Molecular research and the control of Chagas disease vectors

Abstract

Chagas disease control initiatives are yielding promising results. Molecular research has helped successful programs by identifying and characterizing introduced vector populations and by defining intervention targets accurately. However, researchers and health officials are facing new challenges throughout Latin America. Native vectors persistently reinfest insecticide-treated households, and sylvatic triatomines maintain disease transmission in humid forest regions (including Amazonia) without colonizing human dwellings. In these scenarios, fine-scale vector studies are essential to define epidemiological risk patterns and clarify the involvement of little-known triatomine taxa in disease transmission. These eco-epidemiological investigations, as well as the planning and monitoring of control interventions, rely by necessity on accurate taxonomic judgments. The problems of cryptic speciation and phenotypic plasticity illustrate this need -and how molecular systematics can provide the fitting answers. Molecular data analyses also illuminate basic aspects of vector evolution and adaptive trends. Here we review the applications of molecular markers (concentrating on allozymes and DNA sequencing) to the study of triatomines. We analyze the suitability, strengths and weaknesses of the various techniques for taxonomic, systematic and evolutionary investigations at different levels (populations, species, and higher taxonomic categories).Key words: molecular systematics, DNA, allozymes, Triatominae, Chagas disease. CHAGAS DISEASE: BURDEN, CONTROL EFFORTS AND NEW CHALLENGESChagas disease (CD), caused by Trypanosoma cruzi and transmitted by triatomine bugs, is a major public health problem in Latin America. An overall prevalence of ∼ 18 million infections was estimated in the late 1980s, with 90 million people living at risk and 45000 deaths/year. CD was the most important parasitic disease in Latin America in terms of its impact on national economies and public health systems (WHO 1991, World Bank 1993.In the absence of vaccines or adequate drugs for large-scale treatment, the reduction of disease burden critically depends on the control of transmission by triatomine vectors and infected blood transfusion. Several multinational initiatives have been launchedAn Acad Bras Cienc (2005) 77 (3) 438FERNANDO ABAD-FRANCH and FERNANDO A. MONTEIRO with that purpose. Ten years of concerted action in the Southern Cone have resulted in the elimination of transmission by Triatoma infestans (the most widespread domestic vector) from vast areas of the region. Incidence dropped by an average of 94% in the area, and by 65% in Latin America (WHO 2002). Recent estimates show however that substantial efforts are still necessary: overall prevalence remains over 12 million, with 200000 new cases/year in 15 countries (Morel and Lazdins 2003).The success of the Southern Cone initiative was eased by the fact that the main vector species, T. infestans, is entirely domestic throughout most of its range, making reinfestation of insectic...

Chromosomal divergence and evolutionary inferences in Rhodniini based on the chromosomal location of ribosomal genes

“…2E, F). Molecular comparisons with mitochondrial sequences (cytochrome b) also showed markedly divergent haplotypes in the populations from these countries, suggesting that these populations represent discrete groups or incipient species (Abad-Franch et al 2003, Abad-Franch & Monteiro 2005. Considering that both populations show different affinities for domestic and sylvatic habitats (Grijalva et al 2012), their identification as distinct genetic groups has important epidemiological consequences for vector control programmes (Abad-Franch et al 2001).…”

“…Both cases of intraspecific variation detected in triatomines reflect recent divergence processes, which could lead to incipient speciation (R. ecuadoriensis) or a marked population differentiation (T. infestans). In addition, closely related species, such as R. colombiensis and R. pallescens (Abad-Franch & Monteiro 2005), can be differentiated using this marker (Table), suggesting that the 45S chromosomal localisation of the ribosomal clusters could be an appropriate marker for the recognition and detection of recently diverged species or populations. FISH analyses of R. robustus and/ or Triatoma sordida (Noireau et al 1999) cryptic species could help to test this hypothesis.…”

Estructura poblacional y variabilidad genética de Rhodnius prolixus (Hemiptera: Reduviidae) procedente de diferentes áreas geográficas de Colombia

“…Los resultados obtenidos por la técnica de RAPD para las poblaciones domiciliadas concuerdan con lo reportado en estudios anteriores realizados por morfometría y patrones isoenzimáticos, en los cuales se demuestra que aunque R. prolixus presenta baja variabilidad genética, se observan algunas diferencias significativas entre poblaciones locales y otras variantes geográficas (25)(26)(27)(28); otros estudios realizados por medio de análisis de secuencias de ADN mitocondrial y ADN nuclear reportan resultados similares a los obtenidos por medio de las otras técnicas, mostrando que R. prolixus es una especie muy homogénea genéticamente (29)(30)(31)(32); sin embargo en ninguno de estos estudios se incluyen las poblaciones silvestres colombianas recientemente reportadas e incluidas en el presente estudio, razón por la cual encontramos una variabilidad genética moderada entre las poblaciones silvestres y las domiciliadas.…”

Genetics and evolution of triatomines: from phylogeny to vector control

“…Recently diverged species can sometimes be differentiated (Gurgel-Goncalves et al, 2010) or be remarkably isomorphic, making the detection and description of cryptic taxa difficult (Abad-Franch and Monteiro, 2005;Nouvellet et al, 2011). A good example of the underlying taxonomic structure, which could not be resolved by morphological studies alone, is the Triatoma sordida species complex whose Argentinean, Brazilian and Bolivian member species could only be described thanks to additional tools such as cytogenetics and molecular markers (Dujardin et al, 1999b).…”