The pathogenesis of amoebiasis.

Amoebiasis, the infection of humans with Entamoeba histolytica, has a worldwide distribution; humans are the main reservoir and source of infection(1), although some other primates can also be infected. The motile trophozoite of E. histolytica (Fig. 1) lives in the lumen of the large intestine where it multiplies and eventually differentiates into cysts which are shed in the faeces and are responsible for transmission of infection. Two forms of amoebiasis are recognized: luminal amoebiasis where no clinical signs or symptoms are apparent, and invasive amoebiasis where the trophozoites invade the intestinal mucosa to produce dysentery or amoeboma, and can spread in blood to give extraintestinal lesions such as liver abscess. Isoenzyme markers for pathogenic and non-pathogenic types of E. histolytica are well documented, but there is some debate (see Parasitology Today, vol. 3, 37-43) about whether the two types represent completely separate entities or if they can change from one type to the other under certain circumstances (Box 1). Nonpathogenic types produce no apparent symptoms; in this article Adolfo Martínez-Palomo discusses the pathology associated with pathogenic types.