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Protozoal Microorganisms as Intestinal Parasites 51

ENTERIC AMOEBAE pathognomonic (diagnostic) for pathogenic Entamoeba
histolytica but is not always present. Ribosomes can be
Intestinal amoebiasis is a potentially serious infection, arranged in characteristically shaped elongate bars with
although trophozoites may inhabit the intestines for rounded ends that are called chromatoid bodies.
years without causing damage or symptoms, during
which time the person infected is an asymptomatic car- Differentiation Between
rier. The majority in whom amoebal infections exist will
fall into this group and they may spend most of their lives E. histolytica and Harmless
unaware of the infection. But some who are infected will Amoebae
develop amoebic colitis or fulminant colitis in which
The intestines often yield several species of harmless
overt symptoms will be found. The organism chiefly
commensal (living together as nonparasitic) amoebae.
responsible for amoebiasis is that of Entamoeba histolytica.
The organisms that are often observed but that are con-
Another similar amoeboid organism, Entaboeba dispar
sidered nonpathogenic are described in this section.
is considered nonpathogenic, although it inhabits the
It is necessary to compare organisms as a technique
colon of many people. These two organisms are found
to enable a laboratory worker to differentiate by com-
worldwide, especially in tropical countries and in those
parison between those amoebae that are known to be
with low sanitation standards. As many as 50 million
pathogenic and those that are somewhat debatable as
new cases of amoebiasis appear per year in the world,
to their pathogenicity. Differentiation of the Entamoeba
which result in the death of possibly up to 100,000 peo-
histolytica organism and other nonpathogenic amoebae
ple annually.
is extremely important to rule out other causative factors
of intestinal problems that might lead to unnecessary
ENTAMOEBA HISTOLYTICA therapy (Table 3-2). These nonpathogens and the des-
ignation as a pathogen or nonpathogen is arguable even
In cases where amoebic dysentery is suspected, a fresh among experts, and often confuse both experienced and
fecal sample is necessary. If a rectal ulcer is present, a
inexperienced personnel who are performing diagnostic
swab from either the stool or the site of the ulcer should testing of stool samples.
be examined with the use of a microscope, via a wet
mount. A fresh stool while still warm should be exam-
ined quickly in order to see the colorless and motile tro- Morphology
phozoites. Motility increasingly disappears as a specimen
Amoebic colitis is the term for generalized inflammation of
cools and when this happens the parasites are difficult to
the colon and is often used to describe an inflammation
recognize. It is important to distinguish these organisms
of the large intestine, which includes the colon, cecum,
from motile macrophages that may also be in the site as
and the rectum.
an immune reaction. The motile form of the trophozo-
ite has one nucleus but in a fresh specimen, the colorless
nucleus is barely discernible if at all. However, staining
the specimen gives moderate visibility of the nucleus. TABLE 3-2 Common Nonpathogenic Amoebal
Parasites Encountered
Lugol’s iodine is frequently used and the stain kills the
parasite almost immediately, upon which the motility
consequently disappears. PARASITE PATHOGENICITY
Stained Entamoeba histolytica trophozoites have Entamoeba hartmanni Debated among
a transparent outer border (ectoplasm) and an opaque professionals
inner border (endoplasm). The border between endo- Entamoeba coli Nonpathogenic
plasm and ectoplasm is not distinct in Entamoeba coli. Entamoeba polecki Most often considered as
This is a physical feature that allows differentiation nonpathogenic
between E. histolytica and E. coli. The trophozoite of Endolimax nana Nonpathogenic
E. histolytica measures 20 to 40 μm and may contain Iodamoebic butschlii Nonpathogenic
red blood cells (unlike other amoebae). The last detail is