My workgroup gets together every Friday for lunch in our break room. Invariably, we end up discussing some wild topic and today it was the cleanliness of a dog's mouth. Somebody started the whole conversation by talking about a woman friend who would let her dog lick her front teeth. Needless to say, everybody was disgusted, except for a few who insisted that it was healthier than deep kissing another human. We all agreed to e-mail you and drop this in your lap. So, is a dog's mouth cleaner than a person's?
Note to self: Do not open e-mail while eating. This appetite-dampening question unexpectedly makes me speculate on various possibilities. Allowing a dog to lick your teeth, I suppose, does show a certain amount of foresight and ingenuity. Considering the tradition of breeding for specific purposes, one could imagine dog shows filled with Floss Terriers and Old English Plaquedogs. But, since my background is in microbiology, not genetics, I'll leave these details to my South Korean research team.
The old saying, a dog's mouth is cleaner than a human's, likely comes from the observation that a canine will ceaselessly lick his wounds until they healor until a veterinarian fits him with a plastic lampshade. Reflecting on the adage, I instantly remember a relevant fact: a dog's mouth is filthy. While alternating between licking the kitchen floor and his private parts, a dog's idea of a winning lotto ticket is a partially decayed squirrel. If the prize can't be released from the pavement for a victory lap, an old-fashioned wallow and roll will do nicely. Later in the afterglow, his thorough squirrel-juice-removing tongue bath is rewarded by a couple of cat box tootsie rolls. After a day like this, it seems doubtful a dog's mouth could be any cleaner than a human's (with the obvious exception of Colin Farrell).
This bit of folklore is so universally repeated it must have had some help along the way. I found that assistance came from well-intentioned doctors who published that infections developed from human bites were usually more severe than those from domestic animals. This conclusion was based primarily on examination of hand wounds following fights in which a blow was struck to another's mouth, which resulted in a cut from the probable loser's teeth. The authors generally failed to consider that those types of infections are usually quite far along before the injured party seeks help and, typically, the wounds are poorly cared for. The opposite is true of most animal bites. In recent years, doctors have recognized that infection rates are similar for animal and human bites in all places except the hand.
To truly answer your question, one must decide whether clean means sanitary (lack of bacteria) or non-infectious (not causing sickness). The primary investigations into the sanitary issue have not been prestigious university studies, but grade school science projects and fluff local news reports. Generally, the experiments consist of swabbing the cheeks of human volunteers and canine conscripts, then smearing the goop onto agar plates. Bacteria amounts can be fairly accurately estimated from the number of colonies that grow after incubation. Alas, lunchroom science fairs and morning happy news segments are not subject to the peer-review process, but most concludeby actual bacterial numberspeople and dogs rate similarly foul.
The other half of this equation is the infectious nature of those microorganisms. There is good evidence that most beasties present in the dog's mouth are species-specific, meaning they are relatively incapable of causing human illness. We are not exactly impervious to canine microbes, but short of a massive exposure, people with normal immune systems are reasonably resistant. A notable exception is the canine roundworm, Toxocara. Present in the majority of puppies, the eggs of this intestinal parasite are jettisoned into the pup's feces and can quickly end up on the more attractive end of the animal. If ingested by a human, the eggs hatch into larvae that wander aimlessly through your internal organs, never becoming adult wormsa condition called visceral larval migrans. Unless a huge number are swallowed, however, you are unlikely to have symptoms or even know it happened. Still, you'd be wise to use the same caution with a puppy's lips as you might with Mr. Farrell's.
The safety of whom you choose to kiss depends on your concerninfectiousness or mere pollution. The human probably wins Most Likely to Disease, but the canine scores Overall Disgusting. But, if I had to choose, I'd rather get kissed by Eddie than by Frasier, though it would take a huge reality show paycheck before I'd let either one lick my teeth.