Polymicrobial Arthritis Following a Domestic Cat Bite Involving Rahnella aquatilis in an Immunocompetent Patient.

Cat bites frequently lead to polymicrobial infections due to deep puncture wounds that inoculate oral flora into poorly oxygenated tissues. While Pasteurella multocida is the most commonly implicated organism, environmental and atypical pathogens may also play a role, yet often go unrecognized. This article reports a rare case of polymicrobial septic arthritis caused by a domestic cat bite in an immunocompetent adult, with isolation of Rahnella aquatilis, a freshwater-associated Enterobacterales species not previously reported in this context. A 33-year-old immunocompetent male presented with acute hand swelling, pain, and functional impairment within 24 h of the bite. Emergency surgery revealed purulent tenosynovitis and arthritis. Intraoperative cultures identified R. aquatilis, P. multocida, and Pantoea agglomerans. Identification was performed using MALDI-TOF MS. The R. aquatilis isolate was susceptible to beta-lactams (excluding ampicillin), quinolones, and co-trimoxazole. The patient received amoxicillin-clavulanic acid and fully recovered within two weeks. This is the first reported case of joint infection involving R. aquatilis following a cat bite. It highlights the importance of considering environmental Enterobacterales in animal bite wounds, and the utility of advanced microbiological tools for detecting uncommon pathogens. Broader awareness may improve diagnosis and guide targeted therapy in polymicrobial infections.