Arbovirus Testing

Arbovirus testing is used to determine whether a person with signs and symptoms, plus a recent history of potential exposure to a specific arbovirus, has been infected. Testing can be important to distinguish an arbovirus infection from other conditions causing similar symptoms, such as bacterial meningitis, and can help guide treatment.

Typically, the individual test requested is specific for a particular arbovirus, such as West Nile Virus (WNV) or dengue fever, depending on the person's symptoms and likely exposure. Individual test selection depends on a person’s symptoms and where they have been living or travelling. Sometimes, a panel of tests may be carried out to determine which arbovirus is causing the infection.

Two types of tests are available:

Antibody Tests
Antibody testing detects antibodies produced in response to a specific arbovirus infection. There are two classes of antibodies that may be detected:

• IgM antibodies are produced first and are present within a week or two after the start of symptoms. Levels rise for a few weeks, then taper off, becoming undetectable after a few months. IgM antibody testing is the primary test performed on the blood or cerebrospinal fluid of symptomatic people.
• IgG antibodies are produced after IgM antibodies. Typically, the level rises with an acute infection, stabilises and then persists long-term. 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 (titre). This may help determine whether antibodies are from a recent or past infection.

Antibody tests may cross-react with viruses that are similar, so a second test that employs a different method may be used to confirm positive results.

Nucleic Acid Amplification Test
A nucleic acid amplification test (NAAT) amplifies and measures the arbovirus's genetic material to detect the presence of a specific virus. 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.

Testing is primarily requested when a person has signs and symptoms suggesting a current arbovirus infection, especially if the person lives in or has recently travelled to an area where a specific arbovirus is endemic.

In temperate areas an arbovirus infection may be suspected when symptoms arise during mid to late summer. In warmer areas, infections may occur year-round.

Some signs and symptoms may include:

• Fever
• Headache
• Muscle weakness and pain
• Joint pain
• Nausea
• Skin rash

A small percentage of people, especially the young, elderly, and immunocompromised, may have more serious symptoms associated with meningitis and encephalitis. These symptoms may include:

• High fever
• Severe headaches
• Convulsions
• Confusion
• Stiff neck
• Muscular paralysis

Antibody tests may be requestetd 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 cerebrospinal fluid as well as blood.

Results of arbovirus testing require careful interpretation, taking into consideration the individual's signs and symptoms as well as risk of exposure to the insect vectors.

Antibody Tests
Antibody tests may be reported as positive or negative, or may be reported as less than or greater than a certain titre. For example, if the established threshold is a titre of 1:10, then a result less than this is considered negative while a titre greater than this is considered positive.

If IgM or IgG antibody is detected in the cerebrospinal fluid (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 significantly between an initial sample and one taken 2 to 4 weeks later, then it is likely that a person has had a recent infection.

Negative results for IgM and/or IgG antibodies may suggest that symptoms are due to a different cause, such as bacterial meningitis. However, 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. If suspicion of arbovirus remains high, antibody testing may be repeated at a later time or a NAAT test may be done as follow up.

The following table summarises results that may be seen with antibody testing:

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), or repeat testing on a later sample, confirms the diagnosis.

Nucleic Acid Amplification Testing (NAAT)
A positive NAAT for an arbovirus indicates infection with that specific virus.

A negative NAAT means there is no virus present in the sample tested or 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 the person's prognosis.

Other tests, such as antigen tests for dengue fever and viral cultures, 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.

In the UK, the tests are often performed at a central national laboratory NAAT testing may be used to screen donated blood or organs, or to test the tissues of a person who has died (post mortem) to determine whether a specific arbovirus may have caused or contributed to the person's death.

Arbovirus testing can also be performed on suspected host animals and insect 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.

In general, there is no need. Most people who become infected have few to mild symptoms and are only exposed to those arboviruses that are present in the areas where they live or travel. Testing is not usually done on asymptomatic people.