Infective arthritis due to Blastocystis hominis.

A patient with rheumatoid arthritis taking prednisone developed Blastocystis hominis acute diarrhoea, which was associated with increased inflammation and effusion of the left knee. B hominis organisms were found in synovial fluid from the left knee. The patient responded dramatically to metronidazole treatment. B hominis may become disseminated in immunosuppressed patients with diarrhoea and may cause infective arthritis.