Tracking Birth Defects Potentially Linked to Zika Virus in 15 U.S. Jurisdictions, 2016

Zika virus infection during pregnancy can cause serious birth defects, including microcephaly and other brain abnormalities. During the 2016 Zika outbreak in the Americas, existing population-based birth defects surveillance systems in 15 U.S. states and territories were quickly adapted to track every fetus and infant with a birth defect that might be linked to Zika — whether or not anyone knew the mother had been exposed, and whether or not she had been tested.

That is what sets such systems apart from the U.S. Zika Pregnancy and Infant Registry, which follows women with laboratory evidence of possible Zika infection and their infants. Surveillance of all births captures the full picture, including defects with other causes.

How it worked

Staff abstracted medical records on birth defects, congenital infections, pregnancy outcomes, head size, survival and Zika test results, using a standard case definition, for all live births and pregnancy losses from January 1 to December 31, 2016. Cases were sorted into four groups:

1. brain abnormalities or microcephaly;
2. neural tube defects (NTDs) and other early brain malformations;
3. eye abnormalities without a brain abnormality; and
4. other signs of central nervous system dysfunction — joint contractures and congenital deafness — without brain or eye abnormalities.

Because the evidence linking NTDs to Zika is weak, results were calculated both with and without them. Jurisdictions were grouped by Zika exposure: those with local transmission in 2016; those with “higher” rates of travel-associated Zika (one or more confirmed cases per 100,000 residents); and those with “lower” or no travel-associated cases.

Overall results

The jurisdictions covered nearly 1 million live births — about a quarter of all births in U.S. states and territories. They found 2,962 fetuses and infants with birth defects potentially related to Zika, 3.0 per 1,000 live births — close to the 2.9 per 1,000 baseline measured in 2013–2014, before Zika reached the Americas.

Of the 2,962, 2,716 (92%) were live births. Only 45 (1.5%) had laboratory evidence of possible Zika infection; 96 tested negative, and 2,821 (95.2%) were never tested or had no results.

A rise where Zika spread locally

In the three jurisdictions with local transmission, the rate of all monitored defects rose from 2.8 to 3.0 per 1,000 between the first and second halves of 2016 — not a significant change. But excluding NTDs, the defects most strongly linked to Zika rose significantly, from 2.0 to 2.4 per 1,000: 169 cases in the second half of the year against 140 expected, 29 more than expected. Of the 393 cases in these areas, only 32 had laboratory evidence of possible Zika infection, and most were never tested.

Jurisdictions with local transmission: defects potentially related to Zika excluding NTDs, and NTDs, per 1,000 live births, by quarter of 2016. Image from CDC’s page.

In “higher” travel-prevalence jurisdictions the rate held steady at 3.0 per 1,000. In “lower” prevalence jurisdictions it fell significantly, from 3.4 to 3.0 — for reasons that are unclear, though surveillance data are usually not final until about 24 months after the birth year, and these came after only 12, so late-2016 cases were likely undercounted.

All monitored defects by quarter of 2016 and jurisdiction group. Image from CDC’s page.

Why keep watching

The peak of these birth defects was expected in 2017 births: Zika transmission in Puerto Rico peaked in August 2016, local transmission appeared in southern Florida and, in November 2016, in southern Texas — and most Zika-affected pregnancies ended in 2017. Continued surveillance helps communities plan care for affected children and families and lays groundwork for following the children’s health and development. The registry, with more than 7,000 pregnancies with laboratory evidence of Zika, and population-wide surveillance together offer a model for responding to emerging threats.

Limitations

• The three local-transmission jurisdictions differed: Puerto Rico had a widespread outbreak from early 2016, while Texas’s local transmission was confirmed only in November. They also had high rates of travel-related Zika.
• Greater awareness in affected areas may have sharpened case-finding in late 2016 — though NTD rates did not rise, suggesting this does not explain the increase.
• Jurisdictions differ in demographics and case-finding, so the analysis compared each group’s two half-years rather than groups with each other.

Sources

Based on Delaney A, Mai C, Smoots A, et al., "Population-Based Surveillance of Birth Defects Potentially Related to Zika Virus Infection — 15 States and U.S. Territories, 2016," Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.