On Thursday, federal health officials reported the first case of Zika transmitted through anal sex: a Dallas man who infected his male partner after a trip to Venezuela.



“Both vaginal and anal sex is an emerging mode of Zika virus infection that might contribute to more illness than was anticipated,” said U.S. Centers for Disease Control and Prevention researchers in their report.

The CDC has suggested that both straight and LGBT couples use condoms after traveling to regions afflicted by Zika, which now includes 33 nations in the Americas. In the current outbreak of the virus, marked by an alarming increase in severe brain birth defects in Brazil, there have been at least six confirmed sexually transmitted cases, including the Dallas one.

In January, one of the men had returned from a trip to Venezuela, where an outbreak is now ongoing. He had anal sex with his long-time partner without a condom one day before and one day after developing symptoms of Zika — a purple rash and fever. His partner developed Zika symptoms seven days later, confirmed by lab tests.

“I was, in fact, expecting the first report to come out about this scenario soon,” infectious disease expert Brian Foy of Colorado State University told BuzzFeed News by email. “This is not surprising to me at all.”

In 2011, Foy reported that he had infected his own wife with Zika after returning from a trip to Africa infected with the virus, in the first scientific report of its sexual transmission. “If virus is in the semen, anyway it gets into the body of the sexual partner — vaginal intercourse, anal intercourse… perhaps even via oral sex — it has the chance, or is likely, to foster an infection of that sexual partner.”

Only the semen of the man who had been to Venezuela showed some signs of containing Zika virus within three weeks of his return. It did not appear in either man’s saliva or urine. The men had been monogamous for 10 years and had none of the Aedes aegypti mosquitoes in their home that more typically carry the virus.

Although Zika can be spread sexually, mosquitoes still represent the biggest risk, Peter Hotez of the Baylor College of Medicine told BuzzFeed News. “At this point, around the time of acute illness there is a small risk of sexual transmission.”

Notably, all men in the case reports and semen studies experienced symptoms of Zika, which may only appear in 1 in 5 infections. The pressing public health question in sexual transmission of Zika is exactly how long the virus can live in semen, mosquito-borne virus expert Nikolaos Vasilakis of the University of Texas Medical Branch in Galveston told BuzzFeed News. In March, the CDC called for men with pregnant partners who have traveled to Zika-afflicted regions to continue using condoms for the entire duration of a pregnancy, out of an abundance of caution.