[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 4 The Flagellates
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characteristic that distinguishes flagellates from
the other groups of protozoans. All flagellate life
cycles consist of the trophozoite form. Cysts, on
the other hand, are not known to exist in several
of the flagellate life cycles discussed in this
chapter. The morphologic forms of each flagellate
life cycle are noted individually for each
organism.
The general characteristics of the flagellate
trophozoites are similar to those of the amebic
trophozoites, with one major exception. In those
flagellate life cycles with no known cyst stage,
the trophozoite is considered to be more resistant
to destructive forces, surviving passage into the
stomach following ingestion. In addition, these
trophozoites also appear to survive in the outside
environment. As with the amebas, nuclear characteristics
of trophozoites are basically identical
to those of their corresponding cysts.
In flagellate life cycles that consist of both the
trophozoite and cyst, the processes of encystation
and excystation occur, similar to those of the
amebas. Unlike the amebas, however, flagellates
reside mainly in the small intestine, cecum, colon
and, in the case of Giardia intestinalis, the duodenum.
The flagellate cysts, like those of the
amebas, are equipped with thick, protective cell
walls. These cysts may survive in the outside
environment, just like those of the amebas.
The typical intestinal flagellate life cycle is
similar in process to that of the typical amebas
and thus does not appear under the discussion of
each individual parasite. Only notes of interest
and importance are noted, when appropriate. As
with the amebas, the life cycles of the atrial flagellates
differ from those of the intestinal flagellates.
The atrial flagellate life cycles are, therefore,
discussed on an individual basis in this chapter.
LABORATORY DIAGNOSIS
Stools submitted for parasite study that contain
flagellates may reveal trophozoites and/or cysts.
Like the amebas, flagellate trophozoites are
typically seen in loose, liquid, or soft stool specimens,
whereas flagellate cysts are more common
in formed stools. The morphologic forms seen in
specimens other than stool vary and are discussed
on an individual basis. As in the case of
the amebas, the presence of either or both flagellate
morphologic forms is diagnostic.
Nuclear characteristics, such as number of
nuclei present and the presence and positioning
of the nuclear structures, are helpful in differentiating
the flagellates. Proper identification of
structures specific to select flagellates, such as a
finlike structure connected to the outer edge of
some flagellates known as an undulating membrane
and axostyle (a rodlike support structure
found in some flagellates), is often even more
crucial in determining proper parasite identification.
It is important to note that although the
flagellate trophozoites technically possess flagella,
these structures are not always visible, thus
making the other visible flagellate structures
important identifying features.
The use of saline and iodine wet preparations,
as well as permanent stains, results in the same
benefits in flagellate identification as those
described for the amebas. Again, it should be
noted that the permanent smear procedure may
shrink flagellate parasites, resulting in smaller
than typical measurements. Representative laboratory
diagnostic methodologies are provided in
Chapter 2, as well as in individual parasite discussions,
as appropriate.
Quick Quiz! 4-2
Quick Quiz! 4-1
All flagellate life cycles possess trophozoite and cyst
morphologic forms. (Objectives 4-5A and 4-5B)
A. True
B. False
This flagellate morphologic structure is often not visible
under microscopic examination. (Objective 4-9A)
A. Undulating membrane
B. Pseudopods
C. Flagella
D. Axostyle