Also Known As
Formal Name
West Nile Virus
This article was last reviewed on
This article waslast modified on 23 March 2023.
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)

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 (CSF) collected from a spinal tap and/or a blood sample taken from a vein in your arm

Test Preparation Needed?


On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

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 (CSF) is collected from a spinal tap and/or a blood sample is taken 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 in the US and Canada 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 titres 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. 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 blood sample should be collected and tested a couple of weeks later.

    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.

  • When is it requested?

    Antibody testing may be requested 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 travelled to areas where WNV is currently present. IgM antibody testing may be primarily requested 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 follow up 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 requested 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 titres in subsequent 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 UK Health Security Agency (as of 20th August 2018), there have been occasional cases of infection in the UK associated with travel. Previously, the main risk of WNV for UK residents has been for those travelling abroad. However, in 2010, the mosquito species responsible for transmission of the virus (between birds, horses and humans) was detected for the first time in the UK since 1944. This finding may increase the risk of WNV being transmitted in the UK.

    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 blood products as safe as it can possibly be for the recipients. As an additional tool in reducing WNV in the blood supply, blood collection centres have recently started asking potential donors during WNV season if they have travelled to the USA or Canada within the previous 28 days.