Pop quiz: Which bite will most likely cause serious infection if it breaks the skin?

A. A bite from your little sister

B. A bite from a healthy (non-rabies infected) dog

One of the most common old wives’ tales is “A dog’s mouth is cleaner than a human’s mouth.” This is said despite the fact that dogs spend most of their time licking and eating suspicious objects. How can this be? Do dogs have fewer naturally-occurring microbes in their saliva than humans? Or, does one of those microbes have an antiseptic property that works to kill any microbes the dog ingests? Or, indeed, is this just an old wives’ tale that does not hold a grain of truth?

Perhaps the old wives observed that a dog’s wound tends to heal faster when the dog licks it. However, the expression would then be an incorrect interpretation – the dog’s lick most likely heals because it removes dead tissue, not because it sterilizes the wound.

By “cleaner,” does the expression  mean “contains less bacteria?” The human mouth contains more than 100 bacterial cells adhering to each cell of the tongue, 10-20 bacterial cells adhering to each cheek epithelial cell, and 100 million in every milliliter of saliva. As of August 2002, scientists at the Forsyth Institute have found more than 615 different species of bacteria living in the human mouth.

The mouth is an ideal place for bacteria to grow and thrive: it is a constant 35ºC, an excellent climate for mesophiles (bacteria that grow best in warm temperatures), has a large surface area to cling to, and contains a steady consumption of carbohydrates and sugars to feed the bacteria. Bacteria enjoy similar conditions in a dog’s mouth where the temperature is 38ºC, so dogs’ mouths are hardly cleaner.

Or perhaps by “cleaner” the expression means “contains less harmful bacteria?” This suggestion does not really work either, as most bacteria are species-specific and bacteria that are harmful to dogs are not harmful to humans.

However, the fact that bacteria are mostly species-specific has implications on the correct answer to our quiz. While a dog bite by a rabid dog may transmit rabies, a dog that is rabies-free will deliver a less harmful bite than a human bite. The bacteria in the human mouth are adapted to living inside their human hosts and thus have a greater potential to cause infection in a human than dog bacteria, which may not be able to survive in the human host.

After dogs and cats, human bites are believed to be the third most common bite wounds and approximately 10-15% of human bites become infected. Furthermore, human saliva can transmit such diseases as hepatitis B, hepatitis C, herpes, and syphilis, just to name a few. Bites to the hand (such as those that occur when someone punches another person in the mouth) are the most serious as the bacteria can cause the most damage via the tendons in the hand. Both the National Institute of Health and the American Academy of Orthopedic Surgeons warn that a bite from another human should never be ignored, especially if the bite broke the skin.

It is important to remember that while a human bite may pose a larger threat than a dog bite, a dog bite should never be ignored either and one should seek medical attention if bitten, particularly because dogs lick everything. A dog uses its mouth similarly to how humans use hands. Take a moment to list all of the things your hands do from the moment you wake up. Now imagine that you never washed them. This is similar to the conditions in a dog’s mouth.

So, is there any truth to the old wives’ tale “a dog’s mouth is cleaner than a human’s mouth”? Not really. I advise that our love for our furry friends should stop just short of sharing a plate with them.

Bibliography

Cromie. William J. “Discovering Who Lives in Your mouth: Bacteria Give Clues to Cancer and Gum Disease.” Harvard University Gazette. August 2002. Accessed: 4/29/11 <http://news.harvard.edu/gazette/>

Glass. Don. “Dog Mouth vs. Human Mouth.” September 2003. Indiana Public Media: A Moment of Science. Accessed 4/29/11 <http://indianapublicmedia.org/amomentofscience/>

Science Daily. “New Bacterial Species Found in Human Mouth.” August 2008. Accessed 4/29/11. <http://www.sciencedaily.com/>

Tamari, Nastassia. “Is Your Dog’s Mouth Really Cleaner Than yours?” ConnectAmmarillo.com. February 2011. Accessed 4/29/11. http://www.connectamarillo.com/

Van Houte, Johannes. “Bacterial Adherence in the Mouth.” Reviews of Infectious Diseases, Vol. 5, No. 4 (1983) S659-S669