Describe the structure and life cycle of Entamoeba histolytica.
Ans. Classification in Outline:
Phylum : Protozoa
Sub-phylum: Sarcomastigophora
Super-elass: Sarcodina
Class: Rhizopodea
Order: Amoebiba
Genus : Entamoeba
Species: Histolytica.
Schaudion established the specics Entamoeba histolytica in 1903 and
differentiated the pathogenic and non-pathogenic types. It is a parasite that
inhabits the mucous and submucous layers of the large intestine of man. It may
also occur in the liver and lung and rarely invades brain, splccn etc, producing
ulcers. Entamoeba histolytica has been recorded from Gorilla, Chimpanzee,
Gibban; Monkey, Dog, Cat, Rat etc.
The parasite is worldwide distribution and the disease caused by the parasite
is known as 'Amoebic dysentery'.
Morphology = Trophic the trophozoates vary in size from 15 to 40
micra, the average being 18 to 25 micra. Dobell (1919) and others have shown
that the parasite has got two races, one large and the other small. The trophozoite
of Entamoeba histolytica in living condition shows, two distinct portions,
ectoplasm and endoplasm. The ectoplasm after contains injested red blood
corpuscles. The pseudopodia may be long, finger-like and rounded in shape.
The nucleus is the distinct in living condition but when stained with
haematoxylin it shows a small dot-like central karyosome a uniform ring of
small peripheral chromatin granules and at times some chromatin granules in
between them. The nuclear membrane is very delicate. The size of the nucleus
is about 4 to 6 micra in diameter.
Cystic : The cysts of both races of Entamoeba histolytica vary in size
from 10 to 20 micra in diameter. In hacmotoxylin stained preparation a matured
cyst looks spherical and quadrinucleate. Its cytoplasm is clear and often contains
black rod-like chromatoid bodies with rounded ends.
The young cysts are uninucleate or binucleate and their nuclcár structure
is just like that of the trophozoites. But the nucleus of a mature cyst is very
small and its detail structure is difficult to differentiate, particularly of the
small race. It shows a very small central karyosome and a delicate nuclear
membrane.
In young cysts glycogen is present in a diffused state and can be
demonstrated in prepa ion with Lugol's lodine solution producing brownish
colour
Life Cycle : Entamoeba histolytica multiplies by binary fission in the
trophozoite stage. They have the capacity to encyst. Prior to encystment, the
parasite rounds up and eliminates food vocuole. A cyst wall develops and the
nucleus divides first into two and then into 4 small nuclei. At this stage the cyst
is infective to a new host. Through contaminated food or drink the infective
cysts pass into the lower portion of the small intestine of the new host. There
the cyst wall becomes permeable by the action of the intestinal enzymes and 4
nucleate amoeba emerge out from the cyst. In the 4-nucleate amoeba, both the
nuclei and cytoplasm divide and as a result 8 small amoebulae are produced.
These are called metacystic trophozoites. They are motile and penetrate the
mucous membrane. Inside the tissues the trophozoites multiply and produce
the characteristic lesions of amoebic dysentery.
Transmission Cysts of Entamoeba are transmitted from one individual
to another in a variety of ways the cysts are generally transmitted with food or
drink House-flies and cockroches may transmit cysts mechanically. Raw
vegetable is also another source of infection. In many countries human faeces
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