Arbovirus testing is used to determine whether a person with symptoms and a recent history of potential exposure to a specific arbovirus has been infected. Testing can help diagnose the cause of meningitis or encephalitis, distinguish an arbovirus infection from other conditions causing similar symptoms, such as bacterial meningitis, and can help guide treatment. Testing may be performed on blood to detect antibodies to the viruses and/or may be performed on a sample of cerebrospinal fluid (CSF) to determine if an infection is present in the central nervous system.
Typically, the individual test ordered is specific for a particular arbovirus, such as West Nile Virus (WNV) or dengue fever, depending on the person's symptoms and likely exposure. Sometimes, a panel of tests may be used to determine which arbovirus is causing the infection.
Most often, testing involves the measurement of specific arbovirus antibodies produced in response to an infection. In certain cases, testing may involve direct detection by testing for the genetic material (nucleic acid) of the virus.
Antibody Tests Antibody testing is primarily used to help diagnose a current or recent infection. There are two classes of arbovirus antibodies produced in response to infection: IgM and IgG. IgM antibodies are produced first and are present within a week or two after infection. Levels in the blood rise for a few weeks, then taper off. After a few months, IgM antibodies fall below detectable levels. IgG antibodies are produced after IgM antibodies. Typically, the level rises with an acute infection, stabilizes, and then persists long-term.
IgM antibody testing is the primary test performed on the blood or cerebrospinal fluid of symptomatic people. IgG tests may be ordered along with IgM testing to help diagnose a recent or previous arbovirus infection. Sometimes testing is done by collecting two samples 2 to 4 weeks apart (acute and convalescent samples) and measuring the IgG level (titer). This may help determine whether antibodies are from a recent or past infection.
Antibody tests may cross-react with viruses in the same family, so a second test that employs a different method, such as nucleic acid amplification test (NAAT) or a neutralization assay, is used to confirm positive results. These confirmatory methods are specialized tests that may be performed at a public health laboratory or the CDC. They must be done before a diagnosis is established and officially reported to the CDC.
Nucleic Acid Amplification Test A nucleic acid amplification test (NAAT) amplifies and measures the arbovirus's genetic material to detect the presence of the virus. This type of testing is done only at a few specialized laboratories. It can detect a current infection with the virus, often before antibodies to the virus are detectable, but there must be a certain amount of virus present in the sample in order to detect it. For most arboviruses, virus levels in humans are usually low and do not persist for very long.
Nucleic acid testing can also be used to screen for an arbovirus such as WNV in donated units of blood, tissue, or organs. It may be used to test the tissues of a person who has died (post mortem) to determine whether a specific arbovirus may have caused or contributed to their death.
Testing can also be performed on suspected host animals and mosquito pools to detect the presence and spread of an arbovirus in the community and region. This information can be used to help investigate outbreaks, identify and monitor infection sources, and to guide efforts to prevent the spread of the infection.
Antibody tests are primarily ordered when a person has signs and symptoms suggesting a current arbovirus infection, especially if the person lives in or has recently traveled to an area where a specific arbovirus is endemic.
In the U.S., an arbovirus infection may be suspected when symptoms arise during mid to late summer. In warmer areas, infections may occur year-round.
Antibody tests may be ordered within the first week or two of the onset of symptoms to detect an acute infection. An additional blood sample may be collected 2 to 4 weeks later to determine if the antibody level is rising. When an infection of the central nervous system is suspected, antibody testing may be performed on CSF as well as blood.
Nucleic acid amplification testing (NAAT) is not ordered as frequently as antibody testing but may sometimes be ordered when a person has symptoms of an arbovirus infection. NAAT testing is also now routinely used in the U.S. to screen units of donated blood for WNV and may be performed on the blood of tissue and organ donors prior to transplantation.
Results of arbovirus testing require careful interpretation, taking into consideration the individual's signs and symptoms as well as risk of exposure.
Antibody Tests Antibody tests may be reported as positive or negative, or may be reported as less than or greater than a certain titer. For example, if the established threshold is a titer of 1:10, then a result less than this is considered negative while a titer greater than this is considered positive.
If IgM or IgG antibody is detected in the CSF, it suggests that an arbovirus infection is present in the central nervous system. If a CSF antibody test is negative, then it suggests that there is no central nervous system involvement or the level of antibody is too low to detect.
If IgM and IgG arbovirus antibodies are detected in an initial blood sample, then it is likely that the person became infected with the arbovirus within the last few weeks. If the IgG is positive but the IgM is low or negative, then it is likely that the person had an arbovirus infection sometime in the past. If the arbovirus IgG antibody titer increases four-fold between an initial sample and one taken 2 to 4 weeks later, then it is likely that a person has had a recent infection.
If the tests are negative for IgM and/or IgG antibodies, the person may still have an arbovirus infection – it may just be that it is too soon after initial exposure to the virus and there has not been enough time to produce a detectable level of antibody. A negative result may also suggest that symptoms may be due to a different cause, such as bacterial meningitis.
The following table summarizes results that may be seen with antibody testing:
Low or negative or not tested
Four-fold increase in samples taken 2-4 weeks apart
Low or negative
Too soon after initial exposure for antibodies to develop
Symptoms due to another cause
A positive result on an initial test for IgM arbovirus antibody in blood or CSF is considered a presumptive positive since antibodies to viruses in the same family may cross-react. It suggests a diagnosis, but it is not definitive. A positive result on a second test using a different method (NAAT or neutralization assay) confirms the diagnosis.
Nucleic Acid Amplification Testing (NAAT) If a NAAT blood, CSF, or tissue test is positive for an arbovirus, then it is likely that that specific virus is present in the sample tested. A positive NAAT for arbovirus in an animal or mosquito pool sample indicates that the virus is present in the geographic location where the sample was collected.
A NAAT may be negative for an arbovirus 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 an arbovirus.
The presence of arbovirus antibodies may indicate an infection but cannot be used to predict the severity of an individual's symptoms or their prognosis.
Other tests, such as antigen tests for dengue fever, and viralcultures may be used in some instances. NAAT and viral cultures may be used in research settings and by the medical community at a national and international level to identify and study the strains of arboviruses causing infections. Different strains have been isolated and associated with regional epidemics.
This article was last reviewed on April 16, 2012. | This article was last modified on January 27, 2016.
The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.
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