Mycobacterium marinum arthritis mimicking rheumatoid arthritis.

Mycobacterium marinum is an atypical mycobacterium found in salt and fresh water. M. marinum infection occurs following skin trauma in fresh or salt water and usually presents as a localized granuloma or sporotrichotic lymphangitis. It rarely affects the musculoskeletal system. We describe a patient who presented with subcutaneous nodules and an inflammatory arthritis that was thought to be rheumatoid arthritis, and was treated as such with corticosteroids, methotrexate, and anti-tumor necrosis factor-alpha therapy, with worsening of his arthritis.