How Zika Virus Is Diagnosed

Knowing When to Test and What the Results Mean

The Zika virus caused worldwide panic in 2015 when the mosquito-borne disease swept across much of South and Central America right up to the southern part of the United States. While the disease usually only causes mild, flu-like illness, it can become devastating if it occurs during pregnancy, leading in rare cases to a birth defect, called microcephaly, in which the baby is born with an abnormally small head and brain.

It is for this reason that diagnosis is imperative for mothers who may have been exposed to the Zika virus through a mosquito bite. Moreover, because the virus can be passed from man to women during sex, the test—comprised of a combination of blood and urine tests—can identify an infected partner and possibly prevent transmission.

zika virus diagnosis
© Verywell, 2018

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.

Even if you are infected, you won’t necessarily have any symptoms. If you do, they would generally be mild and may include fever, headache, joint pain, muscle pain, swollen lymph glands, and possibly a mild rash.

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

With that being said, 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 in 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 without symptoms who may have been recently exposed to the virus but are not at an ongoing risk. Zika infections generally pose the greatest risk to the fetus during the first trimester and up to eight weeks prior to conception.

On the other hand, 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 who can refer you to an approved laboratory.

Lab Tests

There are two different tests used to diagnosed Zika virus, one that looks for the genetic footprint of the virus and the other which looks for evidence of the infection by way of defensive proteins called antibodies.

The two tests are performed concurrently to make the diagnosis. This is because the genetic test, known as RNA nucleic acid amplification testing (NAT), is more is specific in its ability to detect the virus but less sensitive (meaning that it is prone to false-negative results).

By contrast, the immunoglobulin (IgM) antibody test is far more sensitive but less specific (meaning that it is less able to distinguish Zika from similar viruses).

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 genetic particles in a blood, urine, and other body fluids from a few thousand to over a billion. By doing so, the lab can see up-close if there is any genetic evidence of a Zika infection. If you are suspected of having the Zika virus, NAT testing must be performed on both a blood and urine sample taken concurrently.

The advantage of NAT testing is that is can be performed soon after symptoms appear. With that being said, the level of viral RNA will decline rapidly as the immune system starts to gain control over the infection. As such, NAT testing is only considered useful if tested 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 to deliver an accurate result. Testing too early can result in a false-positive result.

Zika antibody levels tend to increase in tandem with declines in viral RNA. As such, 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.

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, a test may occasionally return an incorrect result. This is among the reasons why confirmatory testing is so important.

Due to its high sensitivity, a negative IgM test can be considered definitive.

Plaque Reduction Neutralization Test

Plaque reduction neutralization test (PRNT) is a test that measures the level of neutralizing antibodies in the blood. Neutralizing antibodies are a subset of immunoglobins whose role it 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 in Pregnancy

The guidelines for testing in pregnancy vary based on whether you, as the mother, have symptoms and are at an ongoing risk of infection. The CCD currently recommends:

  • If you are pregnant and have symptoms of Zika, NAT and IgM testing can be performed concurrently within 12 weeks of the appearance of symptoms.
  • If you do not have symptoms but suspect you have been exposed, you should be tested within two to 12 weeks of returning either from an endemic region or having had sex with a man diagnosed with the Zika virus.
  • 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 be given both a NAT and IgM test. NAT testing of amniotic fluid may also be performed.

Differential Diagnosis

Because the Zika virus is similar in molecular structure and/or symptoms to other insect and non-insect-borne diseases, other causes may need to be ruled out if your test results are anything less than conclusive. They might 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 no less than six months. This is especially true if your partner is either pregnant or in her child-bearing years.

If you get a positive result during pregnancy, it doesn't necessarily mean that your baby will get a birth defect or that you will have a miscarriage. The majority of these infections do not result in either of these complications. Instead, routine ultrasounds will be performed to monitor your baby’s progress and check for any signs of trouble.

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 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 hearing nerves

If your baby is born with a defect of any sort, whether minor or major, referrals would be made to neurologists, ophthalmologists, 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.

What You Can Do to Treat Zika Fever Symptoms
Was this page helpful?
Article Sources