A recent case report in the journal American Journal of Case Reports (Garza et al. 2023) implicated a pet cat as the cause of a person’s recurrent Clostridium (Clostridioides) difficile infection.

Spoiler alert: it’s a pretty weak link. They have a catchy title: “Look what the cat dragged in! Recurrent Clostridioides difficile infection from a household cat,” but unfortunately don’t have the quality of work to back up the story.

Clostridiodes difficile (formerly known as Clostridium difficile) is an important cause of intestinal disease in people. I’ve worked on animal aspects of this bug for years, and it can be found in an impressive range of species. In some, it’s an important cause of disease (e.g. horses, pigs). In others, it’s debatable whether it does anything of consequence (e.g. dogs, cats). There are lots of strains of C. difficile, and those that infect animals tend to be the same as those that infect people. How often or whether it’s transmitted animal-to-human (or vice versa) has been harder to sort out.

As I mentioned already, the report in question is really weak, so it’s hard to say whether there’s anything to the story about the cat at all. Here’s a quick summary:

  • A person developed C. difficile infection, presumably triggered by a course of antibiotics (classic situation).
  • She was treated for the infection, but had a recurrence 2 months later (also a common problem with this kind of infection). She was treated again but responded poorly, continuing to have profuse diarrhea.
  • She then took her cat to a veterinarian. The cat was healthy, but was tested (method not reported) and found to be positive for C. difficile.
    • That’s not surprising since we know a small percentage of cats are shedding this bacterium at any time, and in a household with an infected person the risk is likely even higher (because it also could have come from the person).
  • The cat was then treated, with… something.
    • With what? For how long? Who knows… they left that part out.
  • One month later, the patient was still sick and was treated with a new drug, bezlotoxumab. She then responded to treatment.

Just looking at the timeline, it’s hard to implicate the cat. The person didn’t get better after treatment of the cat. A month later, she still had diarrhea and didn’t get better until she received a new drug.

Also, the main issue wasn’t recurrence, where it would be make sense to look for sources of reinfection. The main problem was that she didn’t respond to treatment during her second episode. It’s hard to implicate the cat in that scenario.

It’s also rather astounding they claim “Ultimately, the patient was not able to achieve long-term resolution of her symptoms until her newly adopted pet cat was treated by a veterinarian.” She more likely responded to a new drug, not treatment of her cat a month earlier. That statement is a major stretch and it’s particularly concerning since it’s in the abstract (which is often the only part of the paper that people read).

More importantly, there was no testing of the strains of C. difficile from the cat and the person. That’s a huge flaw. We know from work that we and others have donethat when we find C. difficile in a person and pet in the same household at the same time, it’s often not the same strain, so the infections are just coincidental, not linked. In this case, we have no idea if the strains were related because they didn’t bother to look.

It’s good that there’s awareness of the potential for pets to play a role in human infection (and vice versa) but it’s frustrating to see publication of yet another poor effort that didn’t investigate things properly, but jumped to a conclusion that could raise unnecessary concern about animals and lead to their unnecessary treatment (or worse).

Was the cat the source? Maybe, but probably not. Even if the cat really was positive for C. difficile (and that’s still be a big “if”), it was presumably either an unrelated infection, or it was infected by the owner and was an innocent bystander of the owner’s disease process.

I’m all for investigating pets as sources of recurrent disease. I get involved in these investigations. However, we need to do it right and not jump to conclusions that are unsubstantiated and potentially harmful. The last thing we need is for even more physicians to ask for unnecessary antimicrobial treatment of animals, which is also an issue with which I contend regularly.