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arly ultrasounds may not pick up signs of microcephaly in pregnant women infected with Zika virus, a newly published paper suggests.

The study, published in the New England Journal of Medicine, reports on the case of a 33-year-old Finnish woman who was infected with Zika during the 11th week of her pregnancy.

She terminated the pregnancy at week 21 after ultrasounds and an MRI scan showed abnormalities in the brain of her fetus. Analysis of the brain tissue after the termination showed it was infected with Zika virus.

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Microcephaly may only become apparent late in a pregnancy. But other signs of brain damage may be visible by ultrasound earlier, said lead author Dr. Rita Driggers, director of maternal fetal medicine at Sibley Memorial Hospital in Washington, D.C.

“The point we’re trying to make is: Don’t just look for microcephaly and calcifications, because those are late [pregnancy] findings,” she told STAT. “You need to look at the anatomy of the brain. That was what my clue was. … It wasn’t normal.”

The study provides additional evidence that Zika infection in pregnancy can damage fetal brains, Driggers said.

The woman — who lives in Washington, D.C. — traveled in Belize, Guatemala, and Mexico last November. After her return, she experienced symptoms of Zika infection and blood testing confirmed the diagnosis.

The woman’s blood was assessed at several points during the pregnancy and the virus remained in her blood throughout, a fact that surprised her doctors, said Driggers.

Driggers said the team treating the woman believes she had virus in her blood because it was multiplying in the fetus and the placenta.

The woman’s first ultrasound showed the fetus’s head was within the normal range, in the 47th percentile. Four weeks later it was still considered normal, but had slipped to the 24th percentile.

Had the fetus been carried to term, the baby would probably have had microcephaly, said Driggers.

Ultrasounds did not reveal calcifications — scarring, a sign of tissue destruction — which is commonly seen in the brains of babies infected with Zika in the womb. But other features of the brain were not normal. There was a buildup of fluid, and the cortex — the outer layer of neural tissue — was very thin.

With a single case, it’s impossible to tell whether this would be a consistent finding in pregnant women infected with Zika. But if it is observed elsewhere, doing blood tests on pregnant women after infection might provide a way to discern which ones are carrying fetuses infected in the womb, Driggers said.

Dr. Karin Nielsen-Saines, a professor of pediatric infectious diseases at the David Geffen School of Medicine at UCLA, described the finding as very interesting. Nielsen-Saines is involved in a study in Brazil where doctors are following more than 300 pregnant women infected with Zika.

It has been assumed virus is only found in the blood during active infection. Nielsen-Saines said blood tests were only done once on the women in her study, but perhaps additional tests are warranted. “This suggests we should be following Zika virus RNA levels in pregnant women over time,” she said.

Another study published Wednesday may offer some clues as to how Zika is affecting fetal brains. The work, in the journal Cell Stem Cell, was conducted by a team of neuroscientists at the University of California, San Francisco.

The scientists reported that a protein the virus uses to infect skin cells — a symptom of Zika is a rash — is found in high concentrations in neural stem cells, the building blocks of the developing brain.

Alex Pollen, a co-first author of the paper, said the team believes the virus uses this protein, called AXL, to infect neural stem cells.

AXL is also found in the retina; a study from Brazil has shown some infants infected in the womb have vision-threatening ocular lesions. Pollen said the hypothesis needs to be tested, work that could be done in animal models.

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