The CDC Is Now Certain That Zika Virus Causes Microcephaly and Other Severe Birth Defects

This story is part of a partnership between MedPage Today and VICE News.

The Centers for Disease Control and Prevention (CDC) has confirmed that the Zika virus causes fetal brain abnormalities, including microcephaly, which causes babies to be born with abnormally small heads.

The finding follows months of nervous speculation about whether the mosquito-borne virus causes problems in unborn children when their mothers become infected during pregnancy. The Zika virus has also been linked to a rare autoimmune disorder called Guillain-Barré syndrome, which affects the nervous system and can cause paralysis.

"This study marks a turning point in the Zika outbreak. It is now clear that the virus causes microcephaly," CDC director Dr. Tom Frieden said in a statement. "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."

Brazilian health officials first noticed a link between the arrival of the virus and an uptick in microcephaly cases in November, prompting public health investigations around the world. By January, countries had begun to issue travel warnings to pregnant women or to advise women to delay having children altogether.

But the virus is new to the Americas had never been linked to microcephaly before. Zika was first discovered in 1947 when a monkey in Uganda became infected, but it wasn't reported in humans for another 20 years. The virus remained in Africa and Asia as part of small outbreaks until 2007. It arrived in Brazil in May and in Puerto Rico in January.

To date, there have been 346 Zika virus cases reported in the United States, but these were all cases where patients were bitten by infected mosquitos while traveling. Another 354 cases were reported in US territories, including 351 where the virus was acquired locally.

The World Health Organization (WHO) announced last week that Zika causes microcephaly and Guillain-Barré syndrome, a conclusion it said was based on "a growing body of preliminary research."

Similarly, the CDC based its conclusion announced today on the summation of recently published studies. It will continue to launch studies to determine whether the babies born with microcephaly as a result of the virus are "the tip of the iceberg" in terms of future developmental problems.

The CDC has not changed its current guidance regarding Zika, which still stands at advising pregnant women to avoid areas where the virus is spreading. It has suggested that pregnant women who must travel to these places take preventive measures to avoid mosquito bites, and that couples there who aren't expecting consult their doctors to learn about family planning and risks associated with Zika.

Although Zika causes microcephaly, the CDC is not saying that all women who are infected with Zika while pregnant will have babies with microcephaly. Instead, they have an increased risk of having babies with microcephaly.

Follow Sydney Lupkin on Twitter: @slupkin