Trypanosoma parasites are flagellate protozoa belonging to the family Trypanosomatidae, which includes blood and tissue parasites of vertebrates, usually transmitted by blood-feeding vectors. Six trypanosome species are known to infect dogs: T. brucei, T. caninum (unknown pathogenicity), T. congolense, T. cruzi, T. evansi and T. rangeli (non-pathogenic).

Trypanosomes are characterised by a longitudinal, fusiform appearance, a flagellum which arouses from the basal body moving forwards, an undulating membrane and a kinetoplast, lying behind the basal body.

The developmental cycle of trypanosomes is typically heteroxenous (that is, part of the cycle occurs in a vertebrate and part in an invertebrate host), although the cycle of one species (T. caninum) is still unknown. A cyclic development takes place in the insect host. Trypomastigote forms are taken up by the insect during feeding. Depending on the species, the trypanosomes multiply within the insect host in different locations. At the end of the development, metacyclic forms are produced which are transmitted to a new host during feeding.

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