Intestinal protozoal infection produces substantial morbidity and mortality in people of all ages, particularly in tropical and subtropical parts of the world. Amebiasis, giardiasis, cryptosporidiosis, and those infections associated with acquired immunodeficiency syndrome (AIDS) are important problems for health in many parts of the world, but some protozoa found in the human digestive system do not cause disease. Vaccines are not yet available for protection against these infections, and many are difficult to treat. The intestinal protozoa that produce important human infections are summarized in Table 201.1.

ENTAMEBA HISTOLYTICA

Entameba histolytica causes dysentery, chronic colonic amebiasis, and hepatic amebiasis. The last topic is dealt with in Chapter 202, Extraintestinal Amebic Infection. Amebic dysentery is a syndrome of bloody diarrhea caused by invasion of the colonic wall by trophozoites of E. histolytica. It is common in many parts of the world, especially West and southern Africa, Central America, and south Asia. In the United States, 3000 to 4000 cases are reported each year. There is now consensus that the species formerly recognized as E. histolytica in fact comprises two species: E. histolytica and E. dispar. The first is the pathogenic protozoan long associated with human invasive disease and with hepatic amebiasis, and the latter is a morphologically identical nonpathogenic protozoan first recognized as the nonpathogenic zymodeme of E. histolytica. The latter does not require treatment, but it cannot be differentiated from E. histolytica morphologically.