If I’m talking to veterinarians or veterinary students about infectious diseases and I mention Capnocytophaga canimorsus, I usually get a blank stare (or “capno-what?”). If I’m talking to physicians, it’s usually the same response, unless they’re infectious disease physicians or trainees (but I still get “capno-what?” from some of them on occasion).
Capnocytophaga canimorsus is a bit of a niche zoonotic bacterium, but it’s an important one. It can be found in the mouths of most dogs, so people are really commonly exposed, but human infections are rare. However, when infection does happen, it tends to be very severe or even fatal. Not having a spleen (e.g. if a person had to have their spleen removed at some point) is a huge risk factor for infection. Other types of immunocompromising disorders and alcohol abuse are other known risk factors. For more details on Capnocytophaga, check out the infosheet in Worms & Germs Resources – Pets page.
A recent case report (Sorensen et al, 2025) reminds us that the risk of infection with this bacterium is no solely from dogs; the report describes a severe Capnocytophaga infection in a person linked to a cat bite. Cats can also carry Capnocytophaga in their mouths, but it’s much less common than it is in dogs.
The affected patient was a 53-year-old man who (unsurprisingly) didn’t have a spleen and was heavy alcohol user (two big risk factors for infection). He had a two-day history of fever, chills, diarrhea and lethargy, with progressive weakness and decreased mental acuity. He then developed a purplish discolouration on his face, back, arms, legs and abdomen. He was taken to the emergency room, where a small cat bite was noticed. He then deteriorated quickly.
It’s not clear whether seeing the bite wound changed their initial approach, but he got the infectious disease kitchen sink treatment: ceftriaxone, vancomycin, azithromycin and atovaquone. Despite this, his disease progressed to severe septic shock, DIC, purpura fulminans and multiorgan failure. Blood cultures were negative, so antibiotics were changed to meropenem, vancomycin, doxycycline and penicillin, and molecular testing was done to look for a fastidious bacterium. That’s how they detected C. canimorsus.
Although the patient gradually improved, the severity of infection resulted (as is common with this disease) in the need for multiple amputations. Both his legs were amputated below the knee, along with multiple fingers. (There are photos included in the case report.)
The authors concluded “Although C. canimorsus is typically associated with dog bites and scratches, it can show up after exposure to cats, as seen in this case, and should be considered along with more common pathogens such as Pasteurella multicoda and Bartonella henselae.”
A common issue with C. canimorsus infections is not considering it until it’s too late. I’ve talked to many family members of people who had this infection, and a common theme is that there was no querying of animal contact or animal bites until after the diagnosis was eventually made, and the family didn’t think to report any animal bites (when they knew about them).
Animal contact and bite history can be really important when someone gets sick, and is takes seconds to ask, but too often the question is never asked. That’s a healthcare gap. At the same time, we need better education of the public so that people can advocate for themselves and know to tell healthcare providers about animal contacts, especially animal bites.