An immunocompromised woman’s cat stirred up a lot more trouble than usual (for a cat). In a recent case report, doctors detail how the feline likely spread a seldom-seen bacterial infection that landed its owner in the hospital.

Doctors in Slovenia wrote the report, published in the June issue of Emerging Infectious Diseases. The 56-year-old woman, a kidney transplant recipient, developed a severe, rare bout of Mycoplasma arginini infection that was eventually traced back to her cat. Thankfully, the woman’s illness was identified in time for proper treatment, and she ultimately recovered.

According to the report, the woman first sought medical care after three weeks of swelling, redness, and pain in her left forearm. Though she was prescribed an oral antibiotic, her condition continued to worsen, and five days later, she was admitted to the hospital. By that point, the swelling and redness had spread to her left wrist, which she was barely able to move. Tests confirmed the infection had spread through the main wrist joint, tendons, and skin surrounding her hand.

Doctors quickly suspected the woman had some kind of unusual germ in her arm, and testing confirmed the presence of M. arginini bacteria. Mycoplasma bacteria in general are pretty weird, since they’re incredibly tiny and lack a cell wall (some species can also invade and hide inside our cells to replicate, much like viruses). This lack of a cell wall makes them invulnerable to many conventional antibiotics, likely explaining why the woman’s first treatment failed. The woman also reported that she had recently been bitten by her cat on her left arm.

With the mystery solved, she was switched to more effective antibiotics. And though she did briefly develop other complications, she and her arm eventually improved enough to be discharged from the hospital. Later follow-up visits confirmed her infection had cleared.

M. arginini bacteria are commonly found in the respiratory and genital tracts of many animals, pets included. The report authors collected samples from the woman’s three cats and dog. They found some sort of mycoplasma bacteria in one of the cats but couldn’t definitely confirm whether it was M. arginini. Still, given the recent cat bite and lack of other plausible explanations, the doctors concluded that her cat was the likely source of infection.

Though there are some common mycoplasma infections in humans, such as the sexually transmitted M. genitalium, M. arginini isn’t one of them. The doctors were only able to find a handful of other cases documented in the medical literature. And as with this case, most involved people with weakened immune systems. People who receive donated organs need constant immunosuppressing drugs to prevent rejection, and the woman had received two kidney transplants, the latest occurring in 2022. While the average person doesn’t have anything to fear from M. arginini, doctors treating immunocompromised patients should be aware of its “potential as a pathogen,” the report authors say, especially if their patients are regularly exposed to animals.

Cat bites in general aren’t something to dismiss, since our furry roommates can carry all sorts of germs capable of sickening even the healthiest people. So you should always quickly wash a cat bite or scratch with running water and soap for at least five minutes. You should also keep a close eye out afterward for signs of infection, such as redness and swelling, and see a doctor immediately if they appear (if you’re in poorer health or were bitten by an unfamiliar animal, you should seek medical care regardless, just in case).