[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 3 The Amebas
life cycle notes does not appear under the discussion
of each individual parasite. Only notes of
interest or importance are noted, when appropriate.
The life cycles of the extraintestinal ameba
differ from those of the intestinal amebas; these
are discussed individually in this chapter.
Quick Quiz! 3-1
Amebas transform from trophozoites to cysts on
entry into an unsuspecting human. (Objective 3-5A)
A. True
B. False
LABORATORY DIAGNOSIS
Amebic trophozoites as well as cysts may be seen
in stool samples submitted for parasite study.
Trophozoites are primarily recovered from stools
that are of soft, liquid, or loose consistency.
Formed stool specimens are more likely to
contain cysts. The morphologic forms present in
samples other than stool are noted on an individual
basis. It is important to point out that the
presence of either or both morphologic forms is
diagnostic.
Proper determination of organism size, using
the ocular micrometer (see Chapter 2), is essential
when identifying the amebas. The appearance
of key nuclear characteristics, such as the
number of nuclei present and the positioning of
the nuclear structures, is crucial to differentiate
the amebas correctly. The presence of other
amebic structures and characteristics, such as
cytoplasmic inclusions and motility, also aids in
the identification of the amebas. Standard microscopic
procedures include examination of specimens
for amebas using saline wet preparations,
iodine wet preparations, and permanent stains.
The saline wet preparation and iodine wet
preparation each have an advantage that supports
their use. Saline wet preparations are of
value because they will often show motility of
the amebic trophozoites. The internal cytoplasmic,
as well as the nuclear structures, may be
more readily seen with the use of iodine wet
preparations.
It is important to note that permanent smear
procedures of samples suspected of having
amebas must be performed to confirm parasite
identification. In most cases, the key identifying
characteristics cannot be accurately distinguished
without the permanent stain. The permanent
stain allows for many of the otherwise refractive
and invisible structures to be more clearly visible
and thus easier to identify. The permanent smear
procedure may, however, shrink amebic parasites,
causing measurements smaller than those
typically seen in wet preparations.
Alternative laboratory diagnosis techniques
are available for three of the amebas covered in
this chapter—Entamoeba histolytica, Naegleria
fowleri, and Acanthamoeba spp. Representative
laboratory diagnostic methodologies are presented
in Chapter 2, as well as in each individual
parasite discussion, as appropriate.
Quick Quiz! 3-2
Formed stool specimens are more likely to contain
which of the following? (Objective 3-8)
A. Trophozoites
B. Cysts
PATHOGENESIS AND
CLINICAL SYMPTOMS
A number of patients infected with intestinal
amebas are asymptomatic. Amebas are often discovered,
however, in patients suffering from diarrhea
without an apparent cause. Diagnosis of
nonpathogenic amebas is important because this
finding suggests that the ingestion of contaminated
food or drink may have occurred. The
presence of pathogenic amebas in addition to
nonpathogenic amebas is also possible because
the transmission route of both amebic groups is
identical. Thorough screening of such samples is
crucial to ensure proper identification of all parasites
that may be present. Infection is most
common in people who live in underdeveloped