Patient Test Information

West Nile Virus Testing

Also known as:

WNV Tests; WNV by NAAT

Formal name:

West Nile Virus, IgM and IgG Antibodies and Nucleic Acid Amplification Tests

Related tests:

CSF Analysis, Arbovirus Testing, Dengue Fever Testing

Why Get Tested?

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 to Get Tested?

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

Sample Required?

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

Test Preparation Needed?

None

How is it used?

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.

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 Tests
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.

    • IgM antibodies are the first to be produced by the immune system in response to a WNV infection. They are present in most individuals within a week or two of the initial exposure. The level of antibody (titer) continues to rise for a short time period and then will taper off. Eventually, after several months, the IgM antibodies fall below detectable levels.

IgM WNV antibody testing is the primary test performed on the blood or cerebrospinal fluid of symptomatic people. The WNV tests available may be positive both with WNV and with any related arboviruses such as the 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.

  • Testing for WNV IgG antibodies can sometimes be used in conjunction with IgM testing to help detect the presence of a recent or previous WNV infection. The IgG WNV test may be ordered once with the IgM test or ordered initially and then again 2-4 weeks later to determine if titers are rising or falling.

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.

When is it ordered?

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:

  • Headache
  • Fever, chills
  • Nausea, vomiting
  • Muscular weakness
  • Skin rash on the back or chest

Signs and symptoms associated with more serious WNV that has central nervous system involvement may include:

  • High fever
  • Extreme muscle weakness
  • Neck stiffness
  • Stupor, disorientation
  • Tremors, convulsions
  • Coma
  • Paralysis (rarely)

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 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.

What does the test result mean?

Antibody Tests
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 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 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
Positive Negative Current infection
Positive Positive Recent infection
Low or negative or not tested Four-fold increase in samples collected 2-4 weeks apart Recent infection
Low or negative Positive Past infection
Negative Negative
  • Too soon after initial exposure for antibodies to develop
  • Symptoms due to another cause

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.

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. 

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.

For the most current numbers of confirmed human cases of WNV in the US 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 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.

What is being tested?

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).

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:

  • Antibody testing–detects either IgM or IgG class of WNV antibodies
  • Nucleic acid amplification (NAAT)–detects the genetic material (RNA) of WNV

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.

  1. Is there a vaccine for West Nile virus (WNV)?

    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.

  2. What can I do to protect against West Nile virus?

    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.

  3. Are people who have no symptoms ever tested for WNV?

    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 he 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).

  4. Is it safe to donate and receive blood?

    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).