Septic monarthritis due to Pasteurella multocida after a cat scratch in a patient with rheumatoid arthritis.

A 60-yr-old woman with a history of seropositive rheumatoid arthritis and using a low dose of corticosteroids presented with a very painful, swollen, red coloured knee with limited range of motion. Physical examination revealed a scratch on her left lower leg without discolouration or induration. Synovial fluid cultures yielded Pasteurella multocida. The pathogenetic significance of this commensal of cats and dogs in man is discussed.