How Zika Virus Is Diagnosed

Knowing When to Test and What the Results Mean

The infection caused by the Zika virus became a significant concern in 2015 when the mosquito-borne illness swept across much of South and Central America up to the southern part of the United States. It usually only causes mild, flu-like symptoms, but the infection can become devastating if it occurs during pregnancy, potentially causing microcephaly, in which the baby is born with an abnormally small head and brain.

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.

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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.

Zika Virus Symptoms

If you are infected with Zika virus, you won’t necessarily have any symptoms.

If symptoms appear, they are generally mild, and may include:

  • fever
  • headache
  • joint pain
  • muscle pain
  • swollen lymph glands
  • a mild rash
Image of face with zika virus rash

Reproduced with permission from ©DermNet NZ www.dermnetnz.org 2022

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.

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 region
  • Pregnant women with Zika virus symptoms who have had possible exposure to the virus
  • Pregnant women without symptoms who live in an area where the Zika is endemic or are at ongoing risk of exposure
  • Pregnant 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.

Zika generally poses the greatest risk to the fetus during the first trimester, starting up to eight weeks prior to conception.

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.

Due to the limitations of the test, a negative NAT result does not exclude a Zika virus infection.

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.

Due to its high sensitivity, a negative Zika IgM can be helpful in excluding acute Zika infection.

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:

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.

If you test positive for Zika during pregnancy, it doesn't necessarily mean you will have a miscarriage or give birth to a baby with birth defects.

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.

Zika Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Woman

Frequently Asked Questions

  • How is Zika virus diagnosed?

    There are three types of tests authorized for use in diagnosing Zika virus infection:

    • 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).
  • How is Zika testing performed?

    Zika NAAT tests can be performed on blood, urine, cerebrospinal fluid, or amniotic fluid samples. Zika IgM and PRNT are both blood-based tests.

  • Can you diagnose Zika based on symptoms?

    No. In fact, most Zika infections don't cause symptoms. Of those that do, the symptoms tend to be mild and similar to those of other illnesses.

    Common symptoms include:

  • Who should be tested for Zika virus?

    The Centers for Disease Control and Prevention (CDC) recommends Zika testing to prevent transmission of the virus to pregnant women in whom the infection can cause birth defects.

    Zika testing is recommended for:

    • Anyone with symptoms and risk factors for a recent Zika exposure
    • Pregnant people with symptoms who have had a suspected exposure
    • Pregnant people without symptoms who are at ongoing risk of exposure
    • Pregnant people with a suspected exposure whose fetus shows signs of a congenital Zika infection, including microencephaly and other birth defects
8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Centers for Disease Control and Prevention. Testing for Zika.

  3. Testing & Diagnosis for Zika Virus | CDC. Centers for Disease Control and Prevention.

  4. Centers for Disease Control and Prevention. Testing guidance.

  5. Clinical Evaluation & Disease | Zika Virus | CDC. Centers for Disease Control and Prevention.

  6. Mead PS, Hills SL, Brooks JT. Zika virus as a sexually transmitted pathogen. Curr Opin Infect Dis. 2018;31(1):39-44. doi:10.1097/QCO.0000000000000414

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By James Myhre & Dennis Sifris, MD
Dennis Sifris, MD, is an HIV specialist and Medical Director of LifeSense Disease Management. James Myhre is an American journalist and HIV educator.