Federal health officials confirmed Wednesday that the Zika virus causes a rare birth defect and other severe fetal abnormalities, marking a turning point in an epidemic that has spread to nearly 40 countries and territories in the Americas and elsewhere.
Scientists at the Centers for Disease Control and Prevention conducted a careful review of existing research and agreed that the evidence was conclusive, Director Thomas Frieden said. It is the first time a mosquito-borne virus has been linked to congenital brain defects.
"It is now clear, and CDC has concluded, that the virus causes microcephaly," Frieden said. CDC is launching more studies to determine whether children with that rare condition, which is characterized at birth by an abnormally small head, represent the "tip of the iceberg of what we could see in damaging effects on the brain and other developmental problems."
The outcome validates the growing research of past months that strongly implicated Zika as the culprit behind a broad set of complications in pregnancy. The pathogen is also increasingly linked to neurological problems in adults. The CDC report, published in the New England Journal of Medicine, focused only on reviewing the evidence linking Zika and fetal anomalies.
Global health officials had already assumed the virus was to blame for the problems being seen in various countries. Since January, many have advised women who were pregnant or hoping to become so to avoid travel to Zika-affected areas or to take steps to avoid Zika infection. That medical advice expanded over time to include women's partners, especially as it became clear sexual transmission of the virus was more common than had been known.
The research released Wednesday won't change that advice, officials said. But they are hoping it will help educate the public about the virus and its potential for harm — particularly in the United States.
"We do know that a lot of people aren't concerned about Zika infection in the United States, and they don't know a lot about it," said Sonia Rasmussen, director of CDC's division of public health information. "It's my hope that we can be more convincing that Zika does cause these severe birth defects in babies and hope that people will focus on prevention more carefully."
The research is likely to help scientists developing a vaccine for Zika, she said.
Researchers said there was no "smoking gun" or single definitive piece of evidence that confirmed the virus as causing microcephaly, calcifications within the brain and severe vision and hearing problems. Rather, the findings of recently published studies and a thorough evaluation by CDC researchers using established scientific criteria led them to the conclusion. Frieden likened the process to putting together pieces of a puzzle.
The World Health Organization had said in recent weeks that there was scientific consensus about the virus and microcephaly as well as Guillain-Barre syndrome, a neurological condition that can lead to paralysis. Researchers in Brazil, the hardest-hit country, said this week that Zika also may be associated with a second serious condition similar to multiple sclerosis. In a few cases with adults, swelling of the brain and spinal cord involving the coating around nerve fibers was seen.
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CDC officials said they worked independently of the WHO. The U.S. agency is conducting studies on Guillain-Barre syndrome, and officials said they are also likely to make a conclusive link between that condition and Zika.
There are still many outstanding questions about the risk facing pregnant women infected with Zika. A limited number of studies show the risk ranges from about 1 percent chance of having a baby with microcephaly to almost 30 percent. A study in Brazil identified that upper range, with pregnant women having "grave outcomes," Rasmussen said.
Brazil normally has an average of 163 cases of microcephaly each year. But since October, officials have confirmed at least 944 cases of microcephaly or other neurological problems, according to the WHO.
After a Zika outbreak in French Polynesia, that island also had an increase in microcephaly cases. It normally has no more than two cases a year, but it saw eight cases during a four-month period in 2014. A recently published study using data from the island estimated the risk for the rare birth condition to be 95 cases of microcephaly for each 10,000 pregnant women infected in the first trimester.
As of Wednesday, 85 cases of Zika have been reported in Florida, including 3 in Hillsborough County, according to the state Department of Health.
A total of 346 cases have been reported nationwide, according to the CDC. All are associated with travel.