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

The notion that a dog's mouth is inherently cleaner than a human's is a widely held belief, often perpetuated by the observation that dogs frequently lick their wounds (Review), which sometimes appear to heal without complication. This has led many to assume that canine saliva possesses superior antiseptic qualities, leading to the old adage that a dog's mouth is somehow purer than our own. The myth also persists because people generally don't contract common human illnesses, like the flu, from dog kisses, further reinforcing the idea of a distinct and perhaps cleaner oral environment.

However, scientific evidence tells a different story. Both human and dog mouths are teeming with hundreds of different species of bacteria, with studies showing remarkably similar total numbers of bacterial types, around 600 to 700 in each. The key difference lies in the specific types of bacteria present. While many dog-specific bacteria are not harmful to humans, others certainly are. For instance, bacteria like Capnocytophaga canimorsus and Pasteurella canis, commonly found in dog saliva, can cause serious and even life-threatening infections in humans, particularly those with compromised immune systems, if transmitted through bites or even licks on open wounds.

The misconception also arises from the fact that most bacteria found in a dog's mouth are not zoonotic, meaning they are not typically transmissible between species. This provides a false sense of security, as it doesn't account for the diverse pathogens dogs encounter and ingest through their natural behaviors, such as exploring their environment with their mouths, sniffing other animals' waste, or eating spoiled food. Therefore, while a dog's mouth may not transmit human-specific illnesses, it harbors its own unique collection of microorganisms, some of which can pose significant health risks to people.