The distinct life cycles of tritryp parasites.

Panels A–C show the life cycles of T. cruzi, Leishmania spp, and T. brucei, respectively. In each panel, some of the parasite stages present in their insect vectors, T. cruzi epimastigotes, Leishmania promastigotes, and T. brucei procyclic forms, are shown on the left. Different sand fly species of the genera Lutzomyia and Phlebotomus are vectors for Leishmania. Triatoma infestans and Rhodnius prolixus are the most important vector species in the transmission of T. cruzi to man, whereas different species of Glossina, also known as tse-tse fly, are vectors of African trypanosomes. Leishmania and T. brucei parasites move from the fly midgut up to the mouthparts before being inoculated into the human host as metacyclic, infective forms. Although Leishmania promastigotes achieve their journey in sand flies by being regurgitated from the stomodeal valve to the mouthparts, T. brucei epimastigotes do not stay in the mouthparts, as they have to first migrate from the proventriculus to the salivary glands where they develop into metacyclic forms and are expelled with the insect saliva. In contrast, T. cruzi infective metacyclic trypomastigotes develop in the hindgut of the triatomine bug and, after being excreted with the insect feces, gain access to the mammalian host bloodstream through skin wounds or the mucous membranes. On the right side of each panel, parasite forms present in the mammalian host, T. cruzi trypomastigotes, and intracellular amastigotes, Leishmania intracellular amastigotes, and T. brucei bloodstream forms are shown. Whereas Leishmania promastigotes are internalized by host phagocytes and reside into the phagolysosome, T. cruzi trypomastigotes actively invade a variety of nonphagocytic cells and are able to escape from the phagocytic vacuole and multiply in the host cell cytoplasm. Although distinct developmental forms of T. brucei are found in the mammalian host, namely stumpy and slender trypomastigotes, they remain extracellular during the entire parasite life cycle and were represented here as bloodstream trypomastigotes. Panel D shows a phylogenetic analysis inferred from glycosomal glyceraldehyde 3-phosphate dehydrogenase (gapdh) nucleotide sequences from 16 trypanosomatid species, with the species that have an intracellular stage shown with a light blue color. The maximum likelihood tree was constructed with 849 nt (80% of gapdh coding sequences), using SeaView v.04 and rooted at the Crithidia fasciculata/A. deanei clade, with the bootstrap values for 1,000 replicates shown in the major basal nodes.