West Nile Virus

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.

• 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.

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.

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.