Earlier this week, the first case of Zika was reported (pdf) in the US in Dallas County, Texas. But unlike other cases of the virus, the county’s health department claims that transmission occurred between two people who had sex.
The pandemic is concentrated in Latin America occurs from a mosquito bite. Pregnant mothers can pass it on to their babies, which has been connected to brain shrinkage or microencephaly. But it’s exceedingly difficult to prove that Zika virus, or any pathogen, is transmitted sexually.
P. Frederick Sparling, a former infectious disease specialist at the University of North Carolina, told Quartz that even if an individual who contracted an illness had sexual contact with someone carrying it, sex may not be the mode of transmission. “Intercourse by itself doesn’t absolutely prove that the virus was sexually transmitted,” he told Quartz. “Who’s to say that he didn’t kiss her and transmit it like mono through kissing?”
“The gold standard for sexual transmission is finding infectious virus in the donor, and finding very similar viral genomes in the recipient,” Vincent Racaniello, a virologist at Columbia University, told Quartz. He explained that you’d have to find an entire virus—and not just partial genetic material—in both patients, be able to confirm that there were absolutely no other vectors (like mosquitoes) present, and confirm that the individuals had sexual contact.
Even if all these criteria are met, however, it doesn’t mean sexual contact is the dominant mode of transmission. “We should distinguish between sexually transmitted viruses [and] sexually transmissible—viruses that now and then might be sexually transmitted under certain circumstances, but which normally are transmitted by another route,” he says.
Though most viruses have a preferred vector, sometimes they can be spread through other means. Ebola is primarily transmitted through human to human contact, but fragments of the virus have been found in the semen of men who have contracted the virus, and there is a strong possibility it could be transmitted sexually.
“There are a lot of infections that can occasionally be transmitted by sex,” Sparling says. Though it’s highly unusual, yeast infections can be transmitted through sexual contact.
This isn’t the first time that Zika virus has been thought to have been transmitted sexually. In 2008, Brian Foy, an immunologist at Colorado State University, reported strong circumstantial evidence that he gave his wife the disease after conducting research in Senegal. In 2013, doctors detected (pdf) Zika in the semen of a man in Tahiti who had contracted the virus, but it’s unclear he passed the virus on to anyone else.
According to a spokesperson from Dallas County Health and Human Services, one patient had received treatment for Zika after traveling to Venezuela, and the other patient had no history of travel to a Zika-affected country or mosquito bites.
Tom Frieden, the director of the CDC, confirmed today (Feb. 5) that they are working with DCHHS to conduct an “ongoing investigation.”
The CDC has issued recommendations suggesting that men who have traveled to countries where Zika is present use condoms or abstain from sex if their partners are pregnant.
Even if Zika can be transmitted sexually, it’s not the dominant way the virus spreads: “Zika virus is transmitted among humans by mosquitoes. If sexual transmission occurs, it is very rare, given the number of Zika virus infections that have been documented,” says Racaniello.