www2.provlab.ab.ca/.../parasite/ artifact/bhominis.htm

12 Blastocystis hominis - phase contrast x 400  Blastocystis hominis


Parasite: Blastocystis hominis


Disease caused by this parasite: Blastocystosis


Introduction:  A member of the complex group of protozoa called stramenopiles (along with other protistes such as brown algae and diatoms), B. hominis is not proven to be an infectious agent.  Most positive diagnostic tests for B. hominis are asymptomatic.  Complaints from symptomatic cases vary from watery diarrhea and abdominal pain/cramps, to fever and rectal bleeding, although it is argued that such symptoms could be associated with another causal agent. 


Agent:  Blastocystis hominis has previously been considered as yeasts, fungi, or ameboid, flagellated, or sporozoan protozoa.  Recent molecular studies dealing with the sequence information on the complete SSUrRNA gene, however, have placed B. hominis with an informal group: the stramenopiles (Silberman et al. 1996).   Stramenopiles include unicellular and multicellular protistes including brown algae, diatoms, chrysophytes, water molds, slime nets, etc. (Patterson, 1994).



Domain/Superkingdom                    Protista

Subkingdom                                      Protozoa

Phylum                                                Sarcomastigophora

Order                                                  Amoebida (or Blastocystea)

Family                                                 Blastocystidae

Genus                                                 Blastocystis

Species                                              Hominis


Relationship of Blastocystis Species Among Eukaryotes

Relationship of Blastocystis species among eukaryotes as determined by phylogenic analyses of 16S-like rRNA gene sequences. (Silberman, JD, et.al.).



The disease is called Blastocystosis.


History of Discovery:  B. hominis was first described in 1911, but may have been mistaken for “cholera bodies” as early as 1949.  B. hominis was thought to be a yeast or a fungus until 1996, when a small piece of ribosomal RNA analysis placed it in the group of protozoa known as stramenopiles.  Aside from resemblance of its rRNA, B. hominis does not share many similarities with the other stramenopiles.  More recent analysis support the Stramenopile classification.


Clinical Presentation in Humans:

Blastocystosis, as well as its diagnosis, is controversial because it has not been proven that associated symptoms come from Blastocystis hominis itself.  It is argued that many people found to harbor blastocystis might have other sources of their symptoms.   Part of this debate spurs from the fact that blastocystis is found mostly in asymptomatic people, and only a minority experience symptoms.  Those who do claim to experience symptoms related to blastocystosis claim the following:

Transmission: Transmission is thought to be through the fecal-oral route, much like that of other better-known GI protozoa, although there is no experimental confirmation.  It is probable that the infectious form is the cyst form.

Reservoir: None

Vector: None

Incubation Period: not available – disease not studied long enough (most life cycle information is somewhat-researched postulation, so time between entry of the parasite and onset of symptoms of the disease (if it is a disease) is unknown.

Morphology:  The vacuolated form (that is found in stool samples used for diagnostics) is the most common form found in the host.  Unicellular, it is 5-30 microns in diameter, with the usual range being 8-10 microns.  Blastocystis is usually spherical, oval, or ellipsoidal, with usually one, but sometimes two to four nuclei located in the rim of the cytoplasm.  In bi-nucleated cells, the two nuclei might be at opposite poles.  Cells contain a large central body, or vacuole, with a thin rim of cytoplasm around the periphery.  Occasionally, a ring of granules can be found in the cytoplasm, and the cell appears to have a “beaded” rim.

At least five other forms are said to exist, many of which can also be found in the fecal material of infected individuals.  A summary of the morphology of the other five forms follows:


Size (mm)


Central vacuole

Surface coat

Number of nuclei

Other remarks


2 - >200

Culture, feces


Present (thin) or absent


Central vacuole occupies most of cell volume



Culture, feces


Present (thin) or absent


Granules in central vacuole; morphology similar to vacuolar form



Feces, culture


Present (thick)

1 or 2

Multiple small vacuoles (may be too small to resolve by light microscopy)



Intestine, feces



1 or 2

Rarely reported



Feces, culture



1 or 2

Rarely reported; conflicting information on morphology



Feces, culture


Present or absent


Cyst wall present (beneath surface coat)

(Stenzel and Boreham 1996)


Blastocystis can easily be confused with other protozoa found in stool specimens.  The CDC division of parasitic diseases provides the following chart to differentiate Blastocystis from these other protozoa:


Differential Morphology of Protozoa Found in Stool Specimens of Humans: Ciliates, Coccidia, and Blastocystis






Number of Nuclei

Other Features


Balantidium coli



50-70 mm or more. Usual range, 40-50 mm.

Ovoid with tapering anterior end.

Rotary, boring.

1 large, kidney shaped macronucleus. 1 small subspherical micronucleus immediately adjacent to macronucleus.  Macronucleus occasionally visible in unstained preparations as hyaline mass.

Body surface covered by spiral, longitudinal rows of cilia.  Contractile vacuoles are present.


45-65 mm. Usual range, 50-55 mm.

Spherical or oval.


1 large macronucleus visible in unstained preparations as hyaline mass.

Macronucleus and contractile vacuole are visible in young cysts.  In older cysts, internal structure appears granular.


Isospora belli

Oocyst: 25-30 mm. Usual range, 28-30 mm.




Usual diagnostic stage is immature oocyst with single granular mass (zygote) within.  Mature oocyst contains 2 sporocysts with 4 sporozoites each.






Mature oocysts with thin wall collapsed around 2 sporocysts or free fully mature sporocysts with 4 sporozoites inside are usually seen in feces.


13-17 mm. Usual range, 14-16 mm.


11-15 mm.
Usual range, 12-13 mm.


Oocyst: 3-6 mm.
Usual range, 4-5 mm.

Spherical or oval.



Mature oocyst contains 4 "naked" sporozoites.  No sporocysts are present.


Blastocystis hominis2


Vacuolated Form

5-30 mm.
Usual range, 8-10 mm.

Spherical, oval, or ellipsoidal


1, usually, but 2-4 may be present.  Located in "rim" of cytoplasm.  In binucleated organisms, the 2 nuclei may be at opposite poles.  In quadri- nucleated forms, the 4 nuclei are evenly spaced around periphery of cell.

Cell contains large central body, or "vacuole" with a thin band, or "rim" of cytoplasm around the periphery.  Occasionally a ring of granules may be seen in cytoplasm and the cell appears to have a "beaded rim".


Center for Disease Control – Division of Parasitic Diseases.  http://www.dpd.cdc.gov/dpdx/HTML/MorphologyTables.htm.


Life Cycle:  The life cycle of blastocystis remains poorly understood.  Through experimentation done by Stenzel and Boreham (5), it has been suggested that binary fission is the only possible means of reproduction.  The avacuolar cell, without a surface coat, is swallowed and travels to the intestines.  As it travels through the intestines, it morphs into its multivacuolar form, which has a thick surface coat.  The cyst develops beneath the coat, which then sloughs off.  The cyst is the probable infectious agent, and the cycle begins again with ingestion of the cyst.  Excystation, probably induced by stomach acids, changes the cyst back into the avacuolar cell without a surface coat found in the intestines in the beginning of the life cycle.  An amoeboid form is thought to exist, but its place in the life cycle is not well known, and possibly arises from the avacuolar form.

Proposed life cycle for B. hominis (Stenzel and Boreham 1996)

Diagnostic Tests:  B. hominis is usually diagnosed by microscopic examination of fecal material stained with iodine or trichrome.  Permanent stained smear is preferred because fecal debris might be mistaken for the organism in wet preparations.  Both ELISSA and fluorescent-antibody tests have been shown to detect the serum antibody in a limited number of tests.

Although the most prominent form in the feces is the vacuolar form, all forms have been found.

www2.provlab.ab.ca/.../parasite/ artifact/bhominis.htm                                               www2.provlab.ab.ca/.../parasite/ artifact/bhominis.htm                                               www2.provlab.ab.ca/.../parasite/ artifact/bhominis.htm

40 - B. hominis - Trichrome stain                2 B. hominis - phase contrast x 400                9 B hominis cysts - phase contrast x 400

Granular form (Trichrome stain x 1000)             Phase contrast microscopy x 400                      Wet mount shows spherical cyst form

CDC DPDx Image Library

Blastocystis hominis, iodine stain

Cyst form stained in iodine

Management  and Therapy:   Many believe that this disease is self-limiting and therefore should not be treated.  However, upon diagnosis with the disease, patients are usually treated with Metronidazole, which has been effective, but studies have also suggested resistance to this drug. 

Epidemiology: This disease is not well-tracked, but appears to occur worldwide.  Originally reported as being associated with diarrhea in the tropics and subtropics, more recent reports have show that infections are common in residents of tropical, subtropical, and developing countries.  Immigrants, refugees, and adopted children from developing countries seem to have a higher incidence of infection than adults and children raised from birth in their new community do.  Numerous studies have provided limited evidence to support that B. hominis is frequently acquired during travel to tropical countries.  Lower socioeconomic groups or those with lower standards of hygiene have been shown to have higher prevalence rates than the rest of the community.  Young adults appear to have the highest rate of infection.  E. nana and E. coli show age prevalence similar to those of B. hominis.  Limited studies are available on the prevalence and significance of B. hominis infections in immunocompromised patients. 

Country Information: B. hominis may be more common in tropical countries, and is certainly more prevalent in poorer countries.  Poorer areas have prevalence rates of 30-50%, compared to 1-10% in wealthier parts of the world.

Public Health and Prevention Strategies:  Crowded areas and unsanitary conditions breed higher rates of infection.  Education in personal hygiene and improvement in community sanitary facilities have been suggested as good ideas for public health involvement, but because of the unconfirmed infective nature of the disease, such efforts have not been reported as of yet.

The CDC lists the following ways to prevent infection with B. hominis:

Useful Web Links: 


-A study done on Blastocystis hominis by a Berkeley student.  Very helpful, comprehensive, and well-summarized information.



-Center for Disease Control and Prevention (CDC) – Division of Parasitic Diseases (DPDx) – Laboratory Identification of Parasites of Public Health Concern – another very well-summarized and comprehensive website with information organized by the following sections: causal agent, life cycle, geographic distribution, clinical features, laboratory diagnosis, and treatment.



-A website to assist microscopists in the differentiation of clinically important parasites from artifact material regularly seen in fecal samples and specimens from other body sites.  Good pictures of different types of stains for B. hominis diagnosis.



-Fact Sheet distributed by the CDC – practical information about blastocystis hominis and blastocystosis for the everyday person





1.  Arisue N. et al.(2002). Phylogenetic Position of Blastocystis hominis and of Stramenopiles Inferred from Multiple Molecular Sequence Data. Journal of Eukaryotic Microbiology 2002: 49, No.1 42-53.


2.  Centers for Disease Control and Prevention

3.  Patterson DJ. Protozoa, evolution and systematics. In: Housmann K, Hulsmann N, editors. Progress in Protozoology. Stuttgart: Fischer; 1994. p. 1-14.

4.  Silberman JD, Sogin ML, Leipe DD, Clark CG. Human parasite finds taxonomic home. Nature 1996;380:398.

5.  Stenzel, DJ et al. (1996) Blastocystis hominis revisited. Clinical Microbiology Reviews. Oct: 563-584.

6.      Nelson, Kara L.  Department of Civil & Environmental Engineering, UC Berkeley.  CE210A: Control of Water-related Pathogens http://www.ce.berkeley.edu/~nelson/ce210a/Blastocystis/Blastocystis.htm


This Page was created by:

Jennifer Miller (jennm@stanford.edu)

Parasites & Pestilence: Infectious Public Health Challenges

Dr. Scott Smith

Stanford University