Officials at the Centers for Disease Control and Prevention said on Wednesday that there is now enough evidence to definitively say that the Zika virus can cause unusually small heads, brain damage and other neurological problems in infants born to infected mothers.
"Most of what we've learned is not reassuring," said Dr. Anne Schuchat, the principal deputy director of the Centers for Disease Control and Prevention. "Everything we look at with this virus seems to be a bit scarier than we initially thought."
As summer approaches, officials are warning that mosquito eradication efforts, lab tests and vaccine research may not be able to catch up. There are 346 cases of Zika confirmed in the continental United States — all in people who had recently traveled to Zika-prone countries, according to the most recent CDC report. Of those, 32 were in pregnant women, and seven were sexually transmitted.
But in Puerto Rico, the Virgin Islands and American Samoa, the virus is now being transmitted locally.
Schuchat said the virus has been linked to a broader array of birth defects throughout a longer period of pregnancy, including premature birth and blindness in addition to the smaller brain size caused by microcephaly.
Zika has also been linked to an autoimmune disorder that is similar to multiple sclerosis.
Scientists at the Centers for Disease Control and Prevention (CDC) have concluded, after careful review of existing evidence, that Zika virus is a cause of microcephaly and other severe fetal brain defects. In the report published in the New England Journal of Medicine, the CDC authors describe a rigorous weighing of evidence using established scientific criteria.
"This study marks a turning point in the Zika outbreak. It is now clear that the virus causes microcephaly. We are also launching further studies to determine whether children who have microcephaly born to mothers infected by the Zika virus is the tip of the iceberg of what we could see in damaging effects on the brain and other developmental problems," said Tom Frieden, M.D., M.P.H., director of the CDC. "We've now confirmed what mounting evidence has suggested, affirming our early guidance to pregnant women and their partners to take steps to avoid Zika infection and to health care professionals who are talking to patients every day. We are working to do everything possible to protect the American public."
The report notes that no single piece of evidence provides conclusive proof that Zika virus infection is a cause of microcephaly and other fetal brain defects. Rather, increasing evidence from a number of recently published studies and a careful evaluation using established scientific criteria supports the authors' conclusions.
The finding that Zika virus infection can cause microcephaly and other severe fetal brain defects means that a woman who is infected with Zika during pregnancy has an increased risk of having a baby with these health problems. It does not mean, however, that all women who have Zika virus infection during pregnancy will have babies with problems. As has been seen during the current Zika outbreak, some infected women have delivered babies that appear to be healthy.
Establishing this causal relationship between Zika and fetal brain defects is an important step in driving additional prevention efforts, focusing research activities, and reinforcing the need for direct communication about the risks of Zika. While one important question about causality has been answered, many questions remain. Answering these will be the focus of ongoing research to help improve prevention efforts, which ultimately may help reduce the effects of Zika virus infection during pregnancy.
At this time, CDC is not changing its current guidance as a result of this finding. Pregnant women should continue to avoid travel to areas where Zika is actively spreading. If a pregnant woman travels to or lives in an area with active Zika virus transmission, she should talk with her healthcare provider and strictly follow steps to prevent mosquito bites and to prevent sexual transmission of Zika virus. We also continue to encourage women and their partners in areas with active Zika transmission to engage in pregnancy planning and counseling with their health care providers so that they know the risks and the ways to mitigate them.
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