Life Science

Is it really true that a dog’s mouth is cleaner than a human mouth?

- Asks Debbie from Medford, NJ

April 21, 2008
What kinds of germs are harbored in there? [Credit: Kayleigh Jane]
What kinds of germs are harbored in there? [Credit: Kayleigh Jane]

Here’s the myth that makes dogs sound like a dental miracle: Despite all the leftover macaroni, rubber bands and dead squirrels they chew, our canine friends still maintain better oral hygiene than human beings do, no matter how studiously we floss and how often we visit our dentists.

Could this really be true?

Well, sadly, no. In short, a dog’s mouth is besieged by its own legions of germs, roughly as huge in population as those living in the human mouth and causing a similar array of dental illnesses.

“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.

One example is the Porphyromonas, a family of rod-shaped bacteria known for causing periodontal disease, a serious gum infection that leads to the loosening and, eventually, detachment of teeth in both humans and animals. Scientists have spotted two distinct species within the family: P. gingivalis was found in the human dental plaque, while its sibling, P. gulae, was found in dogs. Both bacteria thrive on periodontal tissues, eating up the gums and reducing well-rooted teeth to shaky cavities.

Although there are no theories so far to correlate breed and a dog’s proneness to periodontal disease, small and old dogs generally have higher risks of developing a serious form of the disease.

Another common dental disease in humans, however, has largely spared dogs. Dental caries (tooth decay), which according to a 2003 World Health Organization report may affect 90 percent of schoolchildren around the world, hits only about 5 percent of dogs. As complicated as the reason may be, most scientists, including Harvey, point to the scarcity of a bacterium in dogs’ mouths as the major explanation.

The culprit bacterium, S. mutans, eats a big sugar molecule by chopping the sugar into two slightly smaller molecules. This process produces acid as a byproduct. Therefore, the bacterium has evolved to require a slightly acidic habitat, and if lucky, it ends up in the more acidic human mouth rather than the more alkaline dog’s mouth.

One of the rumors related to the cleanliness of a dog’s mouth is the idea that human bites are more infectious than dog bites. However, this too doesn’t hold up to scrutiny. According to Jeein Chung, a veterinarian at Hoboken Animal Hospital in New Jersey, the danger of both human and dog bites depend on the kinds of bacteria in the mouth and the depth of the wound. The bottom line: Cleanse as thoroughly as possible after getting bit, and go to an emergency room if you feel the wound go anywhere beyond the muscles.

As for dogs’ favorite archrivals — I mean, besides squirrels — cats are found to be largely in an identical condition to that of dogs. “We haven’t done as much research on cats,” says Harvey. “But to the extent of what’s been studied, they are [almost] the same.”

About the Author

Jessie Jiang

Jessie has a B.S. degree in chemistry from Peking University in China, where she initially did research on molecular magnetic materials but gradually switched her interest to science writing. Before joining SHERP, she held internships as a reporter at the English-language Shanghai Daily, and at Ogilvy Communications’ Beijing office. She loves writing, and hopes to become a science editor someday as her father has been for decades.



Jeein Chung says:

This is Dr. Chung from Hoboken Animal Hospital, and I am appalled that you did not obtain my legal consent to publish this information because I explicitly asked you if this would be published or not. This is totally unprofessional journalism. Additionally, you did not do any research on the validity of your claim, for there is a wealth of literature in scientific articles that compares and contrasts this claim about the specific organisms that reside in a a dog’s mouth. Looking at the relevant veterinary journals, you would have found a more complete discussion which you totally undermine in your posting especially via my statement. I am also involved in public health and find it important to give accurate advice to the public, and I find it offensive that NYU would endorse such superficial journalism.

greg says:

Can a human transmit hepatitis c to a dog??

thank you!!

David says:

Why don’t myths just die? There are just so many of them and just as many studies proving them wrong. Like the ‘we use only 10% of our brain’ and ‘gum stays in your stomach for 7 years’. Good gracious, how can people still think them as being valid?

Oivia says:

Do you know where I can find a book on that???
Thanks for all the other info but i need it in a book please….

I too think it inappropriate to publish material without the express permission of the person being quoted.. Very unprofessional and it questions the validity of the whole article.

Brea says:

I dont like having my dogs around my new born baby because they have fleas and all they do is chew on themselfs and scratch and lick themselfs. And my husband lets them right by the baby and let them lick him after i asked him not to let them do that because their mouths are gross and that our

Brea says:

Female dog is in heat and the male has been licking her personal area and he let him lick our new born baby. Is that safe i think it is the most nasty thing ever. I dont even let the dogs lick me because i think its gross. And they have nasty mouths but he thinks they have the most clean

Brea says:

Mouth out of all the animals and thinks its ok to let them in the house ans that close to him to be able to do that. Its flipping nasty!!!!!!!!!!

Jdude says:


Kay Latona says:

Brea, if you took the appropriate steps, your dogs would not have fleas, then you wouldn’t need to be concerned about your baby getting fleas. There are 4 monthly prescription medications that kill fleas at all stages. You can get the Rx from any vet, and give your dog (NOT your cat!) a dose once a month and your flea problem will be gone. The names of these Rx medications are Advantix II, K9 Advantax II, Frontline Plus and Revolution Plus. Read the instructions carefully and follow them exactly. Even though my neighbors on both sides have dogs with fleas, my dogs continue to remain flea-free, as they have for years,

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