New research sheds more light on the mysterious epidemic that has swept more than two dozen countries in the Western Hemisphere, alarming citizens and unnerving public-health officials.

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The Zika virus damages many fetuses carried by infected and symptomatic mothers, regardless of when in pregnancy the infection occurs, according to a small study released Friday by Brazilian and U.S. researchers.

In a separate report published Friday, other scientists suggested a mechanism for the damage, showing in laboratory experiments that the virus targets and destroys fetal cells that eventually form the brain’s cortex.

The reports are not conclusive, but the studies help shed light on a mysterious epidemic that has swept across more than two dozen countries in the Western Hemisphere, alarming citizens and unnerving public-health officials.

In the first study, published in The New England Journal of Medicine, researchers found that 29 percent of women who had ultrasound examinations after testing positive for infection with the Zika virus had fetuses that suffered “grave outcomes.”

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They included fetal death, tiny heads, shrunken placentas and nerve damage that suggested blindness.

“This is going to have a chilling effect,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. “Now there’s almost no doubt that Zika is the cause.”

The small size of the study — which looked at 88 women at one clinic in Rio de Janeiro — was a limitation, Fauci added. From such a small sample, it is impossible to be certain how often fetal damage may occur in a much larger population.

Still, the high percentage of fetuses damaged is “very concerning,” he said.

Two fetuses died even though the mothers were infected relatively late in pregnancy, at 25 and 32 weeks — and even after earlier ultrasounds had shown the fetuses to be normal.

“We were just blown away by that,” said Dr. Karin Nielsen-Saines of UCLA’s David Geffen Medical School and one of the lead authors. “We weren’t expecting to find problems in all trimesters.”

Other fetuses whose mothers were infected late in pregnancy suffered brain calcifications or abnormally slow growth. Doctors in Brazil had previously said the worst damage appeared in fetuses whose mothers were infected in the first trimester.

The World Health Organization is awaiting results from a similar but much larger study involving 5,000 women.

The Brazilian study, which continues, followed 88 pregnant women who visited a fever clinic at the Oswaldo Cruz Foundation hospital in Rio de Janeiro, one of Brazil’s leading research hospitals, between September and February.

Every woman with a rash, the most characteristic symptom of infection, was tested for the Zika virus, and 72 were positive. The tests looked for the virus itself; they work only if done early in an infection, but are considered more accurate than later tests for antibodies.

Forty-two of the infected women agreed to have a series of ultrasound scans, as did all 16 uninfected women.

The bleakness of the results was startling. Of the 42 infected women receiving regular ultrasounds, a dozen had babies who died in utero or suffered serious birth defects. Only eight of the babies carried by the 42 women have been born so far, but the ultrasound scans for them turned out to be accurate. None of 16 uninfected women had problems.

The new study does not answer an important question: whether Zika infections so mild that they produce no symptoms in the mother can damage a fetus. Doctors believe four out of five Zika victims have no symptoms. When they do, the most common are rash, joint pain, red eyes and headaches.

In the second study, published in the journal Cell Stem Cell, researchers at Johns Hopkins and elsewhere cultured several types of cells present in early fetal development, including so-called cortical neural progenitor cells, which form the cortex, the outer brain layer responsible for many higher functions.

Three days after they infected all the cells with the Zika virus, 90 percent of the progenitor cells were damaged; they were unable to divide normally and often died. By comparison, the other two types of fetal cells were much less affected.

“The cell types responsible for forming the cortex are the target of the Zika virus,” said Hongjun Song, a neurologist at Johns Hopkins University School of Medicine and senior author of the study.

Some experts cautioned that, because the experiment was conducted with laboratory-grown cells, the results might not apply to humans. “It might be that the results wouldn’t be the same in a living system of actual cortical stem cells,” said Dr. Catherine Spong, of the National Institute of Child Health and Human Development.

Dr. William Dobyns, a pediatric neurologist at Seattle Children’s Research Institute, called the work “highly significant.”

If the cells that should form the brain cortex grow too slowly, he said, “You get a small brain, but on top of that there’s cell death, which means whatever the size the brain is, it will shrink.” That reduction may lead to a conspicuous space between the skull and brain.

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