Maybe you’ve heard an old saying that goes like this: Dogs’ mouths are cleaner than humans’ mouths. It’s a provocative claim. Dogs—creatures who will sniff other dogs’ behinds by way of introduction and who will grab a moldering chicken wing off a summer sidewalk if given the chance—have mouths that are “cleaner” than those belonging to humans—creatures who brush twice a day (and floss when they remember to do so)? With all due respect to the coiners of old sayings, it seems like a stretch.
And it is a stretch.
“I don’t think anybody has a ‘clean’ mouth,” says Brian Collins, a veterinarian with the Cornell Riney Canine Health Center. “We can find hundreds and hundreds of kinds of bacteria in dogs’ mouths or peoples’ mouths.”
One type of mouth is not necessarily cleaner than the other. In fact, dogs’ and humans’ mouths are so different that they’re practically incomparable. A 2012 study by Harvard professor and oral geneticist Floyd E. Dewhirst found that only 16.4% of dogs’ oral bacteria overlaps with that in humans. The research found that though the two mammals have a similar number of bacteria in their mouths, they have largely different microbiomes, so it’s nearly impossible to determine which one is “cleaner.”
“Dirty or clean is very subjective,” Collins says. “There are bacteria there. They live there. Our goal is not to remove them because they are there for a reason. It’s just that in the modern world we can do things to promote oral health and prolong lives.”
Why do we care so much about dogs’ mouths being clean—so much so that we repeat that comforting old adage? Well, mainly, because of all those kisses.
Is it so bad when we let our dogs kiss us?
Spend any amount of time on social media as a dog person, and you’ll see endless idyllic scenes of dogs and babies. The ones where dogs rest their heads on babies’ heads, babies giggle at puppies, puppies crawl all over babies, and, yes, dogs lick babies.
“I don’t want to be the world’s biggest party pooper,” Collins says, “but when I see videos of people letting their dogs lick their kid’s face, it’s kind of cute for a second, but for a number of reasons I’m just not sure it’s the best thing to do. I wouldn’t encourage it.”
Everyone lives with some risk of contracting zoonotic diseases from bacteria in dogs, like bartonella or pasteurella. These, however, are called “opportunistic pathogens” because, more typically, they take advantage of rare conditions like a weakened immune system or an altered microbiome. Children, the elderly, and the immunocompromised are among those who should be extra careful. The risk of infection is higher for children under five than it is for adults as their immune system hasn’t fully developed, Collins explains. Older individuals’ immune systems are less robust, and immunocompromised people, like people with certain cancers or who are living with H.I.V., do not have the defenses against bacteria they need to always feel safe swapping spit with a dog.
In extremely rare cases, people have died from exposure to Capnocytophaga, a group of bacteria commonly found in the saliva of dogs and cats. It’s good to remember that specific conditions have to align in order for this bacteria to have such an impact, like if a person has a compromised immune system. The bottom line is that the immunocompromised should be especially wary of dog kisses—especially those on the mouth—and dog bites.
For the most part, though, when a healthy dog licks a healthy adult with a normal immune response to bacteria, there is a low risk of danger, and you can simply avoid touching your own eyes or mouth until you’ve washed the area and your hands. The risk is slightly elevated when the dog licks you directly on or in the mouth or eyes, but, as Collins points out, it’s difficult to quantify how much more elevated.
There are two major caveats that increase the risk of infection in both people with a normal immune responses and the immunocompromised alike. One is if the dog eats raw meat, either as a one-off or for their daily diet. They could impart salmonella and listeria with those kisses. Another is if the dog has eaten fecal matter, or even has brushed up against stool with their snout. They could carry harmful foreign bacteria, like E. Coli, which can cause a whole host of dangerous oral-born diseases.
Can dogs lick my wounds?
It gets more complicated when a dog licks an open wound. For our dogs’ ancestors, licking their own wounds was a one-stop shop for veterinary health, since they couldn’t swing by the clinic for on-demand treatment. It can still be an effective practice for modern dogs, and may contribute to the idea that dog’s mouths are “cleaner” than humans.
“Saliva does have antibacterial properties. It promotes cells to proliferate [and] reduces enzymes that cause tissue damage,” Collins says. “There are some benefits to it, but I think it all depends on, essentially, the dose.”
If the dog goes overboard and licks the wound too much, then it could prevent the wound from healing or lead to infection. This largely applies to dogs licking their own wounds, though, not a human’s. The risk of infection increases when a dog’s saliva gets in a person’s open wound.
“If we’re talking about dogs licking human wounds, I think it’s safe to say that’s never recommended,” Collins says. “It’s not a good practice to let your dog lick your wounds because we have better ways to treat wounds now.”
If you get bitten by a dog or if your dog licks an open wound, clean the wound out with soap and warm water. It’s also a good idea to have a doctor look at it no matter how minor the wound appears.
Rabies is the most serious disease that’s communicable through a dog’s saliva. It’s viral, rather than bacterial, and it attacks the nervous system of those who contract it. Dogs who have it will show anxiety at first before developing aggression and becoming uncoordinated and confused. Most people know that rabies is transmitted from infected animals from bites and scratches, but it can also be transmitted when an infected animal licks your open wound, mouth, or eye. Rabies is fatal in both dogs and humans if left untreated. If you suspect a dog that bit, licked, or scratched you may be rabid, seek treatment immediately.
Can keeping up my dog’s dental health help?
Poor dental hygiene can cause periodontal disease, which, when caused by a bacterial infection, increases the risks that those kisses present, mostly for the immunocompromised. These dental diseases also pose a danger to your dog, since bacteria from these unhealthy teeth or gums can get into their bloodstream and travel to the heart. To mitigate the chance of your dog developing dental issues, brush your dog’s teeth daily, and keep up vet appointments.
Dog’s mouths can be many things—smiley, toothy, goofily ferocious—but they are not necessarily “cleaner” than the mouths of humans by any measurable standard. The old saying gets at our concern for safety when it comes to living in such close proximity with our beloved dogs and their (mostly delightful) slobber. It’s easier if we can just consider their mouths clean. While being on the receiving end of your pup’s kisses does carry some risks, it’s up to you, the lick-ee, to know your own health and your dog’s habits. You ultimately get to decide how much those kisses are worth.
