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How Zika gets into a baby's brain
Scientists still aren't sure how the Zika virus gets through the placenta, but they're beginning to piece together how it penetrates a baby's brain. - photo by Jennifer Graham
Harry Truman was president when the virus named for the Ziika Forest in Uganda was discovered. But scientists are just now figuring out how Zika crosses the placenta and gets into a baby's brain.

Writing for The Atlantic, journalist Veronique Greenwood explained that although the Zika virus was identified in 1947, it wasn't until an alarming number of babies were born with small heads and brain defects in 2015 that the virus was definitively linked to microcephaly.

Most people with the virus present only mild symptoms: rash, fever, muscle aches and conjunctivitis. Why do some people recover in days, and others sustain brain damage? And how does the virus cross a sturdy placenta to attack a baby's developing brain?

Scientists don't have all the answers, but they have learned that Zika needs a receptor named AXL to spread. And Arnold Kriegstein, a neurobiologist at the University of California, San Francisco, had an aha moment when he read the paper that identified AXL as a Zika enabler.

"His lab had been studying stem cells found inside the developing brain, and he knew that AXL receptors coated these cells in a thick carpet. The receptor was also present in a limited number of other cells, including ones that make up the blood vessels heading into the brain and neural support cells like astrocytes and glia," Greenwood wrote.

Using brain matter from aborted babies, Kriegstein and other researchers have found that neural stem cells, not mature neurons, are vulnerable to a Zika invasion, and once inside, Zika prevents cells from growing and dividing. But much is still unknown, including exactly how the virus uses the receptor, The Atlantic's report said.

"Another pressing question is how the placenta, a tissue with such a remarkable ability to withstand infection that only a small number of pathogens are known to cross from the mother to the fetus, lets Zika through. And why do some babies born to Zika-infected mothers suffer from microcephaly, others from retinal deformities, while others have no apparent damage?" Greenwood wrote.

Meanwhile, the UCSF researchers have had some success treating cells infected with Zika with a commonly prescribed antibiotic, azithromycin. Known to consumers are Zithromax and Zmax, it is used to treat conjunctivitis, bronchitis and sexually transmitted diseases, and it's thought to be safe during pregnancy. But although azithromycin seems to keep the virus from multiplying in a laboratory dish, it has yet to be tested on animals.

Researchers are working days, nights and weekends to find a way to cure, or at least slow the spread, of what has been called a global health emergency, Greenwood said.

The latest research comes amid news that the virus has been found in more than one type of mosquito. It is known to be spread through the Aedes aegypti mosquito, identifiable by its striped legs, but Brazilian researchers announced July 22 that its presence has been confirmed in a common house mosquito that is more plentiful than Aedes aegypti.

No mosquito carrying Zika has been found in the U.S., although two Florida cases are under investigation in Broward and Miami-Dade counties.
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