I’m not sure what to think about the recent increase in scientific papers about Staphylococcus pseudintermedius infections in people. This dog-associated bacterium has been well known for quite a while, and human infections have been sporadically reported, but it seems like there has been a big increase in reported cases over the past year.

The latest case, published in the Journal of Clinical Microbiology (Chuang et al 2010) describes a bloodstream infection in a 6-year-old boy. The infection was associated with an intravenous catheter site. The bacterium was initially misidentified as Staphylococcus aureus, a related bacterium that is more commonly found in people. It seems that the patient’s history of having contact with dogs led to further testing of the bacterium. That’s pretty surprising (and encouraging) from a few standpoints:

  • The attending physicians asked about pet contact. That’s not always done.
  • The physicians recognized the potential for dog-human transmission of bacteria and considered the possibility that there was a misidentification by the lab. I’m quite surprised that they did further testing, since S. aureus is so common.

Unfortunately (as is commonly the case), they didn’t take the investigation any further. It would have been nice for them to have tested the patient’s dogs to see if they carried the same strain of S. pseudintermedius, to provide more evidence that the infection was truly from the dogs.

Concern has been raised before regarding the potential that S. pseudintermedius infections might be misdiagnosed as S. aureus, such that we don’t know the true extent of the problems caused by the dog-associated bacterium. The ability of medical diagnostic labs to differentiate these two bacteria is something that needs to be investigated to help determine whether there may be more going on than we realize.

The increase in reports of S. pseudintermedius infections in the literature could also just be because infections that have always been occurring at a low level are being properly diagnosed, and people are bothering to write them up. The fact that people are still finding single cases of this infection noteworthy suggests that it’s still a very uncommon condition. When you consider that the majority of dogs are carrying this bacterium, and millions upon millions of people have close contact with dogs on a regular basis, it’s clear that people get exposed to this bacterium very often. The fact that infections appear to be so rare indicates that the risks to humans is likely quite low.

It’s also possible that there truly has been an increase in these infections. It’s hard to think of a reason why that might be the case. There’s no evidence that the types of S. pseudintermedius have changed such that current strains are better able to infect people than older strains. Most likely, this is still a rare infection in humans that is often associated with dogs, but is of pretty low risk for the average dog owner. Regardless, continued study in the area is required, to make sure that this is not an emerging problem, especially when you consider that multidrug-resistant forms of this bacterium are also becoming much more common in veterinary medicine. Increased physician awareness about pet contact and zoonotic diseases is required to properly diagnose this and other potentially zoonotic diseases.