Também conhecido como
Nome formal
West Nile Virus
Este artigo foi revisto pela última vez em
Este artigo foi modificado pela última vez em
12 de Janeiro de 2018.
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
When To Get Tested?
When a patient has symptoms suggesting WNV such as headache, fever, stiff neck, and muscular weakness and a diagnosis of encephalitis and/or meningitis; also used as a screen for WNV in donated units of blood
Sample Required?
Cerebrospinal fluid collected from a spinal tap and/or a blood sample drawn from a vein in your arm
Test Preparation Needed?
What is being tested?
West Nile virus (WNV) is an infection that is transmitted to humans primarily by mosquitoes. It is not usually transmitted person-to-person, but there have been cases of WNV being passed on to others through blood donations, organ transplants, and rarely from a mother to child through breast milk. About 80% of the 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 patients frequently 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 your arm.

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

No test preparation is needed.
Accordion Title
Common Questions
  • How is it used?
    West Nile virus (WNV) testing is used to determine whether someone is currently or has recently been infected with WNV. Testing of symptomatic and seriously ill patients can help distinguish WNV from other conditions (such as bacterial meningitis) causing similar symptoms. WNV testing is also used to screen units of blood for the virus, to detect WNV infection in the blood of living tissue and organ donors, and to track the spread of WNV through a community and across the country. Detecting the presence of WNV in the community can alert health providers and promote prevention measures.

    Testing involves measurement of WNV antibodies, specific proteins created by the body’s immune system in response to a WNV infection, or measurement of WNV nucleic acid, genetic material from the virus itself.

    Antibody Testing
    There are two types of WNV antibodies: IgM and IgG. IgM antibodies are the first to be produced by the body in response to a WNV infection. They are present in most individuals within 8 days of the initial exposure. Antibody titers continue to rise for a short time period and then will taper off. Eventually, after several months, the IgM antibodies fall below detectible levels.

    IgM WNV antibody testing may be performed on the blood or cerebrospinal fluid of symptomatic patients as an initial test. The WNV tests available may be positive both with WNV and with any related flaviviruses (viruses in the same family, such as St. Louis Encephalitis virus and Japanese Encephalitis virus). For this reason, most positive WNV IgM tests must be confirmed by another method before a diagnosis is established and officially reported to the Centers for Disease Control and Prevention (CDC).

    If the IgM test is negative, but symptoms and clinical signs still suggest WNV, the test may be repeated on a new specimen collected a few days later.

    IgG WNV antibody testing can be used, along with IgM testing, to help confirm the presence of a recent or previous WNV infection. If the IgG test is positive, then another convalescent blood sample should be collected and tested a couple of weeks later.

    If someone who has received donated blood or an organ transplant within the previous month becomes ill and tests positive for WNV, the blood or organ is traced back to its donor, who is then tested for IgM and IgG WNV antibodies to determine whether they are the source of the recipient’s WNV. 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). Since the majority of patients who become infected with WNV have no symptoms and no associated health problems, antibody testing is not used as a general screening test on asymptomatic people.

    Nucleic Acid Testing
    Nucleic acid testing involves amplifying and measuring the West Nile virus’s genetic material to detect the presence of the virus in blood or tissue. 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 incidental 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, 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. This gives the community health services an opportunity for prevention by alerting people to its presence and the need for preventive measures, such as the use of bug repellents, limiting exposure to mosquitoes at dusk and dawn (when mosquitoes are most active), and community spraying for mosquitoes.

    It also may be useful when ordered post mortem on a patient’s blood or tissues to determine whether WNV may have caused or contributed to the patient’s death.

  • When is it ordered?
    Antibody testing is usually ordered during the WNV season (the peak mosquito season is generally July to October, but in some regions they may be present year-round) and when patients have traveled to areas where WNV is currently present. IgM antibody testing is primarily ordered when a patient has new symptoms suggesting a current WNV infection such as:
    • headache
    • fever, chills
    • nausea, vomiting
    • muscular weakness
    • skin rash on the back or chest.

    Two to four weeks after a positive WNV test, IgM and IgG WNV tests may be ordered on a convalescent blood sample. If an initial IgM test is negative but symptoms persist and other conditions are ruled out, another IgM test may be ordered a few days later to determine whether IgM WNV antibodies are now present.

    Testing is not usually done on 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 they were the source of the infection.

    Nucleic acid testing is now 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.

  • What does the test result mean?
    Antibody Testing
    If the IgM WNV antibody is positive in blood or cerebrospinal fluid (CSF) and confirmed by another method, then it is likely that the patient has a current WNV infection, or that they 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 patient 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 detectible, then it is most likely that the patient was previously exposed to WNV but is not currently infected. Also, 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 presence of WNV antibodies may indicate an infection, but they cannot be used to predict the severity of an individual patient’s symptoms or their prognosis.

    Nucleic Acid Testing
    If a nucleic acid test is positive for WNV, then it is likely that the virus is present in the sample tested (donated blood; blood from a living donor; 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 nucleic acid test 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.

  • Is there anything else I should know?
    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. According to the Centers for Disease Control and Prevention (as of September 18, 2007), there have been 1,982 confirmed human cases of WNV in the U.S. in 2007 and 49 deaths attributed to it as the cause.

    Nucleic acid testing and viral cultures are used in research settings to identify the strain of virus causing the infection and to study its attributes. Different strains of WNV have been isolated and associated with different epidemics around the world.

  • Is there a vaccine for West Nile virus?
    Not for humans yet, but there may be one or more vaccines available in the next few years. A vaccine has been developed by mixing West Nile virus with a vaccine for yellow fever, altering the proteins coating the established vaccine. This new vaccine has been successfully tested for safety and effectiveness in animals and is now being tested in humans.

    Another potential West Nile virus vaccine that uses an inactive protein (instead of a live virus) also has been developed and undergone some initial testing. It would have the advantage of being able to be given to anyone, even children, pregnant women, or those who are immunosuppressed.

  • Is it safe to donate and receive blood?
    Yes. There is no risk for the donor, and WNV nucleic acid testing has 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).
View Sources
Thomas, Clayton L., Editor (1997). Taber’s 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 [26 paragraphs]. Annals of Internal Medicine Volume (137) 3:173-179 [On-line journal article]. Available FTP: http://www.annals.org/cgi/content/full/137/3/173

(2003 September 24, Modified). West Nile Virus (WNV) Infection: Information for Clinicians [17 paragraphs]. CDC, Division of Vector-Borne Infectious Diseases, West Nile Virus [On-line Fact Sheet]. Available FTP: http://www.cdc.gov/ncidod/dvbid/westnile/resources/fact_sheet_clinician.htm

(2003 September 10). Testing and Treating West Nile Virus in Humans [18 paragraphs]. CDC, Division of Vector-Borne Infectious Diseases, West Nile Virus, Questions and Answers [On-line Information]. Available FTP: http://www.cdc.gov/ncidod/dvbid/westnile/qa/testing_treating.htm

(2003 July 15). Update on West Nile Virus [100 paragraphs]. CDC Telebriefing Transcript [On-line News Conference]. Available FTP: 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 [77 pages]. CDC [On-line Guidelines]. Available FTP: 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 [8 paragraphs]. CDC, Division of Vector-Borne Infectious Diseases, West Nile Virus [On-line Clinical Guidelines, Published in MMWR (53) 7]. Available FTP: 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 [17 paragraphs]. Journal of Clinical Microbiology (38) 5:1823-1826 [On-line journal]. Available FTP: http://jcm.asm.org/cgi/content/full/38/5/1823

(2003 July 9). FDA Clears First Test for West Nile Virus [10 paragraphs]. FDA News [On-line News Release]. Available FTP: 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 [43 paragraphs]. FDA [On-line Guidance for Industry]. Available FTP: http://www.fda.gov/cber/gdlns/wnvguid.htm

(2002 December) Arbovirus IFA Slides [22 paragraphs]. PANBIO [On-line package insert]. Available FTP: http://www.panbio.com.au/prodinfo/I-WNV01X%20-%2004DEC02-005.pdf

(2003 July 17, Revised). West Nile Virus IgM Capture ELISA [64 paragraphs]. PANBIO [On-line package insert]. Available FTP: http://www.panbio.com.au/prodinfo/E-WNV01M.pdf

Bren, L. (2003 January-February). West Nile Virus: Reducing the Risk [39 paragraphs]. FDA Consumer Magazine [On-line article]. Available FTP: http://www.fda.gov/fdac/features/2003/103_virus.html

(2004 February 23). US Startled by Extent of West Nile in Blood Donors [12 paragraphs]. Reuters by Yahoo! Health [On-line article]. Available FTP: http://health.yahoo.com/search/healthnews?lb=p&p=id%3A54298

(2003 December 11). West Nile Virus IgM Capture ELISA [91 paragraphs]. FOCUS Technologies, Package Insert [On-line information]. Available online

(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

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 [7 paragraphs]. FDA News [On-line press release]. Accessed on: 9/09/07. Available FTP: http://www.fda.gov/bbs/topics/NEWS/2007/NEW01691.html

(2007 March 2). FDA Approves First Fully Automated Test to Screen for West Nile Virus in Blood and Tissue Donors [5 paragraphs]. FDA News [On-line press release]. Accessed on: 9/09/07. Available FTP: http://www.fda.gov/bbs/topics/NEWS/2007/NEW01578.html

(2006 August 25). Recommendations for Protecting Laboratory, Field, and Clinical Workers from West Nile Virus Exposure [19 paragraphs]. National Institute for Occupational Safety and Health [On-line information]. Accessed on: 9/09/07. Available FTP: http://www.cdc.gov/niosh/topics/westnile/reclab.html

(2005 September 4). What You Need to Know about Mosquito Repellent [19 paragraphs]. CDC West Nile Virus Fact Sheet [On-line information]. Accessed on: 9/09/07. Available FTP: http://www.cdc.gov/ncidod/dvbid/westnile/mosquitorepellent.htm

(2005 September 27). West Nile Virus Fact Sheet [20 paragraphs]. CDC West Nile Virus Fact Sheet [On-line information]. Accessed on: 9/09/07. Available FTP: http://www.cdc.gov/ncidod/dvbid/westnile/wnv_factsheet.htm

(2007 August 1, Reviewed). Testing and Treating West Nile Virus in Humans [18 paragraphs]. CDC Questions and Answers [On-line information]. Accessed on: 9/09/07. Available FTP: http://www.cdc.gov/ncidod/dvbid/westnile/qa/testing_treating.htm

(2007 June 22). West Nile Virus Sequelae Can Be Long-Term [10 paragraphs]. Medscape Reuters Health Information [On-line information]. Accessed on: 9/09/07. Available FTP: http://www.medscape.com/viewarticle/558721

Murray, K. et. al. (2007 April 05). Depression after Infection with West Nile Virus [16 paragraphs]. Medscape from Emerg Infect Dis. 2007;13(3): 479-481. [On-line journal article]. Accessed on: 9/09/07. Available FTP: http://www.medscape.com/viewarticle/554199

(2007 May 1). West Nile virus [30 paragraphs]. MayoClinic.com [On-line information]. Accessed on: 9/09/07. Available online