I doubt you’ll be shocked to hear that the normal host of Staphylococcus felis is cats. It’s a bacterium that can often be found in healthy cats and periodically causes disease in cats (e.g. urinary tract infections). Overall, though, it’s a pretty innocuous bug. Human health risks related to S. felis haven’t been well investigated, but it’s been reasonable to assume there’s a potential (but low) risk of infection in people.

Some supportive evidence of this is provided in a recent case report about suspected transmission of S. felis from a cat to its owner in the form of a wound infection (Sips et al, J Med Microbiol 2023).

The case involves a 58-year-old woman who had back surgery. Two weeks after surgery, she delevloped a mild surgical site infection, with some drainage from the incision and mild redness. She was otherwise fine. They did a routine culture of the wound that identified Staphylococcus felis that was susceptible to all of the antibiotics tested. She received some wound care and was sent home with antibiotics.

While Staphylococcus spp. are leading causes of surgical site and wound infections, S. felis isn’t one we’d expect to find here. It’s a coagulase-negative species, part of a large group of staph that typically only cause infections in higher-risk situations. Coagulase-negative staph can also be found as contaminants in cultures when samples pick up bacteria that just happen to be present on the skin, but aren’t actually part of the problem. So, it’s not definitive that S. felis was the cause of the wound infection in this case, but it’s pretty likely.

When they got the culture results, medical staff queried animal exposure and it turned out that the patient had three cats.

Too often, case reports in medical journals stop here. They say “the infection happened in a person… this bacterium is usually found in animals… the person lives with an animal… they must have gotten it from the animal. Eureka, my job is done, let’s write a paper.” In many cases the link is probably real, but no always, so we need to investigate such things properly.

Fortunately, this case report highlights a great investigation of the situation.

After the owner was diagnosed, they tested her three (healthy) cats. Not surprisingly, all three cats had positive oral swabs for S. felis, and there were mixed results from axillary (armpit) and perineum (bum region) samples as well.

But they didn’t stop there. They looked at the genetic sequences of the bacteria and found that the three cats harboured different strains of S. felis. One cat had the same strain as the owner, so that animal was presumably the source of the wound infection. The others were innocent bystanders, even though they harboured the same bacterial species.

What does this change?

Not much, but it’s a good reminder that infections from various bacteria that are present on/in healthy animals can happen, and that proper investigation is required to truly understand any transmission links.

Staphylococcus felis infections haven’t been reported before in people, but I’d guess they have occurred. Until recently, most identification of bacteria in diagnostic labs was done with biochemical tests. Often, testing would stop at “coagulase-negative Staphylococcus spp” on the assumption that the bug was probably not overly relevant and/or it didn’t really matter which one of those different coag-negative staph it was for treatment purposes. In recent years, more advanced testing has become standard, so we now get more specific identification of bacteria that otherwise might have been missed or generically reported. As a result, my guess is that previous incidents of such infections were just not identified. Regardless, it’s still fair to assume this is a rare event.

There’s no mention of what happened to the cats. Hopefully, nothing. I wouldn’t treat the cats in a case like this because we have no evidence it helps. It’s hard to eliminate a bacterium from its normal ecological niche, and treatment might cause more problems, such as selecting for resistant bacteria that could pose a bigger risk to the owner. Staphylococcus felis is just one of many bacteria present in these cats that can potentially cause disease, and even if was eliminated, the risk posed by the cats wouldn’t drop by much. In these types of situations, I focus on basic hygiene and infection control, and owner awareness about risks.