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The following points highlight the three important species of Entamoeba for which man is host. The species are: 1. Entamoeba Histolytica 2. Entamoeba Coli 3. Entamoeba Gingivalis.
Species # 1. Entamoeba Histolytica:
It was discovered by a Russian zoologist, Friedrick Losch in 1875. This is an endoparasite common in man, apes and monkeys, and may also be found in pigs, cats and rats.
E. histolytica is a microscopic endoparasite and is the causative agent of amoebic dysentery in man. It is seen inhabiting the upper part of large intestine (colon). There are two races or strains of this parasite, a smaller, common, nonpathogenic form and a much larger pathogenic form.
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About 600 million people around world are infected by this parasite. Infection of this parasite is high in India, China, Philippines, Thailand, Mexico and parts of South America. A higher prevalence occurs in males, but usually the more serious and fatal complications affect women and children.
Morphology:
The most active, motile and feeding form is called trophozoite, and has average about 25 um in diameter. The cytoplasm consists of a clear ectoplasm and a finely granular endoplasm. The endoplasm contain food vacuoles filled with RBC in different stages of digestion. The nucleus is single and vesicular, measures about 3 to 5 um in diameter.
Within it lies the endosome (nucleolus), which may be a single granule or a closely packed cluster of minute granules. A ring appears to surround the endosome. Spoke-like lines radiate from the endosome to the nuclear membrane.
Trophozoite produces finger-like pseudopodium and movement is irregular. As there used to be only one pseudopodium, it is typically monopodial. The type and rapidity of movement varies depending upon the consistency of surrounding medium, age of the parasite, temperature etc.
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The nutrition in trophozoite is holozoic. It feeds by phagocytosis. Food usually consists of bacteria or other organic material found in the intestine. RBCs are found only in the food vacuole of pathogenic forms.
Pathogenesis:
The common, smaller, non-pathogeic minuta form lives in the lumen of the large intestine of man. It is harmless and feeds on bacteria. After a period of feeding and reproduction, vacuoles disappear. The Entamoeba becomes rounded. Soon a cyst wall begins to form, these uninucleated stages are precysts.
If the resistance of gut is lowered in infected people, nonpathogenic form may change to pathogenic magna form. It can also be produced by an alternation of host’s diet, increase of host cholesterol, change in the level of host sexual hormones and enzymatic action of the parasite. Magna form is invasive, and it penetrate the intestinal wall by secreting the histolytic enzymes.
They invade the submucosa by dissolving the mucosa of intestinal wall. They multiply by binary fission and produce flask-shaped ulcers containing cellular debris, RBCs, lymphocytes and bacteria. These ulcers rupture and discharge blood and mucus into the intestine that pass to outside with the stool. This results in amoebic dysentery or amoebiasis.
Other symptoms of amoebiasis are vomiting, mild fever, diarrhoea, tenderness over the sigmoidal region of the colon and hepatitis. In the stools of the patient, swarms of Entamoeba and RBCs are found. Stool is usually acidic. The patient is discomforted due to intense abdominal pain with the passing of blood and mucus with stools after every few minutes.
E. histolytica may not be confined to the intestine. Once a lesion is made in the intestinal wall, the amoebae can travel through blood or lymph circulation of host and can invade other tissues like liver, spleen, lungs, skin, gonads and rarely brain.
In all these soft tissues, Entamoeba multiply and cause formation of abscesses. The most common extraintestinal locus of infection in the liver, particularly the right lobe. Hence results in hepatitis. Formation of abscesses in skin result in cutaneous amoebiasis. Abscesses in brain usually result in the death of the host
Treatment:
Many drugs are available for the treatment of amoebic dysentery. Alkaloid Emetine is effective in giving temporary relief. Dehydroemetine, a synthetic derivative of Emetine is equally effective. Chloroquine is used to treat amoebic hepatitis. Other medicine are iodine compounds such as. Chinioform, Diodoquin, Iroform, Humat in, Mexaform and Vioform.
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Antibiotics useful in treating amoebiasis are Aureomycin, Fumagillin, Erythromycin and Terramycin. Recently both acute intestinal amoebiasis and amoebic hepatic abscess are treated with Metronidazole (1-(2- hydroxyethyl) -2 methyl 5 nitroimidazole.). Selection of drugs depends on the location of infection and condition of the patient.
Control:
1. Hands should be properly washed before handling food materials and eating the food.
2. Finger nails should be cut regularly and kept clean.
3. Uncooked vegetables like salad should be eaten only after through washing.
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4. Avoid irrigation of vegetable by contaminated water.
5. Protection of food from contamination from housefly, cockroach etc.
6. Vegetables should be washed before cooking.
7. Avoid defaecation on vegetable fields and open grounds.
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8. Quick and proper disposal of night soil.
9. Proper sanitation of surroundings.
10. Chlorination of drinking water supply.
11. Proper disposal of sewage.
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12. Chemical treatment of human faeces to be used as fertilizer.
13. Covering of eatables by traders.
14. Killing of cysts with alcohol, phenol, formalin, mercuric chloride hydrochloric acid, caustic soda and potassium permanganate.
Species # 2. Entamoeba Coli:
This is found in human large intestine and is nonpathogenic. It has a worldwide distribution and the incidence of infection is more in warm countries like India, Pakistan, Burma and Srilanka.
Life cycle of this parasite differs from E. histolytica. E. coli does not invade host tissue. Trophozoite is smaller in size, ranges from 20 to 30 um in diameter. It is more sluggish than E. histolytica. The cytoplasm usually appears much more dense and crowded with food vaculoes. Food vacuoles contain cyst of other protozoa, bacteria, yeast cells etc., but devoid of red blood cells.
The nucleus possesses heavier peripheral chromatin and an eccentrically placed endosome. Young cysts contain a large glycogen vacuole. They may possess few nuclei. Mature cysts are slim with pointed or irregular ends. The cysts of E. coli are not killed by drying, this results in high incidence of infection in comparison to E. histolytica.
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Cysts are excreted outside the host body through its faeces which contaminate food and water. The infection takes place when cyst contaminated water is ingested by human beings. E. coli occurs in the intestine of 50% of human population. Monkeys and apes also share this parasite with man.
Pathogenesis:
Occasionally red blood corpuscles are reported from the food vacuoles of E. coli. Cysts are found to occur in the stools of diarrhoeic patients. Still there is no evidence to consider this parasite as a pathogenic form.
Treatment:
Owing to nonpathogenic nature, no medical attention is required. However, the drugs available to treat amoebiasis are effective in killing this parasite.
Control:
1. Proper disposal of sewage.
2. Quick and proper disposal of human faeces.
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3. Purification of drinking water.
4. Proper washing of vegetables used in salads.
5. Cysts can be killed by Hydrochloric acid, phenol, caustic soda, cresol etc.
Species # 3. Entamoeba Gingivalis:
It inhabits in the mouth of about 70% of human population. E. gingivalis lives in the gingival areas around the teeth of man, other primates, dogs and cats. It is commonly found in pus-pockets of teeth of pyorrhoea infected persons and in the crypts of infected tonsils. It is the smallest species of Entamoeba.
There are no cysts and no intermediate hosts. It represents the simplest kind of life cycle. It is directly transmitted from mouth to mouth by contact during kissing, in feeding and cough into food at the time of dishing.
The size of E. gingivalis ranges from 5 to 35 um in diameter, with an average of about 15 um. It is appears similar to E. histolytica, but contain many more large food vacuoles, contain bacteria and white blood cells of the host. Ectoplasm is clear and central granular endoplasm is vacuolated. Many broad and blunt pseudopodia are used in locomotion.
The nucleus is lined with beaded peripheral chromatin. It contains an endosome with in a single or many closely grouped granules. Spoke-like fibrils connect the endosome with the nuclear membrane.
Pathogenesis:
There is a high incidence of E. gingivalis in those patients of pyorrhoea. It was believed that the parasite is the causative agent of pyorrhoea. But at present, it is known that pyorrhoea is due to a bacterial infection. E. gingivalis increase the disease by damaging tissue in the gums. The actual pathogenecity of this parasite is still to be established.
Some species of Entamoeba found in other animals:
