Clinical Features:
Whether Blastocystis hominis can cause symptomatic infection in humans is a point of active debate.  This is because of the common occurrence of the organism in both asymptomatic and symptomatic persons.  Those who believe symptoms could be related to infection with this parasite have described a spectrum of illness including watery diarrhea, abdominal pain, perianal pruritus, and excessive flatulence.

Laboratory Diagnosis:
Diagnosis is based on finding the cyst-like stage in feces.  Permanently stained smears are preferred over wet mount preparations because fecal debris may be mistaken for the organisms in the latter.  Do not wash specimens in water (e.g., during concentration procedures) as this will lyse the organisms, resulting in false negatives.

Diagnostic findings

• Microscopy
• Morphologic comparison with other intestinal parasites
• Bench aid for Blastocystis

Treatment:
Despite the controversial clinical significance of this organism, metronidazole or iodoquinol has been reported to be effective.  For additional information, see the recommendations in The Medical Letter (Drugs for Parasitic Infections).