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Organism Index (A-Z)

Blastocystis spp. (Pathogen)

This organism is a member of the Stramenopile group (diatoms, brown algae, chrysophytes, xanthophytes)(may be reclassified in its own group) is a potential pathogen in some patients, and can cause disease.  The central body form is most commonly seen (usual size, 6-40 µm) and can be found in clinical specimens.  What is called “Blastocystis hominis” is a group of subtypes or species, about half of which are pathogenic and half are nonpathogenic.  Pathogenicity differences cannot be determined on the basis of morphology.

Permanent stained slide: Central Body form         Iodine: Central Body form

Life Cycle:
Intestine, organisms passed in feces

Fecal-oral transmission via central body form; contaminated food and water

Worldwide, primarily human-to-human transmission. Blastocystis appears to be worldwide the most common parasite isolated from stool, with Giardia and Dientamoeba switching between second and third in terms of incidence.

Clinical Features:
Infection with Blastocystis may be the cause of diarrhea, cramps, nausea, fever, vomiting, and abdominal pain and may require therapy.  The incidence of this organism appears to be higher than suspected in stools submitted for parasite examination; it is considered the most common protozoan worldwide (review of published literature). In symptomatic patients in whom no other etiologic agent has been identified, Blastocystis should certainly be considered the possible pathogen. When a symptomatic Blastocystis infection responds to therapy, the improvement may also represent elimination of some other undetected pathogenic organism (E. histolytica, G. lamblia, D. fragilis).

Clinical Specimen:
Intestinal:  Stool

Laboratory Diagnosis:
Intestinal:  Ova and Parasite examination (concentration, permanent stained smear); identification based on morphology; fecal immunoassay currently under development.

Organism Description:
Central body form:  Central clear area (can be clear, or stain red/green/blue with trichrome stain); small nuclei are around the outside of the clear area.

Laboratory Report:
Blastocystis hominis (indicate quantity:  rare, few, mod, many); quantity may be linked to presence or absence of symptoms; quantitation should be performed on the permanent stained slide.

Report Comment: 
The name Blastocystis hominis contains approximately 10 different organisms, none of which can be differentiated on the basis of organism morphology; some are pathogenic and some are non-pathogenic.  If no other pathogens are found, Blastocystis may be the cause of patient symptoms
Other organisms capable of causing diarrhea should also be ruled out.

Garcia, L.S. 2016.  Diagnostic Medical Parasitology, 6th ed., ASM Press, Washington, D.C.

Improved hygiene, adequate disposal of fecal waste, adequate washing of contaminated fruits and vegetables

There are multiple subtypes, thus providing an explanation for why some patients are symptomatic and some are asymptomatic; if multiple species are confirmed, some may be pathogenic and some nonpathogenic.  This is probably the most common intestinal parasite found in humans, and is often seen in a higher percentage of patients (as compared with other intestinal protozoa).