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