W

omen infected with Zika can pass the virus to sex partners, the Centers for Disease Control and Prevention reported Friday, citing the first known case, detected in New York City.

While the case involved a woman infecting a man, the CDC said that transmission suggests spread between two women can also occur. The agency urged heightened caution for pregnant women with female sex partners who have traveled to or lived in a place where Zika is spreading.

Dr. John T. Brooks, a senior medical adviser in the division of HIV/AIDS prevention, said the discovery wasn’t surprising.

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“Most STDs” — sexually transmitted diseases — “are transmitted both from men to women and women to men,” Brooks noted.

It’s been known for quite some time that men infected with Zika can spread the virus to sex partners. In 2008, an American researcher got infected while working in Senegal, and his wife developed symptoms a short time after his return.

Then, earlier this year, it was reported that a man who had traveled to a country where Zika is spreading infected his male partner through sex.

Brooks said the US has detected 14 cases of sexual transmission of Zika originating in a man. Ten other countries have also reported sexual transmission from men to sex partners.

The New York case was reported in the CDC journal Morbidity and Mortality Weekly Report.

The woman, in her 20s, had traveled to an area where Zika is spreading. She had sex with a male partner the day she arrived home. They had vaginal sex and did not use a condom.

Seven days later he developed symptoms and also tested positive for the infection.

Earlier this week, researchers from Pointe à Pitre University Hospital in Guadeloupe reported that genital and cervical swabs taken from a woman with Zika tested positive for the virus, as did cervical mucus.

And research in primates has shown that fragments of Zika virus can be detected in vaginal fluid up to seven days after they have been infected.

“The bottom line message is: If you’re going to share body fluids with Zika, there’s a risk,” said Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.

Osterholm wondered how the researchers concluded the transmission occurred during intercourse, and not, for instance, through kissing. The report makes no reference to kissing.

The Zika virus has been found in saliva but it is not yet clear if transmission can occur through kissing alone.

Brooks said, though, that if transmission can occur through kissing, you would expect the number of cases of transmission from men to their partners and women to their partners to be more or less equal.

Every new and unusual discovery about Zika appears to ratchet up concern about the virus. But Brooks said this new discovery doesn’t have huge implications for most people, who have very mild illness or no symptoms at all if they are infected.

The real priority remains preventing transmission to a developing fetus, he said.

“In terms of risk to a developing fetus, sexual transmission from a woman to her sex partners really is only potentially relevant to the small number of pregnant women who have female sex partners who have recently traveled to or lived in an area of Zika transmission,” he said.

The CDC is working on updating its recommendations for ways to prevent sexual transmission of the virus.

Brooks said the new guidance will urge pregnant women with female partners who have traveled to or lived in places where Zika is spreading to use barrier methods during sex or abstain from sex for the duration of the pregnancy.

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Some people have expressed concern that Zika infection will have long-term implications on women’s reproductive health, potentially damaging future offspring.

But Brooks said the discovery that vaginal fluids can transmit infection does not mean women need to fear that future pregnancies will be affected.

The immune systems of infected women who are not pregnant appear to clear out the virus pretty quickly. And once a woman has been infected, it is believed she will be immune for life, Brooks noted.

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