Zika’s role as a cause of severe birth defects confirmed

It’s official: Zika virus causes microcephaly and other birth defects.

A new analysis by the U.S. Centers for Disease Control and Prevention confirms what many earlier studies had suggested: The virus, typically passed via the bite of an infected mosquito, can travel from a pregnant woman to her fetus and wreak havoc in the brain. 

“There is no longer any doubt that Zika causes microcephaly,” CDC director Tom Frieden said in a news briefing Wednesday. The findings, reported April 13 in the New England Journal of Medicine, follow a March 31 report from the World Health Organization that concluded nearly the same thing.

Because the connection between a mosquito-borne illness and such birth defects is so unprecedented, the CDC took time to carefully weigh the evidence, Frieden said. “Never before in history has there been a situation where a bite from a mosquito could result in a devastating malformation.”

In the NEJM analysis, researchers factored in molecular, epidemiological and clinical data, including recent reports of babies born with microcephaly in Colombia. The country has been suffering from a Zika outbreak for months, and thousands of pregnant women have been infected with the virus. Based on what scientists know about the virus, now is about the time they would have expected to see birth defects, said CDC public health researcher and study coauthor Sonja Rasmussen. WHO reports 50 cases of microcephaly in Colombia, seven of which have a confirmed link to Zika.

Researchers still can’t pin down the odds that an infection during pregnancy will lead to microcephaly, though. “What we don’t know right now is if the risk is somewhere in the range of 1 percent or in the range of 30 percent,” Rasmussen said.

Scientists do believe, however, that women who aren’t pregnant would probably clear a Zika infection within eight weeks, and not have problems with future pregnancies, Rasmussen said.