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Border Collie with a stick in its mouth outdoors.
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You’ve probably heard the expression “a dog’s mouth is cleaner than a human’s mouth” at least once in your life. Most of us have just accepted this as fact, but have you ever wondered if it’s actually true?

Here’s a hint: the answer is no.

Apples and Oranges

Comparing a dog’s mouth to a human’s mouth is “like comparing apples and oranges,” according to Colin Harvey, a professor at the University of Pennsylvania’s School of Veterinary Medicine and the executive secretary at the American Veterinary Dental College.

This is because both dog and human mouths are full of microbes. While there is some overlap in the types of bacteria between species, there are also a host of different dental bacteria in your dog’s mouth that you won’t find in yours.

Take the bacterial family known for causing periodontal disease in humans and dogs, Porphyromonas. Researchers discovered that dogs have a type of Porphyromonas called P. gulae, whereas human mouths contain its relative, P. gingivalis. Both bacteria are what most of us would consider “dirty,” and can cause problems for dog and human teeth.

In fact, dogs have more than 600 different types of bacteria in their mouths, a similar number to the 615 and counting types of bacteria Harvard researchers have found in human mouths. Other bacteria that we (humans and dogs) pick up from our environments can also get added to the mix.

Cavalier King Charles Spaniel getting its teeth brushed by a woman at home.
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Can Humans Get Dog Germs?

Perhaps part of the reason the idea that “a dog’s mouth is cleaner than a human’s mouth” came to be so widely believed is that we don’t typically swap diseases with our dogs when we swap saliva. You’re not going to get the flu from a dog kiss, but you might get it from kissing a human loved one.

Most of the bacteria in your dog’s mouth aren’t zoonotic, which means you probably won’t get a disease from a big old doggy kiss. There are exceptions to this. Dogs that eat a raw diet are at an increased risk of contracting salmonella, which can be spread to humans. You also probably shouldn’t share kisses with a dog that regularly raids the litter box.

In other words, kissing your dog is less risky than kissing another human, but that doesn’t mean that your dog’s mouth is necessarily cleaner than a human’s — they just have a mostly incompatible set of germs.

Can Dog Saliva Heal Wounds?

While we’re on the subject of dog mouths, there is another folk belief that you’ve probably heard before about dog mouths: dog saliva helps heal wounds.

This gets a little more complicated. Most mammals, humans included, lick their wounds. Historically, ancient cultures even believed that dog saliva had curative powers. The Greeks and Egyptians both used dog saliva in healing practices and featured dogs in their religious healing rites.

They may have been on to something. The act of licking, alone, offers some benefits to wound healing. The tongue removes dirt and debris from the wound site, which lowers the risk of contamination and infection. Of course, too much licking can lead to self-trauma, as in the case of hot spots, and can actually make things much worse.

Border Collie with a stick in its mouth outdoors.
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But what about the saliva itself?

As it turns out, there are certain proteins in saliva called histatins that can ward off infection. Further research revealed that there are other beneficial chemical compounds in saliva that can help protect cuts from bacterial infections. As if that wasn’t enough, there’s even evidence that suggests licked wounds heal twice as fast as unlicked wounds.

Dog saliva isn’t alone in these properties. Human and other mammal saliva show similar wound-healing activity. This might help explain why we instinctively hold a cut to our mouths and kiss “boo-boos.”

Does this mean that you should have your dog lick their wounds, or that you should lick your own wounds?

Maybe not. Not all of the research about saliva was good. Curative properties aside, saliva has its risks. Take the bacterium Pastuerella, for example. This bacterium is harmless in the mouth. But it can lead to serious infections if introduced into an open wound, resulting in sickness, amputation, and even death. Plus, there are other germs we can pick up from our environments that we don’t want a wound exposed to. Excessive licking of a wound can also lead to infection or self-mutilation.

In short, while there’s some truth to this folk remedy, you’re probably better off treating your dog’s wounds with more conventional care. If you have more questions about whether you should let your dog lick their wounds, contact your doctor or your veterinarian for professional medical advice.

Mixed breed getting its teeth checked at the vet.
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Oral Hygiene

Comparing the cleanliness of human and dog mouths misses a major point: oral hygiene.

Both dogs and humans are equally susceptible to dental disease and benefit from good oral hygiene practices to keep their mouths clean and healthy. Regular brushing and dental cleanings help humans and dogs keep harmful bacteria, like the kind that cause periodontal disease, in check and are an important part of a daily routine.

You can begin brushing your dog’s teeth when they’re a puppy. This will make it easier when your dog is older and full of firm ideas about what they like and dislike. Training your dog to enjoy tooth brushing is just as important as getting them comfortable with the process. Talk to your veterinarian about ways to make tooth brushing enjoyable. Be sure to use a toothbrush and toothpaste designed for dogs. Never use human toothpastes, which can contain harmful substances, such as xylitol.

Your dog’s mouth might not be cleaner than yours, but keeping your dog’s mouth healthy will make you feel better about those sloppy, wet dog kisses.

This article is intended solely as general guidance, and does not constitute health or other professional advice. Individual situations and applicable laws vary by jurisdiction, and you are encouraged to obtain appropriate advice from qualified professionals in the applicable jurisdictions. We make no representations or warranties concerning any course of action taken by any person following or otherwise using the information offered or provided in this article, including any such information associated with and provided in connection with third-party products, and we will not be liable for any direct, indirect, consequential, special, exemplary or other damages that may result, including but not limited to economic loss, injury, illness or death.

Related article: Periodontal Disease in Dogs: Signs, Causes, Treatment