Blastocystis hominis is a protozoan organism frequently found in the human intestinal tract. Controversy surrounds the clinical consequences of infection with this organism: Some researchers strongly believe B. hominis is pathogenic, others believe it is not responsible for clinical symptoms when detected, and still others believe the question of pathogenicity is unsettled [1-5]. We report a case of hemorrhagic proctosigmoiditis believed to be caused by B. hominis infection.

A 76-year-old man presented with generalized abdominal cramps, grossly bloody diarrhea, tenesmus, and fecal urgency that had lasted 24 hours. The patient did not have a fever. He had not drunk water from any source other than the public utility, eaten processed meat, or travelled to foreign countries. No other family members had similar symptoms. A stool specimen was positive for fecal leukocytes and contained numerous B. hominis organisms (30 organisms per 40 times-magnified field); no other ova or parasites were seen. Stool cultures were negative for Clostridium difficile, Salmonella, Shigella, Yersinia, Campylobacter, and Vibrio organisms; Clostridium difficile toxin was not detected. The stool was not examined for viruses or Shiga-like toxins. Results of a serologic test for human immunodeficiency virus was negative. Sigmoidoscopy showed marked diffuse erythema, and examination of multiple rectal and sigmoid biopsy specimens showed moderate acute inflammation not consistent with idiopathic inflammatory bowel disease. Results of mesenteric and celiac arteriography, barium enema, and abdominal ultrasonography were normal.

After treatment with metronidazole, 750 mg three times a day for 7 days, the symptoms resolved. Stool examinations done 2 and 7 weeks after treatment showed no evidence of B. hominis or other enteric pathogens. The patient remained well 2 months after therapy, and findings of a second sigmoidoscopy with biopsy were normal.

We believe this case provides further evidence to support the pathogenicity of B. hominis and that this organism should be considered a possible additional cause of hemorrhagic colitis in both immunocompetent and immunocompromised persons.