Although more than 40 cases of Zika virus have been carried to the U.S. by unwitting travelers, today officials in Dallas County, Texas, announced the country’s first case of local transmission in the ongoing outbreak.

The patient acquired the virus through sexual transmission, the Dallas County Health and Human Services department said after receiving confirmation of the infection from the U.S. Centers for Disease Control and Prevention. The newly infected individual had sex with a person who had acquired Zika virus while traveling outside of the U.S., it said.

Prior to this incident there had been one other documented case of apparent transmission of the mosquito-borne Zika virus via sexual contact in 2008 from Colorado State University biologist Brian Foy to his wife. Foy, who specializes in insect-borne diseases, contracted the virus in rural Senegal and a few days after he became ill his wife (who did not go on the trip) also became symptomatic with the disease.

Public health officials are advising individuals infected with Zika virus to use protection such as condoms to block potential disease transmission. The virus is believed to clear from the blood in about a week but there is no available data about how long it would take for it to clear from sexual fluids. “Men who are diagnosed can now be followed over time to answer this question,” says Scott Weaver, an expert on mosquito-borne viral diseases at The University of Texas Medical Branch at Galveston, via e-mail.

Although the mosquitoes that are biologically capable of transmitting Zika virus are present in Texas there are currently no reports of the virus being transmitted by the mosquitoes in Dallas County or anywhere else in the U.S.