The main groups of trypanosomes recognized in molecular phylogenetic analyses
A brief description of the main trypanosome clades follows (Fig. 15.1A):
• The Aquatic clade: This clade comprises trypanosomes of mainly aquatic and amphibious vertebrates, including all species from fish and several from amphibia. The vertebrate hosts also include reptiles (caiman, chameleon, turtle) and a mammal (platypus).25’21,28’68-10 Many of these species are transmitted by proboscid leeches.11-11 The platypus trypanosome has also been found in a leech, possibly implicating it as a vector.18 There is evidence that some of the trypanosomes from amphibious vertebrates are transmitted by insects,16 and the chameleon trypanosome presumably also has an insect vector. Considerable diversity within this clade has been found in amphibians and sandflies in South America.19
• The T. cruzi clade: See “T. cruzi Clade” section.
• The T. pestanai clade: This clade contains trypanosomes from the Eurasian badger (T. pestanai),8,80 ticks from Japan,81 Australian marsupials, T. copemani, T. gilletti and T. vegrandis,8,25,26’62’82’83 and Brazilian dogs, T. caninum.84 There is some evidence implicating fleas in the transmission of T. pestanai80 and evidence implicating ticks in the transmission of these Australian marsupial trypanosomes.85
• The T. lewisi clade: This clade contains trypanosomes from a wide range of rodents. It also contains trypanosomes from a lagomorph and insectivores.86-88 Each trypanosome species in this clade is thought to be specific to its vertebrate host. The vast majority of the vectors of these trypanosomes are fleas, although the lifecycles of some species are not completely known.1 One exception is T. talpae, from a mole, that is believed to be transmitted by mites.89 The clade contains T. lewisi, the type species of the subgenus Herpetosoma, thus the name Herpetosoma is occasionally used for this clade.16
• The T. irwnini clade: This clade contains T. irwnini from an Australian marsupial (koala), T. bennetti from an American kestrel,26 and T. minasense from a South American primate (red-handed tamarind, Saguinus midas), imported into Japan90; see “T. cruzi Clade” section— T. rangeli for further discussion on this species. The presence of both mammalian and avian trypanosomes within this clade demonstrates that host-switching between different vertebrate classes has occurred, which is unusual among trypanosomes of terrestrial vertebrates.
• The crocodilian clade: This clade comprises crocodilian trypanosomes from Africa and South America and includes T. grayi, transmitted by tsetse flies in Africa.60’91 A recent analysis found some strains in South America and Africa to be closely related, suggesting dispersal after continental separation.92 This clade is more closely related to T. cruzi than to the other trypanosomes transmitted by tsetse flies in the T. brucei group.15
• The T. brucei clade: This clade contains mostly trypanosomes of African mammals. Two subspecies of T. brucei—T. b. gambiense and T. b. rhodesiense—are also human pathogens. Many of these species are pathogens of domestic livestock (T. brucei, T. congolense, T. evansi, T. godfreyi, T. simiae, T. suis, and T. vivax). The majority of trypanosomes in the T. brucei clade are transmitted by tsetse flies (genus Glossina) via the saliva when the fly takes a blood meal. T. congolense and T. simiae are further split into “types” that are, arguably, sufficiently different to warrant species status. gGAPDH sequences from tsetse flies in East Africa suggest the existence of two further species related to T. vivax,30 and 18S rDNA sequences from tsetse flies in Central Africa demonstrate the existence of a further undescribed species.19 Two trypanosomes in the clade, T. evansi and T. vivax, are known from South America, and are believed to have been accidentally introduced into the continent by humans in domestic animals.1,93 DNA microsatellite data support a single introduction of T. vivax into South America from West Africa.94
• The T. theileri clade: This clade contains trypanosomes from marsupial and placental mammals (deer, cattle, primates). It includes T. theileri, the type species for the subgenus, that is commonly found in domestic cattle across the world.56,95 There are two distinct subgroups of Megatrypanum, TthI and TthII, which are arguably different species, with considerable genetic diversity in each.95Megatrypanum in both subclades, when found in cattle is generally referred to as T. theileri11 even though each subclade includes isolates from various other hosts including deer and duikers. T. (Megatrypanum) melophagium a parasite of sheep transmitted by the sheep ked (Melophagus ovinus) is in one of the subgroups and appears to be host specific.96,91 T. theileri is known from South American cattle and buffalo, with distinct strains in each.95 Tabanid flies act as the principal vectors of T. theileri, although ticks also have the capacity to transmit this trypanosome,1,95,98-100 and a high prevalence in tsetse flies raises the question of whether they also act as vectors.19 Stevens et al.12 found a monkey trypanosome, T. cyclops from Southeast Asia, to be related to T. theileri, but fell outside the Megatrypanum group. Subsequently this trypanosome formed a distinct subclade with trypanosomes from Australian marsupials (wallabies) and terrestrial bloodsucking leeches (Haemadipsidae). These leeches are common in tropical forests across Asia and Australia implicating them in the transmission of this trypanosome subgroup.56 The name Megatrypanum has been used for the T. theileri clade 16,95 but is now more commonly used for the two subclades that contain T. theileri.11
• The lizard/snake clade: This clade contains trypanosomes of lizards and snakes.61 Sandflies are the only known vectors of trypanosomes in this clade.91’101
• The avian clade: The two main subclades of avian trypanosome, the T. avium and the T. corvi clades, ’ first grouped together in this clade in a tree produced from combined data from gGAPDH and 18S DNA sequences.47 A wide range of insect vectors transmit the trypanosomes in this clade, including black flies,104’105 hippoboscid flies, and mosquitoes.106’107 Both subclades have a wide distribution, e.g., the T. corvi clade is found in both Australia and Europe.56
The T. cruzi clade
This clade contains trypanosomes that are parasitic in a diverse range of mammals, including two human-infective parasites: T. cruzi and T. rangeli, both of which are restricted to the New World. It also contains trypanosomes from Chiropteran (bat) hosts from both the Old and New Worlds (T. erneyi, T. cruzi T. marinkellei, T. dionisii, T. livingstonei, T. vespertilionis, and several unnamed species). Other trypanosomes within the clade are T. conorhini, a rat parasite, and trypanosomes from African terrestrial mammals (monkey, civet)62,108 and Australian marsupials.8,23,25,27,28,83 The majority of known invertebrate vectors of these trypanosomes are bugs (suborder Heteroptera; order Hemiptera). Three species are transmitted by triatomine bugs: Trypanosoma rangeli and Trypanosoma cruzi by a wide range of species, and Trypanosoma conorhini by Triatoma rubrofasciata. The bat trypanosomes are also thought to be transmitted by bat-feeding bugs. For instance, infections of T. cruzi marinkellei have been described in the bat-feeding triatomine bugs of genus Cavernicola,109 while cimicid bugs in the genus Cimex are frequently found infected with trypanosomes and have been implicated in the transmission of three species of bat trypanosome: T. dionisii, T. incertum, and T. vespertilionis110-112 An exception to bug-mediated transmission may be the Australian marsupial group trypanosomes, which have been identified in tabanid flies, potentially implicating them as vectors.113 The vectors of the trypanosomes from the African terrestrial mammals and other bats are as yet unknown. Interestingly, Salazar et al.114 found that the common bed bug, Cimex lectularius, has the capacity to transmit Trypanosoma cruzi, although the epidemiological importance of this finding is not known.
The T. cruzi clade contains several trypanosomes originally placed within the subgenus Schizotrypanum on the basis of their morphology and lifecycles, prior to phylogenetic evidence. The close relationship between trypanosome species originally classified in subgenus Schizotrypanum is not surprising, as the subgenus has well defined morphology and development within the vertebrate host. The subgenus comprises small trypanosomes that are very difficult to distinguish morphologically from the type species, T. cruzi. They have a voluminous kineto- plast and, typically, curved bloodstream forms in the shape of a C or S, with a short pointed posterior end, which constitute distinctive morphological characters.1 Within the vertebrate host, multiplication occurs within various tissues and organs, rather than in the blood (like many other trypanosome species). Several T. cruzi- like trypanosomes from non-bat South American wild mammals have been described using traditional parasitological techniques: T. (S.) lesourdi Leger and Porry, 1918 from a Spider monkey; T. (S.) prowazeki Berenberg-Gossler, 1908 from a Uakari (a species of New World monkey); and T. (S.) sanmartini Garnham and Gonzales-Mugaburur, 1962 from a Squirrel monkey. It is also possible that several (and perhaps all) of the morphologically described Schizotrypanum species in South American terrestrial mammals represent synonyms of T. cruzi (see next section of this chapter). Likewise, a range of bat trypanosomes have been classified within the subgenus Schizotrypanum using parasitological techniques, including two from Australia, T. (S.) pteropi Breinl, 1921 and T. (S.) hipposideri Mackerras, 1959; and T. (S.) hedricki, T. (S.) myoti and T. (S.) dionisii from elsewhere in the Old World. Morphological similarities between these bat trypanosomes have made it difficult to delineate species and to match molecular data with old parasitological descriptions.
Trypanosomes within the T. cruzi-clade are listed below (Fig. 15.1B):
• T. cruzi cruzi: Also called T. cruzi sensu stricto.
• T. cruzi marinkellei: This bat trypanosome is apparently restricted to South America and was sufficiently divergent to warrant subspecies status.115 Its close relationship to T. cruzi cruzi has been verified using 18S rDNA,8gGAPDH gene,10’47 and kinetoplast Cyt b gene13 sequences. The genome of this species has been sequenced and analyzed.116
• T. dionisii: This bat trypanosome was first described from Europe.117,118 A strain of this species has been found in South America13,119 and is closely related to one of the two known European strains.62
• T. erneyi: This bat trypanosome was described from Molossidae bats captured in Mozambique, Africa and is the closest known living relative to T. cruzi.67
• T. sp. (civet) and T. sp. (monkey): These trypanosomes were isolated in a study that examined trypanosome diversity in a wide range of wild vertebrates in Cameroon, West Africa.108 In that study they remained unidentified, but later characterization, by sequence analysis of their 18S rDNA and gGAPDH genes, demonstrated—for the first time—the presence of the T. cruzi clade in non-bat hosts in Africa.62
• T. conorhini: A trypanosome found worldwide in rats and transmitted by the triatomine bug T. rubrofasciata.1 The origin of T. conorhini is far from clear, but evidence suggests an Old World origin for this species. Its vector, T. rubrofasciata, is part of a group with very close affinity to an Old World genus, Linshcosteus.120 Interestingly, trypanosome species that resemble T. conorhini have been described in Indonesian primates121; under laboratory conditions, they developed in a triatomine bug, T. rubrofasciata,122 a natural vector of T. conorhini. This led Hoare1 to argue that these Indonesian trypanosomes were primate-adapted strains of T. conorhini. On the other hand, Weinman122 argued that Indonesian monkey trypanosomes are unlikely to be T. conorhini, as T. rubrofasciata is rat-specific and, while it has been found in cities, it has never been found in the tropical rainforests of Asia.
• T. livingstonei: This trypanosome was described from bats (Rhinolophus landeri and Hipposideros caffer) captured in Mozambique in Africa and is the most divergent member of the Trypanosoma cruzi clade.123
• T. vespertilionis: This is a widely distributed trypanosome of bats. The single isolate included in phylogenetic trees is from Europe.8’10’47 There is some debate as to whether the isolate used to obtain sequences for phylogenetic trees represents the originally described species on the basis of morphology.124
• T. rangeli: This trypanosome is restricted to South America and has a wide mammalian host range including humans’ although it is not pathogenic to human hosts. A high prevalence of T. rangeli in humans has been reported in Central America and northwestern South America.125 The only known vectors are triatomine bugs of the genus Rhodnius. The inclusion of this species within the T. cruzi clade66 resolved the debate regarding the classification of this species; its ability to develop in salivary glands and be transmitted through saliva of triatomine bugs (although it is also transmitted via feces), had led to it being classified within the Salivaria’ while resemblance of the bloodstream forms to T. lewisi led to classification within the subgenus Herpetosoma.1 Comparisons of a wide range of isolates of this species using sequences of the spliced-leader gene, 18S rDNA and ITS regions, and fluorescent fragment length barcoding (FFLB)22 have revealed four lineages in terrestrial mammals (Lineages A, B, C, and D) and Lineage E, which is apparently restricted to bats.126,127 Lineage divergence appears to be associated with species of Rhodnius, without any clear association of trypanosome lineages with particular vertebrate hosts.
• An unnamed bat trypanosome: This trypanosome was isolated from a fruit bat Rousettus aegyptiacus from Gabon in West Africa. Sequence analysis based on 18S rDNA indicated that it was only distantly related to the other bat trypanosomes in the
• Australian mammal group: A trypanosome from a kangaroo was isolated in Australia.25 Analysis by Stevens et al.8 provided strong support for placing this taxon within, but at the extremity of, the T. cruzi clade; as such, this taxon played a major part in hypothesizing the origin of the T. cruzi clade (see next section). Related trypanosomes that are sufficiently different to warrant being classified as separate species have subsequently been found in a native Australian rodent, Rattus fuscipes,128 possums, and quokka (Setonix brachyurus) ; see Thompson and Thompson for a full listing of trypanosomes identified from Australian mammals. Related trypanosomes have also been identified in tabanid flies in Australia, potentially implicating them as vectors.113
• Madagascar group: Trypanosome 18S rDNA sequences were recently identified in transcriptomes produced from the blood of two endangered wild lemurs (Indri indri and Propithecus diadema) in Madagascar.63 These sequences are most similar to those of the Australian group, differing by only 1.7%.63
• T. spp. Neobats: This group of three closely related bat trypanosomes was first described in bats from the Panama Canal Zone. In this study phylogenetic analysis placed the group near the base of the T. cruzi clade, although resolution within the clade was insufficient to determine its exact relationship with T. livingstonei.129 Later this group was named T. spp. Neobats.64
• T. wauawu: This recently described trypanosome has, as yet, only been found in Pteronotus bats (Mormoopidae) in South America from Guatemala to the Brazilian Atlantic Coast. T. wauwau is a sister taxon to T. spp. Neobats.64 Together these are related to the Australian mammal group in this clade, providing additional support for the bat origin of the T. cruzi clade.