Etiology

The parasite exists in two forms during its life cycle. Active adults, trophozoites, multiply in the lumen of the colon. They frequently live there harmlessly, feeding on the contents of the intestine. Some strains are generally commensal; others are highly pathogenic. Under conditions of stress, lowered host resistance, or when a particularly pathogenic strain is involved, amebas invade the intestinal wall and cause abscesses. As they pass lower into the large intestine, the drier environment stimulates them to form a cyst wall. The original cell nucleus divides twice, producing four daughter nuclei. Cysts are passed with the feces and are infective when swallowed. Excystation takes place in the small intestine, and the young trophozoites, four from each cyst, are carried in the fecal stream to the large intestine. When dysentery occurs, trophozoites are swept out too rapidly to encyst. Even though huge numbers of amebas may be passed, they die quickly and are not infective. Persons with mild or no symptoms produce infective cysts, and it is they, not the patients with dysentery, who spread the disease.

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