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Distribution of Triatominae in the Americas

Genus Rhodnius

At the moment, the genus Rhodnius consists of 18 valid species (16 from Galvao et al.2 plus two additional recent additions). All Rhodnius species have been primarily associated with palm trees even though some species were found in other syl- vatic habitats. Thus, R. domesticus has been reported as colonizing bromeliads and hollow trees in Amazonia, and specimens of Rhodnius species have been found in the cactus mandacaru (Cereus jamacaru) in the Caatinga of northeastern Brazil.1,25,26 Some Rhodnius species may be associated with particular types of palms, such as Rhodnius brethesi with Leopoldinia piassaba and Rhodnius ecuador- iensis with Phytelephas species. Consequently, the distribution of sylvatic Rhodnius broadly coincides with the distribution of palm trees.25 Recent work suggests that deforestation and the associated loss of habitat and host diversity might increase the frequency of Rhodnius—human contact.22,27

The genus Rhodnius encompasses a variety of species, including one primary domestic vector (R. prolixus), various synanthropic species that invade and sporadically colonize man-made ecotopes (R. pallescens, R. neglectus, R. nasutus, R. stali, and R. ecuadoriensis), species that invade but do not colonize houses (R. robustus, R. pictipes, and R. brethesi), and strictly sylvatic species.27 Rhodnius naturally occurs from Central America to northern Argentina, and species richness is highest in Amazonia.

R. prolixus is a domestic vector of T. cruzi to humans in Venezuela, Colombia. It was an important vector in some countries of Central America, but national control programs successfully interrupted T. cruzi transmission by R. prolixus, and even eliminated the species in several regions.28 R. prolixus was likely erroneously recorded from Bolivia, Brazil, Ecuador, French Guiana, Guiana, Panama, and Suriname due to confusion with R. robustus (all countries) and R. neglectus (Central Brazil). R. prolixus is no longer found in its previously reported collection areas in Mexico (Oaxaca and Chiapas), Guatemala, El Salvador, and most of Honduras. Sylvatic populations have been captured in palm trees in Venezuela and the Orinoco region in Colombia but they have never been found in Central America.29—31 Moreover, neither sylvatic nor domestic populations have ever been collected in Panama or south of Costa Rica. The discontinuity of the distribution area combined with the genetic homogeneity of R. prolixus in Central America suggest that domestic vectors from the South American populations have invaded several countries of Central America.32 Two explanations have been put forward. One

explanation suggests that these vectors have been dispersed through passive carriage of eggs and young nymphs in the plumage of storks (Mycteria americana), which are known to migrate between the two regions and nest in palm trees.33 The second explanation suggests that these vectors have spread due to an accidental escape of laboratory-bred insects.34

The species forming the prolixus group (R. prolixus, R. robustus, R. neglectus, and R. nasutus) are particularly difficult to distinguish. The specific status of R. robustus, which is virtually indistinguishable from R. prolixus using morphological characters, has been clarified by the use of modern techniques of species characterization, such as the sequence analysis for a fragment of the mitochondrial cytochrome b. R. robustus currently includes four cryptic species. R. robustus I, which occurs in Venezuela (Orinoco region), is more closely related to R. prolixus than to the other three cryptic species found in the Amazon region.35 The near-sibling R. neglectus and R. nasutus are Brazilian species. The geographic distribution of R. nasutus is restricted to the northeastern region, particularly to the semiarid Caatinga. R. neglectus has a wider distribution across the Cerrado and the adjacent regions of Central Brazil.2,11 Recent studies show that both species are sympatric in the northern Bahia State.27

R. pallescens has been reported in Belize, Nicaragua, Costa Rica, Panama, and Colombia, where it inhabits sylvatic environments and it is often found in human dwellings although without building intradomestic colonies.36 The palm tree Attalea butyracea is its primary biotope. This species is characterized by its sporadic presence inside dwellings in Panama, where it is the only vector of T. cruzi to humans.37 R. stali is a fairly unknown Bolivian species that has been historically confused with R. pictipes. The distribution of Rhodnius stali closely matches that of Attalea phalerata palms in the southwestern fringe of the Amazon biome. R. stali is able to establish colonies in domestic and peridomestic habitats, and the observation of Chagas disease seropositivity in the indigenous population of the Alto Beni region strongly suggests the presence of an ongoing anthropozoonotic disease transmission cycle.38

R. ecuadoriensis survives under a wide range of climatic conditions in Ecuador and northern Peru.27 Sylvatic populations of this species have been mainly found living in the Phytelephas aequatorialis palm trees in northern Ecuador.1,27 The strong synanthropic behavior of R. ecuadoriensis and the absence of palm trees in southern Ecuador and northern Peru suggested that this species might have spread to the region through association with humans.39,40 The recent detection of R. ecua- doriensis in sylvatic habitat (squirrel nests) in the southern Highlands of Ecuador contradicts this hypothesis.41

R. brethesi is a species occurring in the Brazilian Amazon that seems to be tightly associated with L. piassaba palm trees. T. cruzi transmission in the Negro River region (Amazonas, Brazil) is caused by invasion of human dwellings by the vectors. Other observations attribute the attack of wild triatomines to the collectors of fronds from the piassaba palm.42 R. pictipes has a wide geographic distribution throughout the Amazon basin (north and northwest South America) in association with various species of palm trees.11 The sporadic invasion of houses by light-attracted adult R. pictipes may be promoted by the presence of palm trees near households. The ingestion of palm fruit juices contaminated with crushed vectors was documented in some outbreaks of acute oral Chagas disease, which is the main transmission mechanism of T. cruzi in the Amazon region.43

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