Scientists have suggested that oral sex may be yet another possible way Zika can be transmitted from person to person, raising concerns that more cases of the virus may be transmitted through semen.
In a letter to the New England Journal of Medicine, a group of French researchers detailed a case that involved a 24-year-old Parisian woman and a 46-year-old man who had traveled to Brazil.
The woman turned up at a Paris hospital in February with Zika symptoms — a fever, rash, and muscle pain. She had not been anywhere where the virus was transmitting. But she did report having "seven episodes of both vaginal sexual intercourse, without ejaculation and without the use of a condom, and oral sex with ejaculation" with the man, who had recently traveled to Brazil and contracted what sounded like another case of Zika.
The researchers concluded that the case seemed like sexual Zika transmission from the man to the woman, but they weren't sure of the route: "We cannot rule out the possibility that transmission occurred not through semen but through other biologic fluids, such as pre-ejaculate secretions or saliva exchanged through deep kissing," they added. So Zika may have been passed on through pre-ejaculation in vaginal sex or oral sex — or even through saliva during kissing.
There have been more than 20 cases of sexually transmitted Zika reported during this outbreak
Though this is the first description of potential oral transmission of Zika, doctors have noted several cases that demonstrate Zika can be spread through sex.
In the first case of sexual transmission of Zika, a man who traveled to Senegal in 2008 contracted the virus and 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.
This year, with the world's largest Zika epidemic, researchers have been finding even more evidence that sex spreads the virus.
The first case of sexual transmission in the United States during this current outbreak was documented in Dallas in February. A traveler returned from Venezuela, where the virus was circulating, and infected his or her partner.
Since then, there have been at least 23 cases of probable sexual transmission in 10 countries, according to the European Center for Disease Prevention and Control.
Researchers also reported the first male-to-male sexual transmission of Zika through anal sex, which occurred in Texas in January.
All reported cases have involved symptomatic men spreading the virus to their partners
The best evidence so far suggests infectious Zika virus can last in semen for at least 24 days after infection. (In the UK, researchers found non-infectious virus in a man's semen 62 days after he got sick.)
So far, all reported cases of sexually transmitted Zika originated in a male partner who showed symptoms of the virus. (In most cases of Zika infection, people don't experience any symptoms.)
It's possible that men without symptoms can pass on the virus, or that women can give the virus to their sex partners — there are just no documented cases of this.
So there's still a lot of uncertainty here about how Zika spreads through sex. Researchers aren't sure of exactly how long infectious Zika can remain in semen, when people are most at risk of passing on the virus, what types of sex acts are more likely to spread the virus, or whether symptom-free people who are infected with the virus can spread it.
And when people have sex, they don't typically just engage in one act, which also makes it difficult to suss out how the virus was spread in any case.
Sexually transmitted Zika is thought to be rare compared with mosquito-borne Zika — but there's a lot we don't know
Researchers still consider mosquitoes the primary transmitters of Zika virus, but there's a lot they don't know about how common sexual transmission is.
Nikolaos Vasilakis, a professor and researcher at the University of Texas Medical Branch, pointed out that in places where the virus is transmitting through mosquitoes, the role of sex is "masked by the mosquito-to-human transmission."
That said, the current outbreak is the largest the world has known, and there's a lot more to learn about the virus. In countries where Zika is being spread through mosquitoes, it can be extremely difficult to determine whether a person contracted the virus through a mosquito or sexual intercourse and how big the risk of the latter is.
For now, the Centers for Disease Control and Prevention recommends that everyone, particularly the partners of pregnant women who may have been exposed to the virus, take precautions. They suggest using condoms or abstaining from sex for between six weeks and eight months (depending on the likelihood of infection).