Compared to babies without microcephaly, a condition characterized by an abnormally small head, the babies with microcephaly were 55 times more likely to have been infected with Zika in utero. The report was published in the journal Lancet Infectious Diseases.

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For months, scientists with the Centers for Disease Control and Prevention and the World Health Organization have concluded that Zika causes microcephaly and a range of other birth defects. They have cited strong circumstantial evidence: the clustering of microcephaly in the places where Zika was taking place in many countries; the timing of babies born with microcephaly peaking about six months after the peak of Zika infections; evidence of Zika in the blood and spinal fluid of some babies with microcephaly.

But the latest data come from a case control study, where scientists try to match a baby with an abnormality to a baby with similar characteristics but no abnormality to find the differences that could explain the birth defect.

“Although there is a strong scientific consensus that Zika virus is a cause of microcephaly, the early findings from this case control study are the missing pieces in the jigsaw in terms of proving the link,” said Laura Rodrigues, an infectious disease expert at the London School of Hygiene & Tropical Medicine and an author of the study.

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The report is a preliminary analysis. The full study, which will include 200 babies with microcephaly and 400 without, will help quantify the risk more precisely and provide more information about other potential factors, the researchers said.

Still, the early findings are striking, said Andrew Pavia, a pediatric infectious disease specialist at the University of Utah who was not involved in the research. He likened the search for a causal link to hunting a murder suspect.

“The evidence was very strong before this, enough to get a conviction out of most juries,” he said about Zika and microcephaly. “Now they have essentially found the gun in the defendant’s glove compartment. There is overwhelming evidence and there is really no room for doubt.”

The study included 32 infants born with microcephaly between January and May 2016 at eight hospitals in Recife, Brazil, and 62 babies without microcephaly born the following morning. The mothers in both groups had similar characteristics. Most mothers had Zika virus infections. Although many mothers also had other infections, such as dengue, those infections weren't associated with microcephaly in the study, and there was no significant difference between the mothers of the two groups.

Researchers found that about half of the babies with microcephaly had laboratory-confirmed Zika infections in their blood or spinal fluid. By comparison, none of the babies in the healthy control group tested positive for Zika.

The study was requested by the Health Ministry in Brazil, which has more than 1,800 confirmed cases of Zika-related microcephaly.

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As a result of the findings, Zika virus should be officially added to the list of congenital infections alongside others such as syphilis, rubella and cytomegalovirus, said Thália Velho Barreto de Araújo, an expert at the Federal University of Pernambuco in Recife.

Researchers don’t know what proportion of women infected during pregnancy pass the virus on to the fetus, and of those, what proportion get microcephaly. Mothers who had evidence of a Zika infection but had healthy babies may have had Zika before they became pregnant, or if they had the virus during pregnancy it apparently did not cross the placenta.

There have been 2,004 babies born with Zika-related microcephaly or other birth defects around the world, according to the latest WHO report. In the United States, 23 pregnancies have resulted in Zika-related birth defects. Officials are monitoring more than 731 pregnant women with Zika in the 50 states and the District of Columbia and another 1,156 in U.S. territories, most of them in hard-hit Puerto Rico.

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In Florida, where local mosquitoes are actively transmitting the virus in an area north of downtown Miami and in Miami Beach, state officials reported seven new cases Thursday, bringing the total in Florida’s outbreak to 87.