[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 3 The Amebas
Prevention and Control
Several steps may be taken to prevent E. histolytica
infections. Uncontaminated water is essential;
this may be accomplished by boiling or
treating with iodine crystals. It is interesting to
note that the infective (quadrinucleated) cyst is
resistant to routine chlorination. A water treatment
regimen that includes filtration and chemical
treatment is necessary to ensure a safe water
supply. Properly washing food products, avoiding
the use of human feces as fertilizer, good
personal hygiene and sanitation practices, protection
of food from flies and cockroaches, and
the avoidance of unprotected sexual practices
serve as a means to break the transmission cycle.
Notes of Interest and New Trends
Several discoveries during the late 1880s led to
the confirmation that E. histolytica was indeed a
pathogen. Of particular note was the work of
Loesch, who studied the stool of a patient suffering
from dysentery. The ameba-infected stool
from this patient was transferred to a dog for
further study.
The overall prevalence of Entamoeba infection
in the United States is approximately 4%.
Entamoeba spp. infect approximately 10% of
the world’s population. The pre valence of infection
is as high as 50% in areas of Central and
South America, Africa, and Asia.
Of all of the cases of E. histolytica worldwide,
only 10% progress to the invasive stage. Invasive
and noninvasive strains of E. histolytica may be
distinguished by performing isoenzyme electrophoresis
and examining the zymodemes (isoenzyme
patterns). These analyses are conducted
primarily for epidemiologic studies of the organism.
Applications of isoenzyme electrophoresis
are not, however, useful in routine laboratory
testing.
The World Health Organization (WHO) recommends
that intestinal infection be diagnosed
with an E. histolytica–specific test, thus rendering
the classic stool ova and parasite examination
obsolete in this setting. However, finding quadrinucleated
cysts or trophozoites containing
ingested erythrocytes in stool is considered by
many to be diagnostic for amebic colitis.
Several identification methods have been developed,
including specific immunologic tests and
new techniques (see Chapter 2), all of which have
shown promising results. A monoclonal antibody
ELISA assay to detect antigen of E. histolytica in
stool samples has been developed and experimentally
tested. Similarly, DNA hybridization probe
testing using feces samples has been developed.
Molecular analysis by polymerase chain reaction
(PCR)–based assays is the method of choice for
discriminating between E. histolytica and nonpathogenic
amebas. In addition, latex agglutination
testing for the presence of serum E. histolytica
antibodies has been studied and appears to be a
useful screening method. Epidemiologic studies
may benefit from an E. histolytica skin test that
has been formulated.
A nonpathogenic ameba, known as Entamoeba
dispar, has been identified that is morphologically
identical to E. histolytica. Thus, it
is often impossible to distinguish these two
ameba based on morphology alone. Because of
this inability to distinguish these two like parasites,
the laboratory often reports both names if
trophozoites that lack RBCs and/or cysts are
recovered. If however, trophozoites are seen that
contain ingested RBCs, it is then appropriate to
report them as E. histolytica. In cases for which
identification is not apparent, speciation requires
specialized testing methodologies that include
DNA probes and electrophoresis techniques
designed to target enzymes.
Quick Quiz! 3-4
Which of the following structures is (are) typical in
trophozoites of E. histolytica? (Objective 3-9A)
A. Single nucleus with a small karyosome
B. Unevenly distributed peripheral chromatin
C. Chromatoid bars
D. Glycogen mass