Entamoeba histolytica and Other Intestinal Amebas

Causative agents of amebosis (entamebosis, amebiasis)

■ Of the various amebic species that parasitize the human intestinal tract, Entamoeba histolytica is significant as the causative agent of the worldwide occurring entamebosis, a disease particularly prevalent in warmer countries. The vegetative stages (trophozoites) of E. histolytica live in the large intestine and form encysted stages (cysts) that are excreted with feces. The infection is transmitted by cysts from one human to another. The trophozoites of E. his-tolytica can penetrate into the intestinal wall and invade the liver and other organs hematogenously to produce clinical forms of amebosis, most frequently intestinal ameboses (amebic dysentery) and hepatic amebosis ("amebic liver abscess"). Diagnosis of an intestinal infection is primarily confirmed by detection of the parasites in stool. If an invasive, intestinal or extraintestinal infection with E. histolytica is suspected, a serological antibody test can also provide valuable information. Morphologically, E. histolytica is indistinguishable from the apathogenic Entamoeba dispar (collective term for both species: E. histolytica/E. dispar complex). ■

Occurrence. In endemic areas in Africa, Asia, and Central and South America up to 70-90% of the population can be are carriers of E. histolytica /E. dispar, in the USA and Europe about 1-4%. Worldwide the annual number of new cases is estimated at 48 million, with about 70 000 lethal outcomes (WHO, 1998).

Parasites. The causative agent of amebosis is the pathogenic E. histolytica. This species is morphologically identical with the apathogenic E. dispar. They can be differentiated by means of zymodeme and DNA analysis and with monoclonal antibodies. The two species occur in the form of trophozoites (vegetative stages) and cysts (Figs. 9.10 and 9.11c).

■ The trophozoites of E. histolytica are cells of variable shape and size (1060 im) that usually form a single, broad pseudopod (protrusion of cell membrane and cytoplasm) that is often quickly extended in the direction of movement. Stained preparations of the genus Entamoeba show a characteristic ring-shaped nucleus with a central nucleolus and chromatin granula on the nuclear membrane. Trophozoites that have penetrated into tissues often contain phagocytosed erythrocytes.

■ The spherical, nonmotile cysts (10-16 im) have a resistant cyst wall. At first each cyst contains a uninucleate ameba, with glycogen in vacuoles and the so-called chromidial bodies, which are cigar-shaped. The nucleus divides once to produce the binuclear form and later once again to produce the infective tetranuclear cyst (Fig. 9.11c). The cysts are eliminated in the stool of infected persons, either alone or together with trophozoites.

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