The long-standing belief that a dog’s mouth is inherently cleaner than a human’s is a common cultural myth not supported by biological science. Both canine and human mouths host complex microbial ecosystems, but comparing them directly is like comparing two distinct environments. The simple answer is no: a dog’s mouth is not cleaner than a human’s; it is simply populated by different types of microorganisms suited to a different host. This difference in microbial inhabitants is key to understanding the potential health implications of close contact with our canine companions.
Defining “Clean”: The Microbial Difference
The concept of oral “cleanliness” depends entirely on the host species, as both humans and dogs possess mouths teeming with bacteria. A dog’s mouth contains approximately 600 different species of germs, while the human mouth harbors around 615 varieties. The sheer number of microorganisms is comparable, but the composition of the flora is what distinguishes the two.
Canine mouths host bacteria such as Pasteurella canis and various species of Porphyromonas that are part of their normal, healthy flora. These organisms are typically harmless to the dog but are foreign to the human body. Conversely, a large percentage of microbes in a human mouth are not shared with dogs; one study suggested only 16.4% of identified microbes are common to both species.
The presence of bacteria like Porphyromonas causes periodontal disease in both species, demonstrating that neither mouth is immune to microbial issues. Therefore, a dog’s mouth is “clean” only in the sense that its specific bacterial population is naturally balanced for the dog itself.
Functions of Canine Saliva
The myth of a dog’s superior mouth cleanliness likely stems from observations of their self-grooming and wound-licking behaviors. Canine saliva is a complex fluid that plays several biological roles beyond digestion and lubrication.
It contains components that offer mild antimicrobial properties, including the enzyme lysozyme, which breaks down bacterial cell walls. Defensive proteins, such as lactoferrin and immunoglobulin A (IgA), also inhibit bacterial growth and provide a first line of defense against pathogens. This composition helps the dog maintain oral health and aids in the initial cleaning of minor wounds.
Dog saliva tends to have a more alkaline pH (7.5 to 8) compared to human saliva (6.5 to 7). This higher alkalinity is one reason dogs experience fewer dental cavities, as it helps buffer the acids produced by cavity-causing bacteria. However, these properties do not sterilize the mouth or eliminate the hundreds of bacterial species present.
Zoonotic Potential: Bacteria Transferred from Dogs to Humans
The primary concern with the dog’s oral flora is its zoonotic potential: the possibility of transmitting bacteria from animals to humans. Transmission occurs primarily through bites, but it can also happen if dog saliva contacts broken skin, mucous membranes, or an open wound.
One widely discussed bacteria is Capnocytophaga canimorsus, a commensal organism found in the mouths of up to 74% of dogs that rarely causes illness in the animal. In humans, particularly those with weakened immune systems, a C. canimorsus infection can lead to severe complications like sepsis, gangrene, and organ failure. While severe infection is rare, healthy individuals can still become gravely ill, with some cases progressing rapidly to septic shock.
Another common pathogen is Pasteurella multocida or Pasteurella canis, which is frequently isolated from dog bite wounds in humans. These bacteria cause localized wound infections and are a significant reason for medical consultation following a bite. Even simple exposure, such as a dog lick contacting an open sore, can transmit these foreign organisms.