I’ve written about this topic before, but it’s an important (and increasingly common) issue to understand, so bear with me while I address the subject again.

I typically get multiple case consults in person, by phone or by email about methicillin-resistant (MR) staphylococci every day. A lot of these start with "I have a case with an MRSA infection..." While trying not to be rude, I tend to interrupt the conversation at that point with "Is this actually Staph aureus or another staph?"

I do this for a few reasons:

  • A few years ago, the vast majority of "MRSA" infections in dogs, cats, horses and other companion animals were actually MRSA – that is methicillin-resistant Staphylococcus aureus. However, in the past few years, there’s been a tremendous upsurge in other MR-staph, particularly booming numbers of MR-Staphylococcus pseudintermedius (MRSP) infections in dogs. These days, if it’s a dog or cat, when I ask the "What staph is it?" question it’s usually not actually MRSA. We’re  starting to see more MRSP in horses too, complicating things in that species as well.
  • Staph are divided into two groups, coagulase positive species (which include S. aureus and S. pseudintermedius) and coagulase negative species. The coagulase negative species are commonly found in or on healthy animals and are often methicillin-resistant, but they are not very virulent and don’t usually cause disease outside of very high risk populations (e.g. very sick animals in a veterinary hospital). If a MR coag-negative staph is isolated, I am far from convinced it’s the culprit, and typically the real cause of the problem still needs to be found.
  • MRSA is much more of a concern from a public health standpoint, as it can move between animals and people. While MRSP can cause human infections, these are extremely rare.
  • MRSA is not really adapted to live in dogs, cats, horses and many other animals. It can, for a while, but doesn’t do so longterm, and the vast majority of MRSA carriers will get rid of it on their own. In contrast, it appears that MRSP (at least in dogs) can stay with the animal for a very long period of time. Therefore, an animal that has had an MRSP infection has a reasonable chance of shedding the bacterium for a long period of time, which might be of relevance for its health in the future.
  • The two main MR-staph of concern in companion animals are MRSA and MRSP. Some diagnostic labs still don’t try to differentiate the two, despite the fact that there are different guidelines for determining whether they are methicillin-resistant. If someone has a result that doesn’t differentiate MRSA from other staph, I tell them their lab isn’t doing things right and they need to talk to them so they can have confidence in the results.

More information about MRSA and MRSP can be found on the Worms & Germs Resources page.