A Dallas man who contracted Zika in Venezuela transferred it to a male sexual partner after returning home in January, the Centers for Disease Control and Prevention confirmed Thursday.
The case was identified by a local health care provider earlier this year and investigated by Dallas County Health and Human Services before being referred to the CDC.
"At this time, there had been one prior case report of sexual transmission of Zika virus. The present case report indicates Zika virus can be transmitted through anal sex, as well as vaginal sex," the CDC said in a statement Thursday.
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Two days after the man returned to Dallas from a one-week trip abroad, he began to show symptoms consistent with Zika infection — subjective fever, pruritic rash on his upper body and face, and conjunctivitis lasting three days, the CDC said.
During the investigation into the infection, officials learned the man had unprotected anal sex one day before and one day after the onset of symptoms. Seven days after first showing symptoms, the man's partner began to show symptoms of Zika infection as well.
"On Day 7, patient B developed a subjective fever, myalgia, headache, lethargy, and malaise; a few days later, he developed a slightly pruritic rash on his torso and arms, small joint arthritis of his hands and feet, and conjunctivitis," the CDC said.
After a week, all of the symptoms had subsided.
The man who traveled to Venezuela said multiple people living in the area he visited were experiencing symptoms of Zika infections. The man's monogamous partner in Dallas had never traveled to Venezuela and has not traveled to any area with known cases of Zika.
Dallas County health officials sent specimen samples to the CDC for analysis, and it later confirmed the man who traveled to Venezuela had contracted both Zika and dengue, while his partner had only contracted Zika.
The department's director called the developments a game changer.
"Surveillance is going to be on two fronts - one in terms of individuals who travel...and get a mosquito bite and those who travel and engage in sexual activity," said Dallas County health director Zach Thompson.
Thompson said confirmation that Zika can be sexually transmitted should put pressure on federal lawmakers to approve emergency funding for accine research.
"We need a vaccine," Thompson said. "The funding that's being held up in congress is going to hold up whether or not you can do the research. Right now, unless congress kind of moves on giving President Obama what he asks for, there may be some delay in seeing Zika funding."
Further information about the patients is not yet known and is not expected to be released, citing privacy concerns.
Zika virus infection has been linked to increased risk for Guillain-Barré syndrome and adverse fetal outcomes, including congenital microcephaly.