In this article we will discuss about the life history of Entamoeba histolytica (explained with diagram).
Amoebiasis or more correctly it should be called as amoebic dysentery is caused by a protozoan parasite known as Entamoeba histolytica. This kind of protozoan parasite resides in the mucous and sub-mucous layer of the large intestine specially at the iliocaecal junction and cause dysentery, diarrhoea, hepatitis and liver abscess in man.
E. histolytica is a monogenetic parasite as its entire life history is completed in a single host. The parasite is discharged from the body of a host to the exterion in order to enter into the body of a new susceptible host for propagation of the species and particularly when the original host has acquired tolerance against the harmful effects of the parasite.
Man is the usual host of E. histolytica but the parasite is also reported to infect other vertebrate host like cat, dogs, monkeys or rats, pigs etc.
(a) Binary fission:
Trophozoite stage of E. histolytica undergoes binary fission (asexual multiplication) within the wall of large intestine. The nucleus of it undergoes mitosis and this is followed by cytokinesis. As a result, two daughter organisms are formed which grow rapidly in size, feeding upon bacteria and host tissue elements and in their turn, again multiply by binary fission. Some of them invade fresh host cells while others become the pre-cystic or minute forms.
(b) Encystation:
When the parasite cannot continue its life cycle solely in the trophozoite stage it produces such form which will save the parasite from extinction. It is the process of transformation of trophozoites to cysts and takes place inside the lumen of the intestine of an infected individual.
The entire process occurs within a few hours and life span of a mature cyst inside the lumen of the bowel of the original host is only 48 hours. But the cysts are not developed inside the tissues of man, neither in the intestinal wall nor in the areas of metastatic invasion (liver, lungs or other organs). It is to be noted that cysts are highly resistant to desiccation and certain chemicals like acetaldehyde, ethyl-alcohol etc.
The pre-cystic or minute forms encyst only in the lumen of the intestine. During encystation they round up and secrete a thin, refractile, tough and flexible cyst wall around them. The cysts at this stage are uninucleate. The mature cyst is a quadrinucleate body because during encystment the nucleus has undergone multiplication and given rise to four daughter nuclei.
Different Morphological Forms in the Life History of E. histolytica:
Following are the morphological forms during different stages in its life cycle.
In life cycle, it occurs in three distinct forms:
1. Trophozoite or adult form or magna form or feeding form.
2. Pre-cystic or minuta form and
3. Cystic form.
1. Trophozoite or magna form:
The active trophozoite of E. histolytica ranges in size from 10-60 μm. It is the most active, motile and feeding form which is pathogenic to man. It resembles the common amoeba in all structural details. A freshly expelled vegetative form is very active, and moves with the help of a prominent lobe-like pseudopodium.
The morphological characteristics are as follows:
i. Constantly changing body form owing to random formation of small and blunt pseudopodia from the different regions of the body. They move by producing a single broad and blunt pseudopodium at the advancing end.
ii. Mononucleate body with cytoplasm distinguishable into outer ectoplasm and inner granular endoplasm.
iii. Endoplasm containing numerous ingested erythrocytes.
iv. Nucleus is slightly eccentrically placed and is formed of nuclear membrane, karyosome and linin network.
Nuclear membrane is delicate limiting membrane of the nucleus lined with a single layer of uniformly distributed fine chromatin granules internally.
Karyosome or endosome is a small dot like mass formed of plastin and chromatin, often surrounded by a clear area or ‘halo’.
The space between the nuclear membrane and karyosome is traversed by a fine thread of linin network having a spoke-like radial arrangement.
2. Pre-cystic form:
When the host becomes resistant to harmful actions of trophozoites, the trophozoites are converted into the pre-cystic forms inside the lumen of the colon of the host body.
They are characterised by the following features:
i. Smaller in size than the trophozoites. It measures 12-15 μm in diameter in the magna form, in the small race it is about half the size.
ii. Round or slightly ovoid in form with a broad and blunt pseudopodium projecting constantly from the end.
iii. Less active than trophozoites.
iv. Endoplasm comprises of a single nucleus identical to that of trophic forms but endoplasm is devoid of erythrocytes and other food particles (Fig. 6.3).
3. Cystic forms:
They are produced inside the lumen of the large intestine by the pre-cystic forms and liberated from the host body finally along the faeces. They are infective forms of Entamoeba and are able to infect a new host.
Following are the characteristic features:
i. Round body form and it becomes surrounded by a highly refractile membrane—cyst wall.
ii. A mature cyst is a spherical body and it measures about 10 to 12 μm in diameter.
iii. In early cystic forms the cytoplasm contains one or two glycogen masses.
iv. Endoplasm is devoid of erythrocytes and other food particles.
v. Nucleus may be one, two or four in number in mononucleate, bi-nucleate and tetra-nucleate cystic forms respectively (to start with, the cyst is uninucleate but its nucleus divides to form bi-nucleate and finally a tetra-nucleate or quadrinucleate cyst).
vi. Nucleus of the mature cystic forms are smaller in size than trophic and pre-cystic forms.
vii. In mono or bi-nucleate cystic forms usually two deeply staining chromatoid bodies or chromidial bars with rounded ends are visible. According to Pilellea (1963) the chromidial bars are composed of ribonucleoprotein but these bars are never found in the tetra-nucleated or mature cystic forms. Neel (1966) is of opinion that the less of disappearance of chromidial bars in mature cystic forms occur due to uniform dispersion of ribonucleoprotein throughout the cytoplasm.
(c) Transmission:
Infection depends upon intake of food or water contaminated with tetra- nucleate cysts. Cysts are destroyed by drying. The mature cystic forms of E. histolytica after coming to the exterior along with host’s faces enter into alimentary canal of new host with contaminated food and drink.
Untreated human faeces voided by children and adults on open ground or in crop field are a common source of infection. It is known that houseflies and cockroaches are caprophagous in habit (feeding upon faecal matter), carry viable cysts on their legs and transfer them to unprotected food stuffs.
(d) Excystation:
After its entry into the alimentary canal of a host the tetra-nucleate cystic forms pass through the stomach in an unchanged condition as the gastric juice of the host has no action on the cystic forms. Through the contaminated food and drink when the infective cysts pass into the lower portion of the small intestine, the cyst wall becomes very delicate (dissolved) inside the small intestine by the action of the pancreatic juice.
After five or six hours excystation occurs as the cyst wall is digested by trypsin in small intestine and a metacystic amoeba with four nuclei is liberated within the lumen of the ileum. This is called excystment (Fig. 6.5).
(e) Metacystic development:
Each metacystic amoeba or excystic amoeba undergoes binary fission. Its nuclei divide in a specific pattern, accompanied by simultaneous cytoplasmic division to form 8 smaller uninucleate amoebulae or metacystic trophozoites.