You’ve probably seen those people who give their dog wet, slobbering kisses right on the mouth – or maybe you are one of those people! Many people defend this behaviour with the explanation that a dog’s mouth is cleaner than a human’s.
Although few actually pause to try to back up this belief, some claim that a dog’s mouth contains special substances that ward off infection. The truth of the matter is that all mammals have these substances. Pretty much all saliva contains enzymes that help to protect the body from infection. Dogs don’t have any special advantage over people in this regard. And dogs don’t have cleaner mouths.
This myth probably originated from the medical literature. Early studies showed that wounds caused by human bites were more likely to become infected than dog bites. This led many people to assume that the mouths of dogs had fewer bacteria than the mouths of people. This, however, was a leap of faith, not of science. You see, later research showed that this belief was mistaken for two reasons. First, human mouths contain ‘human’ bacteria for the most part, and dog mouths contain ‘dog’ bacteria. Humans are much more likely to become infected by human bacteria, and vice versa. If you count the actual number of bacteria, a human mouth does not contain more bacteria than a dog mouth. They just have different bacteria.
The second problem with those early studies is the manner in which people are bitten by others. It turns out that most bites sustained by people from other people heal without incident. Most human bites don’t cause any more problems than dog bites. One sort of bite, however, has a much higher risk of infection, and this skews the numbers. Bites on the hand, sustained through a ‘clenched fist’ interaction, are significantly worse, and account for many of the human-bite infections. In other words, if you get a bite on your hand because you punched someone in the mouth, this is a bad bite likely to get infected. This is probably due to the mechanism and depth of wound sustained by this interaction, not because of the dirtiness of a human’s mouth. Human-bite wounds anywhere else on the body are no more likely to become infected than dog bites. If you punched a dog in the mouth, you might have problems too, but those have not been studied.
It comes down to how you define clean. If you believe ‘clean’ refers to the types of bacteria and their ability to cause infection in deep wounds sustained in bar fights, then dogs might win. If, however, like most of us, you define ‘clean’ to mean having fewer bacteria in your mouth and not having recently used your tongue as toilet paper, human mouths win out in the cleanliness department every time.