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Can Zika virus infection during pregnancy cause microcephaly in the developing fetus? The evidence is pointing more and more firmly to yes, a leading expert said Friday.

“We’ve not seen any data that would suggest that it’s not true,” Dr. Lyle Petersen, director of the division of vector-borne diseases at the Centers for Disease Control and Prevention, told STAT.


“So I think the simple answer is that we’re accumulating data day by day that the evidence is getting stronger and stronger of a causal link. And it’s quite strong at this point.”

Petersen’s remarks were among the strongest suggestions yet from a top CDC scientist that Zika infection during pregnancy can — at least in some cases — cause microcephaly, a condition in which infants are born with abnormally small heads and possibly brain damage.

Traces of the virus have already been found in amniotic fluid and placental tissues drawn from women who were carrying fetuses that developed microcephaly. The virus was also found in brain tissues of infants born with the condition who died shortly after birth.


Researchers have been reluctant, nonetheless, to say definitively that Zika is connected to the birth defect, in part because very little is known about the virus.

Brazil reported last fall that it was seeing a startling increase in babies born with microcephaly, months after Zika virus first started to sweep through the country.

An increase in microcephaly cases also occurred — although it was only noted after the fact — in French Polynesia, which  experienced a Zika outbreak in 2013 and 2014.

One infant has been born in the United States with microcephaly that may have been linked to Zika infection. The baby’s mother was living in Brazil last May in the early stages of her pregnancy. She gave birth to the baby in Hawaii.

On Friday, Dr. Thomas Frieden, the director of the CDC, reported that among the 51 Zika cases detected in the US since 2015, six have been in pregnant women.

Most of the US cases were acquired outside the country, athough there has been a cluster of locally acquired cases in Puerto Rico and one case in Dallas in which a person was infected by a sexual partner who had recently returned from a place where Zika is circulating.

Zika is proving to be a wily and perplexing foe. In most people, infection doesn’t even produce symptoms. Only one out of five infected experience flu-like symptoms, as well as a raised red rash and conjunctivitis, known as pink eye.

The virus had been thought to be inconsequential to humans until scientists began to suspect a link to microcephaly. The virus may also be behind an apparent rise in the number of cases of Guillain-Barré syndrome, a progressive paralysis.

Most people recover but many need intensive care and some must be placed on ventilators — breathing machines — for periods of time. French Polynesia, Brazil, Colombia, and El Salvador have reported increases in GBS cases associated with Zika outbreaks.

Still, Petersen said the case for a causal link between GBS and Zika infection is not yet as strong as the one for microcephaly.

“GBS occurs after the infection, and so trying to retrospectively identify the infection is a little complicated. But I think the evidence is also accumulating that there’s a fairly strong causal link,” he said.

A team of CDC scientists has been in Brazil working with researchers there looking at the GBS question. They have been conducting what’s known as a case control study, in which people who develop GBS are compared with people who didn’t to try to identify what might have triggered the condition.

That work is just finishing up and the picture will be clearer when the team’s data can be analyzed, Petersen said.

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