Zika virus infections late in pregnancy may pose less risk to the fetus than widely feared. Researchers report today that they found no overt birth defects among 616 babies in Colombia whose mothers showed symptoms of Zika virus disease in their third trimester of pregnancy.
The data, published in The New England Journal of Medicine, are preliminary, cautions Margaret Honein, an epidemiologist at the U.S. Centers for Disease Control and Prevention in Atlanta, one of the authors of the report. Data from other countries have suggested that the virus is most dangerous to a fetus early in pregnancy, so Colombia may still face a wave of birth defects in the coming months. “It’s somewhat reassuring” that few severe problems have appeared in the babies born so far, Honein says, “but this is by no means final.”
Zika virus, which emerged in Brazil last year and has been spreading quickly across Latin America, usually causes mild symptoms, if any. However, a spike in Brazil in cases of a birth defect called microcephaly, in which a baby’s head is smaller than it should be, raised alarm that the virus might be dangerous for pregnant women and their fetuses. So far Brazil has reported more than 1500 cases of microcephaly associated with Zika infection. Animal, laboratory, and clinical data have confirmed that the virus easily infects and kills developing brain cells. In Brazil and elsewhere, doctors report that babies infected with Zika in utero display a range of problems at birth including brain, eye, hearing, and limb defects. One study of women in Rio de Janiero, Brazil, reported that nearly a third of women infected with Zika during pregnancy had some sort of adverse outcome. That study found problems even among women infected in the third trimester: Among five women with confirmed infections, there were two stillbirths and one case of fetal distress.
After Brazil, Colombia has the most cases of suspected Zika virus infection. Since August 2015, health officials there have logged more than 65,000 suspected cases. Researchers are following thousands of women who have had symptoms consistent with Zika virus disease during pregnancy to try to better understand the risk the virus poses. But so far the country has officially reported only seven cases of microcephaly related to Zika infection. That may be temporary. The data so far reflect outcomes only after a pregnancy is completed, Honein says. Suspected Zika cases peaked in January, and many of the women who have been infected are still pregnant. Although the number of cases of birth defects appears lower in Colombia so far, “it is premature to conclude there is a difference” in the rates of affected babies, she says.
The new Colombia data are interesting, but should be treated with caution, says Nikolaos Vasilakis, a virologist at the University of Texas Medical Branch in Galveston. Accurately detecting and diagnosing Zika virus disease is still a huge challenge, he notes, because Zika’s symptoms can be confused with those of other common diseases in the region, including dengue and chikungunya.
Honein says longer-term studies of pregnant women and their babies are needed. “We know that it [Zika] has caused devastating damage” in many infected fetuses, she says. “It’s really, really important to follow up those infants,” even those without obvious problems at birth.