Are Dogs' Mouths Cleaner Than Humans?

By Dr. Carly I. O'Malley December 02, 2019

We have all heard the saying that a dog’s mouth is cleaner than a human. This may have been heard in response to a dog licking your face or licking a cut you had on your hand. However, dogs are notorious scavengers who will eat garbage, roadkill, and even feces. Watching a dog eat these types of items definitely raises some serious doubts about the cleanliness of their mouths. What is the truth? Do dogs have cleaner mouths than humans? What defines the cleanliness of a mouth? Let’s answer some of these questions.

Are Dogs’ Mouths Cleaner Than Humans?

The answer to the age-old question about whether dogs have cleaner mouths than humans is, probably not. However, determining the cleanliness of a mouth is complicated, therefore the answer is not that simple.

When thinking about mouth cleanliness, we often think about the microbes or bacteria found in our mouths. Dogs have 600 different types of bacteria in their mouths compared to the 615 found in the mouths of humans (Burke, 2017). Determining who has a cleaner mouth between dogs and humans is complicated as we both have evolved for different environments, lifestyles, and diets. Having bacteria in your mouth does not necessarily mean your mouth is dirty, as microbes and bacteria play an important role in breaking down food.

Ultimately, the reason we think dogs’ mouths are cleaner than ours is because we rarely get any diseases from our dogs’ saliva, even though we frequently get sick from sharing saliva with other humans. The reason we do not share colds, the flu, and other diseases with our dog is simply because they have canine-specific bacteria whereas we have human-specific bacteria. Another reason that humans have spread the rumor that dogs’ mouths are cleaner than ours is because dogs, like other animals, lick their wounds in order to clean them. This led us to believe that dog saliva has healing properties in it, and therefore their mouths must be clean. Actually, saliva does have healing powers, but those powers are not exclusive to dogs (Burke, 2017).

What Factors Determine Cleanliness?

One of the main factors determining the cleanliness of one’s mouth, whether human or canine, is your overall oral health. Oral health can be determined by dental hygiene habits and by genetics. Regular dental hygiene helps reduce the number of harmful bacteria in our mouths, regardless of genetics. Irregular or improper oral hygiene can increase the risk of diseases, which can spread to other parts of the body. To reduce the number of harmful bacteria in your dog’s mouth, schedule a regular dental routine that includes annual veterinary check-ups and brushing their teeth. Some dogs may not enjoy getting their teeth brushed but introducing this practice with a little positive reinforcement training is a great way to get them to enjoy it. Dental disease and mouth cleanliness are big problems in the veterinary community. VCA pet hospitals report that 80% of dogs have dental disease by the time they are 3 years old. The most common dental disease seen in dogs is periodontal disease which is caused by plaque and tartar around the tooth. The longer the plaque stays on the tooth, the more likely it is that harmful bacteria will flourish in the mouth (Bellows, 2017).

The other factor that determines mouth cleanliness is the rate of infection that occurs after bites. Besides the pain that comes along with getting bit by an animal, one of the major issues with getting bit is watching for infection in the bite wound. Each animal has different bacteria in their mouths, and that will determine the risk of infection after a bite. Infection from dog bites is actually pretty rare compared to the risk of infection following a bite from a cat or human. Bites to the hand present the highest risk of dangerous infection that will require medical attention (Griego et al., 1995).

What Research Has Been Done To Determine Cleanliness Using Those Factors?

Over the years, a number of researchers have strived to scientifically answer the question about whether a dog or a human’s mouth is cleaner. Other researchers have compared bite infection data or bacteria profile between species to better understand mouth cleanliness.

One study by Griego and colleagues in 1995 compared bite data between dogs, cats, and humans. They found that dog bites account for 80-90% of all bite injuries, but that only 2-20% of dog bites result in infection. Cats are the culprits in 5-15% of bite injuries but are far more responsible for bite-related infections than dogs, with 30-50% of cat bite victims reporting infections. Surprisingly, humans come in third behind dogs and cats for incidence of bite injuries, with 4-23% of bites coming from humans. Risk of infection from human bites is high, with 10-50% of bite victims suffering from subsequent infection.

One high school student conducted a study to find out whether humans or dogs had cleaner mouths. She collected saliva samples from her neighbors and their dogs and then tested the bacteria found in the samples. The student found that the human average for total bacteria was 3 and the dog average was 2.7, both of which are considered moderate levels of bacteria. When she looked at the different types of bacteria present in the saliva, she found that humans and dogs had similarly low levels of gram-negative bacteria but that humans had a higher occurrence of gram-positive bacteria than dogs. The student concluded that it was hard to make meaningful comparisons between dog and human bacteria because the type of bacteria present in each species was very different and each mouth was cleaner in some ways and dirtier in other ways (AMNH, 2011). 

What Are The Types Of Bacteria That Exist In A Dog’s Mouth?

Every animal, regardless of how clean their mouth is, has bacteria in their mouth. Bacteria is not always bad. Different types of bacteria play different roles in your dog’s overall mouth health and cleanliness (NIH, 2019). The kinds of bacteria found in a dog’s mouth is much different than the kinds found in human mouths. While domestic dogs are considered omnivores, they have recently evolved from carnivores. Therefore, their mouth bacteria are designed to break down protein and bone from animal carcasses. Humans are omnivorous, so our mouth bacteria help us break down plant and animal materials. Humans are also much more likely to snack on sweets, which can contribute to more dental issues when compared to dogs.

  • Helpful Bacteria

Helpful bacteria are those that are specially designed for the mouth of the species they live in. These types of bacteria work to help digest food and control populations of harmful bacteria (NIH, 2019). Regularly brushing your dog’s teeth, bringing them for dental cleanings at the veterinarian, and providing a high quality, species-appropriate diet can help promote helpful bacteria. However, disrupting the natural flora of bacteria found in the mouth can cause issues, as this allows harmful bacteria to grow and invade the mouth.

  • Harmful Bacteria

Harmful bacteria are those that are not beneficial to the health and functioning of the mouth and digestive system. Harmful bacteria lives in the tartar and plaque on the teeth. Not addressing these issues, along with feeding a poor diet, can cause the harmful bacteria to flourish. Harmful bacteria can cause other issues like tooth decay, dental disease, and can even spread to cause heart disease.


In conclusion, there are a number of factors that can be used to compare the cleanliness of animal mouths, particularly those of dogs and humans. Oral hygiene can be taken into account when determining cleanliness. If we consider that, humans would probably have cleaner mouths since we brush our teeth twice a day and regularly visit the dentist. The number and type of bacteria found in the mouth could be a measure of cleanliness, but as humans and dogs have evolved to consume different diets, direct comparisons are difficult to make. In terms of bacteria, human and dog mouths can be cleaner in some ways and dirtier in others, so it is a tie. You can also consider the rate of infection after a bite injury. Humans are far more likely to get an infection from the bite of another human or cat than they are from a dog, so perhaps a dog’s mouth is cleaner. Based on this evidence, it is hard to say whether a dog’s mouth is cleaner than a human. The world may never know the true answer to this question.

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Works Cited

AMNH. 2011. Are dogs’ tongues really cleaner than humans? American Museum of Natural History Young Naturalist Awards Curriculum Collection. Retrieved November 28, 2019.

Bellows, J. 2017. Dental disease in dogs. VCA Hospitals. Retrieved November 28, 2019.

Burke, A. 2017. Is a dog’s mouth cleaner than a human’s mouth? AKC. Retrieved November 28, 2019.

Griego, R.D., Rosen, T., Orengo, I.F., and J.E. Wolf. 1995. Dog, cat, and human bites: A review. Journal of the American Academy of Dermatology 33(6):1019-1029.

NIH. 2019. Mouth Microbes: The helpful and the harmful. NIH News in Health. Retrieved November 28, 2019.