Blastocystis hominis is usually considered a nonpathogenic, commensal intestinal protozoa in man. Occasionally cases are encountered where a person with Blastocystis has gastrointestinal symptoms that resolve in conjunction with loss of Blastocystis in the stool.
Criteria for a considering Blastocystis to be a potential pathogen in a patient:
(1) abundant Blastocystis in the stool, with > 5 per high dry microscopic field (x400 magnification)
(2) no other pathogenic organism found, despite multiple cultures and stool examinations
(3) clinical resolution of symptoms in parallel with clearance of Blastocystis following therapy with Bactrim (trimethoprim-sulfamethoxazole) or other regimen to which the organism is susceptible
Recurrence of symptoms on re-exposure to Blastocystis or relapse after stopping the drug regimen would further support the diagnosis.
Limitations:
• Some authors believe that the antibiotic used to treat the blastocystis actually eliminates some other pathogen that has gone undetected.
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