information]. Available online through http://www.focusanswers.com.

(2003 June 25). Focus Submits West Nile Virus Diagnostic Kits to FDA for 510 (k) Clearance [5 paragraphs]. FOCUS Technologies [On-line Press Release]. Available online through http://www.focusanswers.com.

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. [On-line journal article]. 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.