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The evidence that Zika causes fetal brain damage is now 'awfully strong'

A report describes an abnormal pregrancy following a Zika infection

A fetus that was aborted weeks after the mother was infected with Zika provides striking evidence that the virus causes fetal brain abnormalities, researchers say. The report, published today in the New England Journal of Medicine, isn't the first to document a case of a mother passing on the virus to a fetus, but it does provide the most detailed look yet at changes that occurred in the fetus' brain following a mother's Zika infection.

In the study, researchers describe the case of a 33-year-old Finnish woman who was almost three months pregnant when she became infected with Zika during a trip to Guatemala. Fetal ultrasounds later showed that her fetus' head wasn't growing at a normal rate; its brain also began to display abnormal anatomy, researchers say. After the pregnancy was terminated, scientists found Zika virus in the fetus' brain, as well as in the umbilical cord and placenta. Because researchers were able to follow the pregnancy every step of the way, scientists think this case lends strong support to the idea that Zika virus causes fetal brain damage.

"That’s pretty much a smoking gun."

"What makes this study so convincing is that this one case was so thoroughly studied from the time of infection to the ultrasound studies," says Lee Norman, an intelligence officer in disaster medicine planning in the United States Army National Guard, who didn’t work on the study. Because the woman was so closely followed by doctors, researchers were able to eliminate other factors that could have played a role in the fetal brain abnormalities and focus in on the Zika virus. "Finding the virus in the brain of the fetus in the NEJM study that’s pretty much a smoking gun," he says.

In most cases, the Zika virus isn't dangerous. About 80 percent of people infected with the virus never develop symptoms, which resemble the flu and last no more than a week. But an outbreak of the virus in Brazil has caused concern among health officials who suspect Zika might be responsible for stillbirths, problems with the placenta that may harm a developing fetus, and a birth defect that can affect the brain size of newborns, microcephaly. There's no cure or treatment for the virus. And even though Zika is mostly transmitted through mosquito bites, it is possible to become infected through sexual contact. Because of this, the US government recommends that women with Zika should wait eight weeks before trying to conceive, and men should wait at least six months after their symptoms first appear to have unprotected sex. Both recommendations were made despite the fact that scientists have yet to demonstrate a definitive link between Zika and fetal brain abnormalities. Now, scientists are working to document the effects of the virus in expecting mothers.

The anatomy of the fetus' brain was abnormal

The woman's pregnancy was normal prior to her Zika infection. But three months in, during a trip to Guatemala, she began to show Zika-like symptoms. Four weeks after she had recovered, the woman tested positive for antibodies that fight off Zika; another batch of tests eliminated other possible culprits, like dengue. Then, between her 16th and 19th week of pregnancy, the fetus' head stopped growing at a normal rate; its head went from being in the 47th percentile to the 24th percentile, meaning that only 24 percent of fetuses have a head that size or smaller at that stage. A fetal ultrasound also showed that the anatomy of the fetus' brain was abnormal. "Given the grave prognosis," the mother decided to have an abortion, the researchers wrote in the study; she was five months pregnant by then.

"I think that the evidence is now awfully strong."

The report provides "very striking evidence" that Zika virus infection causes severe abnormalities, says Sara Cherry, a microbiologist at the University of Pennsylvania who didn't work on the study. But a single case isn't enough to prove Zika's role in fetal brain abnormalities, she says. What matters is the picture that data from this study and others paint. Emerging and aggregate data "really do show that Zika virus infection can lead to developmental defects in the fetus," she says. Harvard University immunologist Eric Rubin put things more bluntly. The study "doesn't prove the link between Zika and microcephaly," he told The Verge. "However, particularly given the other recently published study, that shows that women who develop Zika during pregnancy are at high risk for fetal abnormalities, I think that the evidence is now awfully strong."

Earlier this month, another study showed that women who are infected with Zika during the first trimester of pregnancy face a 1 in 100 chance that their fetus will develop microcephaly. According to Hongjun Song, a neuroscientist at Johns Hopkins University, the only way scientists will be able to definitely prove that Zika causes fetal brain abnormalities is by conducting a large, controlled study. Norman, for his part, says scientists need to test the effects of Zika on animals.