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It’s confirmed. Zika virus causes microcephaly and other birth defects, CDC says

Today, the Centers for Disease Control and Prevention definitively confirmed what may have seemed a forgone conclusion: Zika virus causes microcephaly and other neurological birth defects.

“This study marks a turning point in the Zika outbreak. It is now clear that the virus causes microcephaly,” CDC director 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.”

The health agency’s assessment is not based on a single piece of evidence, but rather months of careful evaluation and the growing number of reports showing a biological connection between the brain disorders. A synopsis of those findings has been published in the New England Journal of Medicine.

The researchers reached this conclusion by using Shepard’s criteria — an accepted set of seven rules used to identify causes of birth defects, also known as teratogens.

“Causality is established when either criteria one, three, and four…or criteria one, two, and three are fulfilled,” according to the report. Research into Zika virus has satisfied five of the seven criteria. One sticking point was the abundance of microcephaly observed in fetuses and newborns that had been exposed to the virus in the womb.

Shepard’s criteria for proof of Zika-related birth defects in humans as applied to the relationship between Zika virus infection and microcephaly and other brain anomalies. Photo by Rasmussen SA et al., New England Journal of Medicine, (2016)

Shepard’s criteria for proof of Zika-related birth defects in humans as applied to the relationship between Zika virus infection and microcephaly and other brain anomalies. Photo by Rasmussen SA et al., New England Journal of Medicine, (2016)

Of the two unfulfilled criteria, one involves performing tests in animal models, which have not been developed, and the other only applies to chemical agents.

For now, the CDC’s guidance for pregnant women remains the same. They recommend that pregnant women avoid travel to Zika-hit regions. Infected men, with or without symptoms, can pass Zika virus to sexual partners. So, the CDC recommends abstinence or condom usage for couples if a male partner has traveled to a Zika-impacted region.

The Zika outbreak has afflicted 33 countries and territories in the Americas, according to the World Health Organization.

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