[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 4 The Flagellates
93
TABLE 4-8
Parameter
Size range
Shape
Nuclei
Other structures
Well-defined
nuclear membrane
Well-defined
cyst wall
Enteromonas hominis Cyst:
Typical Characteristics at
a Glance
Description
3-10 µm long
4-7 µm wide
Oval, elongated
One to four
Binucleated and
quadrinucleated nuclei
located at opposite ends
Central karyosome
No peripheral chromatin
None
Cytoplasm
Size range: 3-10 m by 4-7 m
Average length: 5-8 m
FIGURE 4-11 Enteromonas hominis cyst.
Nuclei
of the trophozoite. Four flagella originate from
the organism’s anterior end. Three of these flagella
are directed anteriorly; the fourth is directed
posteriorly. The posterior end of the organism
comes together to form a structure resembling
a small tail. These trophozoites are simple, relatively
speaking, in that structures such as an
undulating membrane, costa, cytostome, and
axostyle are absent.
Cysts. The typical oval to elongated E.
hominis cyst measures 3 to 10 µm long by 4 to
7 µm wide, with an average length of 5 to 8 µm
(Fig. 4-11; Table 4-8). On first inspection of these
organisms, yeast cells may often be suspected.
Further investigation, however, reveals one to
four nuclei. When more than one nucleus is
present, these structures are typically located at
opposite ends of the cell. Although binucleated
cysts appear to be the most commonly encountered,
quadrinucleated forms may also occur.
The nuclei resemble those of the trophozoites in
that each consists of a well-defined nuclear membrane
surrounding a central karyosome. Peripheral
chromatin is again absent. The cysts of E.
hominis are protected by a well-defined cell wall.
Fibrils and internal flagellate structures are also
not seen in the cyst form. It is important
to note that the size range of E. hominis cysts
overlaps that of Endolimax nana cysts. A high
frequency of binucleated cysts seen on a stained
preparation indicates probable E. hominis because
the probability of finding binucleated E. nana
cysts is extremely rare.
Laboratory Diagnosis
Examination of stool samples is the laboratory
diagnostic technique of choice for identifying E.
hominis trophozoites and cysts. Unfortunately,
this organism is difficult to identify accurately
because of its small size. Careful screening of
samples is recommended to prevent missing an
E. hominis organism.
Epidemiology
E. hominis is distributed worldwide in warm and
temperate climates. Ingestion of infected cysts
appears to be the primary cause of E. hominis
transmission.
Clinical Symptoms
Infections with E. hominis are characteristically
asymptomatic.
Treatment
E. hominis is considered to be a nonpathogen.
Treatment for E. hominis infections is, therefore,
not indicated.