Dog vs. Human Mouths: A Scientific Comparison

Dog vs. Human Mouths: Not Cleaner

The widespread belief that a dog's mouth is inherently cleaner than a human's is a common cultural myth. Scientific research consistently demonstrates that this old wives' tale is incorrect. Both canine and human mouths are teeming with microorganisms, forming complex ecosystems rather than sterile environments.

Key Takeaways

• Both dog and human mouths contain hundreds of different types of bacteria.
• The bacterial species found in dogs are largely different from those in humans, not fewer in number.
• Comparing the cleanliness of a dog's mouth to a human's is "like comparing apples and oranges," according to Colin Harvey, a professor at the University of Pennsylvania’s School of Veterinary Medicine (Source 3).
• Neither mouth is sterile; both are complex microbial environments.

Bacterial Differences & Similarities

While both species host a vast array of microbes, the specific composition of these bacterial populations differs significantly.

Dog Mouth Bacteria

Dogs have approximately 600 different species of bacteria in their mouths (Source 3, 5). These include organisms like Pasteurella canis and various species of Porphyromonas, which are considered part of their normal, healthy flora (Source 5).

Human Mouth Bacteria

Human mouths contain a similar number, with approximately 615 different species of bacteria identified by Harvard researchers (Source 3, 5).

Species Specificity

Most bacteria are host-specific, meaning they thrive in either a dog or a human, but not typically both. For example, the bacterial family Porphyromonas causes periodontal disease in both species. However, dogs primarily host Porphyromonas gulae, while humans typically have Porphyromonas gingivalis (Source 3, 4). This highlights that while the type of issue might be similar, the specific bacteria causing it are often distinct. One study suggested that only 16.4% of identified microbes are common to both species (Source 5).

Shared Issues

Despite the differences in specific bacterial species, both dogs and humans are equally susceptible to dental diseases. These conditions, such as gingivitis and periodontal disease, are primarily caused by bacterial plaque buildup (Source 3, 4). Regular oral hygiene practices are essential for both species to manage these harmful bacteria (Source 3).

Why the Myth Persists

The persistent belief that a dog's mouth is cleaner than a human's stems from several common observations and folk beliefs that lack scientific backing.

Disease Transmission

One major reason for the myth is the observation that humans typically do not catch common human illnesses, such as the flu, from dog saliva (Source 3). This lack of shared disease transmission can lead people to falsely conclude that a dog's mouth is inherently "cleaner." However, this simply means that most dog-specific pathogens are not equipped to infect humans, and vice-versa.

Wound Licking

Another contributing factor is the folk belief that dog saliva possesses healing properties for wounds (Source 3). While a dog's rough tongue might help clean debris from a wound, and their saliva does contain some antibacterial compounds, it also introduces a multitude of bacteria that can lead to infection. Relying on a dog's lick for wound healing is not recommended.

Risks of Dog Licks

While many dog-specific bacteria are harmless to humans, there are definite risks associated with dog licks, especially in certain situations.

Zoonotic Bacteria

Although most bacteria in a dog's mouth are host-specific and not harmful to humans, some zoonotic bacteria can be transmitted. These are bacteria that can pass from animals to humans. Individuals with compromised immune systems, the elderly, or very young children are particularly vulnerable to potential infections from these bacteria (Source 4).

Skin Irritations

Dog saliva can sometimes cause skin rashes or irritations in sensitive individuals. This is often due to allergens present in the saliva or the physical action of the licking itself.

Open Wounds

Allowing a dog to lick open cuts, scrapes, or wounds on a human can introduce harmful bacteria directly into the bloodstream or underlying tissues. This significantly increases the risk of infection, which can range from mild to severe.

Maintaining Oral Hygiene for Dogs

Just like humans, dogs require consistent oral hygiene to maintain a healthy mouth and prevent dental disease. Good dental care can significantly reduce the bacterial load and prevent serious health issues.

Daily Brushing

Daily brushing is the most effective way to remove plaque and food debris from your dog's teeth. It is crucial to use dog-specific toothpaste, as human toothpaste contains ingredients like fluoride and xylitol that are toxic to dogs. A toothbrush designed for dogs, or even a finger brush, can be used.

Dental Chews/Treats

Various dental chews and treats are designed to help reduce plaque and tartar buildup. These products often have abrasive textures or contain enzymes that help clean teeth as the dog chews. However, they should be used as a supplement to brushing, not a replacement.

Professional Dental Cleanings

Regular veterinary check-ups should include an assessment of your dog's oral health. Professional dental cleanings, performed under anesthesia by a veterinarian, are crucial for removing hardened tartar above and below the gum line. The AKC emphasizes that both dogs and humans benefit from good oral hygiene practices to keep their mouths clean and healthy (Source 3).

Diet

Certain diets can support dental health. Some kibbles are specifically formulated with a larger, more abrasive texture to help scrape plaque off teeth as the dog eats. Discussing dental-specific diets with your veterinarian can be beneficial.

When to See a Vet

Recognizing the signs of dental problems in your dog is essential for early intervention and preventing more severe conditions. If you notice any of the following symptoms, a veterinary visit is warranted:

• Bad breath (halitosis): While some dog breath is normal, unusually foul or strong odors can indicate bacterial overgrowth, dental disease, or other health issues.
• Red, swollen, or bleeding gums: These are classic signs of gingivitis, the early stage of periodontal disease.
• Excessive drooling: Increased drooling, especially if it's bloody or discolored, can signal pain or infection in the mouth.
• Difficulty eating or chewing: Reluctance to eat, dropping food, or chewing on one side of the mouth can indicate tooth pain.
• Loose or missing teeth: These are advanced signs of dental disease and require immediate veterinary attention.
• Pawing at the mouth or face: This behavior can indicate discomfort or pain in the oral cavity.