How Many Children Are Actually Infected With Zika?

Many infants born in 2017 were not tested for Zika virus

new born baby, healthy

The Zika virus remains a health threat in the United States, and around the world.

Nearly 70 years after the Zika virus was first identified, there is an obvious need to protect expectant mothers and their infants from infectious diseases.

Zika can cause serious damage to the developing brain of infants when the virus infection occurs during pregnancy.

And, now, previously identified healthy infants are reporting developmental problems caused by the Zika virus.

A recent report from the CDC focused on determining how many children remain undiagnosed for the Zika virus.

This question remains unanswered because many women who were exposed to Zika while pregnant were not tested.

Or, they were not tested in a timeframe that enabled Zika identification.

On January 25, 2018, the US Centers for Disease Control and Prevention (CDC) released the first report of population-based birth defects associated with congenital Zika virus infection.

Congenital microcephaly is defined as:

  • diagnosis or mention of microcephaly or small head in the medical record and,
  • for live births, head circumference less than the third percentile for gestational age and sex at birth or within the first 2 weeks of life or,
  • for non-live births, prenatal head circumference more than 3 SDs below the mean on prenatal ultrasound

About 5% to 10% of pregnancies with laboratory evidence of possible Zika virus infection have been initially reported to have birth defects.

This report published in JAMA online examines about 7,000 pregnancies completed from January 2016 through December 2017, with laboratory evidence of possible Zika, reported to the US Zika Pregnancy and Infant Registry.

When limiting this CDC report’s data to the birth defects strongly linked to Zika virus infection, there was a significant 21 percent increase (from 2.0 to 2.4 per 1000 live births) in the latter half of 2016 compared with the first half.

Researchers believe the actual number of congenital Zika virus infections in the Americas is likely much higher because many infants born in 2017 may not have been identified with Zika.

Most of the fetuses and infants described in the CDC surveillance report had no laboratory evidence of congenital Zika virus infection and therefore, no Zika virus testing performed.

For some of these infants, Zika virus testing would not have been indicated because of lack of possible maternal exposure or identification of other etiologies.

Most Zika virus infections are asymptomatic and detecting Zika virus RNA is complicated. Moreover, Zika virus infection during pregnancy cannot be ruled out by negative nucleic acid testing results.

Serologic testing is affected by the timing of specimen collection, and interpretation of positive results is complicated by cross-reactivity, especially in persons previously infected with a related flavivirus, such as Yellow Fever or dengue.

Additionally, there are also case reports of infants born with normal head circumference who developed postnatal-onset microcephaly, following congenital Zika virus infection.

The full range of developmental disabilities and other adverse early childhood outcomes associated with congenital Zika virus infection in the United States can only be determined by following the infants as they develop.

Understanding what is happening with those infants might have far-reaching implications for other exposed infants whose congenital infection was not identified during pregnancy or at birth.

To prevent these Zika related birth defects, the CDC continues to recommend that pregnant women not travel to areas with risk of Zika transmission including Texas, Florida, Mexico, certain Caribbean islands, and South American countries, such as Brazil.

People should take precautions even during the winter months because it often stays warm enough in certain locals for mosquito activity to continue through much of the winter.

Specifically, residents of the Rio Grande Valley in south Texas should remain on alert for Zika.

And, international travelers should avoid mosquito bites while abroad and for 21 days after returning, in case they have been exposed to the virus.

Furthermore, healthcare professionals should remain vigilant and consistently consider possible exposure to Zika virus during pregnancy, regardless of the availability of testing results.

Laboratory testing for congenital Zika virus infection is recommended for infants born to mothers with laboratory evidence of Zika virus infection during pregnancy, and for infants who have abnormal clinical findings suggestive of congenital Zika virus syndrome and a maternal epidemiologic link suggesting possible transmission, regardless of maternal Zika virus test results, says the CDC.

For infants born to mothers with risk factors for maternal Zika virus infection assessment of the infant, including comprehensive physical exam and careful measurement of head circumference, neurologic assessment, as well as a newborn hearing screen, should be performed.

In addition, healthcare providers should consider whether further evaluation of the newborn for possible congenital Zika virus infection is warranted, in which case a head ultrasound and ophthalmologic assessment should be considered.

Based on results of this evaluation, testing of the infant for Zika virus infection could be considered.

Testing of placental tissue specimens by Zika virus reverse-transcription polymerase chain reaction (RT-PCR) is conducted at CDC’s Infectious Diseases Pathology Branch (IDPB). Pre-approval is required prior to submission of any tissue specimens. For pre-approval please contact [email protected].

Corresponding Author: Margaret A. Honein, Ph.D., MPH, Birth Defects Branch, Centers for Disease Control and Prevention, 1600 Clifton Rd, Mailstop E-86, Atlanta, GA 30333 ([email protected]). Published Online: January 25, 2018.

Conflict of Interest Disclosures: Brenda Fitzgerald, MD, Coleen Boyle, Ph.D., MS, Margaret A. Honein, Ph.D., MPH have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported.