Page 14 - Medical Parasitology_ A Textbook ( PDFDrive )

6 3 Amoebae

a b

Fig. 3.1 Entamoeba histolytica. (a) Trophozoite, (b) Cyst

Its nucleus is spherical and contains central karyosome. The nuclear membrane
is lined by a rim of evenly distributed chromatin. It reproduces by binary fission. It
is killed by drying, heat and chemical sterilization.
The trophozoites undergo encystment in the intestinal lumen. Before encyst-
ment, the trophozoite extrudes its food vacuoles and rounds up to form a precystic
stage, measuring 10–20 μm in size. It contains a large glycogen vacuole and chro-
matoid bars. It secretes a cyst wall to become cyst.
The cyst is spherical in shape. Immature cyst contains a single nucleus, a glyco-
gen vacuole and chromatoid bars which are cigar shaped with rounded ends. The
chromatoid bars are visible in saline. With iron haematoxylin stain, nuclear chroma-
tin and chromatoid bodies appear deep blue or black. When stained with iodine, the
glycogen mass appears golden brown while the nuclear chromatin and karyosome
bright yellow. Mature cyst contains 4 nuclei. It measures 10–20 μm in size
(Fig. 3.1b). The glycogen mass and chromatoid bars disappear in mature cyst. The
cyst wall is highly resistant to gastric juice and unfavourable environmental
conditions.

Life Cycle (Fig. 3.2)
(1) The cysts (usually found in formed stools) and trophozoites (in loose stools) are
passed out in faeces of infected human. (2) Cysts are ingested via contaminated
food or water. (3) In the intestine, the cysts undergo excystation and form trophozo-
ites (4). (5) As the trophozoite passes down the intestine, it undergoes encystation
and is excreted in the faeces.
Entamoeba histolytica completes its life cycle in human host. In the majority of
cases, E. histolytica remains as a commensal in the large intestine. They are carriers
or asymptomatic cyst passers and are responsible for maintenance and transmission
of infection in the community.