Chronic monoarthritis of the knee in systemic lupus erythematosus

We describe a middle-aged lady with systemic lupus erythematosus who presented with chronic left knee monoarthritis without constitutional symptoms. The histology of synovial tissue taken at arthroscopy showed acute inflammation and Erysipelothrix rhusiopathiae, identified with some difficulty, was isolated from the enrichment broth only. Blood cultures were negative. Her history revealed significant exposure to pond fish. She responded well to intravenous penicillin and remains well 12 months later.