Are dogs mouths cleaner than humans?

Fighting Dirty: Are Dogs’ Mouths Cleaner Than Humans’?

We’ve all heard the saying that a dog’s mouth is cleaner than a human’s mouth. It’s a quip that’s often met with skepticism — and we don’t blame the nonbelievers.

After all, your dog spends a lot of time licking his own butt and has a penchant for eating things like garbage, roadkill, and cat poop. Then there’s the matter of oral care: unlike us, he doesn’t brush his teeth twice a day.

So how clean can his mouth really be? And how sanitary is it compared to yours?

Let’s find out once and for all whether dogs’ mouths are cleaner than humans’. Get your mouthwash ready — what you’re about to learn may make you need it!

Bacterial Breeding Grounds

Dog is opening his mouths

When we talk about oral cleanliness, what we’re really talking about is the amount and type of bacteria living in the mouth.

Bacteria aren’t always bad: certain types help with food digestion, and others can protect our gums and teeth. But some species are responsible for many of the nasty things we associate with poor oral hygiene: bad breath, gum disease, and tooth decay.

Both humans and dogs have both beneficial and harmful oral bacteria. Around 700 different types of bacteria have been identified in human mouths; for dogs, that number hovers around 600.

In that regard, our mouths are pretty similar in terms of bacteria quantities.

But our two species share just 15% of bacteria species. In other words, 85% of the bacteria found in your dog’s mouth is completely foreign to your body.

This is both a blessing and a curse. Some of the bacteria found in your dog’s mouth are species-specific and don’t affect humans at all — but some can, and because our immune systems aren’t used to them, the effects can be disastrous.

The Kiss of Danger

We don’t usually consider helpful bacteria to be “dirty,” but harmful bacteria most definitely are. And many of the bacteria in your dog’s mouth are harmful to humans.

All the gross things your dog’s mouth comes in contact with really add up. When your dog licks you, he’s leaving behind traces of that turd he tasted on his walk, his latest butt-cleaning session, and the ground residue he licked from his paws.

The kinds of bacteria found in those things are the last thing your immune system needs to deal with. But if your dog licks an open wound or a mucous membrane like your mouth or your nose, that’s exactly what happens.

For example, you’ll often find Pasteurella multocida in a dog’s mouth, but if it makes its way to your bloodstream, it causes infection — or worse. In one case, an infant contracted a serious case of meningitis as a result of a dog lick and P. multocida.

Capnocytophaga canimorsus is another species of bacteria that’s common and harmless in dog saliva, but for humans, it’s a nightmare.

In 2007, a woman became infected with C. canimorsus after being licked by a dog. The infection did not respond to antibiotics and caused her to go into septic shock; eventually, her left leg, right foot and all fingers and toes had to be amputated.

Aren’t Dog Tongues Antibacterial?

teeth cleaning dog with toothbrush

If dogs’ mouths contain so many hazards to human health, how do you explain the commonly-held belief that dog saliva has healing powers? And why does your dog instinctually lick both his and your wounds?

The truth is that dog saliva does have certain healing properties. It’s been found to have minor antibacterial qualities and can promote quicker wound healing.

But those benefits occur mainly when a dog licks himself or another dog. And they’re pretty minimal benefits, too: the constant presence of bacteria in your dog’s mouth indicates that the antibacterial effect of dog saliva can’t be too powerful!

For humans, on the other hand, a dog lick on an open wound introduces hundreds of foreign bacteria species into your bloodstream. Whatever benefits the saliva itself offers are negated by the influx of new bacteria that your immune system must now destroy.

The Final Verdict: Not Cleaner!

Cleanliness is tough to quantify, but it’s safe to say that your dog’s mouth is nowhere near as clean as the old sayings would have you believe. And the fact that we humans practice good oral hygiene gives us an edge over our furry friends.

We may not be too different from dogs in terms of oral bacteria quantity. But the deadly nature of many species of canine oral bacteria means that, for our safety, we need to think of dogs’ mouths as dirty and avoid letting them lick our mouths and wounds.

"If you think dogs can’t count, try putting three dog biscuits in your pocket and then give him only two of them."
--Phil Pastoret

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