A few weeks ago, Dr. James Bale saw a series of MRI images in a medical journal of MRI scans of babies infected with Zika in the womb.
They scans showed something Bale had seen only a few times in his 30-year career: a phenomenon called fetal brain disruption sequence.
As the fetus's brain starts to grow, it creates pressure, which pushes on the skull and causes it to grow. But if something stops brain growth — such as a virus — pressure on the skull drops. And the skull can collapse down onto the brain.
The skin around the head continues to grow, Bale says. So the baby is born with wrinkles of skin at the back of the neck and a tiny skull. In some cases, the baby's head is as small as an orange, or about half the size of a healthy baby's head.
"It's quite remarkable what the Zika virus is doing to the brain of young infants," Bale says. "Many of them will die often in infancy, and the majority, if not all, will then have a long-term, severe developmental problems."
Now scientists think they have an understanding about how Zika causes these severe brain malformations. The findings come from a series of mouse experiments, published Wednesday in three leading journals.
"We detected the virus all over the mice and in different regions of the body," Muotri says.
But for some reason — and scientists don't know why yet — Zika is particularly attracted to brain cells. And once inside the cells, Muotri says, Zika turns them into viral factories that start producing huge amounts of virus. Until they burst.
"They explode, and more viral particles are released that can infect other cells. And they can just amplify themselves," Muotri says.
More and more brain cells get infected. More die. This cell death is already a problem for the fetus. It scars the brain and creates inflammation.
But the situation gets worse because the brain cells infected by Zika are extremely special. They're called neural progenitor cells. And they're responsible for building a large portion of the brain.
"These are fast-replicating cells that will give rise to billions of cells in our brains," Muotri says.
So if a fetus loses even just a small percentage of these cells, a portion of its brain will never develop. "And the impact later in life would be dramatic," he says.
A second study, published in the journal Cell Stem Cell, confirmed that Zika destroys neural progenitor cells inside a growing embryo. In that experiment, a team of scientists at the Chinese Academy of Sciences, injected the virus directly into the brains of mice embryos, developing inside their moms.
Muortri says death of brain cells is likely the major way that Zika causes microcephaly in babies. But it isn't the full picture.
In the third study, Indira Mysorekar and her colleagues at Washington University in St. Louis, also infected pregnant mice with Zika.
They found the virus not only damages the brain but also attacks the placenta.
"The nutrient and blood exchange that normally happens between the mother and the fetus is reduced," Mysorekar says. This slows down the baby's growth — and may hurt the brain as well.
Mysorekar and her colleagues published their findings in the journal Cell.
She says mouse experiments can never tell us exactly what's happening in people. Human anatomy is more complicated.
But one thing is clear: Once Zika infects the fetus, "it leaves a lot of havoc and devastation in its wake," she says. "It's almost like a tornado or an earthquake. There is death following Zika."
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. Transcript provided by NPR, Copyright NPR.