Work over the past sixteen years has reclassified Blastocystis hominis as a protozoan, removing it from the yeasts where it has languished for seventy years. B. hominis is now firmly established as a member of the Sporozoa, an intestinal parasite and pathogen of man and primates. When B. hominis is present in large numbers in stool contents, marked diarrhea is usually experienced. The drug of choice is metronidazole. Patients with severe infection have been treated at the Clinical Center. There was one fatal case. This protozoan is now a subject of national and international interest to parasitologists and physicians as a course of diarrhea in man. Treatment is usually with metronidazole. Trimethoprim-sulfamethoxazole is also used successfully. Progress has been slow on research into B. hominis mitochrondria, but we at last have sufficient data for publication. We receive calls from around the United States a number of times each week, to present problems in diagnosis and treatment of blastocystis.
