It is for this reason that diagnosis is imperative for pregnant people who may have been exposed to the Zika virus through a mosquito bite. Moreover, because the virus can be passed from males to females during sex, the test—comprised of a combination of blood and urine tests—can identify an infected partner and possibly prevent transmission.
Indications for Testing
Getting bitten by a mosquito, even in an area where the Zika virus is endemic, does not necessarily mean that you have been infected. The disease is passed by a specific type of mosquito known as the Aedes aegypti, which can be identified by white markings of its legs and a lyre-shape marking on it back.
If you believe that you have been exposed to the Zika virus—either because you have symptoms or have traveled to a high-risk area—there are tests available to diagnose the infection.
If symptoms appear, they are generally mild, and may include:
fever headache joint pain muscle pain swollen lymph glands a mild rash
CDC Recommendations
Zika virus testing is not for everyone. The primary aims of testing are to prevent transmission of infection from mother to child during pregnancy and to prevent sexual transmission of the virus to a woman who is either pregnant or likely to get pregnant.
To this end, the Centers for Disease Control and Prevention (CDC) recommends Zika testing for the following groups only:
Anyone, female or male, with symptoms who may have been exposed to the virus, either because they live in an endemic area, have recently traveled to an endemic region, or had unprotected sex with someone who lives or has traveled to an endemic regionPregnant women with Zika virus symptoms who have had possible exposure to the virusPregnant women without symptoms who live in an area where the Zika is endemic or are at ongoing risk of exposurePregnant women who have had possible exposure to Zika virus and whose fetus has signs of congenital abnormalities like microcephaly on an ultrasound
Testing may also be considered for pregnant women who normally aren’t at risk and don’t have symptoms, but who may have been recently exposed to the virus.
Testing is not recommended for men without symptoms, non-pregnant women without symptoms, or as a form of preconception testing.
If you cannot find a commercial lab near you able to perform the test, call your local or state health clinic, which can refer you to an approved laboratory.
Lab Tests
There are two different tests used to diagnose Zika virus, one that looks for the virus’s genetic material and the other that looks for the presence of anti-viral antibodies.
The two tests are performed concurrently to make the diagnosis:
The genetic test, known as RNA nucleic acid amplification testing (NAT), is more specific in its ability to detect the virus but less sensitive (meaning that it is prone to false-negative results). The immunoglobulin (IgM) antibody test is far more sensitive but less specific (meaning that it is less able to distinguish Zika from similar viruses).
Zika antibody levels tend to increase in tandem with declines in viral RNA. When used together, the RNA NAT and IgM tests provide a high level of accuracy when diagnosing Zika.
RNA NAT Testing
RNA NAT testing is a technology that rapidly amplifies the number of viral genetic particles in the sample from a few thousand to over a billion so, that the lab can see up-close if there is any genetic evidence of a Zika infection.
Your NAT testing must be performed on both a blood and urine sample taken at the same time.
Timing of the Test
The advantage of NAT testing is that is can be performed soon after symptoms appear. The level of viral RNA will decline rapidly as the immune system starts to gain control over the infection.
Limitations
As such, NAT testing is only considered useful within 14 days of the first symptoms. The only exception is in symptomatic pregnant women in whom viral RNA can persist for up to 12 weeks.
IgM Testing
IgM testing is a blood-based test that detects antibodies produced by the body to fight the Zika virus. It can take the body up to two weeks after exposure to produce enough antibodies for the test to be accurate. Testing too early can result in a false-positive result.
Timing of the Test
The IgM test is most useful in the first 12 weeks of infection and sometimes longer. It can also be used to test cerebrospinal fluid in cases where Zika infection has caused brain inflammation.
Limitations and Specificity
Where the IgM test may fall short is in its specificity to the Zika virus. Zika virus belongs to the Flaviviridae family and is closely related to the viruses that cause dengue fever, yellow fever, and Japanese encephalitis. Because of this, the test may return an incorrect result. This is among the reasons why confirmatory testing is so important.
Plaque Reduction Neutralization Test
Plaque reduction neutralization test (PRNT) measures the level of neutralizing antibodies in the blood. Neutralizing antibodies are a subset of immunoglobins whose role is to kill the virus. Unlike non-neutralizing antibodies detected in IgM testing, neutralizing antibodies can remain in the body for years, ready to attack if the virus ever appears.
The PRNT is reserved for confirming tests that are either inconclusive, ambiguous, or presumptive.
Testing During Pregnancy
The guidelines for testing during pregnancy vary based on whether you, as the mother, have symptoms and are at ongoing risk of infection.
The CDC currently recommends:
If you are pregnant and have symptoms of Zika: Testing is done for Dengue and Zika NAAT and Dengue IgM on a serum sample and Zika NAAT on urine sample as soon as symptoms occur, less than 12 weeks from symptom onset. For asymptomatic pregnant women with recent travel to an area with risk or exposure (sex with a man diagnosed with the Zika virus): Zika virus testing is NOT routinely recommended, but NAAT testing may still be considered up to 12 weeks after exposure or travel. If you are pregnant and live in a region where there is an ongoing risk of exposure: You should have an IgM test during your first prenatal visit and two additional visits. If an ultrasound reveals signs of birth defects consistent with Zika virus: You would have both a NAT and IgM test. NAT testing of amniotic fluid may also be performed.
Differential Diagnosis
The Zika virus is similar in molecular structure and/or symptoms to other insect and non-insect-borne diseases. Other causes of your symptoms may need to be ruled out if your test results are not conclusive.
Other possible diagnoses include:
Chikungunya virus Dengue Leptospirosis Malaria Parvovirus Rubella Rickettsial infection Yellow Fever
If You Test Positive
A positive Zika result would confirm that you have the virus. To prevent passing the virus to a sexual partner, you would either need to abstain from sex or use condoms consistently for at least six months. This is especially true if your partner is either pregnant or a female in her child-bearing years.
The majority of these infections do not result in complications, even during pregnancy. Routine ultrasounds will be performed to monitor your baby’s progress.
If your baby is born with no defects, a number of tests will be performed to ensure everything is alright, including:
Zika virus testing at birth A hearing test before discharge from the hospital A head ultrasound within one month of birth A comprehensive eye exam within one month of birth An automated auditory brainstem response (ABR) test within one month of birth to evaluate the baby’s auditory (hearing) nerves
If your baby is born with a defect of any sort, whether minor or major, referrals would be made to a neurologist, ophthalmologist, and other specialists able to treat and manage your baby’s condition.
Developmental and other intervention services would be sought in addition to family support services.
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RNA nucleic acid amplification testing (RNA NAAT) detects the genetic material of the virus. Immunoglobulin M (IgM) testing detects antibodies produced by the body in response to the virus. Plaque reduction neutralizing testing (PRNT) measures the level of neutralizing antibodies (which remain for longer in the body than non-neutralizing antibodies).
Common symptoms include:
FeverRashHeadacheConjunctivitis (“pink eye”)Joint painMuscle pain
Zika testing is recommended for:
Anyone with symptoms and risk factors for a recent Zika exposurePregnant people with symptoms who have had a suspected exposurePregnant people without symptoms who are at ongoing risk of exposurePregnant people with a suspected exposure whose fetus shows signs of a congenital Zika infection, including microencephaly and other birth defects
