Blastocystis hominis: epidemiology and natural history.

To study the demographic profile of Blastocystis hominis carriers from Hamilton, Canada, the Regional Parasitology Laboratory records for 1988 were reviewed, and a prospective study on carriers was conducted to clarify the natural history of the infection and ascertain the role of B. hominis as an intestinal pathogen. Retrospective analysis revealed that 8% of stool samples harbored B. hominis. The median age of the carriers was 37 years; 55% were female. Prospective analysis of 139 patients showed that most (76%) of 86 in whom B. hominis was the sole organism found (and for whom data were complete) continued to harbor the parasite in stool samples submitted a median of 57 days after the first sample. There was no correlation between the presence of B. hominis and symptoms. Thus, B. hominis, though commonly seen in stool samples submitted to this laboratory, is thought to be a commensal organism.