This is one of those questions that catches me by surprise every time someone asks it…worse, it completely floors me on the rare occasion where a client in an exam room states it like a known fact. Are dog’s mouths cleaner than human’s?
As I’m checking in a client who allowed their dog to lick themselves into a major injury, it can be hard to keep a straight face when they tell me they let him lick because “dog’s saliva is sterile.” Ummm, no. Not it isn’t. Not even close. So where does this myth come from? And what’s the truth?
Let’s just start with the correct answer: No.
I mean, have you seen what the average dog puts in their mouth? They gobble down rotting garbage. They eat (and roll in) feces and dead animals. They think drinking dirty puddle water is the best! I won’t even go into butt sniffing and such.
How this can possibly translate into a clean mouth I have never understood!
And yet, this myth persists. I have no proof, but I suspect this is one of those Old Wives Tales that has been passed down from the times before we understood germ theory. Let’s look at this this old myth and put it to rest!
Famous Mythology -- Are Dog’s Mouths Cleaner Than Humans?
Ok, this is my theory as to how this whole thing got started. I am not alone in suspecting this base cause, just in case you thought I was the originator of this idea!
Way, way back in antiquity (many times, likely), someone noticed that their dog had an injury. Given that veterinary care wasn’t really known as a specialty until modern times, this human likely treated the dog similar to how they would have treated a human’s injury.
Maybe they washed the wound with herbs. They likely didn’t try to bandage it, since the dog would have chewed it off anyway. Regardless, at some point the human sees the dog licking the wound. The dog eventually heals from the injury, and the human observer concludes that the dog’s saliva healed the wound!
Correlation= Dangerous Assumptions
Now, there is a famous scientific saying I am going to quote here. “Correlation does not equal causation.” This means that just because two things seem to be related does not mean that one of them is causing the other, or even that they are related at all!.
Real world example. If you poll people in the US and Canada who have been convicted of a major crime and asked them what they ate the day they were convicted, you will probably find bread products listed very high on the list. Toast, sandwiches, rolls etc. One could conclude that eating bread was correlated with getting convicted of a crime.
So, using bad science, a researcher could write a viral internet post stating that eating bread causes someone to get convicted of a crime at trial. Which is clearly absurd. That is the same methodology people use when they ask, “Are dog’s mouths are cleaner than human’s?” after a dog appears to heal a wound with saliva.
Dogs Lick Their Wounds- And Often Make Them Worse
Let’s take on the question of dog’s licking their wounds next. Does it help them heal?
No! It often makes things much, much worse, in fact. I can’t tell you the number of dogs who have come in for a minor wound or injury that was turned into a major wound just from them licking it!
Think about hot spots. Many dogs will get them, and some dogs get them frequently. What is a hot spot?
Hot Spots are Created by Licking!
A hot spot is not a wound or an infection, not at first.
A hot spot is primarily created when your dog starts licking a part of their body. The saliva creates an environment where there is constant moisture and warmth, and bacteria growth flourishes in this area. Soon, you have a patch of irritated, red, oozy skin.
Sometimes hot spots start in areas where there is a minor wound or insect bite. But I have seen plenty that start where there was nothing wrong- the dog started licking and created the entire thing, just from the saliva!
In veterinary medicine, we call these “moist derms,” which is short for the technical term moist dermatitis. In common English, this means wet, inflamed skin. Most of the time, the moisture is caused almost entirely by the constant bath of saliva.
A dog with a minor skin infection may have few symptoms other than some redness of the skin, and maybe a bit of itchiness. Sometimes all they need is a little topical antibiotic ointment, and things get better. Add in a saliva bath, and now you are looking at topical and oral antibiotics…and a cone of shame!
Your vet bill just doubled, at least. From licking.
Dogs Lick People- And Make Them Sick!
In addition to causing damage to themselves, dog’s can transmit infections to humans through licking as well. Sometimes these infections can be life threatening if the bacteria in the dog’s saliva invades the human’s body.
This past summer, a man in the US lost several limbs and almost died when he caught a systemic infection from one of his dog’s licking him.
The culprit was a bacteria called capnocytophaga, which is very common in the canine mouth. Most dogs carry this bacteria, even puppies. Usually, it doesn’t cause any problems to humans.
In the case above, the licking dog managed to get a bit of saliva carrying the bacteria into an area on the man’s skin where there was a defect. The bacteria invaded his bloodstream, causing an almost fatal sepsis that resulted in him losing limbs to save his life.
Ok, this is a rare and drastic case! Most of the time, a dog licking you isn’t likely to have this kind of impact.
But you should be careful to wash your hands if your dogs licks them, and avoid letting a dog lick any open wounds. Dog’s mouths are NOT clean, and certainly not sterile by any stretch of the imagination!
Dog Mouth vs Human: Which is Cleaner?
It actually isn’t very useful to compare a dog’s mouth to a human’s.
Each species has a different group of common oral bacteria found in their mouths, so it’s like comparing apples and bananas. They are both fruit. But they are not really similar beyond that.
Even if dogs had fewer kinds of bacteria in their mouths, would this really translate into “cleaner?” Is 200 kinds of bacteria really cleaner than 400? Does the number really matter?
No, it doesn’t really matter. Dogs do not have clean or sterile mouths. Neither do humans. We all have bacteria in our mouths, all the time.
That’s the key point. Dogs have oral bacteria in their mouths, and the proof of this is right in front of your eyes!
Periodontal Disease -- Are Dog’s Mouths Cleaner Than Human’s?
Lift up the average dog lip and take a good look at those teeth. What do you see?
Ok, not all dogs have bad teeth.
But most dogs over the age of 1 have some level of periodontal disease. Especially smaller dogs and dogs with awkward muzzle designs (chihuahuas, shih tzus, pugs, and English bulldogs all pop to mind).
When you lift up a lip and see thick chunks of brown tartar covering the gum line, you are seeing the results of periodontal disease. And what causes this? Oral bacteria.
It isn’t the point of this post to go into detail about periodontal disease, but the fact is, if dogs had clean mouths, they would never have rotten teeth, swollen gums or heavy tartar build up. They wouldn’t need dentals to keep their mouths healthy.
I can tell you, I have done hundreds of doggy dentals, and every one of those dogs had tartar build-up. Even the ones who had great home dental care! The ones who get regular dentals and home care usually have healthier mouths…but the effects of oral bacteria were still obvious.
A Dog’s Mouth is Not Clean
As I said above, it doesn’t really matter which species has MORE types of bacteria in their mouths. Neither species has a clean mouth, that’s the key takeaway. To read more of the science behind the bacteria in dogs vs humans mouths, you can check out this National Geographic article
Don’t let a dog or a human lick your open wounds! Any bite that breaks the skin can cause an infection.
All mouths carry bacteria that could cause a life-threatening infection under the right circumstances. When asking, “Are Dog’s Mouths Cleaner than Human’s?” take a minute and think about it. Think about all of the weird and unsanitary things that go into, and sometimes come out of, that dog’s mouth.
Don’t let your dog lick their wounds, either. Use an e-collar to prevent this.
I have saved myself hundreds of dollars in vet bills by slapping an e-collar on my dog at the first sign of a problem. E-collar, cleaning and topical treatment, and 95% of the time things heal up without using prescription medications. The key is to catch things early and prevent the licking from making it worse.
Don’t buy into the hype. Dog’s mouths are not clean, they are not sterile, and preventing your dog from licking at a wound is much more likely to help the situation than make it worse.
Jen Clifford has a B.A. in Biology from Reed College. She was a field biologist for several years
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