[trih-pan-uh-sohm, trip-uh-nuh-]
trypanosome, 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. gambiense and T. rhodesiensecauses African sleeping sickness and is 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.

Trypanosomes are a group of kinetoplastid protozoa distinguished by having only a single flagellum. All members are exclusively parasitic, found primarily in insects. A few genera have life-cycles involving a secondary host, which may be a vertebrate or a plant. These include several species that cause major diseases in humans.

The most important trypanosomal diseases are trypanosomiasis (African Sleeping Sickness and South American Chagas Disease); these are caused by species of Trypanosoma. The Leishmaniases are a set of trypanosomal diseases caused by various species of Leishmania.

A variety of different forms appear in the life-cycles of trypanosomes, distinguished mainly by the position of the flagellum:

Amastigote (leishmanial) - reduced or absent
Promastigote (leptomonad) - anterior of nucleus, free from cell body
Epimastigote (crithidial) - anterior of nucleus, connected by a short undulating membrane
Opisthomastigote (herpetomonad) - posterior of nucleus, passing through a long groove in the cell
Trypomastigote (trypanosomal) - posterior of nucleus, connected by a long undulating membrane

Most trypanosomes have at least amastigote and promastigote stages. Trypanosoma appears in all five forms, with the trypanosomal stage occurring in the vertebrate host. Trypanosoma brucei sub-species have two forms in the bloodstream of a vertebrate host, the rapidly dividing long-slender form and the non-dividing short stumpy form. The short stumpy parasites are adapted for uptake into the tsetse fly vector, and are non-proliferative in comparison with the slender forms.

Unique to the African trypanosome Trypansoma brucei is the expression of a variable surface glycoprotein (VSG) coat on the cell surface, which undergoes constant variation in order to evade the humoral immune system and host antibodies. It is thought that recombination from a repertoire of about 100 complete VSG genes, and a large number of VSG-related sequences, is responsible for the vast diversity of the parasite. This recombination would retain effectiveness in immune evasion by maintaining diversity.


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