General Characteristics

 Blastocystis hominis comprises a number of different strains that are indistinguishable morphologically; some of which are pathogenic, and some are nonpathogenic. Although usually listed with the amebae, the organism’s classification is still under review; different strains eventually may be classified as different species. Although the true role of this organism in terms of disease has been controversial, it is now generally considered a causative agent of intestinal disease. The current recommendation is to report the presence of B. hominis and quantitate from the permanent stained smear (i.e., rare, few, moderate, many, packed); this information may be valuable in helping to assess the pathogenicity of the organism in the individual patient.

B. hominis consists of four major forms. The cyst form is the most recently described form of the life cycle stages. Thick-walled cysts are thought to be responsible for external transmission through the fecal-oral route; thin-walled cysts are thought to cause autoinfection. Cysts can vary in shape but are mostly ovoid or spherical. The central vacuole form (also referred to as the central body form) is the most common form found in clinical stool samples. The large central vacuole can occupy most of the cellular volume. The amoeboid form is rarely seen. The granular form can be seen in cultures of B. hominis.

Epidemiology

Infection with B. hominis is acquired by the fecal-oral route from infective forms contained in the feces. The organisms can be ingested in contaminated food and drink or acquired from fomites or through various sexual practices that may include accidental ingestion of fecal organisms. As with E. histolytica, flies and cockroaches can be responsible for mechanical transmission. Human-to human and animal-to-human transmission are probably more common than suspected.

B. hominis is a common intestinal parasite of humans and animals, with a worldwide distribution. Depending on the geographic location, it may be detected in 1% to 40% of fecal specimens. B. hominis may be the most common parasite found in the intestinal tract.

Pathogenesis and Spectrum of Disease B. hominis can cause diarrhea, cramps, nausea, fever, vomiting, abdominal pain, and urticaria and may require therapy. A possible relationship between B. hominis and intestinal obstruction and perhaps even infective arthritis has been suggested. In patients with other underlying conditions, the symptoms may be more pronounced. The incidence of this organism appears to be higher than suspected in stools submitted for parasite examination. In symptomatic patients in whom no other etiologic agent has been identified, B. hominis should certainly be considered the possible pathogen. It has been suggested that proteases of genetic subtype 3 could be considered a virulence factor responsible for protein degradation and subsequent pathogenesis.

Laboratory Diagnosis

Routine Methods. Routine stool examinations are very effective in recovering and identifying B. hominis; the permanent stained smear is the procedure of choice, because examination of wet preparations may not easily reveal the organism. If the fresh stool is rinsed in water before fixation (for the concentration method), B. hominis organisms, other than the cysts, are destroyed, and a false-negative report may result. The organisms should be quantitated in the report (i.e., rare, few, moderate, or many). It is also important to remember that other possible pathogens should be adequately ruled out before a patient is treated for B. hominis.

Antigen Detection. Fecal immunoassays to detect B. hominis antigen have been developed but are not yet commercially available. The technique currently used is the enzyme-linked immunosorbent assay (ELISA).

Antibody Detection. ELISA and fluorescent antibody tests have been developed to detect serum antibody to B. hominis infections. A strong antibody response is consistent with the ability of this organism to cause symptoms. Also, demonstration of serum antibody production both during and after B. hominis symptomatic disease is immunologic evidence for the pathogenic role for this protozoan, although it may take 2 years or longer with chronic infections to develop a serologic response.

Therapy

 Although clinical evidence is limited, studies have been done on the in vitro susceptibility of B. hominis to numerous drugs. Currently, metronidazole (Flagyl) appears to be the most appropriate drug. Diiodohydroxyquin (Yodoxin) also has been effective, and dosage schedules for these two drugs are as recommended for other intestinal protozoa. The development of new drug sensitivity assays may improve researchers’ ability to evaluate the activities of various drugs against this organism.

Prevention

Prevention requires improved personal hygiene and sanitary conditions, in addition to proper disposal of fecal material.

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