ARI SHAPIRO, HOST:
A year ago, few people had heard of the mosquito-borne virus Zika. It was considered a largely harmless disease. All that has changed. Scientists say they are stunned by the kind of damage it can do. So far more than a thousand babies have been born with brain damage linked to Zika. Many more cases are expected. As NPR's Michaeleen Doucleff reports, several studies published today help explain how Zika can cause such horrific consequences for babies. And a warning, some of the details that you're about to hear are graphic.
MICHAELEEN DOUCLEFF, BYLINE: A few weeks ago, Dr. James Bale saw a series of images in a medical journal. They showed MRI scans of babies' brains, babies affected with Zika. His heart sank.
JAMES BALE: It was frightening to me because of the extent of the damage.
DOUCLEFF: Bale is a pediatric neurologist at the University of Utah. These images showed something he had seen only a few times in his 30-year career, a phenomenon called fetal brain disruption sequence. In a normal fetus...
BALE: The brain starts to grow. It creates pressure, which in turn causes the skull to grow.
DOUCLEFF: But if the Zika virus infects the fetus, the brain can stop growing. The skull collapses down onto the brain, and in some cases, the baby is born with a skull no bigger than an orange.
BALE: Many of them will die, often in infancy. And the majority, if not all, will then have long-term severe developmental problems.
DOUCLEFF: Now scientists think they understand exactly what Zika is doing to cause these brain malformations. Alysson Muotri is a professor of molecular medicine at the University of California, San Diego. He led the study published in the journal Nature. He infected pregnant mice with Zika, and looked to see how the virus harmed the fetus.
ALYSSON MUOTRI: We detected the virus all over in different regions of the body.
DOUCLEFF: But for some reason - and scientists don't know why yet - Zika is attracted to brain cells. And once inside the cells, Zika turns them into viral factories that start producing huge amounts of virus until the cells burst.
MUOTRI: So they explode and more viral particles are released that can infect in other cells. And they just amplify themselves.
DOUCLEFF: More and more brain cells get infected, more die. This is already a problem for the fetus, but the situation gets worse. That's because the brain cells that Zika infects, they're extremely special. They're responsible for building the brain.
MUOTRI: These are fast replicating cells that will give rise to billions of cells that we have in our brains.
DOUCLEFF: And so if a fetus loses even just a small percentage of these cells, a major portion of its brain will never develop.
MUOTRI: The impact later on in life would be dramatic.
DOUCLEFF: Indira Mysorekar is a reproductive biologist at Washington University in St. Louis. She let a separate study out today. She says mouse experiments can never tell us exactly what's happening in a people. Human anatomy is more complicated. But one thing is clear - once the virus infects the fetus...
INDIRA MYSOREKAR: It leaves a lot of havoc and devastation in its wake. It's almost like a tornado or an earthquake. There is death following Zika.
DOUCLEFF: So Mysorekar says the best hope right now is a vaccine that protects pregnant women and their babies. Michaeleen Doucleff, NPR News.
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