Zika Resource Center
Zika Resource Center
Perspective from Peter J. Hotez, MD, PhD
January 25, 2018
3 min read

Zika caused more birth defects in US than expected

Perspective from Peter J. Hotez, MD, PhD
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Photo of Brenda Fitzgerald
Brenda Fitzgerald

A new report suggests Zika virus caused more birth defects in the United States than expected, and the number of cases may continue to go up.

Writing in MMWR, CDC and state health officials said there was a 21% increase in the birth defects most strongly linked to Zika infection during pregnancy in the last half of 2016 compared with the first half of the year. The cases were identified in the three areas of the U.S. where there was local transmission of the virus via infected mosquitoes: Puerto Rico and southern areas of Florida and Texas.

Specifically, the prevalence of birth defects strongly related to congenital Zika virus infection rose from 2 to 2.4 cases per 1,000 live births — 29 more cases of microcephaly or other brain abnormalities, eye abnormalities or central nervous system dysfunction than were expected in these areas in the last half of 2016, according to the report.

Researchers expected cases to peak in 2017, and so future data could show another increase, they said.

“Because the peak of local virus transmission in these jurisdictions occurred in the second half of 2016, and most exposed pregnancies were completed in 2017, it is critical that public health surveillance programs continue reporting the occurrence of these birth defects to monitor for trends following the Zika virus outbreak,” CDC Director Brenda Fitzgerald, MD, and colleagues wrote in a related editorial in JAMA.

Photo of Margaret Honein
Margaret A. Honein

When the Zika epidemic struck 2 years ago, officials adapted existing surveillance programs in the U.S. to detect birth defects potentially related to Zika. The new report was based on data from 15 U.S. states and territories conducting population-based surveillance for these cases.

According to the researchers, including Margaret A. Honein, PhD, MPH, acting director of the Division of Congenital and Developmental Disorders in the CDC’s National Center on Birth Defects and Developmental Disabilities, an increase was observed only in the three areas with local transmission and not in any of the other jurisdictions, which included areas with a low prevalence of confirmed travel-associated cases of Zika virus infection. The increase was apparent after other birth defects that are not often associated with Zika, such as neural tube defects, were excluded from the analyses. According to the CDC, it is not known if this increase is due to local transmission of Zika virus alone, or if there are other contributing factors.

Infographic demonstrating increase in Zika-related birth defects
Click to view image.

Overall, about three out of every 1,000 babies born in these 15 jurisdictions — 2,962 in all — had a birth defect meeting the case definition for birth defects potentially related to Zika virus infection during pregnancy. Almost half were born with brain abnormalities or microcephaly, or both. Around 9% had eye abnormalities and more than 22% had nervous system damage, including joint problems and deafness, without brain or eye abnormalities.

The CDC still recommends that pregnant women not travel to areas with risk for Zika transmission. Fitzgerald and colleagues said clinicians should “consistently consider possible exposure to Zika virus during pregnancy, regardless of the availability of testing results” and that “infants with birth defects potentially associated with Zika virus infection and possible maternal Zika virus exposure during pregnancy should be tested for evidence of Zika virus infection, other congenital infections … and other causes of microcephaly or birth defects.”

An earlier assessment showed that children born with microcephaly who tested positive for Zika virus face challenges beyond infancy that range from motor impairment to eating and sleeping difficulties.

“Babies with Zika-related birth defects need all the help they can get, as soon as possible and for as long as they need it,” Fitzgerald said in a statement about the new report. “This report highlights the critical importance of documenting birth defects possibly related to Zika and our need to maintain vigilance.” – Gerard Gallagher


Delaney A, et al. MMWR Morb Mortal Wkly Rep. 2018;doi:10.15585/mmwr.mm6703a2.

Fitzgerald B, et al. JAMA. 2018;doi:10.1001/jama.2018.0126.

Disclosures: The authors report no relevant financial disclosures.