Amebiasis is an infection of the colon caused by a parasitic protozoan, the ameba Entamoeba histolytica. Several species of ameba inhabit the large intestine. Most are harmless commensals or minor parasites, usually causing little or no clinical damage. The closely related species Entamoeba coli and Entamoeba hartmanni are commensals, and infection with E. histolytica is also often asymptomatic. E. histolytica is probably a species complex, with a number of morphologically similar forms with varying degrees of invasiveness. E. hartmanni, formerly believed to be a “small race” of E. histolytica, is now recognized as a separate nonpathogenic species. Pathogenic amebas cause light to severe intestinal damage (amebic dysentery) and sometimes spread to the liver, lungs, brain, and other organs.

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. 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.