Septic arthritis due to Pasteurella multocida complicating rheumatoid arthritis

SUMMARY A patient with rheumatoid arthritis treated with corticosteroids developed a septic arthritis of her right knee. She had been bitten on her right leg by her pet cat 2 weeks earlier. Pasteurella multocida, resistant to penicillin, was isolated from the septic joint and from the oro-pharynx of the cat. The arthritis was successfully treated with ampicillin. No attempt to eradicate the organism from the cat was made because of the expectation of early recolonisation. Case report A 64-year-old woman with a 3-year history of sero-positive, erosive, rheumatoid arthritis was admitted for a carpal tunnel release operation. She was on aspirin, alcofenac, and prednisone. Two days later she developed a high fever with malaise, culminating in severe pain in her right knee. The knee was hot and tender, with swelling extending from the joint to the lower tibia, where there were 2 small dry wounds. These had been caused by a bite from her pet cat 2 weeks earlier. Purulent fluid was aspirated from the right knee. Gram stain showed numerous pus cells and Gram-negative coccobacilli. Horse-blood-agar plate culture yielded Pasteurella multo-cida. Blood cultures were sterile. The white blood count showed a moderate polymorphonuclear leucocytosis and lymphopenia. A fasting glucose was normal. X-ray of the knee showed only the effusion. Antibiotic therapy was begun with flucloxicillin and sodium fusidate, but when bacteriological results became available it was changed to ampicillin 500 mg 4 times a day, with further clinical improve-ment. The cultured organism was sensitive to ampi-cillin, cephalexin, tetracycline, and cotrimoxazole, and resistant to penicillin G, fusidic acid, and cloxacillin. The knee was rested in a back slab splint. After 2 weeks full mobilisation was gradually achieved. With the isolated organism as antigen titres of agglutinating antibody increased to