Aedes aegypti mosquitoes, the Zika virus is primarily spread through the bite of a mosquito. (AP Photo/Ricardo Mazalan, File)

Women are capable of passing a Zika infection to their male sexual partners, according to new research released Friday.

For the first time, public health officials report that a woman transmitted the virus to her male partner during sex. All previously reported cases of sexually transmitted Zika infection have been spread from men.

But according to the Centers for Disease Control and Prevention, a New York City woman in her 20s had intercourse with a male partner, who did not wear a condom, the day she returned from a Zika-affected region outside the United States.

The next day, the CDC report says, the woman developed symptoms of Zika illness: fever, fatigue, rash, joint and back pain, swelling, numbness and tingling in her hands and feet. That same day, the woman also started her menstrual period. Two days later, she went to her primary care doctor, who took blood and urine samples and sent them to the New York City Health Department lab for testing. The tests were positive, showing the presence of virus in her blood and urine.

A week after the couple had sex, the man developed fever, rash, joint pain and red eyes. He went to the same doctor who had diagnosed Zika infection in the woman. The doctor suspected sexual transmission of the virus and alerted the health department. The man, who also is in his 20s, had not traveled outside the country during the year before his illness, did not have other recent sexual partners and had not been bitten by a mosquito in the week before he got sick. Blood and urine samples were collected from the man, who tested positive for the virus in his urine.

New York City health officials who investigated the case said the virus present in the woman's vaginal fluids or menstrual blood was likely transmitted to her male partner through urethral mucous or undetected abrasions on his penis.

Until now, scientists knew that a man with Zika could pass it to a female or male partner during unprotected sex, regardless of whether the man shows symptoms. The virus remains in the semen longer than it does in blood. The latest findings suggest that Zika is spread much like other sexually transmitted diseases.

"It would not be unexpected to see more cases of women transmitting it to sexual partners," said John Brooks, the CDC's expert on sexual transmission of Zika. Because Zika infections in pregnant women can cause severe birth defects in babies, the latest research means pregnant women who have female sex partners who live in or have traveled to Zika-affected areas also need to take prevention measures, he said.

The CDC is updating its recommendations to address sexual transmission of Zika for women, even though no cases of woman-to-woman transmission have been documented. Pregnant women with female sex partners should use barrier methods every time they have sex, avoid re-using sex toys or avoid sex during the pregnancy, Brooks said.

At the moment, there's not enough evidence to show that sexual transmission goes "beyond a pair of partners," Brooks said. "It's very difficult to figure out in an area of ongoing transmission of Zika, how many infections are due to mosquito bites and how many were due to sexual transmission."

The virus is primarily spread through the bite of an infected mosquito.

Everything you ever wanted to know about the Zika virus and its spread across North and South America. (Daron Taylor,Claritza Jimenez/The Washington Post)

In the New York case, two conditions also existed that increased the likelihood of sexual transmission. The man was uncircumcised, which always raises the risk of acquiring sexually transmitted viral diseases, such as HIV and herpes, Brooks said. The woman was also becoming ill — with a lot of Zika virus in her system — right at the time she had sex.

It's unclear how much virus was in her vaginal fluids, but animal models have shown that the virus can be present in those secretions.

Until now, CDC's guidance to prevent sexual transmission was based on the assumption that any spread occurs from a man to his partners. Based on the latest information, officials are recommending that health-care providers report to local or state health department any patients with illnesses compatible with Zika infection who do not have a history of travel to an area where the virus is spreading locally but who had a sexual exposure to a partner who did travel.

The Zika virus has spread to nearly 50 countries, primarily in South America and the Caribbean.

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