LabCorp and its Specialty Testing Group, a fully integrated portfolio of specialty and esoteric testing laboratories.
To help diagnose an infection caused by West Nile virus (WNV); to determine the cause of viral meningitis or encephalitis or febrile illness that occurs during the summer season; to detect the presence of 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 you have symptoms suggesting WNV such as headache, fever, stiff neck, and muscular weakness and have been exposed to mosquitos or when you have had a diagnosis of encephalitis and/or meningitis associated with the summer season
Cerebrospinal fluid collected from a spinal tap and/or a blood sample drawn from a vein in your arm
West Nile virus (WNV) is an infection that is transmitted to humans primarily by mosquitoes that have bitten infected birds or other infected humans. Testing detects either the West Nile virus directly or antibodies produced in response to WNV infection in blood and/or cerebrospinal fluid (CSF).
West Nile virus is a member of the Flavivirus family, which includes viruses such as Zika, Yellow Fever, and Dengue. WNV also belongs to the Arboviruses group, which are transmitted to people by insects, such as mosquitoes. In 2014, a total of 2,204 cases of WNV disease were reported in the United States, including 1,347 cases affecting the central nervous system (neuroinvasive). More than half of the detected infections were from states of California or Texas.
In addition to birds, WNV may also be transmitted through contact with other infected animals, their blood, or other tissues. The virus is not passed through handling or consumption of infected birds or directly from person-to-person; however, there have been rare cases of WNV being transmitted to others through blood donations, organ transplants, and from a mother to child during pregnancy, delivery, or through breast milk.
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, people with weakened immune systems (immunocompromised), and individuals with underlying conditions such as diabetes or high blood pressure (hypertension). While most symptoms resolve within several weeks, some nerve damage and paralysis may linger or be permanent.
The two types of WNV testing include:
West Nile virus (WNV) testing is used to help determine whether a person with signs and symptoms and a history of recent exposure has an acute WNV infection. WNV is an infection that is transmitted to humans primarily by mosquitoes that have bitten infected birds or other infected humans. Testing of symptomatic and seriously ill people can help distinguish WNV from other conditions causing similar symptoms, such as bacterial meningitis, and can help guide treatment.
WNV testing is also used to ensure donated blood is free of WNV. In 2016, 288 blood donors, who were otherwise asymptomatic, were identified as WNV-positive when screened for the virus. 45 of those donors developed clinical illness with WNV disease.
Testing is not used for screening asymptomatic people. The majority of those who become infected with WNV (about 80%) will have no symptoms and no associated health problems, so testing is not useful in these cases.
Two types of WNV tests are available:
Antibody testing is primarily used to help diagnose a current or recent infection. There are two classes of WNV antibodies produced in response to infection: IgM and IgG.
Nucleic Acid AmplificationTest
A nucleic acid amplification test (NAAT) amplifies and measures the West Nile virus's genetic material to detect the presence of the virus. This test can detect a current infection with the virus often before antibodies to the virus are detectable. While it can specifically identify the presence of WNV, there must be a certain amount (number of copies) of virus present in the sample in order to detect it.
Since humans are secondary hosts of WNV (birds are the primary hosts), virus levels in humans are usually relatively low and do not persist for very long.
Nucleic acid testing is most useful as a screen for WNV in donated units of blood, tissue, or organs, for detecting WNV in the blood of living tissue and organ donors, and for testing birds and mosquito pools to detect the presence and spread of WNV in the community. It is possible to determine that WNV has spread to a particular area and is in the bird and mosquito population before any human cases are identified. It also may be used to test the blood or tissues of a person who has died (post mortem) to determine whether WNV may have caused or contributed to their death.
Antibody tests are primarily ordered when a person has new signs and symptoms suggesting a current WNV infection, particularly if the person lives in or has traveled to an area where WNV is endemic. An infection may be suspected, especially if symptoms arise during the WNV season. In the United States, the peak mosquito season is generally July to October, but in some regions they may be present year-round.
Some signs and symptoms of WNV include:
Signs and symptoms associated with more serious WNV that has central nervous system involvement may include:
Two to four weeks after a positive WNV test, IgM and IgG WNV tests may be ordered on a convalescent blood sample (from recovering patient). If an initial IgM test is negative but symptoms persist and other conditions are ruled out, another IgM test may be repeated a few days later to determine whether IgM WNV antibodies are now detectable.
Nucleic acid amplification tests (NAAT) are routinely used to screen units of donated blood for WNV and may be performed on the blood of tissue and organ donors prior to transplantation.
If the IgM WNV antibody is positive in blood or cerebrospinal fluid (CSF) and confirmed by another method, then it is likely that the person has a current WNV infection, or that the person had one in the recent past. If the IgM antibody is detected in the CSF, it suggests that the WNV infection is present in the central nervous system.
If IgM WNV and IgG WNV antibodies are detected in the initial sample, then it is likely that the person contracted the WNV infection at least 3 weeks prior to the test. If the IgG WNV antibody is positive and the IgM WNV antibody level is low or not detectable, then it is most likely that the person was previously exposed to WNV but is not currently infected. If WNV IgG antibody titers in convalescent samples continue to rise, this change would indicate a more recent infection. If the WNV IgG antibody levels have not changed or have decreased, this would indicate a past but not recent infection.
The following table summarizes results that may be seen with WNV antibody testing:
|IgM Result||IgG Result||Possible Interpretation|
|Low or negative or not tested||Four-fold increase in samples collected 2-4 weeks apart||Recent infection|
|Low or negative||Positive||Past infection|
The presence of WNV antibodies may indicate an infection but cannot be used to predict the severity of an individual person's symptoms or their prognosis.
Due to the possibility of false-positive results due to cross-reactivity of the antibodies with other viruses, or non-specific reaction, all positive WNV antibody tests are usually confirmed using another test. The confirmatory test (neutralization antibody test) is performed at specialized laboratories, such as state public health or CDC laboratories. The neutralization antibody test confirms whether the detected antibodies are capable of binding and inactivating the WNV. Viral cultures and immunohistochemistry can also be used to detect WNV, although they are not used in routine testing.
Nucleic Acid AmplificationTesting (NAAT)
If a NAAT is positive for WNV, then it is likely that the virus is present in the sample tested (donated blood; blood from a donor; CSF; a tissue sample from a human, bird, or other animal; or a mosquito pool sample) and is present in the geographic location where the sample was collected.
A NAAT may be negative for WNV if there is no virus present in the sample tested or if the virus is present in very low (undetectable) numbers. A negative test cannot be used to definitely rule out the presence of WNV. A NAAT may detect WNV as long as the virus is actively replicating in the person.
In some warm areas, WNV is present year-round, but in most regions, it is seasonal: cases occur during the mosquito season. The amount of WNV present depends in part on the number of infected birds and the mosquito population. Prevention depends on controlling individual exposure and on controlling the mosquito population.
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 Centers for Disease Control and Prevention's West Nile Virus web site.
NAAT and viral cultures are used in research settings to identify and subtype the strain of virus causing the infection and to study its attributes. These tests are important in studying the spread and sometimes the source of infections or epidemics. Different strains of WNV have been isolated and associated with different epidemics around the world.
Not for humans yet, but there may be one or more vaccines available in the next few years. Research is ongoing. Vaccines for other flaviviruses, such as yellow fever, have been available for about 70 years and have well-established safety and efficacy records, so researchers are optimistic that a solution can be found.
You can protect yourself from WNV by protecting against mosquito bites. Preventive measures include using insect repellent, wearing long-sleeved clothing and pants when outdoors, staying indoors at dawn and dusk when mosquitoes are most active, and eliminating standing water sources that attract mosquitoes. Communities also take preventive measures by monitoring the seasonal risks and movement of WNV and spraying for mosquitoes as warranted.
Testing is usually not done for asymptomatic people, but when a blood or organ recipient becomes infected with WNV, both IgM and IgG antibodies may be ordered on the donor (who is frequently asymptomatic) to help determine whether that person was the source of the infection.
Similarly, if a breastfeeding baby contracts WNV, the mother will likely be tested to determine whether the infection may have passed to the baby through the mother's milk (a rare but documented event).
Moreover, all donated blood is tested for contamination with WNV by NAAT. Donors suspected of having WNV in their blood are notified and referred for counseling.
Yes. There is no risk for the donor, and WNV nucleic acid amplification tests (NAATs) have been added to the list of extensive testing that is done to make the U.S. blood supply as safe as it can possibly be for the recipients. As an additional tool in reducing WNV in the blood supply, blood collection centers have recently started asking potential donors during WNV season if they have had a recent fever or headache (symptoms of an infection with WNV or other virus).
Sources Used in Current Review
2018 review performed by Yanal M. Murad, PhD, DABMM.
Rutledge TF. Morbidity and Mortality Weekly Report Surveillance for Human West Nile Virus Disease — United States , 1999 – 2008. 2010;59(31):1999-2008.
WHO | West Nile virus. WHO. Available online at http://www.who.int/mediacentre/factsheets/fs354/en/. Published 2016. Accessed February 12, 2018.
West Nile Virus Disease Cases and Presumptive Viremic Blood Donors by State – United States, 2016. 2016. Available online at https://www.cdc.gov/westnile/resources/pdfs/data/WNV-Disease-Cases-and-PVDs-by-State-2016_09292017.pdf. Accessed February 12, 2018.
Centers for Disease Control and Prevention. Diagnostic Testing, West Nile Virus. Available online at https://www.cdc.gov/westnile/healthcareproviders/healthcareproviders-diagnostic.html. Accessed February 12, 2018.
Fda, Cber. Draft Guidance for Industry: Use of Nucleic Acid Test on Pooled and Individual Samples from Donors of Whole Blood and Blood Components for Transfusion to Adequately and Appropriately Reduce the Risk of Transmission of WNV. 2009. Available online at http://www.fda.gov/BiologicsBloodVaccines/GuidanceComplianceRegulatoryInformation/. Accessed February 12, 2018.
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). Mosby's 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.
(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.
(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.
(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.
(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.
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.
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.
(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.
(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.
(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.
(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.
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.
(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.
(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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Accessed January 2011.
(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. Accessed May 2014.
(© 1995–2014). West Nile Virus (WNV) Antibody, IgG and IgM, Serum. Mayo Clinic Mayo Medical Laboratory [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/84186. Accessed May 2014.
(Reviewed 2013 June 7). Diagnostic Testing WNV Antibody Testing. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/westnile/healthCareProviders/healthCareProviders-Diagnostic.html. Accessed May 2014.
Gavin, M. (Reviewed 2012 August). West Nile Basics. TeensHealth from Nemours [On-line information]. Available online at http://kidshealth.org/teen/infections/bacterial_viral/west_nile.html. 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. 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. 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. Accessed May 2014.