A Woman Can Officially Transmit Zika to a Man

Zika has thwarted scientists’ assumptions once again: New York City health officials have just reported that the Zika virus has been sexually transmitted from a woman to a man.

While researchers have known that the virus is sexually transmissible from the beginning of this year, all of the cases have involved men passing the virus onto their partners. Right now, the official CDC statement on Zika still states that we “don’t know if a woman with Zika can pass the virus to her partners, during vaginal or oral (mouth-to-vagina) sex.” There’s a pretty high chance that statement is going to change over the next few days.

A report released by the CDC today describes a case study in which the New York City Department of Health and Mental Hygiene identified a woman who, after visiting a country with known Zika transmission, passed the virus onto her partner. After returning to New York after her trip, the woman began to experience all of the common symptoms of the virus: Fever, fatigue, a rash, and numbness in her hands and feet. Her primary care provider tested her blood and urine and confirmed she had Zika. Just six days later, her male partner started to develop similar symptoms. After similar rounds of testing, it was confirmed that he had the virus, too.

A similar case study has just been published in The Lancet Infectious Diseases: On Monday, researchers from Pointe Pitre University Hospital in Guadeloupe reported that the Zika virus had been detected in the mucus lining a woman’s genital tract, even though her blood and urine were clear. While this report alone wasn’t enough to show that Zika virus could be transmitted from a woman to a man, it did show that women’s bodies also allowed the virus to thrive even after it’s cleared from the blood and urine, similar to the way it can survive in semen.

Researchers have known since the beginning of this year that Zika virus particles survive in semen and can be passed on through unprotected sex. In fact, the virus thrives better in semen than it does in blood.

The New York City case study provides the strongest evidence yet that Zika can be transmitted from women to their partners. Until recently, efforts to contain the Zika virus have relied mostly on a single strategy: Don’t get bitten by mosquitoes. But as researchers discover more about Zika’s ability to be sexually transmitted — and thereby reach areas that are otherwise mosquito-free — controlling its spread will mean we’ll have to be more diligent about wrapping it up.