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

The idea that a dog's mouth possesses a superior level of cleanliness compared to a human's is a widely held belief, often perpetuated by anecdotal observations. For centuries, people have witnessed dogs licking their own wounds (Review), or even those of humans, seemingly aiding in the healing process. This perceived ability to cleanse and heal has likely contributed to the romanticized notion that canine mouths are somehow sterile or antiseptic.

However, scientific understanding reveals a more complex reality. Both human and canine mouths are teeming with a diverse array of bacteria, forming intricate microbiomes. While it is true that dogs and humans host different species of bacteria, neither mouth is inherently 'cleaner' than the other. In fact, a dog's mouth can harbor bacteria such as Pasteurella, Capnocytophaga, and even Salmonella, some of which are zoonotic and can cause significant infections in humans if transmitted through bites, scratches, or even close contact. The notion of canine saliva possessing unique antiseptic properties is largely unfounded.

Despite the scientific evidence, this misconception persists for several reasons. Our deep affection for our pets often leads us to overlook potential risks, and the immediate absence of illness after a casual lick can reinforce the false impression of canine oral purity. Furthermore, the belief might stem from a simpler time when less was known about microbiology, and observable outcomes, like a wound healing after being licked, were attributed to a dog's mouth being clean rather than the body's natural immune response or other factors.