[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 7 Miscellaneous Protozoa
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Nucleii
Vacuole
Size range: 5-32 m
Average size: 7-10 m
FIGURE 7-11 Blastocystis hominis vacuolated form.
FIGURE 7-12 Trichrome stain, 1000×. Typical Blastocystis
hominis vacuolated form.
common form seen and the easiest to recognize
is the vacuolated form. Therefore, only this form
will be described here.
Vacuolated Forms. Although the vacuolated
form of B. hominis may range in size from 5 to
32 µm, the average form measures only 7 to
10 µm (Figs. 7-11 and 7-12; Table 7-6). This
morphologic form is characterized by a large,
central, fluid-filled vacuole that consumes almost
90% of the cell. The remaining 10% assumes the
periphery of the organism; it consists of a ring of
cytoplasm in which two to four nuclei are typically
present.
TABLE 7-6
Parameter Description
Size 5-32 µm
Vacuole
Centrally located
Fluid-filled structure
Consumes almost 90% of organism
Cytoplasm Appears as ring around periphery
of organism
Nuclei
Two to four located in cytoplasm
Laboratory Diagnosis
Stool is the specimen of choice for the recovery
of Blastocystis. In iodine wet preparations, the
peripheral cytoplasm containing one or more
nuclei appears a light yellow in color, whereas
the central vacuole does not stain and appears
clear and transparent. In permanent stain preparations,
the central vacuole may vary in its ability
to stain from not at all to very apparent. The
nuclei located in the peripheral cytoplasm in
these preparations typically stain dark. It is
important to note here that saline, like water,
usually lyses this organism and may lead to a
false-negative result. Therefore, it is important to
screen suspicious samples with an iodine wet
preparation and to use a permanent stain to
confirm the presence of the parasite.
Life Cycle Notes
B. hominis reproduces by sporulation or binary
fission. The organism passes through a number
of morphologic forms during these processes. B.
hominis participates in sexual and asexual reproduction,
and exhibits pseudopod extension and
retraction. A detailed discussion of the B. hominis
life cycle has not been widely presented.
Epidemiology
Blastocystis hominis
Vacuolated Form: Typical
Characteristics at a Glance
Early nonscientific documentation of B. hominis
infections indicated that they occurred as epidemics
in subtropical countries. Select articles on
B. hominis over the past 10 to 25 years or so
suggest that this organism may be found in a