Amoeba falls into the class Lobezea of the Protozoa phylum. The Lobezea are organisms which move with the help of pseudopodia. Reproduction is by binary fission. The production of a cyst is one of the stages in the life cycle. Among the pathogenic species for man is the Entamoeba histolytica. The causative agent of human intestinal amoebiasis, or amoebic dysentery.
Enteroamoeba histolytica occurs in the human body in such forms:
1. Entamoeba histolytica forma magna is a vegetative large tissue form which feeds on the erythrocytes and does not become encysted:
2. Entamoeba histolytica forma minuta is a vegetative small commensal encysted form which lives in the lumen of the large intestine.
3. Cysts which develop from the forma minuta. E.histolytica penetrates into the tissues of the large intestine under the influence of a number of factors (lowered resistance of the human body due to various diseases, intoxications, overheating, overstrian, injuries, substances that cause lysis of cells and tissues. The parasite grows to a size of 30-50 micrometer in diameter and becomes capable of phagocytizing erythrocytes. This vegetative form is known as the tissue form. Entamoeba histolytica forma magna. It is usually found in the mucosanguineous stools (feces with blood and mucus) of patient with amebiasis. The amoeba ectoplasm is translucent, while the endoplasm is granular. The endoplasm contains the nucleus and food vacoules. The nucleus has a central karyosome. Forma minuta is the main form of E.histolitica. Its size ranges from 12 to 25 micrometer. Bacteria in phagocytes are found in small numbers in the endoplasm. Ectoplasm is poorly developed and accumulated in the pseudopodia, motility is decreased. Amoebas inhabit the upper part of the large intestine of a healthy man and known as luminal, cavity or commensal forms. The forma minuta transforms into a cyst. The cyst are spherical in shape with a diameter of 9 to 16 micrometer. They have thin-walled, double membranes, Fully mature cysts contain four nuclei. Immature cysts have one, two and sometimes three nuclei. Cysts are discharged with the feces for a long period of time and sometimes throughout life. They may re-enter the human body with foodstuffs or water and transform into luminal forms int he intestine.
Pathogenesis and disease
Fecal-Oral transmission of cysts involves contaminated food or water, Ingested cysts of E.histolytica excysts in the small intestine. Trophozoites (forma minuta) are carried to the colon, where they mature and reproduce. Forma minuta may live in the lumen as being commensal. Successful colonization and transformation of forma minuta into forma magna depends on factors such as intestinal motility, transit time, the presence or absence of specific intestinal flora, the host’s diet. If mucosal invasion occurs, it may be limited to a few simple superficial erosion or it may progress to total involvement of the colonic mucosa with ulceration, most often in the caecum or sigmoid colon. Patients have acute or chronic diarrhoea, which may progress to hemorragic dysentery. Invasion of blood vessels leads to secondary extra-intestinal lesions. Extraintestinal disease may be present as a complication (peritonitis) or as metastasis (liver, lung, skin or brain abscess).
Fresh stools are examined under the microscope. E.histolytica (forma magna and cysts with 4 nuclei) can be demonstrated in the stools.
All patients are transferred to a hospital and adequately treated. preventive measures against amoebiasis also include protection of foodstuffs and water from flies and contamination with feces. The staff of catering establishments must be examined for cysts carriage. Health education of the population.
Non-pathogenic amoebas can parasite the human gastrointestinal tract. These include Entamoeba gingivalis (Oral Cavity), Entamoeba Coli (large intestine) and E.Coli larger than E.histolytica. Their cytoplasm is granular and the vacuoles contain bacteria, leukocytes, food particles, and glycogen, but no erythrocytes. E.Coli forms cysts with 8 nuclei, E.gingivalis doesn’t form cysts.
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