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©2013 UpToDate ® Print Email Blastocystis stages Knowledge of the life cycle and transmission is still under investigation, therefore this is a proposed life cycle for B. hominis . The classic form found in human stools is the cyst, which varies tremendously in size from 6 to 40 µm (1) . The thick-walled cyst present in the stools (1) is believed to be responsible for external transmission, possibly by the fecal-oral route through ingestion of contaminated water or food (2) . The cysts infect epithelial cells of the digestive tract and multiply asexually (3,4) . Vacuolar forms of the parasite give origin to multi-vacuolar (5a) and ameboid (5b) forms. The multi-vacuolar develops into a pre-cyst (6a) that gives origin to a thin-walled cyst (7a) , thought to be responsible for autoinfection. The ameboid form gives origin to a pre-cyst (6b) , which develops into thick-walled cyst by schizogony (7b) . The thick-walled cyst is excreted in feces (1) . Originally published in: Singh M, Suresh K, Ho LC, Ng GC, Yap EH. Elucidation of the life cycle of the intestinal protozoan Blastocystis hominis. Parasitol Res 1995; 81:449. Reproduced with kind permission from Springer Science + Business Media B.V. Reproduced from: Centers for Disease Control and Prevention. Parasites and Health: Blastocystis hominis infection. Available at: file://www.dpd.cdc.gov/dpdx/html/Blastocystis.htm .