FIG. 1.

Life cycles of Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. Trypanosomatids have digenic life cycles, which alternate between an insect vector, usually responsible for disease transmission, and a mammalian host. In the invertebrate hosts, trypanosomatids infect the gut, wherein they adopt a flagellated form and an extracellular life style. These insect vector stages are designated as procyclic (T. brucei), epimastigote (T. cruzi), or promastigote (Leishmania spp.). In the case of the mammalian stages of these organisms, their morphology and residence varies according to the parasite species. Trypanosoma brucei resides as a flagellated, extracellular cell (the trypomastigote or bloodstream form) in the circulatory and lymphatic systems of its vertebrate hosts. As for T. cruzi, it alternates between an extracellular, flagellated, and nonreplicative trypomastigote form, and an intracellular, aflagellated, and replicative amastigote stage. Almost any cell, phagocytic or nonphagocytic, can be infected by T. cruzi, although the parasite has a tropism for muscle and nervous cells. Finally, Leishmania spp. resides and replicates as nonmotile amastigotes inside the phagolysosomes of macrophages. Also depicted are a tse-tse fly, a triatomine bug, and a sandfly, the insect vectors for T. brucei, T. cruzi, and Leishmania spp., respectively. These organisms cause diseases in humans and also in cattle (T. brucei) and in dogs (Leishmania spp.).