Most amoebae are free-living, but some are endoparasites. They have adapted themselves to live actively in the intestine of many invertebrates and vertebrates. Besides weakening the host body by obtaining nourishment from it, these may cause diseases and. hence, called pathogenic parasites. One of such pathogenic amoebae is Entamoeba histolytica. It causes amoebic dysentery’ or ‘amoebiasis’ that is why it is commonly called ‘dysentery amoeba.
E histolytica also occur in other mammals, lower vertebrates and some invertebrates, often without evident damage to these hosts. The contents of lower intestine may be examined by diluting with physiological saline solution on slide.
E. histolytica is world-wide in distribution. It is known to be widely distributed in tropical, subtropical, temperate regions. The incidence is high in India. China, Mexico and parts of S. America. Craig (1962) estimated that about 10% of the human population is infected by this parasite.
Habits and habitat
As mentioned E. histolytica occurs in man and other mammals like dogs. cats. rats, monkeys, baboon, chimpanzee, gorilla etc. It lives in the upper part of large intestine (colon) of man It lives in the mucous and submucous layers of colon. It secretes a toxic substance which dissolves and destroys the mucous lining.
From here, certain individuals are carried by blood stream to liver, lungs, kidneys, gonads and even the brain.
The parasite feeds on the dissolved tissues, bacteria, and blood corpuscles. It causes a serious and often fatal disease known as amoebic dysentery. Patients of this disease discharge mucous and blood in their stool.
The active motile form of E. histolytica is called the trophozoite which usually ranges in long diameter from 15-25 µm. It resembles Amoeba in its structural details. It is bounded by a thin, flexible, transparent semipermeable membrane, the plasmalemma. The cytoplasm is differentiated into outer ectoplasm and inner endoplasm. The ectoplasm forms a thin, clear, relatively firm outer layer. The endoplasm is granular and more fluid part. A single pseudo- podium is formed of ectoplasm only.
In the endoplasm a nucleus and several vacuoles are present The nucleus is vesicular and rounded. It is bounded by a delicite nuclear membrane, whose inner surface is encrusted with a fine peripheral layer of chromatin granules.
The karyosome or endosome is small and centrally placed in the nucleus. It is often surrounded by a clear area or halo. The size of nucleus is about 4-6n in diameter and it contains six chromosomes.
In the endoplasm, food vacuoles vary in size and number are present. They enclosing red blood corpuscles, white blood corpuscles and debris of epithetical cells and bacteria. The contractile vacuole is entirely absent.