The Zika virus damages a large percentage of the fetuses carried by infected and symptomatic mothers, regardless of when in pregnancy the infection occurs, according to a small but frightening study released Friday by Brazilian and American researchers.
In the study, published online by The New England Journal of Medicine, researchers found that 29 percent of women who consented to ultrasound examinations after testing positive for infection with the Zika virus had fetuses that suffered “grave outcomes.”
“This is going to have a chilling effect,” said Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases. “Now there’s almost no doubt that Zika is the cause.”
The numbers, he said, are “very concerning.”
The small size of the study — which enrolled only 88 women who had visited a clinic in Rio de Janeiro — was a limitation, he added, “but there will be other studies that I believe will corroborate this.”
In two cases, babies 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 the David Geffen Medical School at the University of California, Los Angeles, 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 showed brain calcifications or abnormally slow growth.
Doctors in Brazil had previously said that the worst damage appeared in fetuses whose mothers were infected in the first trimester.
Other scientists echoed Dr. Fauci’s assessment that it added powerful new evidence that the mosquito-borne virus — and nothing else — is behind Brazil’s wave of fetal brain damage.
“It does not close the deal, but it’s a huge step forward,” said Dr. Ernesto T.A. Marques, a Zika researcher at the University of Pittsburgh Medical School who was not involved in the research, which he described as a “mini-case-control study.”
The World Health Organization is awaiting results from a much larger case-control study, involving 5,000 women, mostly from Colombia. Final results are not expected until May or June, when large numbers of the babies in it will come to term.
The Brazilian study, which is still ongoing, included 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, was tested for Zika virus, and 72 were positive.
Forty-two of the 72 infected women agreed to have a series of ultrasound scans, as did all 16 women who were uninfected.
Of the remaining 30 infected women, two miscarried and 28 declined the scans, the authors said.
“Some women who said no said the campus where the ultrasounds were done was too far away,” Dr. Nielsen-Saines said. “But we think it was also not wanting to know. There’s a tendency to be fatalistic, to say, ‘God willing, everything will be fine, or I’ll deal with it when the baby is born.’”
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.
Since all The study does not answer an important question: whether a mild Zika infection can damage a fetus.
Although it was small, “this kind of study is the gold standard,” Dr. Fauci said.