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The obligately anaerobic "Blastocystis hominis" was first described a century ago. Because it may take many different morphological forms in infected individuals, its taxonomic affiliations were uncertain for many years, but molecular systematic study has established that it belongs in the protist group known as the stramenopiles. Silberman et al. (1996) discussed the biological characteristics and phylogenetic placement of this protist. Because of the poorly understood genetic diversity of these parasites, many researchers now refer to Blastocystis or Blastocystis spp. rather than B. hominis (Santin et al. 2011). Tan (2004) reviewed the biology, epidemiology, and management of Blastocystis, but noted that its pathogenicity, its life cycle, and the function of certain of its organelles remained poorly understood, in part because of the lack of a suitable animal host model with which to pursue research on this parasite. Blastocystis infections have a worldwide distribution, but prevalence is highest in areas with poor hygiene and deﬁcient sanitation services and facilities (Tan 2004).
Blastocystis is among the most prevalent single-celled intestinal parasites worldwide, but the clinical significance of its presence remains unclear, despite numerous studies to investigate its prevalence in different populations and its possible role in gastro- and extra-intestinal disease. There are some indications that the clinical significance of infection may depend on the genetic subtype of the infecting organisms. (Stensvold et al. 2009)