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Blastocystis Species
Blastocystis is a protist that parasitizes the lower intestine of humans and a vast range of animals. Although Blastocystis has been implicated as a cause of various gastrointestinal tract symptoms, its role as a pathogen, mechanism of transmission, and life cycle have been subject to much debate. A clinical syndrome of self-limited intestinal symptoms has been described in several series of predominantly adult patients, with infection intensity possibly accounting for differences in clinical presentation. Extraintestinal manifestations, such as urticaria, also have been described. , Blastocystis spp. from humans, other mammals, and birds are classified into subtypes, but because non−DNA-based laboratory methods do not enable distinction between subtypes, the organism should be reported as Blastocystis sp., not as Blastocystis hominis.
Organism
Blastocystis is a protist belonging to the Stramenopiles. It has been isolated from a wide variety of animals and has been reported in human populations from most parts of the world. A total of 9 subtypes (arguably species) have been identified in humans to date, with humans colonized most commonly by subtypes 1, 2, 3, and 4, depending on geographic region. , , Several morphologic stages have been described, including the vacuolar, avacuolar, multivacuolar, granular, ameboid, and cystic stages. The vacuolar form is observed most frequently in clinical samples and in fecal cultures; it is spherical and usually ranges in size from 4–15 μm in diameter. The cyst (2–5 μm) is the transmissible form and is able to survive in water at room temperature for ≤19 days but is fragile at extremes of heat and cold and when exposed to common disinfectants. After ingestion of an infectious cyst, the organism excysts in the lumen of the cecum and proximal colon, developing into vacuolar forms and undergoing encystation before being excreted as the cyst form, which, if eaten, completes the cycle.
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