The Zika virus can be transmitted by anal sex as well as vaginal sex, according to a report issued on Thursday by the Centers for Disease Control and Prevention.

The agency described a case of man-to-man sexual transmission in January.

The case, which was previously disclosed by health officials in Texas without identifying the genders of the partners, was the first known case of sexual transmission of Zika within the United States in the current epidemic.

It involved a Dallas resident who became infected with Zika through a mosquito bite while visiting Venezuela and then infected his male partner through unprotected sex upon his return. Both had relatively mild symptoms, and blood was not detected in either man’s semen.

Previous cases of suspected sexual transmission involved men with blood in their semen, leading doctors to speculate that sexual transmission took place only when the testes or prostate was so heavily infected that bleeding occurred.