AILSA CHANG, HOST:
We've heard a lot about the Zika virus over the past eight months, from South America to Puerto Rico and now a neighborhood in Miami Fla., where mosquitoes are transmitting the disease locally. The CDC says pregnant women should avoid the area. Pregnant women, of course, are the largest concern because the virus can cause severe birth defects, and that raises a question that researchers have been struggling with. We know how women get Zika - mosquito bites. But the babies, how are they getting the virus?
CAROLYN COYNE: Something about Zika was very different.
CHANG: Carolyn Coyne is a microbiologist at the University of Pittsburgh. She studies the virus. Zika is different because scientists had never seen a mosquito-borne virus cross from a pregnant mother to her fetus. And Coyne is one of the few researchers who study the organ that would normally keep a baby healthy - the placenta. It's the bridge and the gate between a mom and her baby.
COYNE: Generally, people are thinking, oh, no, what's going on in the brain of the fetus, which of course is a critical question to answer. But my thinking was even more so than that. You know, the virus first has to reach the fetus.
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COYNE: The mystery to me was really, you know, how is Zika, unlike so many other viruses, able to cross the placental barrier?
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CHANG: Months later, she's still searching for the answer to that question. And so we asked her to explain, how did we get here? Why is it so hard to figure out how Zika gets through the placenta and into the fetus?
COYNE: The placenta, I think, is widely regarded as the most understudied human organ, probably for a variety of reasons. You know, one is that scientists require models, and accessing placental tissue can be difficult. Second to that, I always say, is that I think the placenta does a remarkably good job at what it's supposed to do, which is prevent anything bad from reaching, like, a - like a virus from reaching the fetus. And so, you know, when you have something that does a great job, I think you can kind of overlook it.
CHANG: And the fact that so little is known about the placenta makes the Zika mystery that much harder to solve.
COYNE: One thing that I think that's sort of underappreciated and a lot of people don't know is really how different the human placenta is throughout pregnancy. Zika virus can actually induce damage to the fetus really at all stages of gestation. What I would guess that that really suggests is that the virus probably takes multiple roads into the placenta and that those roads may be quite distinct.
CHANG: When scientists discover those paths, they can start figuring out how to block them off.
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CHANG: For now, there is no vaccine for Zika and no treatment for pregnant women who have the virus. The only advice they can get - don't get bitten. And Carolyn Coyne says creating drugs for pregnant women is also a relatively uncharted frontier.
COYNE: You know, I think I saw that less than 10 percent of FDA-approved compounds have even been tested for their effects on pregnant women.
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COYNE: Going forward, you know, not only do we have to understand how Zika virus accesses the fetus, but I think we need to focus our attention on understanding and developing therapies for pregnant women just more generally.
CHANG: That was Carolyn Coyne of the University of Pittsburgh. Transcript provided by NPR, Copyright NPR.