Health officials in Dallas on Tuesday reported that a patient there had contracted Zika through sex, the first such case to be reported in the United States since the mosquito-borne virus began spreading through the Americas last spring.
The patient, who was not identified, was infected while having sex with someone who returned to the country after having contracted the virus while traveling abroad, officials said.
Sexual transmission of Zika is thought to be rare, although there was at least one such case in Colorado in 2008.
The Dallas patient who contracted the virus had not recently traveled outside the country and is not pregnant.
“Now that we know Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others,” Zachary Thompson, Dallas County’s health and human services director, said in a statement. “Next to abstinence, condoms are the best prevention method against any sexually transmitted infections.”
Confirmation of both infections was made by the Centers for Disease Control and Prevention. The epidemiological investigation to determine if it was a sexually transmitted case was conducted by Dallas County officials.
The World Health Organization on Monday declared a surge in birth defects linked to Zika to be a public health emergency. On its own, officials said, Zika causes mostly mild flu-like symptoms and would not have been deemed an emergency.
Public health officials are investigating the link between Zika and a severe birth defect called microcephaly, in which newborns have abnormally small heads, and Guillain-Barré syndrome, a neurological condition that triggers temporary paralysis.
The Zika virus is normally transmitted by mosquitoes, specificially Aedes aegypti mosquitoes. They become infected by taking a blood meal from a person who is infected with the virus; once infected, they pass it on.
The risk of sexual transmission is not as well understood. In the 2008 case, Brian Foy, a researcher at Colorado State University, contracted the disease while doing fieldwork in Senegal. He only developed symptoms after his return home; a few days later his wife became ill.
Foy wrote about the case in the journal Emerging Infectious Diseases. Another report in the same journal chronicles the discovery of Zika virus in a semen sample take from a man in Tahiti who had recently recovered from Zika infection in late 2013.
It is not actually known how long the virus remains in the semen of a man who has recovered from Zika. The CDC will be issuing recommendations in the coming days geared toward preventing sexual transmission of the virus, a spokeswoman, Kathy Harben, told STAT in an email. She said in the meantime pregnant women should avoid exposure to the semen of a man who has been in area with a Zika outbreak.
The CDC’s counterpart in England, Public Health England, has already recommended that women whose partners have traveled to a country with an active Zika outbreak should avoid getting pregnant for a period of time after the man’s return.
If a man who has traveled to a country with Zika transmission does not have symptoms, any sexual partners should avoid pregnancy for 28 days after his return, the agency said. If he does have symptoms, pregnancy should be avoided for six months after his recovery.
Public Health England said it was issuing the recommendations as “a precaution,” adding it would revise them when more precise information about sexual transmission is known.
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