[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 7 Miscellaneous Protozoa
Double
layered
cell wall
TABLE 7-3
Two mature sporocysts
(each contains
4 sporozoites)
Size range: 25-35 m by 10-15 m
Average size: 30 m by 12 m
FIGURE 7-7 Isospora belli oocyst.
Isospora belli Oocyst:
Typical Characteristics at
a Glance
Parameter
Description
Size range 25-35 µm long, 10-15 µm
wide
Appearance
Transparent
Shape
Oval
Cell wall
Two layered, colorless and
smooth
Developing sporoblast Unicellular with granular
cytoplasm
Young oocyst
Two sporoblasts
Mature oocyst
Two sporocysts, each
containing four
sausage-shaped
sporozoites
morphologic form within the oocyst, known as
a sporoblast, consists of a roundish immature sac
that contains a small discrete nucleus and granular
cytoplasm. As it matures, the young oocyst
divides into two sporoblasts. Each sporoblast
continues to mature and eventually becomes a
sporocyst, which consists of a mature roundish
sac containing four sausage-shaped sporozoites.
This stage is known as the mature oocyst (Fig.
7-7). Throughout its development, the sporoblast
and sporocysts are surrounded by a smooth,
colorless, two-layered cell wall.
Laboratory Diagnosis
The specimens of choice for recovery of the I.
belli oocysts are fresh feces and duodenal contents.
Stool samples may contain oocysts that are
immature, partially mature, and/or fully mature.
In addition, material collected via an Enterotest
may also be used to obtain the oocysts. Intestinal
biopsies collected from infected patients may
reveal the intracellular morphologic stages of the
organism. It is interesting to note that a biopsy
from an infected patient may contain I. belli
oocysts, whereas a stool specimen from the same
patient may be free of the parasites. This occurs
particularly in patients who have only small
numbers of organisms present.
I. belli oocysts may be visible in direct wet
preparations and in those made following the concentration
or flotation procedures. Promising
results have been obtained on stool specimens processed
using the Sheather’s sugar flotation procedure.
It is important to note that I. belli oocysts
appear transparent and may be difficult to recognize
when present in saline wet preparations. The
oocysts are more readily discernible in iodine preparations.
It is therefore important to include an
iodine wet preparation in the standard processing
of samples for parasite study, particularly those in
which I. belli is suspected. In addition, a decreased
microscope light level and proper contrast are necessary
when screening suspicious slides to achieve
the most favorable conditions for parasite recovery.
This is particularly true when screening
samples that have been tested by the zinc sulfate
technique or another concentration procedure following
polyvinyl alcohol (PVA) preservation.
A tentative diagnosis may be made following
preparation and examination of an auraminerhodamine
permanent stain. However, the recommended
permanent stain for successful I. belli
oocyst confirmatory identification is a modified
acid-fast stain. This clearly shows the organism’s
characteristics, as well as those of Cryptosporidium
oocysts, another important member of
the sporozoa (see later). Wet preparations for
Isospora may also serve as adequate confirmatory
tests when necessary.