Infection by any species of the amebas is properly called amebiasis. The term amebiasis includes amebic dysentery and other disease processes due to invasion of the liver and other organs by amebas. The most harmful of the spesies pathogenic to humans bears name Entamoeba histolytica (ent, inside ; lytic, dissolving). The organisms are usually ingested as dormant cysts in feces or sewage-polluted food or water. They soon undergo excystation and grow into fragile, actively multipliying trophozoites, which primarily attack the lining (mucosa) of the intestine, usually the large bowel. These entamebas, by means of tissue-destroying enzymes, burrow into and in places, undermine the intestinal lining and cause ulcers (amebic dysentery). There is little inflammatory reaction unless, as in common, secondary bacterial infection develops.
Entamoeba histolytica in the trophozoite form often burrows through the intestinal lining and deep into the intestinal wall. Occasionally, rupture of the intestine occurs as a result. The patien may then die of peritonitis caused by escape of the bacteria of the feces into the bdominal cavity.
The amebas may also invade the intestinal lymph and blood vessels and then are carried to the liver, lungs, brain, and other organs, where they can become localized and produce large amebic abscesses.
Like many other infections diseases, amebiasis is often chronic and may be present with little definite symptomatology for a long time. Carries of E. histolytica are common in some areas of low grade sanitation. Thus, amebic infection is often unknowingly disseminated by persons with mild cases of the disease or by carries. Amebiasis is comman in all warm regions and is frequently found in temperature zones around the world.
Entamoeba histolytica is eliminated only in the feces and may appear in one or both of two forms.
Cysts. Most amebas have the property of forming rounded, dormant, thick-walled, drough-resistant cysts 10-20 µm in diameter that remain alive and dormant for hours or days in the lumen of the colon, in feces or in moist. polluted soil or water. They are slowly killed by drying and exposure to sunlight, and they are susceptible to heat and vigorous disinfection. Unless active diarrhea is in progress, which quickly flushes the throphozoite from the bowel before encystment can occur, it is the cysts form that is commonly found in stools. These cysts are transmitted from person to person by the well known fecal-oral route of transmission of all types of intestinal infection. Cysts of less than 10 µm in diameter may be found in feces; they are most likely nonpathogenic E. histolytica or subspecies called E. histolytica hartmanni.
Throphozoites. The actively multipliying, fragile, throphozoite form of E. histolytica is excreted only in the watery stools of acute amebic dysentery, is not resistant outside the intestine and quickly dies when cooled or dried. Throphzoite of E. histolytica frequently ingest red blood cells. those of other species rarely or never do. These facts are of diagnostic value.
Throphozoites multiply only by binary fission. They may be easily studied in culture 37 C under partial anaerobiosis. If they produce disease about 10 per cent of infections. they attack the lining of the cecum or lower ascending colon and more distal portions.
I rewrited from microbiology in health and disease book, 15th edition, created by Frobisher and Fuerst.