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54 CHAPTER 3

TABLE 3-3 Comparison of Trophozoites and Cysts

TROPHOZOITES CYSTS
Motility Rapid or sluggish motility Not applicable
Cytoplasmic Appearance Finely or coarsely granular, vacuolated Not applicable
Cytoplasmic Inclusions RBCs, bacteria, or yeasts Not applicable
Nucleus or Nuclei Number present Number present
Karyosome Location and size Location and size
Peripheral Chromatin Present or absent Present or absent
Chromatoid Bodies Present or absent Present or absent
Glycogen Vacuole Present or absent Present or absent
Size Same importance with both stages Same importance with both stages
Shape Shapes vary

swift differentiation between E. histolytica and the non- ENTAMOEBA COLI
pathogenic E. dispar.
It is important to be able to identify E. coli (not to be
confused with the bacteria Escherichia coli), which is
Treatment and Prevention
nonpathogenic by comparing E. coli and E. histolytica.
Parasites in the tissues (intestinal wall) can be treated with The microscopic diagnostic features that follow for
metronidazole or tinidazole. The dose of metronidazole E. coli and E. histolytica will be helpful in differentiating
(Flagyl) is also the treatment for certain bacterial infec- between the two organisms, one of which if pathogenic
tions, as well as for certain amebic parasites. These drugs (E. histolytica) and the other (E. coli) is not.
are rapidly absorbed in the proximal intestine. For this
reason they are not very effective in combating the para- Morphology
sites in the distal intestinal lumen (tubelike opening) near
the anus. These amebic parasites are best treated with Entamoeba coli is one several nonpathogenic varieties
diloxanide furoate, which kills amoebae upon contact. of amoebic parasites, but is medically important not
However, parasites that have invaded the tissues are not because it is pathogenic, but because it can be confused
affected by this drug, as it is difficult for the drug to gain during the microscopic examination of stained stool
contact with the imbedded parasites. Other contact amoe- specimens with the pathogenic Entamoeba histolytica.
bicides are iodoquinol and paromomycine, which result Differentiation between these two species is typically
in a somewhat higher relapse percentage, but this is not accomplished by visual examination of the parasitic
always the case. cysts by use of light microscopy. But new and emerging
Humans act as reservoirs for the infectious methods using molecular biology techniques have been
E. histolytica. The cysts for this organism are extremely developed and are currently on the market, whereas
hardy and can be transmitted by infected water. The others are currently being developed to provide
cyst stages of the organisms can survive environmental improved and more rapid. The identification of E. coli is
extremes and require both filtration and chemical treat- not in itself of enough significance to seek treatment as it
ment of water in order to kill or remove the organism. is considered harmless. But other pathogenic organisms
During military operations where water is taken from may have been ingested at the same time the patient was
wells, springs, lakes, and running water, iodine is used infected by this benign species of Entamoeba because
to ensure safety when drinking the water from local and the route of infection for most species of amoebae is
untreated sources. Sanitary living conditions are man- the same.
datory in order to prevent transmission of this parasitic Entamoeba coli as a nonpathogenic species of
amoeba. Entamoeba that frequently exists as a commensal parasite