In what has been called an unprecedented association, Zika virus is causally related to microcephaly and other severe fetal brain abnormalities, according to the Centers for Disease Control and Prevention (CDC) (N Engl J Med 2016 Apr 14. [Epub ahead of print]).

“This is an unprecedented association,” said CDC Director Tom Frieden, MD, MPH, during a press briefing announcing the report findings. “Never before in history has there been a situation where a bite from a mosquito could cause a devastating malformation. Never before have we seen an illness spread by a mosquito linked to a birth defect.”

Dr. Frieden said scientists at CDC confirmed mounting evidence of the link, “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.”

Accumulating Evidence
The report noted that no single piece of evidence provides conclusive proof that Zika virus infection is a cause of microcephaly and other fetal brain defects. Instead, increasing evidence from several recently published studies and a careful evaluation using established scientific criteria supports the researchers’ conclusions.

This finding means that a pregnant woman who is exposed to Zika virus is at increased risk for having a baby with these developmental problems. It does not mean, however, that all women who have Zika virus infection during pregnancy will delivery a child with neurological problems; some infected women have delivered babies that appear to be healthy, according to the CDC.
Establishing this causal relationship between Zika and fetal brain abnormalities is an important step in driving additional prevention efforts, focusing research activities and reinforcing the need for direct communication about the danger of Zika. Although one important question about causality has been answered, many questions remain, Dr. Frieden said.
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.
“The science now shows what hundreds of … families impacted by Zika virus have suspected all along: Zika virus is causing the tragic increases in microcephaly cases and other serious brain defects,” Dr. Frieden said.
The CDC is not changing its current guidelines as a result of this finding. Pregnant women should continue to avoid traveling 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 speak with her health care provider and strictly follow steps to prevent mosquito bites. In addition, pregnant women should use protection during sexual activity with a man who has been to an area where there is Zika virus, even if he does not show signs of infection. The virus can be transmitted sexually.
The CDC encourages 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 can mitigate the risks.