Dog looks into the lens with tongue out on the background.

Is a dog’s mouth really cleaner than a human’s mouth?

A host of pathogens lurks in each slobbery or scratchy lick. They’re no big deal for Fido—but they could turn deadly in humans.

Cats and dogs—like this handsome Australian shepherd—love to lick their humans. But scientists say you might want to think twice about letting them slobber all over you. Our pets' mouths are filled with hundreds of species of bacteria, including some that are harmful to humans.
Photograph By ROBERT CLARK, Nat Geo Image Collection

When Julie McKenna arrived at the hospital in Mildura, Australia, in 2007, she could barely speak. Her arms and legs were cold and mottled, and her face was turning purple.

Doctors quickly determined that Julie was in septic shock—bacteria in her bloodstream were attacking her from within. Even after starting on antibiotics, the purple kept spreading, and her organs began to fail. Eventually, parts of her arms and legs began turning black.

She had been hospitalized for more than two weeks before doctors were able to identify the bacteria in her blood. It was Capnocytophaga canimorsusa bug commonly found in the saliva of healthy dogs and cats.

Only then did Julie remember that she had scalded the top of her left foot in hot water a few weeks before she got sick. It wasn’t a bad burn, and she hadn’t thought much of it when her fox terrier puppy licked the wound.

Like Julie, most of us don’t really know what’s swimming around in our pets’ saliva, or how dangerous it might be. Our skin and immune systems normally stand between us and our pets’ germs—but those systems can be breached.

About 10 to 15 percent of dog bites become infected, as do up to half of cat bites. Sometimes the consequences are deadly: In one study, 26 percent of people with confirmed C. canimorsus infections died

(In other important questions: Would your dog eat you if you died?)

Now, scientists have begun to describe all the bacterial species living in dogs’ and cats’ mouths and compare them to our own, and their work is revealing a host of potential pathogens lurking in each slobbery kiss or scratchy lick.

In a puppy’s mouth, C. canimorsus is no big deal. At least a quarter of all dogs and many cats carry them. Humans normally don’t, and once the bacteria got in Julie’s bloodstream, her body struggled to fight off infection.

Antibiotics eventually turned the tide, but doctors had to amputate her left leg below the knee, part of her right foot, and every one of her fingers and toes. “It's changed my life in every aspect,” she later told ABC News in Australia.

Here are a few of the myths about our pets’ mouths and the reality of what coats them—and us—with each pass of the tongue.

Takes a licking

If anyone knows what’s in a dog or cat’s mouth, it’s Floyd Dewhirst, a bacterial geneticist at the Forsyth Institute and professor of oral medicine at Harvard. Dewhirst pioneered the study of the oral microbiome—all the bacteria that live in the mouth—in humans, dogs, and cats.

About 400 to 500 bacterial species are common and abundant in the human mouth, he says. So far, Dewhirst and his colleagues have identified around 400 kinds of oral bacteria in dogs and almost 200 in cats, and Dewhirst expects more will be found with further study.

One of the main reasons we can get infections from our pets, he says, is that our bacterial ecosystems are so different from each other.

“If you look at humans and dogs, we only saw about 15 percent that are the same species,” he says. So, many of a dog’s mouth bacteria are less likely to be kept in check by our immune systems and native bacteria. The oral microbiomes of cats and dogs, on the other hand, overlap by about 50 percent. 

(Ancient DNA reveals how cats domesticated themselves.)

“Part of that may be what the bacteria evolved to eat,” Dewhirst says. Human mouths are dominated by streptococcal bacteria, which are good at eating sugars. “Since cats and dogs normally don’t eat too many doughnuts, there’s almost no strep,” he says.

A single lick can deposit untold millions of these unfamiliar bacteria, Dewhirst says, and they’ve been detected on human skin hours later. When studying the skin microbiome of humans, scientists were surprised to find several people with patches of skin covered in dog bacteria.

“So, if you’re licked by a dog, and someone were to take a Q-tip five hours later and rub that spot, they could recover over 50 different species of dog-mouth bacteria,” he says.

Healing paws?

Weirdly, history is packed with lore suggesting that canine saliva can heal rather than harm. Dogs supposedly licked wounds at the ancient Greek temple to Asclepius, the god of healing. And there’s a long-running but unconfirmed tale that Caesar’s army employed wound-licking dogs.

Even if it happened, that doesn’t mean it was a good idea.

There are several antibacterial compounds in dog and cat mouths—including small molecules called peptides—and in humans’ mouths, too. But your pet’s tongue is not a magic source of germ-killers.

You wouldn’t want to rely on these compounds to sterilize an area, Dewhirst says. Plenty of bacteria thrive in the mouth despite their presence.

What’s more, studies of the antibacterial properties of saliva have been taken out of context, says veterinarian Kathryn Primm, who writes frequently about cats and dogs and hosts the radio show Nine Lives with Dr. Kat.

One 1990 study found that dogs’ saliva had slight antibacterial effects when a mother licks herself and her young, Primm points out. And a 1997 study in The Lancet showed that on the skin, nitrite in saliva is converted to nitric oxide, which is antimicrobial.

But both studies involved licking within the same species, not one species licking another with a mouth full of foreign bacteria. By contrast, when a soldier let a dog lick his wound in 2016, he spent six weeks in a coma while bacteria ate his flesh.

(What are flesh-eating bacteria—and how do you fight them?)

Groomed for success

So, if your pet’s mouth is full of bacteria, how clean does it get by licking itself?

The reason housecats spend so much time grooming is that they retain the instinct as predators. Wild cats use their barbed tongues to “clean blood and such from their fur,” Primm says. “They don’t want to be tracked by the smell of their prey.”

Dogs, on the other hand, aren’t so picky. “If you didn’t clean a dog, it would just be dirty,” Primm says. “They aren’t stealthy hunter ninjas like a cat, so it doesn’t matter as much from a survival standpoint.”

As they groom, cats are also coating themselves with bacteria, but they have evolved alongside these microbes and their immune systems are accustomed to them, so it’s not a problem for the cat.

The good news for pet owners is that most of a cat’s oral bacteria aren’t going to survive indefinitely when deposited on cat hair. The bad news is that they don’t die right away: One study found nearly a million living bacteria on each gram of cat hair.

The team also tested how many bacteria were transferred from a cat to a human’s pre-sterilized hands by petting the cat for two minutes. The answer should come as relief to cat owners: just 150 or so bacteria made the trip per petting session.

That doesn’t represent a problem as long as we keep ourselves clean, too: “I always advise people to use standard hygiene,” Primm says. “After an animal licks your hands, it’s a good idea to wash them.”

The main thing pet owners need to watch out for is any chance for those bacteria to get through the skin, Primm says. Once they get inside us, bacteria find a moist, happy environment in which to grow, potentially leading to infection.

As for dogs and their full-face slobber-fests, they’re usually not harmful — as long as your immune system is strong and you don’t have any wounds on your face or mouth that would let bacteria into your bloodstream. “Two different dogs licked me in my mouth this week,” Primm says.

But keep in mind that babies and the elderly can have weaker immune systems than healthy adults. In one case, parents brought their seven-week-old infant to the hospital with a fever and the soft spot on his skull bulging.

It turned out he had meningitis caused by Pasteurella multocida, another pathogen common in cat and dog mouths. His two-year-old brother often let the family’s dogs lick his hands, and he also had a habit of letting his baby brother suck on his little finger.

So, wash those hands, pet lovers, and maybe think twice before letting a dog or cat plant a big wet one on your face.

Editor's note: This story is an excerpt from the book Gory Details: Adventures From the Dark Side of Science by Erika Engelhaupt.

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