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Ask A Vet: Is A Dog’s Mouth Really Cleaner Than A Human’s?

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In light of a recent news story about a woman nearly dying from an infection sourced to her dog’s saliva, “dog licking” is in the news. Some people say that a dog’s mouth is cleaner than a human’s mouth is, but the mouths of both species certainly have what is called “natural flora”. These are bacteria that live inside the mouth and seldom cause any type of issue for the host. The presence of different species of natural flora keeps things in balance and healthy, as they compete with each other for resources and none can overgrow in a normal circumstance.

So if no mouth is truly sterile or free from bacteria, where did the idea that a dog’s mouth is cleaner than a human’s come from? Of course, we should bear in mind that it is a little like saying my trash can is cleaner than your toilet. I do not plan to scoop material from either one and smear it onto an open wound, but if you feel that you must compare dog and human mouths, I might know where the idea came from.

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Consider this: in 1990, there was a study that suggested that there may be some bacterial growth inhibition in dog saliva. It turned out that the inhibition was only specific to certain strains of bacteria and it was only slight, but it was interesting nonetheless. Because the inhibition seemed to affect E. Coli bacteria and not Staph species, these particular researchers felt their findings may explain the greater prevalence of Staph infections in wounds (46%) versus E. Coli  (9-17%). 1

A more recent study warns that because the bacteria in a dog’s mouth is different from that naturally occurring on human skin, it is not a good idea to allow dogs to lick open human wounds because an infection is likely to occur.2

The answer is no, a dog’s mouth is not cleaner than yours. It has its own bacterial flora. The important part is that your dog’s flora are likely to be different than yours, inoculating an open wound with his germs, just adds to the trouble that is already there. This is even more important if you are very young, very old, or immune compromised in any way, like chemotherapy or disease. If your dog licks your face and you have no wounds on it and you are healthy overall, don’t worry too much, but be aware that dog saliva (like your own) is teeming with bacteria… but the ones in his mouth have already been accounted for by his body and not yours!

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  1. Physiol Behav. 1990 Sep;48(3):383-6.Antibacterial properties of saliva: role in maternal periparturient grooming and in licking wounds. Hart BL, Powell KL.
  2. Tijdschr Diergeneeskd. 2012 Sep;137(9):594-6.[Is being licked by dogs not dirty?][Article in Dutch] Overgaauw P, van Knapen F.

 

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Written by Dr. Kathryn Primm
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