Your dog licks your face, you pet its head—this is the language of love you repeat every day. But in this back-and-forth, the bacteria in your mouths are quietly completing a two-way exchange. And the consequences of this exchange took researchers an entire study to fully understand.

In 2024, a South Korean research team conducted a study combining oral examinations and genetic analysis on 19 dog owners and their dogs. They sought to answer a seemingly simple question that almost no one had seriously considered:What impact does your daily intimate contact with your dog have on its oral health?

The results were more striking than anyone anticipated.

The study recruited 19 dogs and their owners—the first question alone was unsettling.

At the start of the study, veterinarians asked the owners a simple question: “How often do you brush your dog’s teeth?” Not a single owner answered “daily.” Only about 58% said they brushed once a week, while the remaining 42% brushed once a month or not at all. This finding prompted the research team to dig deeper—because the subsequent genetic analysis would reveal a truth most owners had never considered.

This is not a criticism, but a very common reality. Many owners know in their hearts that they “should brush their dog’s teeth,” but they often assume the dog will resist, they lack time, or “it looks fine,” so they keep putting it off. The problem is: dogs cannot tell you they have a toothache; they simply endure it silently.

How veterinarians “score” a dog’s teeth

In this study, veterinarians used two simple scores to assess each dog’s oral condition. You don’t need to understand medicine—just remember one thing:The lower the score, the healthier the teeth.

The study found that dogs brushed once a week or more had an average dental calculus accumulation score of 0.74 points, and a gingival inflammation score of 0.54 points. For dogs that were rarely brushed, the two scores were 0.83 points及 and0.61 points

The Other Side of Love

Every face lick is love—but also a moment of bacterial exchange between you

The study found that 15 out of 19 surveyed dogs (approximately 79%) had a habit of licking their owner’s face or mouth. For dogs, this is a natural way to express affection and seek connection. To them, every lick is pure love. But science tells us that each such intimate contact is also an opportunity for bacteria to flow bidirectionally between you. Using genetic analysis techniques, researchers identified the bacteria on the dogs’ tooth surfaces one by one. In samples from 10 dogs, they found a total of。

73 different bacterial species

The Journey of Bacteria

Human “cavity-causing bacteria” had taken up residence in dogs’ mouths The researchers specifically searched for traces of this “human cavity bacterium” in the dental plaque of all 19 dogs. Ultimately, in2 dogs

In simple terms: When you kiss your dog every day or let it lick your face, it is similar to how family members transmit bacteria to one another through daily shared living. The more bacteria you have in your mouth, the more likely it is that your dog will harbor the same bacteria. This is not to tell you to stop loving your dog—but rather to remind you that the way you love your dog can be more thorough.

What Other Bacteria Are in a Dog’s Mouth? Some May Also Affect Humans

In addition to caries-causing bacteria, the study also found several other types of bacteria in dogs’ dental plaque that may, under certain circumstances, affect humans:

“I Know It’s Important”—But Knowing Has Not Made Us Act

One detail in the study is more thought-provoking than any statistic: Among 19 owners, 12 were able to accurately assess whether their dog’s teeth were healthy. In other words, most owners actually have sufficient awareness.

However, the study also found that even when owners clearly knew their dog had dental problems, this “knowledge” did not lead them to brush their dog’s teeth more frequently. There is a deep gap between knowledge and action.

But the insidious nature of oral problems lies precisely in their “silence.” Tartar accumulates slowly, gums quietly become inflamed, and bacteria multiply between the teeth—until one day the veterinarian tells you, “Your dog needs a dental cleaning under general anesthesia,” and you realize the problem has been there all along.

Bad Teeth Are Not Just a Dental Problem

Many people assume that if a dog’s teeth are bad, the worst outcomes are bad breath or difficulty eating. In reality, chronic gum inflammation can have profound effects on a dog’s overall health.

When gum tissue remains inflamed, the immune system stays in a constant “state of alert,” consuming significant energy. At the same time, the bone surrounding the gums may gradually deteriorate, potentially leading to loose teeth or even tooth loss over time.

More seriously, once bacteria from the oral cavity pass through the inflamed and damaged gums into the bloodstream, they can reach vital organs such as the heart and kidneys, causing more severe health problems. This risk is especially significant for older dogs.

A Few Small Things You Can Do for Your Dog Starting Today

The good news is that improving your dog’s oral health does not require you to become a veterinarian or purchase expensive supplements. It simply requires you to establish a few simple daily habits:

• Brush Your Dog’s Teeth 3 to 5 Times Per Week—Use a toothbrush and toothpaste specifically designed for dogs (human toothpaste contains ingredients harmful to dogs; do not use it). If your dog is resistant at first, you can start by gently rubbing the teeth with your finger coated in toothpaste to help your dog get used to it.
• Schedule Regular Professional Dental Cleanings with a Veterinarian—Once tartar has hardened, it cannot be removed by brushing at home; it must be removed by a veterinarian under anesthesia using ultrasonic tools. It is recommended to assess whether a dental cleaning is needed every 6 to 12 months.
• Provide Dental Chews or Dental Toys—While these aids cannot replace brushing, they can help slow down the accumulation of dental plaque between brushings.
• Maintain Good Oral Hygiene Yourself—The study reminds us that, given the bacterial transmission between humans and dogs, brushing and flossing your own teeth diligently every day also indirectly protects your dog.
• Be Mindful of Close Contact if There Are Elderly or Infants in the Home—There is no need to completely prohibit your dog from licking people, but avoiding mouth-to-mouth kisses and licking areas of skin with wounds can significantly reduce the chance of bacterial transmission.