www2.provlab.ab.ca/.../parasite/
artifact/bhominis.htm
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).
Classification:
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.).
Synonyms:
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:
- Watery diarrhea
- Abdominal pain / cramps
- Perianal pruritis (itch)
- Excessive flatulence
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:
|
Form |
Size (mm) |
Source |
Central vacuole |
Surface coat |
Number of nuclei |
Other remarks |
|
Vacuolar |
2 -
>200 |
Culture,
feces |
Present |
Present
(thin) or absent |
1-4 |
Central
vacuole occupies most of cell volume |
|
Granular |
6.5-80 |
Culture,
feces |
Present |
Present
(thin) or absent |
1-4 |
Granules
in central vacuole; morphology similar to vacuolar form |
|
Multivacuolar |
5-8 |
Feces,
culture |
Absent |
Present
(thick) |
1 or 2 |
Multiple
small vacuoles (may be too small to resolve by light microscopy) |
|
Avacuolar |
~5 |
Intestine,
feces |
Absent |
Absent |
1 or 2 |
Rarely
reported |
|
Amoeboid |
2.6-7.8 |
Feces,
culture |
Absent |
Absent |
1 or 2 |
Rarely reported;
conflicting information on morphology |
|
Cyst |
3-10 |
Feces,
culture |
Absent |
Present
or absent |
1-4 |
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 |
|
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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
Granular form (Trichrome stain x 1000) Phase contrast microscopy x 400 Wet mount shows spherical cyst form
CDC DPDx Image Library
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:
- Wash hands with soap and water after using the
toilet and before handling food.
- Avoid water or food that may be contaminated.
- Wash and peel all raw vegetables and fruits
before eating.
- When traveling in countries where the water
supply may be unsafe, avoid drinking un-boiled tap water and avoid
uncooked foods washed with un-boiled tap water. Bottled or canned carbonated
beverages, seltzers, pasteurized fruit drinks, and steaming hot coffee and
tea are safe to drink.
- If you work in a childcare center where you change diapers, be sure
to wash your hands thoroughly with plenty of soap and warm water after
every diaper change, even if you wear gloves.
Useful Web Links:
http://www.ce.berkeley.edu/~nelson/ce210a/Blastocystis/Blastocystis.htm
-A study done on
Blastocystis hominis by a Berkeley student.
Very helpful, comprehensive, and well-summarized information.
http://www.dpd.cdc.gov/dpdx/HTML/Blastocystis.htm
-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.
http://www2.provlab.ab.ca/bugs/webbug/parasite/artifact/bhominis.htm
-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.
http://www.cdc.gov/ncidod/dpd/parasites/blastocystishominis/factsht_blastocystis_hominis.htm
-Fact Sheet distributed by the
CDC – practical information about blastocystis hominis and blastocystosis for
the everyday person
Resources:
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