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Zika virus has been discovered in the placentas of at least two American women who contracted the virus while traveling and miscarried after their return to the United States, the chief pathologist at the Centers for Disease Control and Prevention told STAT on Wednesday.

The finding adds to a growing mound of evidence that Zika infection during pregnancy can pass from the mother to the fetus she carries. It is believed that in some such cases, the virus attacks the developing brain, triggering microcephaly, a condition in which newborns have abnormally small heads.

There have already been reports, from the United States and Brazil, of traces of Zika virus being found in amniotic fluid and placentas from women who were discovered to be carrying fetuses with microcephaly.


And last month the CDC reported it had found the virus in brain tissues from two microcephalic infants from Brazil who died shortly after birth. Those findings, initially reported via the media, were published Wednesday in CDC’s in-house journal Morbidity and Mortality Weekly Report.

Also on Wednesday, researchers from Slovenia reported a supporting finding in the New England Journal of Medicine.


Together, the new reports lend additional weight to the recommendations from public health authorities at the CDC and elsewhere that women who are pregnant should avoid traveling to places where Zika virus is spreading.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions. Alex Hogan/STAT

Health officials have speculated that infection in pregnancy can also lead to an increased rate of miscarriages. The United States has not previously reported miscarriages in American travelers who have contracted the virus, but Dr. Sherif Zaki, the CDC’s chief pathologist, said the agency was now aware of at least two such cases. He said they miscarried early in their pregnancies but did not say where the women lived.

A woman in Hawaii who was infected with Zika virus recently gave birth to a child with microcephaly.

The Slovenian team that found Zika in brain tissue, from the University of Ljubljana and the University Medical Center Ljubljana, performed an autopsy on a fetus with microcephaly.

A woman from Slovenia who had been employed as an aid worker in Brazil for the past couple of years became pregnant in February 2015. A few months later, she contracted Zika and later decided to return to Slovenia.

An ultrasound at 29 weeks of her pregnancy revealed possible brain anomalies in the fetus. Microcephaly was later confirmed and she chose to terminate the pregnancy.

Analysis of the brain tissue showed profound brain abnormalities, said Tatjana Avšič Županc, senior author on the paper. The group also found high concentrations of viral particles in the brain tissue — but no evidence of Zika anywhere else.

“We think that this really represents the most compelling evidence to date that congenital CNS [central nervous system] malformations are associated with Zika virus,” Avšič Županc said.

Dr. Albert Ko, who has been working in Brazil investigating the rise in microcephaly cases, said the Slovenian findings are consistent with what is being seen in that country.

Ko, a professor at Yale’s School of Public Health, said the condition of the fetal brain described in the Slovenian paper was very concerning. He suggested the type of damage seen would likely translate into a poor prognosis for a child.

Ko and some colleagues reported Tuesday in the journal JAMA Ophthalmology that a number of Brazilian babies born with microcephaly suspected to be linked to Zika infection also have damage to their eyes. Ko said he fears there will be a spectrum of birth defects associated with this virus, some of which remain to be identified.

As for the questions of whether Zika infection is causing microcephaly in some infants, Zaki said that while it is important to gather more evidence, finding Zika virus in the tissues of a growing number of malformed brains is very persuasive.

“To me as a pathologist, this is undoubtedly evidence that it’s causing it,” he said.

An earlier version of this story incorrectly said that at least three American women who contracted Zika virus while traveling miscarried after their return to the United States.