Domiciliation process of Rhodnius stali (Hemiptera: Reduviidae) in Alto Beni, La Paz, Bolivia

Authors


Summary

We report a systematic collection of Triatominae inside houses and in the peridomestic environment of Alto Beni, department of La Paz, Bolivia. This area is free of Triatoma infestans and although we detected previously seropositivity for Trypanosoma cruzi, the Alto Beni region is not officially considered as endemic for Chagas disease. From 11 houses of five localities, we collected adults, nymphs and eggs of a Rhodnius species, which was confirmed by morphological and morphometric analysis as Rhodnius stali. This little-known species has long been confused with R. pictipes, and was originally described from museum specimens labelled as R. pictipes. Our data show that R. stali is able to establish colonies in domestic and peridomestic habitats in Bolivia, and it is probably the vector responsible for Chagas disease seropositivity observed in the indigenous population of Alto Beni.

Introduction

The genus Rhodnius (Hemiptera, Reduviidae, Triatominae) comprises 14 species distributed in Central and South America (Dujardin et al. 2000). They are mainly of sylvatic habitat, living amongst the leaves of palms and epiphytic bromeliads, or in nests of birds or small mammals (Lent & Wygodzinsky 1979). Several are also of epidemiological significance as domestic vectors of Trypanosoma cruzi (causative agent of Chagas disease); particularly (i) Rhodnius prolixus in Venezuela, Colombia and some countries of Central America (Schofield & Dujardin 1997); (ii) R. pallescens in Panama (Christensen & Vásquez 1981; Ponce 1995) and northern Colombia (Lopez & Moreno 1995); and (iii) R. ecuadoriensis in southern Ecuador and northern Peru (Defranc 1987; Barreda 1995). Some of the sylvatic species have also been implicated in the transmission of T. cruzi to humans, either by flying directly to feed on people, as R. brethesi in the Amazon region (Coura et al. 1994), or by flying into peridomestic and domestic habitats where they may form small colonies, as R. neglectus in Brazil (Garcia Zapata et al. 1985). This latter process may represent the first step in adapting to domestic habitats, and could be the route towards an increasing vectorial importance (Dujardin et al. 2000). We report here this process in R. stali, a species of previously unknown ecology, originally labelled as R. pictipes (Lent et al. 1993), and we show consistent morphometric differences between the two taxa. Rhodnius stali is probably the vector responsible for Chagas disease transmission among indigenous communities of Alto Beni region of Bolivia.

Materials and methods

Insects

We examined a total of 160 houses, to search for Triatominae specimens, in eight indigenous communities of the Alto Beni, province Sud Yungas, department of La Paz, Bolivia. This is a region of tropical rain forest, at an altitude ranging from 500 to 800 m. Houses in the region are generally built with rustic materials such as palm leaves for roofs, and with walls made of wooden planks and cane shafts. In five of these communities, we detected adults, nymphs and eggs of a species of Rhodnius. Rectal contents of all the bugs of third (N3), fourth (N4) and fifth (N5) instars, and adults were expressed onto slides for microscopical analysis for infection with T. cruzi.

Morphometric analysis

The similarity of collected insects to topotype specimens of R. stali, and divergence from R. pictipes, was examined by a multivariate morphometric study. From the Alto Beni collection, we measured 10 adult specimens (four males and six females). For comparison, we also measured eight specimens of R. stali (four males and four females) from the type locality in the Chapare (Lent et al. 1993) collected by H. Bermúdez (unpublished data), together with eight R. pictipes (five males and three females), originated from the FIOCRUZ reference strain (from Pará state, Brazil), which had been confirmed as R. pictipes by isoenzyme analysis (Chavez et al. 1999).

The heads were cut at the level of the collar and glued to a small triangular plastic support. The wings were mounted on microscopic slides using Hoyer medium. Both structures were stored dry at room temperature for morphometric analysis, while legs and thorax were stored at −20 °C for subsequent isoenzyme studies (see Chavez et al. 1999).

With a camera lucida, drawings of the wings were made at a magnification that allowed maintenance of a consistent plane of focus to control distortion. On the membranous parts of the hemelytra (fore wing), we identified four landmarks (W1 to W4) from which two distances were measured: W2–W4 (an estimation of wing width = WW), and W1–W3 (an estimation of wing length = WL) (Figure 1, top). We also used three head measurements: the anteclypeus width (AC), the length of the antenniferous tubercle (AT) and the length of the second segment of the rostrum (R2) (Figure 1, bottom).

Figure 1.

Measurements taken from head and wings. Top: left hemelytra in dorsal view. W1 and W4 are ‘homologous’ points recognized by the intersection of local venations. W2 and W3 are recognized as extremes of curvatures. The distance W1–W3 is taken as an estimator of wing length, and W2–W4 as an estimator of wing width. Left: head in lateral view. AT is the length of the antenniferous tubercle, and R2 is the length of the second rostral segment. Right: head in dorsal view. AC is the width of the ante-clypeus.

Allometry-free differences

Data were log-transformed prior to analysis (to equalize or homogenize the variance). To reveal relevant metric differences between groups which could be suspected to be conspecific, we used an allometric ‘common principal component’ model which is able to remove the effects of growth variation (Flury 1984; Airoldi & Flury 1988). In this model, the first component is taken to account for allometric growth (Klingenberg & Zimmerman 1992). In the space of log-transformed measurements, conspecific individuals are expected to be found along the straight line defined by this single component. By construction, metric variation orthogonal to this direction is allometry-free, that is it describes metric variation free of growth effects (Klingenberg 1996). Thus, the influences of within-group allometries were removed by using all the common principal components except the first one (Klingenberg 1996). The resulting ‘form’ variables (allometry-free) were submitted to a canonical variate analysis. For comparing overall size between both species, we used as estimator the first common principal component (Flury 1984; Airoldi & Flury 1988). All calculations used the packages NTSYS® (Rohlf 1998), STATA® (Computing Resource Center 1992) and JMP® (SAS Institute Inc. 1995).

Results

Insects

Of 160 houses examined (in eight communities), we found bugs in 11 houses and collected 65 Rhodnius specimens (adult and nymphs) (Table 1) identified as R. stali according to the keys of Lent and Wygodzinsky (1979) and Lent et al. (1993). The insects were collected from five communities: in four houses we found bugs simultaneously in both peridomestic and domestic environments; in four houses only in peridomestic, and in three cases only in the domicile (Table 2). Fifty-six of these specimens were collected from peridomestic environments (animal shelters and cereal stores), and nine inside the dwellings (Tables 1 and 3). None of the bugs were positive for T. cruzi infection.

Table 1.  Number of Rhodnius stali captured in the houses. Domestic, specimens collected inside dwellings; Peridomestic, specimens captured in animal shelters (chicken coop, cereal stores)
LocalityExamined
houses
Infested
houses
Number of R. stali captured
DomesticPeridomestic
Palos Blancos59514
Palmeras90
Alto Remolinos80
Triunfo100
Entre Ríos11113
Luz Porvenir1423
Ingavi161122
Nuevos Horizontes332327
Total16011956
Table 2.  Place of capture of Rhodnius stali in the houses. Domestic, number of houses with specimens collected inside dwellings; Peridomestic, number of houses with specimens collected in animal shelters (chicken coop, cereal stores); D and P, simultaneous presence of specimens in domestic and peridomestic environments
LocalityInfested
houses
Place of capture
DomesticPeridomesticD and P
Palos Blancos514
Palmeras0
Alto Remolinos0
Triunfo0
Entre Ríos11
Luz Porvenir22
Ingavi11
Nuevos Horizontes22
Total11344
Table 3.  Origin and stage of specimens. Stage of specimens: N1, first stage nymph; N2, second stage nymph; N3, third stage nymph; N4, fourth stage nymph; N5, fifth stage nymph
 EggsN1N2N3N4N5Adult
Peridomestic21810131159
Domestic11311022
Total3211111411711

We also collected 32 eggs, 11 adhered to a cane shaft in a house from Luz Porvenir, and 21 glued to the axils of one branch of a ‘motacu’ palm tree (Attalea phalerata) near the house in Palos Blancos (we cut down all branches). These eggs were confirmed as R. stali after rearing to adults in the laboratory. Exuviae were also detected in domestic and peridomestic environments in all the infested houses.

Morphometric analysis

For the discriminant analysis between R. pictipes and R. stali, three measurements of the head and two of the wing were combined (AC and AT with R2; WW with WL). Strong size differences were apparent between the two groups, as illustrated by the first common principal component (see the horizontal axis of Figure 2). The remaining common principal components were used to perform a size-free discriminant analysis, which also showed significant differences between these two taxa, as illustrated by the first canonical factor (see the vertical axis of Figure 2).

Figure 2.

Principal component analysis of Rhodnius stali and R. pictipes. Size and form discrimination. The horizontal axis is the first common principal component (cpc1) which is used here as a general size estimator. The vertical axis is the first discriminant factor (cv1bk) derived from size-free variables (i.e. the remaining common principal component, cpc2 a cpc5). It had a 91% contribution to the total variation. Polygons enclose specimens of R. pictipes, R. stali from Chapare (type locality) and R. stali from Alto Beni (present work). As expected for two conspecific populations, Chapare and Alto Beni completely overlapped.

Discussion

In the last revisions of the Bolivian Triatominae (Bermúdez 1994; Galíndez et al. 1996), the genus Rhodnius is represented by R. pictipes, R. prolixus, R. robustus and R. stali. This latter species was previously misidentified as R. pictipes (Tibayrenc & Le Pont 1984) as its specific status was only recognized in 1993 (Lent et al. 1993). In the Alto Beni of Bolivia, R. stali is the only Rhodnius species found in domestic and peridomestic habitats (R. robustus is present in the region, but has only been found in sylvatic habitats) (Matias et al. 2001). Rhodnius stali is morphologically very similar to R. pictipes, although, our morphometric comparison revealed that the Alto Beni specimens did not differ significantly from topotype specimens of R. stali, but did differ significantly from a reference series of R. pictipes. The metric differences between R. stali and R. pictipes were striking, including differences in form and in size (Figure 2).

Our collections of R. stali from communities of Alto Beni revealed not only adult bugs but also nymphs, exuviae and eggs in peridomestic and domestic habitats, suggesting early stages in the domestication of this normally sylvatic species. This region of Alto Beni has been subject to extensive deforestation and new human settlements over the last 10 years, and the materials used in the construction of dwellings – particularly palm leaves – may have helped in introducing the insects to the artificial ecotopes. As we found eggs of R. stali adhered to the axils of the leaves of Attalea phalerata (motacu palm), we suppose that this palm may be the original sylvatic habitat of R. stali. Its wide distribution in an arc around the southern part of the Amazon region (Henderson et al. 1995), seems to match most of the recorded distribution of R. stali, and it is tempting to speculate that adaptation to this southern Amazonian palm species may have influenced the derivation of R. stali from the more widely distributed R. pictipes (Schofield & Dujardin 1999).

At present, the Alto Beni region of Bolivia is not considered endemic for human Chagas disease, so no routine surveillance is carried out. Moreover, this sample of R. stali from Alto Beni showed negative results for infection with T. cruzi, although previous studies have found natural infection of R. stali by T. cruzi in this region (Tibayrenc & Le Pont 1984; E. Martinez & M. Torrez, unpublished data). In one of the communities studied here (Palos Blancos) we found 4.5% of seropositivity for Chagas disease in 88 people examined (A. Matias, unpublished data). Thus, we believe that the apparent domiciliation of R. stali in the Alto Beni of Bolivia, and the evidence of local human infection, are important data calling for an increased entomological surveillance.

Acknowledgements

This work has benefited from international collaboration through the European Community – Latin America Triatominae research network (ECLAT). Thanks to C. Schofield from LSTMH, UK for the comments about this manuscript.

Authors
J. De la Riva, INLASA, C. Rafael Zubieta N° 1889, La Paz, Bolivia. E-mail: jascemineriv@latinmail.com
J. P. Dujardin, UMR CNRS - IRD 9926, BP 5054, Montpellier Cedex 1, France. E-mail: jzd7@cdc.gov
E. Martinez, UMSA, Facultad de Medicina Av. Saavedra N° 2246, La Paz, Bolivia. E-mail: Eddy.Martinez@uv.es
A. Matias, UMSA, Facultad de Medicina Av. Saavedra N° 2246, La Paz, Bolivia. E-mail: amatiflebo@mixmail.com (corresponding author).
M. Torrez, INLASA, C. Rafael Zubieta N° 1889, La Paz, Bolivia. E-mail: mitorrez@mixmail.com

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