Bacteria in Dog Saliva

Laurie Anne Walden, DVM

Are dogs’ mouths really cleaner than humans’ mouths? Definitely not! You may have read the recent news story of a Wisconsin man who required multiple limb amputations after contracting a bacterial infection from dog saliva. This type of severe infection is extremely rare, but dogs' (and cats’) mouths do harbor quite a variety of bacteria, some of which can make people sick.
 
The organism responsible for the Wisconsin patient’s infection was Capnocytophaga canimorsus, which is common in dog and cat saliva. Many different Capnocytophaga species normally live in the mouths of healthy dogs, cats, and humans. Up to 74% of dogs and 57% of cats have Capnocytophaga in their mouths. The types of Capnocytophaga that live in human mouths are usually different from those in dogs and cats.
 
Capnocytophaga canimorsus is one of the bacteria that cause infections in people who are bitten by dogs. Although it is usually spread through bites, it’s possible for a person to contract the infection after being licked by a dog. The Wisconsin man was reportedly licked, not bitten. A case report published in 2016 described an older woman who developed serious illness from Capnocytophaga canimorsus that likely came from contact with her own Italian greyhound. She recovered fully after 2 weeks of intensive care and antibiotics.
 
The vast majority of people exposed to this bacterium do not become ill. (Think of all the times you’ve been licked by a dog and haven’t gotten sick.) People who have had their spleen removed, use alcohol to excess, or have compromised immunity (from cancer or HIV infection, for example) are at increased risk of Capnocytophaga infection. Infection can cause severe sepsis (blood infection), gangrene, and inflammation of the heart lining, eyes, lymph nodes, and brain membranes. Infection in a pregnant woman can spread to the fetus. A 2015 review article reported a fatality rate of about 26%.
 
Of course Capnocytophaga are not the only bacteria in our pets’ mouths. Oral cavities—everyone’s, humans included—contain hundreds of different species of bacteria. Most of these bacteria are not zoonotic; that is, they are not spread between animals and people. The ones that do cause illness in people are usually transmitted by bites. However, one study found that several oral bacteria species could potentially be transmitted by close contact between pet dogs and their owners.
 
Prevention
For most people, the risk of contracting a severe infection after being licked by a dog is incredibly low. But if your dog has zoonotic parasites like hookworms or roundworms, eats a raw meat diet, or eats feces (including foraging for snacks in the cat’s litter box), it would be a very good idea not to let him lick you in the face.
 
The Centers for Disease Control and Prevention recommends taking extra precautions around animals if you have an increased risk of infection. And everyone should take steps to prevent dog bites, which are much more likely than doggie kisses to cause an infection.
 
Photo by James Barker

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