Protozoan, Intestinal

Abstract

Diarrheal diseases due to intestinal protozoa remain a major public health problem in the developing world. This article reviews pathogenesis of disease due to the major intestinal protozoa that infect humans. Here, intestinal protozoa belonging to the phyla Apicomplexa ( Cryptosporidium species, Isospora belli , and Cyclospora cayetanensis ), Microsporidia ( Encephalitozoon intestinalis and Enterocytozoon bieneusi ), and Sarcomastigophora ( Giardia intestinalis and Entamoeba histolytica ) are discussed. Although these parasites have varied biology, they emerge with major common themes. First, the infectious stage of each parasite is able to survive harsh environmental conditions and transit through the stomach. Second, the vegetative stage of each parasite is able to adhere to host epithelial cells. Third, each parasite has mechanisms to evade the host immune response. Fourth, disease in each case results from a combination of parasite factors (e.g., parasite-mediated cell killing) and immune-mediated tissue damage. And fifth, susceptibility to disease due to intracellular pathogens (i.e., the Apicomplexa and Microsporidia) is greater in individuals with cell-mediated immune defects, but susceptibility to disease due to extracellular parasites (e.g., G. intestinalis and E. histolytica ) appears to be unaffected by defects in cell-mediated immunity. Further studies toward understanding the general mechanisms of pathogenesis and immunity are needed to develop improved methods for treatment and prevention of infection by these important pathogens.