Have you ever heard this before: “A dog’s mouth is cleaner than a human’s mouth.” I’ve heard it many time before, almost always from people I would call “animal lovers.” I heard it again just this past week.
I have always liked dogs. I grew up living with them for most of my childhood and well into my teenage years. I am currently not a dog owner, but I appreciate all the benefits that a dog lends to a household. But even now, when I recall having heard this notion for the first time when I was about eight years old, I knew that it had to be a load of crap. How could a dog’s mouth be cleaner when it uses its tongue to clean itself? I had seen dogs eat their own feces, so how could that constitute cleanliness? Hell, even the bible states that a dog is a filthy animal because “a dog returns to what he has vomited.” As I got older, I became more cautious of letting dogs lick my face, especially around my lips and mouth. I had no science or data to help me decide that a dog’s mouth should not be near my own. I just used common sense.
While common sense is a fair starting point to take a stance on any given item, it is hardly scientific, and by no means a sure indicator of accuracy. So I have decided to set aside my common sense and look for data, for or against, the notion that a dog’s mouth is cleaner than a human’s mouth. To determine what constitutes “clean”, the website A Moment of Science has this to offer:
When we talk about the cleanliness of a mouth, we’re really talking about the amount and type of bacteria it contains. The kind of bacteria found in a human mouth and a dog mouth depend on what’s been there recently. Unlike dogs, humans typically do not eat raw meat, garbage, and small animals in various states of decay. Given a dog that just locked its jaws around a decomposing squirrel, we might say that the dog’s mouth is, for the moment, less clean than a human mouth that has not recently housed a dead squirrel. And since the squirrel-eating dog may pick up disease-causing bacteria from the dead animal, a bite from that dog may pass on dangerous germs.
But insofar as a clean mouth means one that is less likely to cause disease, consider that germs tend to be species specific. Harmful dog germs are usually harmless for humans, unless we’re talking about rabies, a disease that affects dogs and people in equally damaging ways. But generally, humans are immune to most dog germs. Likewise, human germs probably won’t harm a dog.
From our friends at Livescience.com, they have this tidbit to add:
“It’s like comparing apples and oranges,” says Colin Harvey, a professor of surgery and dentistry at the University of Pennsylvania’s School of Veterinary Medicine. He is also the executive secretary at the American Veterinary Dental College. Although there’s a vast overlap of bacteria in the mouths of both species, Harvey considers the question of which one is cleaner to be irrelevant because a) both are teeming with microbes, and b) in many cases, a dog’s dental bacteria differ from their human counterparts.
Ok, so humans have their native oral germs, and dogs have their own. By this measurement, they each could be considered to be as clean (or dirty) as the other. However, by the measurement of cleanliness of what has recently gone into their respective mouths, human must be considered to have cleaner mouths. James McNamara, VMD, of the Bethel Veterinary Hospital here in Connecticut, concurs:
“Dogs mouths are very dirty places. There are literally dozens of awful species of bacteria that live in a dogs mouth. Pasteurella, strep, E coli, fecal contaminant; they all live in spades inside a dog’s mouth.”
And for an extremely graphic example, the experts at Veterinarypeople.com offer this:
Toxocara canis (Roundworm) is the predominant cause of a serious condition called ‘visceral larva migrans’ in humans. Most victims are children. They are infected by inadvertently consuming worm eggs in soil (typically by getting dirty fingers in their mouths). The worm is not present in its correct host but tries to complete its life cycle anyway. The worm gets lost in the human body (classically in the eye), dies, and generates an extreme inflammatory reaction. If the worm dies within the human eye, blindness usually results.
It seems the popular concept of a dog’s cleaner-than-human mouth does not hold up to the magnifying glasses of science. So how did this myth penetrate and stick with our popular culture? According to the folks at About.com, under their ‘Urban Myths’ category, they state the following:
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. Statistics to that effect were published in journals and repeated by medical professionals, and folk wisdom took off from there. 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%.
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.
Despite all of this, people should not panic if a dog licks their lips or mouth. It is rare that people will pick up any infections from the lick of a dog’s tongue, assuming that the person does not have any conditions to exacerbate the exchange of germs, such as open sores on their mouth or people who have weakened immunity systems. The human immune system will fight off those germs very effectively. The risk of infection through a bite is a much more potent means of catching the dog’s germs. People have been bitten in the process of, say, a rambunctious dog eagerly kissing their master on the mouth when a small “nip” occurs on the master’s lip. Then it’s time to consult a physician.
So why do some dog owners let their dogs lick them on the lips and mouth? There are no statistics to answer this question, but according to Dr. McNamara, it would appear to be anthropomorphic:
“People like to believe that dogs are equal to children, and they treat them as children. Because a child’s mouth is cleaner than an animal’s mouth, they don’t think about the consequences. When I was going to veterinary school, I witnessed a first year vet student share her ice cream with her dog; one spoonful for her, followed by one spoonful for the dog, all the while using the same spoon. When I asked her about it, she told me it was “ok, because her dog’s teeth and gums were very clean”, this from a person studying to be a vet!”
The power of anthropomorphism, assigning human qualities to non-human things, could very well be considered the root cause of this popular myth. A person’s passion and love for a dog can cause them to forget or block out the fact that they are still dealing with a canine instead of another human being. Anthropomorphism can be a very strong emotion which, if not carefully considered, can overshadow science, reason and, in this case, common sense.
I sincerely hope this post has not left any animal lovers with a bad taste in their mouth.