Health officials in the United States are reporting the first suspected case of Zika virus being sexually transmitted from a woman to a man.

The twenty-something woman from New York City engaged “in a single event of condomless vaginal intercourse” with a male partner on the day that she returned from a Zika-endemic country, according to a report by the U.S. Centers for Disease Control and Prevention.

Since 2015, the Zika virus has spread to 48 countries and territories, primarily in Latin America and the Caribbean, and has been linked to serious neurological disorders and birth defects.

While most infections are transmitted through mosquito bites, Zika can also spread through sex and the virus can linger in semen for several months. Eleven countries have now reported sexual transmission of Zika, though previous cases have involved men spreading the virus to women or to other men.

The woman — who is not pregnant — reported cramps and a headache while waiting to catch her flight to New York City. The following day, she developed a fever, rash and other symptoms. She also started menstruating and described her flow as “heavier than usual.”

On the third day, the woman saw her doctor and provided urine and blood samples, both of which tested positive for Zika.

The man, who is also in his 20s, developed a fever, rash, joint pain and eye inflammation one week after having sex with the woman. Two days later, he went to a doctor and lab tests detected the virus in his urine samples, though not his blood.

He told health investigators that he had not travelled outside of the U.S. in the last year and didn’t have any other sexual partners or mosquito bites prior to getting sick.

The Zika virus had been previously detected in vaginal fluid and scientists have speculated that the virus could potentially transmit from women to men. It remains unknown whether women can transmit the virus to other women through sexual intercourse or contact.

This latest report, which was published Friday in the CDC’s Morbidity and Mortality Weekly Report, comes just three weeks ahead of the Summer Olympics, which are scheduled to begin on Aug. 5 in Rio de Janeiro.

Brazil has been particularly hard hit by the Zika epidemic and a handful of athletes — including Canadian tennis player Milos Raonic — have pulled out of the Games, citing Zika fears. More than 230 researchers, ethicists and academics have also called for the Olympics and Paralympics to be postponed or cancelled, warning that they could further fuel Zika’s global spread.

The CDC recently published a study rebutting these concerns, however, noting that the Olympics are unlikely to have a major impact in seeding new outbreaks — with the exception of four developing countries that do face substantially-higher risks: Yemen, Chad, Djibouti, and Eritrea.

Health officials stress that they are still learning about Zika and more research is needed to fully understand its risks. The authors of Friday’s report noted that current public health guidelines for preventing Zika are predicated on the assumption that the virus is mostly spread by male sexual partners.

“This case represents the first reported occurrence of female-to-male sexual transmission of Zika virus,” said the CDC report.

“Current guidance to prevent sexual transmission of Zika virus is based on the assumption that transmission occurs from a male partner to a receptive partner. Ongoing surveillance is needed to determine the risk for transmission of Zika virus from a female to her sexual partners.”

The CDC said in a followup statement that it is now changing its current recommendations for preventing Zika transmission through sexual intercourse.

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Its current guidelines is for pregnant women to abstain from sex — or use condoms — if their sexual partners live in a Zika-affected country or have recently travelled to one. This advice is now being extended to female sex partners of pregnant women, the CDC said.

The public health agency is also working on updated recommendations for sexually active people who are not pregnant or planning a family.

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