trypanosome trĭpˈənəsōmˌ [key], microscopic, one-celled protozoan of the genus Trypanosoma, typically living as an active parasite in the bloodstream of a vertebrate; hundreds of species are known. A trypanosome is long and pointed and possesses a flagellum. The flagellum arises at the front, or anterior, end of the parasite and curves back to form the edge of a long, undulating membrane used in locomotion. Trypanosomes have a complex life cycle; most species undergo part of their development in the digestive tract of insects, which spread the parasite by biting.

Many trypanosomes do not appear to harm their hosts, but a number of species cause serious diseases in humans or domestic animals (see trypanosomiasis). T. brucei gambiense and T. brucei rhodesiense cause African sleeping sickness and are transmitted by tsetse flies. T. cruzi is the cause of Chagas' disease, prevalent in South and Central America, which affects the nervous system and heart; it is transmitted by the bite of assassin bugs. Other species, restricted in distribution to Africa and Asia, cause diseases of horses and cattle. Control measures include elimination or reduction of the insect carrier populations and measures to reduce the likelihood of bites.

Trypanosomes are classified in the phylum Mastigophora of the kingdom Protista.

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