We’ve studied Clostridium difficile in my lab for years and we probably have one of the world’s most diverse collections of this important bacterium. We have thousands of isolates from people, pets, livestock and many different wildlife species (as well as from meat, vegetables and water). Most of the focus on C. difficile involves human disease, which makes sense because it’s a very important cause of serious gastrointestinal illness in people. It causes disease in some animal species, but in many, it doesn’t seem to ever cause a problem.
Looking at the genetic makeup of a bacterium, it’s possible to infer how old it is. C. difficile is pretty ancient. It’s been estimated that in emerged 1.5-85 million years ago (He et al. PNAS 2010). That’s well before humans (or potentially even mammals) emerged, so it’s clearly not a human-origin bacterium. Rather, it evolved in wildlife and certain strains have made their way into people.
That’s a long introduction for a paper we just published about C. difficile in polar bears (Weese et al. Anaerobe 2019).
Why polar bears?
Usually, we do “hypothesis driven research,” where we come up with a specific answerable question or problem and try to answer or solve it (or some small part of it).
Sometimes, we’re opportunistic and curious, with less of a real plan. Here, it was basically someone saying “we have polar bear poop… is there anything you’d like to do with it?” Sometimes these curiosity-driven studies are quite rewarding.
We got samples from a large study that was collecting feces from wild polar bears in the Canadian Arctic, bears temporarily housed in the Polar Bear Jail in Churchill, Manitoba (a facility to where nuisance bears get shipped for a while before being released back into the wild), and a few polar bears from a zoo.
Surprisingly, we found C. difficile in almost 17% of samples: 18/120 (15%) from wild bears, 4/7 (57%) from the polar bear jail and 2/16 (13%) from the zoo.
To be honest, my first thought when we had so many positive results was “crap, contamination” (pardon the pun). However, typing of the bacterium showed us this was not the case. We found a variety of C. diff strains, but none of the strains in the in the wild bears were the same as strains we’ve ever seen. Also, the strain distribution was different in bears from the M’Clintock Channel compared to those from Hudson Strait (two different bear populations), despite the fact that the samples were all processed together in the lab. (So it couldn’t have been contamination at the lab level – that made me feel a lot better.)
Were the polar bears sick?
It’s hard to say, but probably not. Fecal samples were collected off the ice or ground, not right from the bear (for pretty obvious reasons). Presumably, like most wildlife species, C. difficile can be present in the gut of polar bears without causing disease.
How do polar bears get this bacterium?
We’ve been able to find C. difficile in most species that we’ve studied. That includes most livestock species, pet species and a range of wildlife, from raccoons to bats. It’s also been found in seals (not in the Arctic, but I’m not aware of anyone looking there). Since seals are a common food source for polar bears, if the bacterium is present in seals in the Arctic, that’s a likely source of exposure.
Does C. difficile-shedding in polar bears pose a risk to people?
I guess. However, there are lots of ways that we can get exposed to this bacterium. It’s possible we get exposed to low levels of it most days. If you’re close enough to the polar bear to get C. difficile from it, you probably have bigger things to worry about.