I get this question surprisingly often. In one way, that’s good because it shows increasing awareness of the potential for interspecies transmission of microorganisms. In some situations, when a person has an infection, the pet should be considered in case it was the source of the infection and/or in case it’s at risk of becoming infected by the person.

Clostridium difficile is a very important cause of diarrhea (and sometimes more severe intestinal disease) in people. Previously it usually only affected people confined to hospitals and people being treated with antibiotics, but it’s now being identified more often in people in the community.

The potential for interspecies transmission of this bacterium is real.

• C. difficile can be found in a small percentage of healthy dogs and cats.
• The strains of C. difficile in dogs and cats are almost always the same as those found in people. My lab has one of the largest collections of C. difficile around and we only have a couple of C. difficile isolates from dogs and cats that we have not found in people.
• Dogs that visit human hospitals are at increased risk of shedding this bacterium in their stool, and certain types of contact with people have been identified as increasing this risk (Lefebvre et al 2009).
• Dogs owned by an immunocompromised person are at increased risk of shedding C. difficile (Weese et al 2010), presumably because the person is more likely to shed the bacterium and subsequently infect the pet.
• Dogs that are owned by people being treated with antibiotics are more likely to shed the bacterium (Lefebvre et al 2009). That’s probably because, as with immunocompromised people, when someone’s being treated with antibiotics, they have a greater likelihood of shedding C. difficile and their dog subsequently becomes infected.

However, there’s currently no indication that testing is needed.

• What would the results tell you? If you identify C. difficile in your dog and you have a C. difficile infection, does that mean that you were infected by the dog, you infected the dog or you were both infected by the same source?
• What would you do with the results? There’s no indication to treat the dog if it’s positive and healthy.
• What testing would you actually get done? Testing for diagnosis of C. difficile disease usually involves trying to detect bacterial toxins in stool. The tests aren’t meant to be used on normal stool. To really know what’s going on, you’d need to have the bacterium cultured from your pet’s stool as well. Not many labs can do that. Furthermore, to get really useful information, you’d also need to get your stool cultured and, if C. difficile was present in both you and your dog, molecular typing would be required to show that they were the same strain. Very few places can do that. Even with that information, at the end of the day, finding the same strain in you and your pet doesn’t tell you more than the fact that the bug probably moved between you and your pet, in one direction or another.

If you have C. difficile, it’s reasonable to take precautions to reduce the risk of infecting other individuals, both human and animal:

• Practice good hygiene. Wash your hands thoroughly after using the washroom.
• Don’t let your dog drink out of the toilet.
• Use antibiotics judiciously. If your pet is being treated with antibiotics and you have C. difficile, there’s probably a greater chance of your pet picking up the bacterium.
• If you have C. difficile and your pet develops diarrhea, tell your veterinarian. It’s important that they know that your pet may be at higher risk of C. difficile infection.

If you have recurrent C. difficile infections, considering the pet as a possible source might be reasonable, but we don’t currently know what role pets may play. As described above, investigating your pet as a possible source would require culturing stool from both you and your pet, having both typed using molecular tests, and a joint effort involving your veterinarian and physician.