People sometimes get freaked out by the concept that they have approximately 10-times as many bacterial cells on them as all their own body cells combined.
- Yes, our cells are a minority in our own bodies, and amongst the trillions of bacteria we carry are many that could kill us given the opportunity.
Yet, we have somehow managed to survive, both individually and as a species. So, keeping things in perspective is important and, in reality, we need much of that bacterial population to keep us healthy.
Just like every person is carrying many bacteria on any given day that can cause illness or infection, every animal is carrying many different microorganisms that can infect a person. While infections from pets do happen, they are uncommon – we’re not seeing dog owners dropping like flies on the street, which is a testament to our immune system and other body defenses and barriers (e.g. intact skin).
So, when studies come out describing various bugs at various sites in various animals, you have to put them into context. It’s not that the studies are bad (my lab does a lot of work trying to define the complex bacterial populations of sites like the intestinal tract, oral cavity, respiratory tract and skin), it’s that we need to think about what the results really mean and avoid sensational headlines in the press.
A recent paper in the Archives of Oral Biology (Yamasaki et al. 2012) is an example of this. The study looked at mouth bacteria in dogs and their owners. They used molecular testing developed for human oral samples and focused on bugs that have been implicated in dental disease – not the range of bugs that are more often associated with zoonotic infection. Not surprisingly, they found lots of different bacteria in the mouths of the dogs, including some bacterial species that were present in both dogs and their owners. No methods were used to type the bacteria to see whether the strains found in dogs and people were the same or whether dogs and people just normally have those bugs present, independently, in their mouths. However, we know that transmission of certain bacterial between people and their pets is a relatively common event, and it wouldn’t be surprising if the same applies for oral bacteria, through direct contact (e.g. kissing/licking) or through indirect contact (e.g. a person touching a dog’s face then his/her own face).
So, this was an interesting study… nothing earth-shattering but the first side-by-side comparison of oral microflora in people and dogs, and it provided some useful information for future research. The authors rightly discuss the limitations and things that need to be done to further investigate this, such as looking at strains and evaluating the bacterial population in relation to types of contact that people and dogs have (e.g. do certain activities increase the likelihood that people and pets share oral bacteria). They conclude by writing "In summary, we found that the distribution of periodontopathic bacterial species in dogs and their owners is diverse, though several species including P. gulae may be transmitted during close daily contact. Therefore, our findings could be significant in understanding the relationship between the oral health of humans and their companion animals"
Yet, headlines like "New study warns against kissing your dog" don’t really reflect the true content… not uncommon but unfortunate.
I get asked about dogs licking a lot. My general line is that I don’t particularly like to be licked but for the average healthy person, I don’t get worked up about it. My kid were playing with puppies yesterday and were getting licked. I didn’t fire up the power washer to hose them down after. If I had an infant, an immunocompromised child or some other high risk person it would be a different story. Licking around the ears is something I like to see avoided because of some links between this activity and certain ear infections in kids, but overall it’s a relatively low risk situation that some people enjoy.