West Nile virus (WMV) test
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
This test checks whether you have been infected with West Nile virus — a virus spread mainly by mosquito bites. It is used when a patient has symptoms of a serious brain or spinal cord infection and may have been exposed to the virus through travel or, rarely, through blood transfusion or organ transplant.
Who needs this test
West Nile virus (WNV) is rare in the UK. Most cases in UK residents are associated with travel to areas where the virus is present, including parts of Europe (particularly southern and eastern Europe), the Middle East, Africa, North America, and parts of Asia.
Testing is most likely to be needed if you have returned from an affected area and then develop symptoms. The virus spreads mainly through the bite of an infected mosquito. It does not pass from person to person in normal contact, though rare cases of transmission through blood transfusion, organ transplantation, and occasionally from mother to baby through breast milk have been reported.
Your doctor may request this test if you have symptoms that suggest your brain or spinal cord may be affected — a condition called encephalitis or meningitis — and your symptoms could be due to WNV. This is more likely if your symptoms developed during or shortly after the mosquito season (typically July to October in temperate regions) and you have been in a country where WNV is active.
People most at risk of serious illness if infected include those aged 60 and over, and those with a weakened immune system.
Preparing for your test
No special preparation is needed. Depending on your symptoms, one or both of the following samples may be collected:
- Blood sample — taken from a vein in your arm. Used to look for WNV antibodies or, in some circumstances, for the virus itself.
- Cerebrospinal fluid (CSF) — collected by a lumbar puncture (spinal tap). If WNV infection of the brain or spinal cord is suspected, CSF testing is important because it can detect antibodies in the fluid directly surrounding the central nervous system.
WNV testing is performed at specialist reference laboratories and is not available at all hospitals. Your doctor will arrange referral to the appropriate service.
Understanding your results
What the test measures
West Nile virus is a flavivirus — a family of viruses that includes dengue fever, Zika, and yellow fever. In humans, WNV is spread almost entirely through infected mosquito bites. Mosquitoes pick up the virus by feeding on infected birds, which are the main carriers. Humans are what is known as dead-end hosts — we do not pass the virus back to mosquitoes or on to other people in normal circumstances.
Most people infected with WNV — around 80% — have no symptoms at all. Of those who do feel unwell, most experience a mild flu-like illness that gets better on its own within a few days to a few weeks. Only about 1 in 150 people infected develops a serious illness affecting the central nervous system.
Two types of test are used:
- Antibody tests (IgM and IgG) — detect the immune response your body makes to WNV. These are the most commonly used tests.
- Nucleic acid test (NAT) — detects the virus’s genetic material directly. Used mainly for blood donation screening or in specialist diagnostic settings.
Positive WNV antibody tests need to be confirmed by a second method before a diagnosis is made, as related viruses can give a similar result.
Results are always interpreted alongside your symptoms, travel history, and the time of year.
A negative result does not always rule out WNV, particularly if tested very early in the illness when antibodies may not yet be detectable.
IgM antibody positive
IgM antibodies are the first antibodies the body makes in response to a WNV infection. They are usually detectable in the blood or CSF within 8 days of infection. A positive IgM result — confirmed by a second test — suggests a current or very recent WNV infection. If IgM is detected in the CSF (cerebrospinal fluid), this suggests the infection has reached the central nervous system.
Because WNV IgM tests can also react with related flaviviruses, a positive result must be confirmed by a specialist reference laboratory before a diagnosis is confirmed.
IgG antibody positive
IgG antibodies develop a few weeks after infection and persist in the blood for months to years. If only IgG is positive (with low or absent IgM), this usually means the infection occurred in the past and you have recovered. If both IgM and IgG are positive together, infection likely occurred at least three weeks before testing. A rising IgG level on a repeat test taken two weeks later confirms a more recent infection.
Nucleic acid test (NAT) positive
A positive nucleic acid test confirms that WNV genetic material was detected in the sample. However, because WNV levels in human blood are typically low and short-lived, this test may be negative even when infection is present. A negative NAT does not rule out WNV.
If results are negative but suspicion remains
If your initial test is negative but symptoms continue and WNV remains a possible diagnosis, your doctor may repeat the test a few days later. IgM antibodies may not yet be detectable very early in the illness. Your doctor will consider other causes of encephalitis or meningitis at the same time.
Questions to ask your doctor
What happens next
If your test is negative, your doctor will look for other possible causes of your symptoms, including other viral and bacterial infections that can cause meningitis or encephalitis.
There is no specific antiviral treatment for West Nile virus. Management focuses on supportive care — rest, fluids, and medicines to manage fever and pain for mild cases. Severely ill patients with encephalitis or meningitis will be managed in hospital, often in an intensive care unit if needed. Most people with the mild form of the illness recover fully. People with the neurological form of the disease may recover more slowly and some effects, such as muscle weakness or memory difficulties, can persist for months.
If WNV is confirmed, your case will be reported to the UK Health Security Agency (UKHSA). This helps public health authorities track the spread of the infection and identify any risk to the blood supply or organ donation.
What can affect your results
- Timing of the test — IgM antibodies take up to 8 days to appear after infection. Testing too early may give a false negative. Repeat testing may be needed.
- Cross-reaction with related viruses — WNV antibody tests can react with other flaviviruses, including those associated with dengue, Zika, yellow fever, and Japanese encephalitis. Confirmation testing is always needed.
- Previous vaccination — vaccines for related flaviviruses (such as yellow fever or Japanese encephalitis) can cause a positive WNV antibody result. Tell your doctor about any vaccinations you have had.
- Weakened immune system — people with a weakened immune system may not produce detectable antibodies even when infected. Nucleic acid testing may be more useful in these patients.
Other tests you might need
When WNV infection is suspected, other tests are likely to be done at the same time to investigate the full picture:
- CSF analysis — a lumbar puncture to check the cerebrospinal fluid for signs of infection and inflammation in the central nervous system.
- Full blood count (FBC) and inflammatory markers — to assess the overall response to infection.
- Tests for other causes of meningitis and encephalitis — including bacterial cultures, herpes simplex virus (HSV) PCR, and other viral tests, as many infections can cause similar symptoms.
- Brain imaging (MRI or CT scan) — may be arranged to look for signs of inflammation in the brain.
About West Nile virus
West Nile virus is a mosquito-borne infection that can, in rare cases, cause serious brain inflammation (encephalitis). It is present in parts of Europe, Africa, the Middle East, North America, and Asia. Most people infected have no symptoms or mild illness. There is currently no vaccine licensed for use in humans. Find out more about West Nile virus on the NHS website and the UK Health Security Agency website.