New York City's health department on Friday reported the first female-to-male transmission of the Zika virus, which is most typically spread by the bite of an infected mosquito.
The U.S. Centers for Disease Control and Prevention said the report is the first documented case of sexual transmission of Zika from a woman to her male sex partner.
All previously reported cases of sexually transmitted Zika virus infection have been spread from men to their sex partners, the Atlanta-based CDC said in a statement.
In the New York case, transmission of the virus occurred on the day that a woman in her 20s returned to the country's largest city from an area with active Zika transmission and had a single event of unprotected sex with a male partner. The man had not traveled outside of the United States in the prior year.
The woman developed fever, fatigue, a rash and body aches the next day, and sought treatment. Health department officials then confirmed her infection.
When her male partner developed symptoms seven days later and sought treatment from the same caregiver, he, too, was diagnosed with Zika.
Health officials from New York reported the case in the CDC's weekly report on death and disease. They said the timing and sequence of events support female-to-male Zika virus transmission through unprotected vaginal sex.
The CDC said it recommends that all pregnant women who have a sex partner who has traveled to or resides in an area with Zika use barrier methods such as a condom every time they have sex, or they should not have sex during the pregnancy. Although no cases of woman-to-woman Zika transmission have been reported, these recommendations now also apply to female sex partners of pregnant women.
CDC said it is updating recommendations for sexually active people in which the couple is not pregnant or concerned about pregnancy and for people who want to reduce personal risk of Zika infection through sex.
U.S. health officials have concluded that Zika infections in pregnant women can cause microcephaly, a birth defect marked by small head size that can lead to severe developmental problems in babies.
The World Health Organization has said there is strong scientific consensus that Zika can also cause Guillain-Barre, a rare neurological syndrome that causes temporary paralysis in adults.
The connection between Zika and microcephaly first came to light last fall in Brazil, which has now confirmed more than 1,600 cases of microcephaly that it considers to be related to Zika infections in the mothers.