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Continued controversy exists about Blastocystis hominis. Long considered a protozoan of worldwide distribution, most likely an amoeba, this strict anaerobe has been classified by small subunit rRNA gene analysis into the heterogeneous group of protists, the Stramenopiles,1 which also includes diatoms and brown algae (kelp). Whether there are single or multiple species is unresolved. No definite life cycle has been delineated. Transmission is presumed to occur by the fecal-oral route from contaminated food or water. The organism’s pathogenicity remains controversial,2-4 and asymptomatic infection is common. Some B hominis cause disease rather than colonization when present in large numbers in the absence of other stool pathogens. Others consider B hominis an enteric commensal and ascribe response to treatment as elimination of other undetected stool pathogens or resolution of noninfectious etiology. Others suggest it is an opportunistic pathogen.

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The most commonly reported symptoms are nonspecific: nausea, mild diarrhea, vomiting, flatulence, and abdominal cramping. Fever, weight loss, and stools with blood, mucus, or leukocytes are uncommon. The ability to cause invasive disease is controversial. Controlled studies fail to confirm a true pathogenic role, although this confusion may be attributed to pathogenic and nonpathogenic ribodeme types, similar to Entamoeba histolytica and E dispar.5 However, it is also difficult to generate valid control groups, because most stools submitted are obtained from symptomatic patients. Identification of B hominis by trichrome staining is complicated by the organism’s great morphological diversity. Other diagnostic approaches used to detect the organism in human stool (indirect fluorescent antibody and, more recently, polymerase chain reaction6,7) or host serological response (ELISA) are not readily available.

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Most patients, adult or pediatric, immunocompetent or immunocompromised, resolve symptoms spontaneously. Some experts recommend that if, after thorough search for other stool pathogens (parasites, bacteria, viruses) and for noninfectious etiologies, the symptoms are protracted and B hominis is found in multiple stool specimens, empirical treatment with metronidazole, iodoquinol, or trimethoprim sulfamethoxazole at antiprotozoan doses can be considered.2,4

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2. Leber AL. Intestinal amebae. Clin Lab Med. 1999;19:601-619.  [PubMed: 10549428]
3. Leder K, Hellard ME, Sinclair MI, Fairley CK, Wolfe R. No correlation between clinical symptoms and Blastocystis hominis in immunocompetent individuals. J Gastroenterol Hepatol. 2005;20:1390-1394.  [PubMed: 16105126]
4. Stenzel DJ, Boreham PFL. Blastocystis hominis revisited. Clin Microbiol Rev. 1996;9:563-584.  [PubMed: 8894352]
5. Kaneda Y, Horiki N, Cheng XJ, et al. Ribodemes of Blastocystis hominis isolated in Japan. Am J Trop Med Hyg. 2001;65:393-396.  [PubMed: 11693890]
6. Stensvold R, Brillowska-Dabrowska A, Nielsen HV, Arendrup MC. Detection of Blastocystis hominis in unpreserved stool specimens by using polymerase chain reaction. J Parasitol. 2006;92:1081-1087.  [PubMed: 17152954]
7. Libman MD, Gyorkos TW, Kokoskin E, ...

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