Current Views on the Clinical Relevance of Blastocystis spp. Kevin S. W. Tan & Haris Mirza & Joshua D. W. Teo & Binhui Wu & Paul A. MacAry Published online: 27 January 2010 # Springer Science+Business Media, LLC 2010 Abstract Blastocystis is an enteric protistan parasite of uncertain clinical relevance. Recent studies indicate that the parasite is a species complex and humans are potentially hosts to nine Blastocystis subtypes, most of which are zoonotic. Subtype 3 is the most common in prevalence studies, followed by subtype 1. Laboratory diagnosis is challenging; the currently recommended diagnostic ap- proach is trichrome staining of direct smears coupled with stool culture. Polymerase chain reaction testing from stools or culture is useful for determining Blastocystis subtype information. The controversial pathogenesis of Blastocystis is attributed to subtype variations in virulence; although current studies seem to support this idea, evidence suggests other factors also contribute to the clinical outcome of the infection. Clinical signs and symptoms of blastocystosis include abdominal pain, diarrhea, bloating, and flatulence. Extraintestinal manifestations, predominantly cutaneous, also were reported. In vitro and animal studies shed new light on the pathobiology of Blastocystis. Keywords Blastocystis . Pathogenesis . Subtypes . Diagnosis . Prevalence Introduction Blastocystis is an unusual protistan enteric parasite classified under a highly diverse group of organisms called strameno- piles, and is the only known member of this group associated with human pathology [1••]. Blastocystis was first described in the early 1900s but continues to be a controversial organism. A renewed interest in this parasite resulted in the revision of its classification, appreciation of its genetic diversity and zoonotic potential, refinement of molecular diagnostic techniques, and development of Blastocystis–host interaction models [1••]. Historically, similar numbers of studies either implicated or exonerated Blastocystis as a cause of human disease, but in recent years an increasing number of reports suggest that Blastocystis is an emerging microbial pathogen [1••]. The discrepancies found in the literature largely result from use of nonstandardized diag- nostic techniques, difficulty in identifying the parasite in the clinical laboratory setting, small sample sizes, and in some cases, misinterpretation of data. Additionally, the parasite’ s extensive genetic diversity probably contributes to the underestimation of its clinical significance. Recent studies that take the genetic diversity of this organism into account suggest that its association with several disorders is subtype- dependent [2–5]. At least nine subtypes (genotypes) of Blastocystis have been described on the basis of small subunit ribosomal RNA gene analysis [6•], and nonhuman primates, mammals, and birds appear to be reservoir hosts for most subtypes (Table 1). In this regard, the use of the term Blastocystis hominis is no longer appropriate; instead, the parasite should be referred as Blastocystis spp or Blastocystis spp subtype n, where n is the subtype number according to the Stensvold classification [6•]. Clinical Presentation Human infections with Blastocystis are called “blastocys- tosis.” A variety of signs and symptoms are associated with blastocystosis, ranging from nonspecific intestinal symp- toms to cutaneous disorders. K. S. W. Tan(*) : H. Mirza : J. D. W. Teo : B. Wu : P. A. MacAry Laboratory of Molecular and Cellular Parasitology, Department of Microbiology, Yong Loo Lin School of Medicine, National University of Singapore, 5 Science Drive 2, Singapore 117597, Singapore e-mail: Kevin_Tan@nuhs.edu.sg Curr Infect Dis Rep (2010) 12:28–35 DOI 10.1007/s11908-009-0073-8