Fig 1. Life cycle variation among African trypanosomes.

The passage and development of African trypanosomes through the fly vector varies by species. The figure shows the route taken by each species through the vector and position of each developmental stage. Insect anatomy is abbreviated thus: proboscis (P), mid-gut (MG), salivary gland (SG) and proventriculus (PV). The life cycle within the fly is defined here by four stages: (1) Ingestion of bloodstream-form parasites; (2) migration of parasites to the insect mid-gut with differentiation into procyclic forms (T. vivax lacks this stage); (3) migration anteriorly to the proboscis (T. vivax), proventriculus (T. congolense) or salivary gland (T. brucei) and differentiation into epimastigote forms; and (4) differentiation into metacyclic forms and inoculation into the vertebrate host upon insect feeding.