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62 CHAPTER 3

immature cysts but are not likely to be observed in ma-
MICROSCOPIC DIAGNOSTIC ture cysts. An elongated chromatoid bar has smoothly
FEATURE rounded and blunt ends that sometimes leads to some
confusion with E. histolytica.

General
Classification Amoeba Symptoms

Organism Endolimax nana Persons infected with E. dispar primarily show no symp-
Specimen Required Stool specimen toms unless concurrent infections with other organisms
are present. But symptomatic patients may suffer from
Stage Trophozoites most
diagnostic stage abdominal pain and diarrhea, which may or may not be
listed as a cause of infection by E. dispar because it is
Size 5–12 μm for both
trophozoite and cyst considered nonpathogenic.
stages
Shape Elongated trophozo- Life Cycle
ites and oval cysts
Motility Sluggish E. dispar is a noninvasive parasite and although it resem-
bles E. histolytica, it does not cause disease. The organism
Nucleus(i) 1 for trophozoites
and 4 for cysts reproduces simply by binary longitudinal fission. Excysta-
tion occurs primarily in the lower ileum and there the cyst
Cytoplasm Vacuoles accompa-
nied with granular develops into the trophozoite form. Trophozoites continue
appearance to multiply in the colon and may become encysted.
Other Features Large and irregularly
shaped karyosomes Disease Transmission
in both stages
Amoebiasis is most often achieved by eating contami-
E. nana nated food or drinking water containing the cysts of the
Trophozoite Cyst various organisms, including E. dispar. Transmission of
E. histolytica and perhaps E. dispar, because they are
virtually indistinguishable from each other, may be con-
tracted by a sexual route.
Delmar/Cengage Learning Laboratory Diagnosis

The morphological characteristics of E. histolytica are
compared with the common and so-called non- pathogenic
species that cause confusion with identification and differ-
entiation of these various species. Entamoeba histolytica
and another amoeba, Entamoeba dispar, which is about
centrally located. Motility is demonstrated by directional 10 times more common, look the same when seen under
and progressive movement. a microscope and require extensive investigation to differ-
The cyst ranges from 10 to 20 μm and the mature entiate between the two (see Table 3-4). This table, when
cyst contains four nuclei, whereas the immature cyst used with a properly calibrated micrometer, will enable
contains fewer than four nuclei. The Entamoeba genus is the medical professional to differentiate between the two
characterized by peripheral chromatin organized evenly species, but a number of organisms must be studied, and
around the nucleus, and this is not true of any other average sizes obtained (Figure 3-18).
genus of amoebae. The karyosome is extremely small Actively motile amoebic trophozoites are most
and centrally located. Glycogen vacuoles may be seen in commonly found in wet mounts from liquid or soft fecal