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The Zika virus can harm developing fetuses even if a pregnant woman’s infection is so mild she shows no symptoms, scientists from Colombia and the Centers for Disease Control and Prevention reported Wednesday.

Four women from Colombia who had asymptomatic Zika infections during pregnancy gave birth to babies who had microcephaly — an abnormally small head. The newborns were confirmed to have the Zika virus in their systems, the researchers reported in an article published in the New England Journal of Medicine.


“We do think the majority of Zika virus infections are asymptomatic. And this does provide some evidence that asymptomatic infections during pregnancy do pose a risk to the fetus,” said Margaret Honein, an epidemiologist and a senior author of the paper.

“I think our level of concern for pregnant women is very high, for both symptomatic and asymptomatic infections with Zika virus,” Honein said.

It is estimated that 4 out of 5 people infected with the Zika virus are asymptomatic.


The same article reported that Zika infection late in pregnancy may not be as dangerous for the fetus as an infection early on. No cases of microcephaly or other obvious birth defects were seen among the babies born to a group of 616 Colombian women who contracted the Zika virus in their third trimester, the researchers reported.

Most of the women, however, were diagnosed by symptoms only, not blood tests — a limitation of the study, said Dr. Karin Nielsen-Saines, a professor of pediatric infectious diseases at the David Geffen UCLA School of Medicine.

Nielsen-Saines has been studying Zika infection in pregnancy in Brazil. She and colleagues reported in early March that as many as 29 percent of women who contract Zika during pregnancy may have a baby with birth defects, of which microcephaly is only the tip of the iceberg. 

Though the women in this study who were infected in the third trimester didn’t have babies with visible birth defects, that can’t been taken as an assurance these infants are all completely healthy. In fact, 2 percent were born with a low birth weight and 1 percent of the women miscarried late in their pregnancies.

As well, the authors didn’t report on brain scans of the infants. It’s known that some infants who were infected with Zika in the womb have tissue scarring in their brains. They may also have underdeveloped brains even though their heads are normal sized.

Dr. Rita Driggers, director of maternal fetal medicine at Sibley Memorial Hospital in Washington, D.C., said the children in the study will have to be followed and tested to see if they experience developmental problems.

Driggers reported in March on a case in which a woman who was infected during her first trimester chose to terminate her pregnancy in week 21. Her ultrasounds hadn’t shown that the fetus was microcephalic, but an MRI showed brain abnormalities.

Honein said more research needs to be done on the babies born to the women infected in their final trimester of pregnancy.

“There hasn’t been a full evaluation done,” she said. “So I think it’s going to be important to follow up to really understand: Are there any eye abnormalities or vision problems? Are there any hearing-related problems? And are there any other developmental issues that might not become apparent immediately.”

The Colombian authors of the study were with the Instituto Nacional de Salud in Bogata and Colombia’s ministry of health and social protection.