Dear Urban Legends:
I have a problem with people who say "a dog's mouth is cleaner than a human's." I have asked for facts from these people to back up their statements but no one has been able to provide them to me.
I believe that dogs and humans have similar enzymes for breaking down food, and I know that my dog licks herself in places I don't, and she always has "doggie breath."
I don't think it's possible for a dog's mouth to be cleaner, unless dogs have some super-secret enzyme that we don't.
What I remember hearing when I was a kid was that a dog's mouth is "sterile." I can even recall someone in my neighborhood attempting to prove this to anybody who'd sit still for it by letting his dog lick the inside of his mouth — not particularly conclusive, but a memorable demonstration just the same.
I believed what I was told at the time in spite of contradictory evidence, namely that people often become quite ill and even die after being bitten by a dog. If a dog's mouth is sterile, how could it transmit rabies, tetanus, pasteurella or any of the other types of infection associated with dog bites?
But I digress. The precise question was: Is a dog's mouth cleaner than a human's? The answer to that is no, too, and basically for the reasons you've already cited. As we all know, dogs aren't particularly fussy about where they put their tongues or what goes into their mouths.
"A dog's mouth contains a lot of bacteria," confirms Dr. Gary Clemons, a veterinarian in Milford, Ohio. "Remember, a dog's tongue is not only his wash cloth but also his toilet paper."
Well, it's true!
So, where did the notion that a dog's mouth is cleaner than a human's come from? Doctors, evidently. It has long been noted in the medical literature that human bites are more likely to become infected than those of other mammals, including dogs. Once statistics to that effect were published in journals and began to be repeated by medical professionals, folk wisdom took off from there.
Bite wounds vs. closed-fist injuries
Lately, however, the accuracy of those statistics has come under attack, with critics objecting that some of the human "bites" compared to animal bites in earlier studies weren't really bites at all. A 1988 review published in the Annals of Emergency Medicine found the following:
Recent study of human bites has shown that the early literature depicting all human bites as having an extraordinarily high infection and complication rate was biased by its emphasis on human bites of the hand that presented late with infection already present. These bites, the so-called closed-fist injuries (CFI), do indeed have a poor prognosis, but it may be as much due to their location and initial neglect as to the source of the injury. Human bites elsewhere do not seem to have any higher risk than animal bites, which have an infection rate of about 10%. (Source)And a 1995 review in the Journal of the American Academy of Dermatology concurred:
Human bite wounds have long had a bad reputation for severe infection and frequent complication. However, recent data demonstrate that human bites occurring anywhere other than the hand present no more of a risk for infection than any other type of mammalian bite. (Source)
My impression based on a quick layman's survey of the literature, therefore, is that although the issue remains scientifically controversial, the revisionists have a very good point. Until recently, the statistics on human bite wounds didn't differentiate between what we would ordinarily consider a bite and so-called closed-fist injuries — the type of hand-wound suffered by a human being who slugs another human being in the mouth.
By their very nature such wounds are deeper and more serious than bites passively sustained, and thus more likely to result in complications. Their inclusion in general bite-wound statistics, some researchers now argue, skewed past pathological comparisons of human bites with animal bites.
If you don't mind my falling back on a gut feeling, I'll conclude by reiterating that I have personally witnessed a human being allowing a dog to lick the inside of his mouth, and it was... unpleasant to behold. Say what you will about the relative cleanliness of the canine mouth vs. the human, I'd rather be French-kissed by a human being than a dog any day. I'm just fussy that way.