“A dog's mouth is cleaner than a human's.”

The idea that a dog's mouth is somehow cleaner than a human's has been a persistent misconception, likely born from observing dogs licking their wounds (Review) and seemingly healing without issue. This led to the belief that canine saliva possessed unique antibacterial or even "magical" healing properties. Another factor contributing to this widespread myth is the common experience that we don't often catch human illnesses from our dogs, leading many to assume a fundamental difference in oral hygiene.

However, scientific evidence reveals a more complex reality. Both human and dog mouths are teeming with hundreds of different types of bacteria. While some studies show similar numbers of bacterial species in both, the crucial distinction lies in the specific kinds of bacteria present. The oral ecosystems of dogs and humans are largely distinct, with only a small percentage of bacterial species being shared between the two. Many bacteria found in a dog's mouth are unique to canines and, while generally harmless to the dog, can be quite dangerous to humans. For instance, bacteria like Capnocytophaga canimorsus and Pasteurella canis, commonly found in dog saliva, can cause severe infections in humans, particularly if transmitted through bites or if saliva enters an open wound.

People continue to believe this myth partly because the types of bacteria in a dog's mouth are so different from those in a human's that common illnesses typically don't jump between species. This means a dog isn't likely to catch your cold, and you won't get the flu from a dog's kiss. However, this difference does not equate to "cleaner." Dogs also explore their world with their mouths, chewing on various objects, licking other animals, and consuming things that would be considered unsanitary by human standards, all of which introduce a diverse array of microorganisms. Therefore, while affectionate licks might seem harmless, it is important to remember the potential for bacterial transmission, especially for individuals with compromised immune systems or when saliva comes into contact with broken skin.