Trypanosomes are parasites of all classes of vertebrates. Within this one genus there is a variety of life cycle patterns. Two major patterns are related to whether the trypanosome is belongs to the salivarian or stercorarian subgroups. The Salivaria are those trypanosomes that transmit from the "anterior station" of the insect host; i.e. they are in the saliva and are inoculated into the host during feeding. The Stercoraria transmit from the "posterior station"; i.e. infective stages are passed in the feces of the insect during feeding. These may contaminate the wound caused by the feeding insect, be scratched into the skin, or enter when transferred to a mucous membrane.
This is an excellent example of two species in the same genus possessing quite diferent life cycles and evenlife cycle stages. This course does not require you to identify parasites to species, but this is also an example of why this is an important ability in a practical situation. You should know that both these types of life cycles exist within this group of parasites.
Trypomastigote forms circulate within the blood of a vertebrate host. They are extracellular parasites. A blood-feeding vector such as a tse-tse fly ingests trypomastigote along with its blood meal. Trypomastigotes continually divide by fission within the midgut over a period of 1-2 weeks. Over the next week they migrate anteriorly in the gut, eventually entering the salivary gland and accumulating there. Within the salivary gland the parasite transforms into an epimastigote form and continues to divide. They eventually transform into a metacyclic trypomastigote and are now infective to another vertebrate host. After inoculation into the blood stream of a new vertebrate host the parasite continues to divide in the trypomastigote form.
Trypomastigote forms circulate within the blood of a vertebrate host. They are extracellular parasites. A blood-feeding vector such as a bed bug ingests trypomastigote along with its blood meal. Trypomastigotes transform into epimastigotes within the midgut After about 1 week they transform into metacyclic trypomastigotes. When the host feeds, it often defecates at the same time, releasing metacyclic trypanosomes that contaminate the wound. Within the vertebrate the trypomastigotes are phagocytosed by host defense cells or they may enter other cells such as cardiac muscle. During this intracellular phase of existence they transform into amastigotes and multiple by fission. Large numbers of amasitgotes accumulate and form pseudocysts. Eventually the amastigotes transform into trypomastigotes, rupture the host cell, and enter the circulation. They may be ingested by a vector or infect a new host cell, again becoming amastigotes and repeating the cycle.