Unlike the bulk of Zika cases so far, which originated with mosquito bites, the Dallas patient was infected with the virus after having sexual contact with a person who had just returned from Venezuela, where Zika is circulating. (For privacy reasons, details about the patients have not been released.)
This is not the first case of sexually transmitted Zika in the US
While this is the first case of the virus being locally acquired in the continental US this year, it's not the first case of Zika sexual transmission.
For years, researchers have known about this possibility. There have been two studies in the medical literature that suggest Zika can be sexually transmitted.
In one case, a man who traveled to Senegal in 2008 and contracted Zika passed it to his wife through intercourse after he returned home to Colorado.
In 2013, Zika was isolated from semen — further demonstrating the potential for Zika sexual transmission. There are also cases of the virus being passed on during labor and blood transfusion, which suggest the virus can be transmitted from person to person through bodily fluids and not just by mosquitoes.
The CDC will issue guidance on preventing Zika sexual transmission
Until now, however, the US Centers for Disease Control and Prevention hasn't issued guidance about preventing Zika sexual transmission, citing a lack of evidence. Researchers aren't sure how long Zika can remain in semen, or when people with the virus are most likely to infect others. And it's not clear how common sexual transmission actually is compared with mosquito spread.
But other health authorities have gone ahead anyway. The Dallas case prompted health officials there to suggest people use condoms in order to prevent the spread of the virus.
"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," said Dallas County Health and Human Services Director Zachary Thompson. "Next to abstinence, condoms are the best prevention method against any sexually transmitted infections."
Public Health England, meanwhile, has been advising men to wear condoms for about a month after traveling in countries where Zika has spread, and those who had unexplained fevers or a diagnosis of Zika to wear condoms for six months.
So far, the CDC has issued travel advice, warning pregnant women to avoid countries where the virus is circulating. They've also suggested that women who are thinking of getting pregnant soon consult their doctors before traveling.
The Dallas case is expected to prompt an official warning, according to CNN. For now, the CDC said in a statement, "Sexual partners can protect each other by using condoms to prevent spreading sexually transmitted infections. People who have Zika virus infection can protect others by preventing additional mosquito bites."