Blastocystis hominis continues to be a subject of controversy. It is the most common single-celled organism detected in human stool samples worldwide. Long considered a protozoan of worldwide distribution, this strict anaerobe has been classified by small subunit rRNA gene analysis into the heterogeneous group of protists, the Stramenopiles, which also includes diatoms and brown algae (kelp). It is most likely an amoeba.
PATHOGENESIS AND EPIDEMIOLOGY
To date, as many as 17 subtypes of Blastocystis species have been isolated. Prevalence greater than 5% in better-resourced countries and as high as 76% in under-resourced countries has been reported. The life cycle of B hominis has not been elucidated clearly. Infectivity studies in mice have shown that transmission occurs via the fecal-oral route by a cyst form. These cysts multiply in the epithelial cells of the digestive tract to form vacuolar and amoeboid forms. The vacuolar forms multiply by binary fission and other modes such as budding and ultimately undergo encystment to form the infective cysts, which are shed in the feces of the host. Blastocystis may exert its pathologic effects via increasing intestinal permeability, epithelial barrier degradation, and cytokine release from colonic cells, although controversy still exists regarding its exact mechanism. Asymptomatic infection is common; however, some B hominis subtypes 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.
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 Entamoeba dispar. However, it is also difficult to generate valid control groups, because most stools submitted are obtained from symptomatic patients.
Diagnosis can be made by visualization of cysts by direct microscopy and with special stains such as Lugol’s iodine and trichome staining of stool specimens (Fig. 339-1). Identification by trichome staining is limited by the organism’s great morphologic diversity and the time-consuming nature of the staining procedure. Other diagnostic approaches used to detect the organism in human stool include in vitro culture, indirect fluorescent antibody, host serologic response (enzyme-linked immunosorbent assay), and more recently, polymerase chain reaction (PCR). In vitro culture is considered to be the gold standard for diagnosis of B hominis. However, direct fluorescent antibody testing is reported to be rapid, practical, and equally sensitive to culture methods. PCR can detect subtypes of isolate; however, it is extremely costly and nonstandardized.