Intestinal protozoan infections involving humans are by no means confined to tropical/subtropical countries; however, it is here that maximal prevalence, and consequently, morbidity, assumes a major practical importance. Coccidial infections (Cryptosporidium spp., Isospora belli, Sarcocystis hominis and Cyclospora spp.), and Blastocystis hominis and Microsporidium spp., which were previously underrecognised, have come to the fore in the present era, largely in association with HIV infection. Nevertheless, these organisms can induce self-limiting infection in travellers and other individuals in countries in which standards of sanitation/public health are less than satisfactory. Overall, however, there can be no doubt that Giardia lamblia (also known as G. intestinalis and G. duodenalis) and Entamoeba histolytica are numerically the most important protozoan parasites to involve the gastrointestinal tract. Whereas trophozoites of the former organism are virtually confined to the small intestinal lumen, the extra-intestinal manifestations of E. histolytica infection are of greater importance than its colorectal pathogenic properties.
This review concentrates on the prevalence of the 2 infections in different populations; this reflects both incidence and outcome, including duration of illness. Epidemiology is also addressed. Routes of transmission of the 2 infections are covered, together with morbidity and, in the case of E. histolytica infection, mortality also.