The common belief that a dog’s mouth is somehow cleaner than a human’s is a persistent misconception. A dog’s mouth is not cleaner, but rather contains a completely different set of microorganisms. Both canine and human mouths are teeming with bacteria, adapted to their specific host.
The Myth vs. The Microbes
Both species host a massive and complex oral microbiome, making the idea of a “cleaner” mouth misleading. A dog’s mouth contains approximately 600 different species of germs, a number comparable to the variety found in a human mouth. The bacterial populations are simply species-specific, meaning the microbes that thrive in a dog’s environment are generally foreign to a human host.
The most prevalent bacteria in a dog’s mouth include species like Pasteurella and Porphyromonas. Pasteurella canis, a common organism in the canine oral cavity, is frequently isolated from infected dog bite wounds in humans. Conversely, the human oral microbiome is dominated by different genera, such as various Streptococcus species.
While some bacterial families, such as Porphyromonas, cause periodontal disease in both species, the specific strains are different. For example, dogs often carry Porphyromonas gulae, while humans are more likely to host Porphyromonas gingivalis.
Practical Health Risks
The species-specific nature of the bacteria means that a dog’s saliva introduces foreign microbes that a human body may not be prepared to handle. The risk of infection from a dog lick is generally low if the saliva only contacts healthy, intact skin. However, the risk increases significantly if the saliva touches mucous membranes, such as the eyes, nose, or mouth, or if it enters an open wound or scratch.
Dog bites present a much higher risk because the bacteria are forcefully injected deep into the tissue. One of the most concerning zoonotic pathogens transmitted through a bite is Capnocytophaga canimorsus, a bacterium found in the mouths of up to 75% of dogs. While infection is rare, C. canimorsus can cause severe, life-threatening sepsis, particularly in individuals with compromised immune systems, though cases have been reported in healthy people.
Any dog bite that breaks the skin requires immediate attention due to the high potential for infection from organisms like Pasteurella canis and C. canimorsus. The wound should be thoroughly washed with soap and water to flush out as much bacteria as possible. Seeking medical evaluation is necessary, as a healthcare professional can determine the need for antibiotics or a tetanus booster to prevent serious complications.