Amoebiasis or amoebic dysentery is caused by a protozoan parasite called Entamoeba hystolytica (also known as E. dysenteriae).
It was first discovered by Losch in 1873 in Russia. It has the same systematic position as Amoeba proteus.
Habit and Habitat:
In human being Entamoeba histolytica occurs in the lower part of the small intestine or the upper part of the large intestine (colon). It is worldwide in distribution but mostly occurs in the tropical countries where sanitary conditions are poor.
Structure:
Adult Entamoeba histolytica are found in two forms active or trophic form and minuta form. The active form measures 20 to 30 H in diameter while the less active minuta form is only 7-10 The trophic form has structure almost similar to that of amoeba and produces a single pseudopodium in the direction of movement (monopodia) (Fig. 6(A).7). In the endoplasm a nucleus is present which contains a distinct nucleolus in the centre. Contractile vacuoles are absent while food vacuole contains R.B.C. and bacteria.
Life Cycle:
It involves three stages:
(a) Adult Trophozoite
(b) Pre-cystic or minuta stage
(c) Cystic stage (Fig. 6(A).8).
The Trophozoite or feeding stage lives in the intestinal wall and dissolve the lining of mucosa by proteolytic (protein digestive) enzymes. It divides by binary fission. It makes flask shaped ulcers in the intestinal wall.
The pre-cystic stage or minuta form feeds on bacteria and lives as commensal. They are round and store food material in the form of glycogen and chromatoid bars. The discharged trophozoite from the ulcers enters into precystic stage.
Each pre-cystic stage produces a resistant wall around itself during unfavourable condition and gives rise to cystic stage. The nucleus divides twice to form four nuclei. The quadrinucleate cysts come out along with human faeces. They are resistant to desiccation, temperature and chemicals and are infective stage of entamoeba.
The cystic stages or cysts from the outer environment are carried to the alimentary canal of healthy man through contaminated food and drinks. In the intestine the cyst wall dissolves (excystation) by the action of trypsin and the tetranucleated form emerges from the cyst. It is known as metacyst. Now each of the four nuclei divides once resulting in an octanucleate form which undergoes multiple fission to form eight, uninucleate amoebulae.
Mode of Infection:
The parasites (quadrinucleate cysts) get into the body of healthy persons by contaminated human food, drinks, uncooked vegetables either directly or through house fly or food handlers etc.
Pathogenecity:
About 90% of the human population is carrier of this parasite. The trophozoite dissolves the intestinal mucosa by producing proteolytic enzymes and form flask shaped ulcers through which blood is discharged. For this reason the stool of the patient contains mucous and blood. There is abdominal pains, headache and nausea. In acute cases the parasite reaches liver, spleen or lung through blood circulation.
Treatment:
The common drugs used for treating amoebiasis are Emetin, Enteroquinol, Enterovioform, Mexaform, Gramoneg, etc. Sometimes the parasite enters the liver through circulation and causes liver abscess. In that case chloroquin is used as an effective drug.
Prevention and Control:
To prevent amoebic infection the following steps may be taken:
(a) Persons should live under good sanitary conditions.
(b) Hands should be properly washed with soap and nails should be cut regularly before handling or taking the food.
(c) Drinking water should be protected against contamination. Unfiltered drinking water should be boiled or chemically treated before use.
(d) Fruits and vegetables should be well washed and food should be properly cooked before eating.
(e) Food articles should be properly covered and protected from flies and cockroaches etc.
(f) Vegetables grown in soil should be soaked in potassium permanganate solution (for about half an hour) or in 5% acetic acid (15 minutes at 30°C).
(g) Human faeces should be chemically treated before being used as fertilizers.
(h) By far the most important measure is to educating illiterate people about simple hygiene and sanitary measures.