CDC confirms Zika causes microcephaly, other birth defects

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Scientific criteria support unprecedented link between vector-borne illness and devastating birth defects, Frieden says, while efforts to quantify risks continue

Saying data and an accumulation of evidence leave no further doubt about links between Zika infection and birth defects, U.S. Centers for Disease Control and Prevention confirmed today that infection with the mosquito-born virus during pregnancy causes microcephaly and other severe neurological impacts in babies.

Experts at the agency applied standardized criteria used by scientists to weigh the likelihood of pathogens or other factors to which pregnant women have been exposed having caused subsequent birth defects in their babies, as well as a growing body of studies examining rising rates of Zika infection and microcephaly in the Western hemisphere. Among the criteria validating their conclusion, CDC staff and scientists said during a press conference today was the combination of exposure to a relatively rare virus and a rare birth defect, and evidence of the virus in the brains of babies who have been born, miscarried or died with signs of neurological damage. Cases of microcephaly caused by Zika are unusually severe, a CDC representative said, with a greater degree of stunted head and brain development than is commonly seen among babies with the birth defect, which is also caused by other factors. A report documenting their findings was being completed during the press conference and was released in the New England Journal of Medicine later today. While the findings will not affect the agency’s recommendations on travel, sexual transmission and pregnancy, including that women who have been exposed to the virus wait at least eight weeks before becoming pregnant, CDC public health information director and MMWR editor Dr. Sonja Rasmussen voiced hope that the conclusions would foster greater attention to risks and preventive measures.

CDC representatives also emphasized today much remains unknown about the impacts of the virus.

“We still need many more answers,” CDC Director Dr. Tom Frieden said. They include the full range of risks posed by the virus, the stage of pregnancy at which risks are the highest, the rate of risks, as well as the risks of other impacts that may surface later, and the impact of other factors in elevating the risks of birth defects. While a study in French Polynesia found the risk of microcephaly to be about 1 percent among babies born to women infected with Zika while pregnant, a Brazil study found a nearly 30 percent risk.

Scientists are still exploring the association of rising rates of Zika infection and rising rates of the paralyzing Guillain-Barre syndrome that have surfaced where the virus is being transmitted, and expect to conclude a causal link exists, a CDC representative said today.

The announcement, accompanying press conference and scheduled article release followed by a week a more low key statement from the World Health Organization, among bullet points in its last Zika situation report that “based on a growing body of preliminary research, there is scientific consensus that Zika virus is a cause of microcephaly and Guillain-Barré syndrome.”

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