The first documented case of the Zika virus transmitted by sexual contact from a woman to a man was reported in New York City, the Centers for Disease Control announced Friday.
The finding is the first time the CDC has documented the illness being spread from a woman to a man. All prior cases of sexually-transmitted Zika involved a man who infected a woman, according to the CDC.
The Zika virus causes only a mild illness, at worst, in most people. But infection during pregnancy can lead to severe brain-related birth defects for the fetus. The New York woman was not pregnant.
While this is the first documented case of a woman spreading Zika through sex, health experts say it is not surprising because most diseases that can be spread through sex can be spread by both men and women. It has likely been happening throughout the recent Zika outbreaks in Brazil, Latin America and elsewhere, though experts say it is probably not very common.
On the day the New York woman returned from a trip to a Zika-infected country, she had vaginal sex with her partner, without a condom, health officials were told.
She went to her doctor three days after her return, after developing common Zika symptoms such as fever, fatigue, rash and back pain. Tests showed Zika infection.
Seven days after they'd had sex, her male partner developed similar symptoms. Two days later, he went to the same doctor. The doctor tested him even though he hadn't traveled from a Zika outbreak area and no cases of female-to-male transmission had been reported. He tested positive for Zika.
They both are in their 20s, but no other details about them were released, including where the woman traveled or when the infection started. Both have recovered, a CDC official said.
The woman began menstruating the day after they had sex. Health official say she may have spread the virus through vaginal fluid or menstrual blood.
The primary concern about Zika infection is the virus's threat to pregnancies, and health officials have issued cautions to pregnant women who have a male sex partner who may have been infected.
Male-to-female transmission is considered far more likely than the other way around, experts said. One reason is that Zika virus has been found to linger in semen for more than two months, but is thought to stay in vaginal fluid no more than two weeks, said Dr. John T. Brooks, a CDC expert on sexually-transmitted diseases who is part of the agency's Zika response team.
The case likely does not complicate efforts to fight the virus or show an important additional pathway for transmission, Brooks said, because female-to-male transmission is relatively difficult.
In the New York case, for example, several factors lined up to allow the disease to spread. The couple had sex just before the woman developed symptoms, a time when the amount of virus in her body may have been particularly high. They had sex just before her period started, so there may have been a small amount of early bleeding. And the man was uncircumcised, and uncircumcised men are considered at higher risk of catching sexually transmitted diseases.
The news comes two days after the city Health Department announced there have been 310 confirmed cases of Zika in the five boroughs, including 36 women who were pregnant at the time of infection.
Nationwide, more than 1,300 people in all 50 states and Washington have been diagnosed with Zika. At least 14 of the patients came down with the illness through sexual contact; all of the others contracted Zika while in another country.
Zika has been linked to microcephaly, a condition in which a newborn's brain is underdeveloped and the head is abnormally small. In addition to sexual contact, the virus is spread through specific species of mosquitoes.
The mosquito species most commonly associated with Zika's spread is not found in the tri-state, but a similar species that scientists think could transmit the disease inhabits the area.
New York City health officials have said they've been monitoring populations of the insect and applying pesticides to keep mosquito-borne diseases like Zika at bay.
All pregnant women with sex partners who travel to or live in Zika-stricken areas should use barrier-based contraception, such as a condom, according to the CDC. The recommendation now also includes women who have sex with women, even though there have been no documented woman-to-woman transmissions.