Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. PARASITOLOGY Blastocystis and its pathogenic potential: latest aspects Constantine M. Vassalos a,b , Nicholas Vakalis a and Chryssanthy Papadopoulou b Blastocystis is a ubiquitous intestinal protozoan isolated from human and other animals and is frequently found in human stools. Various morphological forms and wide genetic diversity have been observed. Research results have been gradually unraveling the conundrums of parasite’s life cycle and pathogenicity. The presence of amoeboid forms in Blastocystis xenic culture might facilitate checking for possible pathogenic isolates between or within subtypes, probably under specific conditions. This review focuses on the latest aspects in Blastocystis pathogenicity in association with interspecific or intraspecific variation. We also focus on differences in morphology, surface properties and between amoeboid and nonamoeboid forms and other putative virulence factors. Asymptomatic infections have been reported; immunocompromised individuals might have diarrhoeal disease related to Blastocystis; and immununocompetent patients with Blastocystis might present with irritable bowel syndrome-like symptoms, skin and other extraintestinal manifestations. Drug susceptibility testing is not currently available; metronidazole is the drug of choice, but resistance has developed. Other therapeutic options include nitazoxanide and trimethoprim – sulfomethoxazole. Paromomycin was reported to be successful in treating urticarial manifestations of Blastocystis infection. Further investigations are required to study virulence regulation at genetic level or by other host factors and to identify new targets for drugs. ß 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Reviews in Medical Microbiology 2008, 19:87–97 Keywords: amoeboid forms, Blastocystis, irritable bowel syndrome-like, metronidazole, pathogenic potential, resistance, subtype 3, utricaria, virulence Introduction Blastocystis is anaerobic intestinal protozoan of humans, other animals and arthropods [1]. Recently, it was demonstrated that Blastocystis could be placed within the stramenopiles, and isolates could be separated into 10 subtypes (1–10) according to phylogenetic trees con- structed from ssrRNA sequences [1,2]. Subtype 3 has been the most common Blastocystis genotype infecting humans worldwide followed by subtype 1 [3]. For the most part of the last century, this ubiquitous organism, appearing more often in tropical and subtropical countries, was thought to be a harmless inhabitant of the human gut, though it has frequently been found in human stool samples [4]. The lack of a suitable animal model has hampered the elucidation of pathogenesis and a Department of Parasitology, Entomology and Tropical Diseases, National School of Public Health, Athens, and b Department of Microbiology, Medical School, University of Ioannina, Ioannina, Greece. Correspondence to Constantine M. Vassalos, Department of Parasitology, Entomology and Tropical Diseases, National School of Public Health, Alexandras Avenue, 196, GR-115 21, Athens, Greece. Tel: +30 6972025952; fax: +30 2107625322; e-mail: conmiva@hotmail.com Received: 24 January 2009; accepted: 26 January 2009. DOI:10.1097/MRM.0b013e32832d87ec ISSN 0954-139X Q 2009 Wolters Kluwer Health I Lippincott Williams & Wilkins 87