Doctors have known for some time that a man can spread the Zika virus to a woman through sex. Now officials have documented the first case in which a woman apparently infected a man through unprotected sexual intercourse.

The case occurred in New York City when a woman in her 20s returned from a trip to a country where Zika is spreading, according to a report released Friday by the federal Centers for Disease Control and Prevention.

The woman, who was not identified, and a male partner had sex without a condom the day she returned home, according to the report.

After the woman developed Zika symptoms, she went to her doctor. Tests showed she was infected with the Zika virus. Her partner, who is also in his 20s, subsequently got sick and tested positive for the virus. He had not traveled outside the United States nor did he have other risk factors for Zika, such as having been bitten by a mosquito.

"The hypothesis is that this was transmitted sexually, and I think that's probably a pretty good case," John Brooks, a medical epidemiologist who is in charge of studying the sexual transmission of Zika for the CDC, told Shots in an interview. "This the first case that we're aware of, anywhere."

The report did not identify the country where the woman had traveled. Both she and her partner recovered, Brooks says. The woman was not pregnant.

Most people who get infected with Zika do not get sick, or they develop only a relatively mild illness. But Zika can cause serious birth defects when pregnant women get infected. And the virus can cause neurological complications in adults in rare cases.

Based on this case, the CDC is revising its recommendations for how people can protect themselves from getting infected with the Zika virus through sexual contact.

Currently the CDC recommends that pregnant women whose male sexual partner has traveled or lived in a place where Zika is spreading use a barrier method of contraception, such as a condom, or refrain from sex during the pregnancy. The CDC now recommends that pregnant women whose sexual partners are female take the same precautions.

In addition, the CDC plans to update its recommendation for sexually active couples in which the woman is not pregnant. Currently, the CDC recommends that men who have traveled to a place where Zika is spreading and develop symptoms should use condoms or abstain from sex for at least six months. If they don't have symptoms, they should take precautions for at least eight weeks.

Brooks says the CDC plans to provide similar advice to women about "what they might want to consider doing to reduce the risk that they may expose someone to Zika or transmit it to someone else."

He noted that mosquitoes remain a major way the Zika virus is spread. No cases of Zika transmission through mosquitoes have been reported in the U.S.

It's unclear exactly how women spread Zika to men, but the virus has been detected in the vaginal fluid of at least one woman and in nonhuman primates.

Brooks noted that the male partner involved in the New York case was not circumcised. Uncircumcised men tend to be at greater risk for sexually transmitted diseases. In addition, the woman involved in the case started her period shortly after they had sex. So there's a chance the virus could have spread through blood.

The report from the New York Department of Health and Mental Hygiene was published in the CDC's Morbidity and Mortality Weekly Report.

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