ATLANTA — For the first time in history, a mosquito-borne virus has been identified as the cause of devastating brain birth defects.

That was the conclusion of the U.S. Centers for Disease Control and Prevention, which on Wednesday confirmed that the Zika virus is to blame for severe developmental and neurological problems in newborns. The announcement came after months of reviewing studies from Brazil to Atlanta that documented the virus’ link to neural stem cell death. Health officials said the last infectious virus to cause “a major epidemic of congenital birth defects” was rubella, or German measles, in the 1960s. At that time 20,000 babies affected by rubella were born each year.

Researchers studying the Zika virus found that within hours of a mosquito bite it begins to kill fetal neural tissue, the cells that are the building blocks of the human brain and nervous system. In clinical studies, including one by Emory University, the virus not only kills neural cells, but transfers the virus to other cells, compromising them at a critical time of a baby’s development. The infection manifests itself in the form of obvious birth defects, such as microcephaly, but has been linked to more subtle developmental abnormalities that may not manifest for months or even years. The virus has also been linked to miscarriages.

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In a paper published Wednesday in The New England Journal of Medicine, the four CDC researchers leading the agency’s Zika response said several studies now prove the link is unmistakable.

“We conclude that a causal relationship exists between prenatal Zika virus infection and microcephaly and other serious brain anomalies,” wrote the four lead doctors of the CDC’s Zika response team.

In a news conference on Wednesday Dr. Sonja Rasmussen, one of the CDC’s Zika researchers said that even with this evidence, doctors still don’t know “the full range” of problems Zika presents in a pregnancy. Researchers continue to believe that the first trimester is the most vulnerable time for a woman to be exposed to Zika, but evidence shows that infection later in a pregnancy can cause poor fetal growth, fetal deaths, or defects that an ultrasound cannot pick up.

“Never before in history has a bite from a mosquito caused such a malformation,” said Dr. Tom Frieden, CDC director.

While the link between Zika and microcephaly, a condition where babies have unusually small heads and underdeveloped brains, has long been suspected, Wednesday’s announcement is the first time the CDC has made the direct connection. The correlation to other birth defects heightens the alarm even as warmer weather ushers in mosquitoes that could spread the virus. The World Health Organization recently said the link appeared strong, though the CDC said it did not work with the WHO on its report.

Wednesday’s news is likely to increase pressure on Congress to approve $1.8 billion in funding sought by the Obama administration to combat the virus.

It has been more than half a century since an infectious disease caused an epidemic of birth defects, the last one being rubella, the authors said. While there is a vaccine for rubella, there is no vaccine yet to prevent Zika. The National Institutes of Health will begin Zika vaccine trials in September, but Dr. Anthony Fauci, head of the NIH’s Institute of Allergy and Infectious Diseases, said earlier this week it could be months if not years before a vaccine is ready.

Zika is transmitted by bite from either the Aedes aegypti or Aedes albopictus mosquitoes, both of which are found in Georgia and nearly 30 other states from California to New York. The disease is also transmitted sexually. The CDC is still advising pregnant women not to go to any region experiencing a Zika outbreak because an infection can occur with a single mosquito bite. Pregnant women or those who want to become pregnant shouldn’t have sex with a man who has traveled to regions with Zika outbreaks. But while Zika remains in the blood longer than the 10 days initially believed, after six months a woman or man who has had the virus should be able to conceive without the potential of microcephaly complications, the CDC said.

Researchers at Emory University contributed to Wednesday’s findings. Working with researchers from Johns Hopkins University and Florida State University, the Emory researchers discovered that neural stem cells exposed to the virus begin to die within hours. The remaining neural stem cells not only retain the virus but spread it to other neural cells that eventually form the brain.

“It’s attacking at the earliest points of development,” said Peng Jin, a genetics professor at Emory University told The Atlanta Journal-Constitution. Jin was one of 15 researchers on the neural report published in Cell Stem Cell journal. “It keeps replicating, at least from our cell culture study, that’s what we’re seeing.”

Jin also said there was another potentially menacing feature of the virus. Babies with Zika exposure could appear healthy at birth, but because their brains continue to develop after birth, developmental abnormalities could show up later. There is also the question of why some babies get microcephaly after exposure and others don’t. Jin said that answer could take years to be revealed.

“Maybe there’s something unique within this particular strain of the virus or there might be genetic factors on how Zika attacks or does not attack stem cells,” Jin said.

Frieden and Rasmussen said no one piece of evidence proved the link, but that “mounting evidence and causal relationships” from a variety of studies taken together proved the relationship. Rasmussen said the agency decided to release its verdict now, rather than wait for results from other, ongoing studies.

“We could wait until all the studies were in, but that could take potentially years,” Rasmussen said.

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