"We know that Zika can live in saliva about as long as it persists in blood, which is an average of seven days," said Dr. John Brooks, senior medical adviser at the Office of Infectious Diseases at the Centers for Disease Control and Prevention. "That might lead someone to worry about whether they can get Zika from kissing."
"But we're talking about deep kissing, or French kissing, where a lot of saliva is exchanged and the tongue goes into the partner's mouth," explained Dr. William Schaffner, medical director at the National Foundation for Infectious Disease. "It would have to be what I call passionate kissing."
Sounds scary. So let's ask the next important question: How likely are you to get Zika from kissing -- even if it's the deep, passionate variety?
"Saying it can be transmitted via saliva is going way past probability," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "More than likely, it's a very rare event if it occurs at all. A mosquito bite is overwhelmingly the main way this virus is transmitted, with sexual acts involving semen a distant second."
"It certainly wouldn't stop me from giving my wife a kiss," Brooks agreed. "While we can never say never when it comes to this virus, if deep kissing alone was a significant means of transmission, we would know by now."
A 'French kiss' in France
The idea that French kissing might play a role in the spread of the Zika virus was brought up in a recent case study published in the New England Journal of Medicine
A 24-year-old woman living in Paris, with no prior exposure to the virus, tested positive for Zika after having sex with her boyfriend, who had just returned from Rio de Janeiro. While in Brazil, the 46-year-old man had Zika-like symptoms including a rash, fever and body aches. Upon his return, the couple had sex seven times, with "ejaculation only during oral sex and not during vaginal sex, using withdrawal of penis prior to ejaculation to avoid pregnancy."
In day three of her illness, the woman's urine and saliva tested positive for Zika, but a vaginal swab was negative.
"In our case, the most likely transmission route was semen via oral sex," lead study author Dr. Eric D'Ortenzio said. But the researchers also couldn't rule out the possibility that Zika was transmitted by other body fluids, such as "pre-ejaculate secretions or saliva exchanged through deep kissing."
Therefore, concludes the study, "Transmission of Zika through saliva is an important question that should be investigated."
Zika in saliva: What we know
We've known for a while that Zika is found in saliva
. During the first major outbreak of Zika in French Polynesia, saliva was used to test for the virus
when drawing blood was difficult, such as with infants. But while that sort of test can detect the presence of Zika, it can't tell whether the virus is alive or dead.
"So it may well be that what you have principally in saliva is dead virus or dead virus remnants," Schaffner said. "It's not been documented yet that viable living virus can be recovered in a sustained fashion from saliva."
Then there are the anatomy issues. Not all parts of the body are equally vulnerable to the virus.
"For example, vaginal or anal tissue is very vulnerable. So is blood-to-blood," Brooks explained. "But if you were to get Zika-infected blood or semen on your skin, that's not a realistic route of transmission, as the skin is not as susceptible."
The mouth and throat contain the same type of mucous membranes that line the vagina and anus, which is why semen deposited via oral sex might be a rare but realistic route of transmission. But when it comes to saliva, there are additional factors at play.
"Saliva, unlike vaginal fluid, has a whole bunch of enzymes in it to initiate the digestive process," Schaffner said. "Plus, it's being constantly swallowed and renewed. And once it gets down through the esophagus and into the stomach, it's hitting a lot of powerful acids that would destroy the virus."
And because we put all sorts of things in our mouths, says Schaffner, cells along the lips, tongue and interior of the cheeks have a protective coating called keratin, designed to prevent injury to those mucous membranes.
"That makes it much more difficult, if not impossible, for a virus to attach to those portions of the cells," Schaffner added. "That does not exist in the vagina or anus."
But despite all of these caveats, experts cannot rule out the possibility that someone, somewhere, might get Zika by kissing. But they come really close.
"I spent most of my career working on HIV/AIDS, and the same fears came up with that virus," Fauci said.
"Is HIV, like Zika, in saliva? Absolutely, it's in saliva. Can it be transmitted by saliva? Almost certainly not. Can you absolutely prove without a shadow of a doubt it can never be transmitted by saliva? No, you can never do that. But the one thing you can say is that if it occurs, it's extraordinarily rare."