Now, as children exposed to the virus during pregnancy start to get older, researchers have started to tease out how common these secondary neurodevelopmental problems may be—and how they can occur even if babies appeared fine at birth.
In a study published Tuesday, researchers from the Centers for Disease Control and Prevention and colleagues reported that 9 percent of 1,450 children examined had at least one neurodevelopmental abnormality possibly tied to Zika, including seizures, hearing problems, difficulties swallowing, and cerebral-palsy-like movement issues. They also found that 6 percent had a Zika-related birth defect and 1 percent had both defects and neurodevelopmental problems.
Taken together, the study reports that 14 percent of babies exposed to the virus in utero—about 1 in 7—appear to have been harmed in some way.
“By continuing to follow these babies as they age and grow, we will learn more about Zika and remain alert to the problems that develop over time,” CDC Director Robert Redfield said on a call with reporters.
The study focused on cases in the U.S. territories, including Puerto Rico, and looked at children likely exposed to the Zika virus during pregnancy who were at least 1 year old. It’s possible that additional neurodevelopmental problems could emerge as the children grow up.
In an interview, Peggy Honein, director of the CDC’s Division of Congenital and Developmental Disorders, said that officials don’t have baseline rates of these neurodevelopmental abnormalities for babies at 1 year old, so it is difficult to know whether Zika is causing all of the problems seen in the children in the sample. But many of the health problems are consistent with what’s known about the Zika virus—that it zeroes in on brain cells and can destroy the tissue. Other viruses that can pass from mother to fetus also lead to neurodevelopmental problems that crop up after birth. And scientists previously reported that Zika-affected babies in other countries have been missing developmental milestones.
The study’s birth defect rate of 6 percent fits with past studies that have found between 5 and 10 percent of affected fetuses are born with defects.
Birth defects caused by Zika include brain damage, eye problems, and, most notoriously, microcephaly—when the brain doesn’t develop fully and the head doesn’t grow properly. But microcephaly can also arise as children age, and about 1 percent of babies in the new report showed signs of so-called postnatal-onset microcephaly.
“Many of these looked healthy at birth,” Honein told STAT. “Their head didn’t grow the way it was supposed to, which is an indication that their brain is not developing properly.”
In January 2016, the CDC issued recommendations for how to evaluate infants possibly exposed to Zika during pregnancy, even if they appear healthy at birth. But the new report showed that not all of these children are getting the advised tests: 60 percent had brain imaging, about half had hearing screens, and just over a third had an ophthalmologic evaluation.
The study included only babies that had some follow-up care reported to the U.S. Zika registry, and there were almost 700 other Zika-exposed babies in U.S. territories who had turned 1 but had not had their care reported.
It’s possible that some babies’ exams weren’t reported, but, Honein said, “We still think there are opportunities for improvement here.”
Identifying neurodevelopmental problems is important because early intervention can improve cognitive and behavioral function, experts say.
Zika is primarily spread by mosquitoes, but it can also be transmitted through sex. And on Tuesday in a separate report, the CDC revised its recommendations for how long men who might have contracted Zika should wait to have unprotected sex to avoid spreading the virus. It is now three months from either symptom onset or the last possible exposure to the virus, which is down from six months and is the result of additional research that painted a clearer picture of how long the virus can linger in a person.
The second report also noted that studies conducted in mice and monkeys indicate that sexual transmission of Zika might be more damaging to fetuses than mosquito-borne transmission, a finding officials flagged to highlight the importance of preventing sexual passage of the virus.
Although it has faded from the headlines, Zika is still circulating at low levels in places around the world, and the CDC continues to emphasize that its travel recommendations for pregnant women, their partners, and others remain in place.
“It’s still a risk to pregnant women and their babies,” Honein said. “The Zika story is not over, particularly for the children who have been affected by congenital infection.”