INTRODUCTION
Blastocystis species (previously referred to as Blastocystis hominis) are anaerobic protozoan parasites found in the human gastrointestinal tract [1]. The organism was initially discovered in 1911 and for many years was considered to be a harmless yeast. Studies in the 1970s demonstrated that Blastocystis spp are protozoans [2,3].Blastocystis spp are the most common eukaryotic parasites found in human stool specimens, but there is considerable controversy regarding whether they represent a commensal organism or a true pathogen. Foci of particular study are how the presence of Blastocystis spp influences immune inflammatory responses in the gut microbiome and whether it is a potential marker of intestinal dysbiosis.
Issues related to nonpathogenic enteric protozoa are discussed further separately. (See "Nonpathogenic enteric protozoa".)
EPIDEMIOLOGY
Blastocystis spp have been observed worldwide. The organism resides in the colon and cecum of children and adults. The mode of and risk factors for transmission are not fully understood. The parasite is ubiquitous in the environment, and contaminated food, water, and soil are potential sources of infection through faecal-oral transmission [4-7]. The occurrence of Blastocystis spp in environmental water sources may be associated with climatic factors (eg, temperature) and water contamination (coliform count, turbidity, and total dissolved solids) [8-10]. Blastocystis spp have also been found in animals including pigs, monkeys, rodents, and poultry. There seems to be poor host specificity; transmission occurs from human to human and between humans and animals [11-13]. Blastocystis is more commonly found among individuals with occupational exposure to animals, supporting the potential for zoonotic transmission.The prevalence of Blastocystis spp varies between countries and between communities, and according to the diagnostic techniques used [1]. In general, the estimated prevalence of Blastocystis spp is higher in resource-limited settings than resource-rich settings (30 to 50 percent versus 5 to 10 percent, respectively). Prevalence is highly variable, which may be related to poor hygiene, animal exposure, and consumption of contaminated food or water [1]. In one study from Senegal, 100 percent of 93 fecal samples were positive for Blastocystis spp [14]. In a study from Canada, 8 percent of stool samples submitted to a reference laboratory were positive for Blastocystis spp [15]. Blastocystis spp are also commonly found in the stool of returned travelers from resource-limited settings [4,16]. In one study of nearly 2000 stool specimens from travelers in Nepal, the prevalence of Blastocystis spp was 30 percent [17] (see "Travelers' diarrhea: Epidemiology, microbiology, clinical manifestations, and diagnosis" and "Evaluation of fever in the returning traveler"). However, Blastocystis carriage in travelers is highly dynamic. In a study including more than 470 Dutch travelers who had stool samples taken before and after travel, 36 percent carried Blastocystis prior to travel; in 28 percent of these travelers, no Blastocystis or a different subtype was detected following travel [18].