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, when we think about it at all, but have you ever wondered if it is 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, which is a similar number to the 615 and counting types of bacteria Harvard researchers have found in human mouths. These bacteria can also be joined by other bacteria that we (humans and dogs) pick up from our environments, adding to the mix.
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 are 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 are not zoonotic, which means you probably won’t get a disease from a big old doggy kiss. There are exceptions to this. Dogs that are fed a raw diet are at an increased risk of contracting salmonella, which can be spread to humans, and you really don’t want to 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 does not mean that your dog’s mouth is necessarily cleaner than a human’s — he just has 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, and 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.
But what about the saliva itself?
As it turns out, there are certain proteins in saliva called histatins that can ward off infection, and further research has 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 is even more evidence that suggests licked wounds heal twice as fast as unlicked wounds.
Dog saliva is not alone in these properties. Human and other mammal saliva show similar wound-healing activity, which 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 your 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 can lead to serious infections if introduced into an open wound, resulting in sickness, amputation, and even death. And there are plenty of other germs we can pick up from our environments in our mouths that we do not want a wound exposed to. Also, excessive licking of a wound can lead to infection and self-mutilation.
In short, while there is some truth to this folk remedy, you are probably better off treating your wounds and your dog’s wounds with more conventional care to avoid any unnecessary risks. If you have more questions about whether or not you should let your dog lick your wounds, contact your doctor or your veterinarian for professional medical advice.
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 his is a puppy. This will make it easier down the road when your dog is older and full of firm ideas about what he does and doesn’t like. Training your dog to enjoy tooth brushing is just as important as getting him used to the process. Talk to your veterinarian about ways to make tooth brushing enjoyable, and be sure to use toothpaste designed for dogs and never 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.