Entamoeba histolytica and Other Intestinal Protozoa. Pathogenesis and Immunology

Abstract

Entamoeba histolytica provides a striking example of how the infliction of harm to its host is altogether counterproductive for the normal propagation of the parasite. Paradoxical as it may appear, people with amebic dysentery and an ulcerated colon are noninfective to others (except under special circumstances to be considered later). True enough, the feces of such a person swarm with amebae, highly motile and full of red blood cells and tissue debris. But such amebae die quickly in the external environment. Even if swallowed immediately by another person, or if fed to a susceptible animal, such as a dog or a kitten, they cannot establish infection since they are killed by the acidity of the stomach. Only the cysts can survive externally for some days in a moist environment at ordinary temperatures, and only the cysts are able to pass through the stomach without injury and to excyst in the intestine (see Chapter 2). For reasons that are not understood, no cysts are formed when there is active amebic disease of the colon. Similarly, the other main manifestation of amebic disease, abscess of the liver, again constitutes a blind alley for the parasite. Cysts are not formed, and even if formed they would have no outlet from the host. It is only in healthy carriers, when relatively smaller amebae live mainly as harmless commensals in the lumen of the colon, feeding largely on bacteria, that large numbers of the cysts essential to infection of other hosts are formed. Over a million cysts per gram of feces may be excreted daily by such a person. What triggers cyst formation remains one of the unsolved major problems in the biology of E. histolytica. As already noted (Chapter 8), in axenic culture encystation has never been induced. The only way to get cysts in vitro is still the method developed years ago by that pioneer protozoologist C. Dobell, requiring cultivation with bacteria without rice starch and the subsequent addition of rice starch. This experimental difficulty in obtaining cysts remains as a hindrance in work with animal models of amebiasis.