Trypanosoma

Trypanosoma is a genus within the kinetoplastids, a group of single-celled parasitic flagellates in the phylum Euglenozoa. The name comes from the Ancient Greek *trypano-* (borer) and *soma* (body), referring to their corkscrew-like motion. Most species are heteroxenous, meaning they need more than one obligatory host to complete their life cycle, and are typically transmitted by a vector. While the majority are spread by blood-feeding invertebrates, transmission mechanisms vary; for instance, *Trypanosoma equiperdum* spreads between horses and other equines through sexual contact. In their invertebrate host, they are usually found in the intestine, but in the vertebrate host, they occupy the bloodstream or an intracellular environment. These parasites infect a wide range of hosts and cause serious diseases, including sleeping sickness in humans (from *Trypanosoma brucei*) and Chagas disease (from *Trypanosoma cruzi*). The mitochondrial genome of *Trypanosoma*, known as the kinetoplast, consists of a complex network of catenated circles and minicircles and requires various proteins for organization during cell division.

The history of the genus begins in 1841, when Gabriel Valentin observed flagellates now classified under *Trypanoplasma* in the blood of trout. The genus itself was named *Trypanosoma* (as *T. sanguinis*) by Gruby in 1843, after finding parasites in frog blood. In 1903, David Bruce identified both the protozoan parasite and the tsetse fly vector responsible for African trypanosomiasis.

Taxonomically, early phylogenetic analyses using 18S ribosomal RNA sequences suggested that *Trypanosoma* (as traditionally defined) was not monophyletic, with the biflagellate Bodonida nested within it. These analyses indicated an ancient split between a branch containing all Salivarian trypanosomes and another containing all non-Salivarian lineages. The latter branch then splits into a clade of bird, reptilian, and Stercorarian trypanosomes infecting mammals, and a clade with fish trypanosomes and reptilian or amphibian lineages. However, more recent analyses combining the glyceraldehyde phosphate dehydrogenase gene with SSU rRNA, whole genomes, and the kinetoplast genome show that *Trypanosoma* (as traditionally defined) is indeed monophyletic, despite the distinct clades formed by *T. cruzi* and *T. brucei*. The division into Salivaria and Stercoria remains valid, as each corresponds to a clade in these analyses.

The Salivaria group includes African trypanosomes, transmitted to the vertebrate host in the saliva of tsetse flies (*Glossina* spp.). Antigenic variation, well-studied in *T. brucei*, is a shared characteristic. Subgenera include *Trypanozoon* (containing *T. brucei*, *T. rhodesiense*), *Duttonella* (containing *T. vivax*), and *Nannomonas* (containing *T. congolense*). The Stercoraria group includes Central and South American trypanosomes, transmitted in the feces of insects from the subfamily Triatominae (notably *Triatoma infestans*). Subgenera include *Schizotrypanum* (containing *T. cruzi* and several bat trypanosomes such as *T. cruzi marinkellei*, *T. dionisii*, *T. erneyi*, *T. livingstonei*, and *T. wauwau*, plus a few unnamed species) and *Herpetosoma* (containing *T. lewisi* and *T. rangeli*, though including both makes this subgenus paraphyletic).

Evolution of the group follows the general structure of these groups and subgenera. Key questions include how trypanosomes first became parasitic, how they became dixenous (parasitizing two hosts), and how they came to infect vertebrates, mammals, and humans. The emergence of parasitism dates to the origin of the order Trypanosomatida, which includes both trypanosomes and *Leishmania*; *Bodo saltans* is a close relative of the common ancestor. A dixenous lifestyle evolved from a monoxenous ancestor at least three times: in *Phytomonas*, in vertebrate-infecting *Trypanosoma*, and in vertebrate-infecting *Leishmania*/*Porcisia*/*Endotrypanum*. The Salivaria split from other trypanosomes about 150 million years ago, likely living in insect guts until tsetse flies provided access to mammalian blood. The common ancestor of the *T. cruzi* clade (approximately *Schizotrypanum*) appeared about 84 million years ago, shortly before the diversification of bats, which likely contributed to the group’s diversification and ultimately to a trypanosome that infects many mammals.

Selected species include *T. ambystomae* (amphibians), *T. antiquus* (extinct, found in Miocene amber), *T. avium* (birds and blackflies), *T. bennetti* (birds and biting midges), *T. boissoni* (elasmobranchs), *T. brucei* (with subspecies *T. b. brucei* causing nagana in cattle; *T. b. equiperdum* causing dourine in horses, spread through coitus, arising separately from *T. b. evansi* in Eastern Africa; *T. b. evansi* causing surra in camels and other animals, with a single human case reported in India in 2005, emerging independently twice in Western Africa; and *T. b. gambiense* and *T. b. rhodesiense* both causing sleeping sickness in humans), *T. cruzi* (causing Chagas disease in humans), and *Trypanosoma culicavium* (infecting birds).