Date Published: March 26, 2008
Publisher: Public Library of Science
Author(s): Juan Carlos Pizarro, Lauren M. Gilligan, Lori Stevens, Rodrigo Correa-Oliveira
Abstract: BackgroundFor Chagas disease, the most serious infectious disease in the Americas, effective disease control depends on elimination of vectors through spraying with insecticides. Molecular genetic research can help vector control programs by identifying and characterizing vector populations and then developing effective intervention strategies.Methods and FindingsThe population genetic structure of Triatoma infestans (Hemiptera: Reduviidae), the main vector of Chagas disease in Bolivia, was investigated using a hierarchical sampling strategy. A total of 230 adults and nymphs from 23 localities throughout the department of Chuquisaca in Southern Bolivia were analyzed at ten microsatellite loci. Population structure, estimated using analysis of molecular variance (AMOVA) to estimate FST (infinite alleles model) and RST (stepwise mutation model), was significant between western and eastern regions within Chuquisaca and between insects collected in domestic and peri-domestic habitats. Genetic differentiation at three different hierarchical geographic levels was significant, even in the case of adjacent households within a single locality (RST = 0.14, FST = 0.07). On the largest geographic scale, among five communities up to 100 km apart, RST = 0.12 and FST = 0.06. Cluster analysis combined with assignment tests identified five clusters within the five communities.ConclusionsSome houses are colonized by insects from several genetic clusters after spraying, whereas other households are colonized predominately by insects from a single cluster. Significant population structure, measured by both RST and FST, supports the hypothesis of poor dispersal ability and/or reduced migration of T. infestans. The high degree of genetic structure at small geographic scales, inferences from cluster analysis and assignment tests, and demographic data suggest reinfesting vectors are coming from nearby and from recrudescence (hatching of eggs that were laid before insecticide spraying). Suggestions for using these results in vector control strategies are made.
Partial Text: Chagas disease is a parasitic disease in which the pathogenic agent, Trypanosoma cruzi is transmitted by hematophagous insects of the sub-family Triatominae. Triatoma infestans is the major vector in the Andean highlands where the disease is endemic and has infected humans for over 9000 years . Chagas disease is the most important parasitic disease in the Americas in terms of mortality and economic impact . In Bolivia the endemic area covers 55% of the country and, in 1985, more than one million people were infected . In 1991 a public health program, the Southern Cone Initiative was launched by the World Health Organization to eliminate vector populations , through spraying of houses and surrounding areas with pyrethroid insecticides . In Argentina, Brazil, Chile, and Uruguay, T. infestans is exclusively domestic or peri-domestic, thus eradication of the vector in these regions, followed by vigilance against re-infestation, has proven largely successful in reducing transmission of T. cruzi and thus the prevalence of Chagas disease . In contrast, in Bolivia the vectors occur in domestic, peri-domestic, and sylvatic environments ; thus, control of T. infestans in towns and homesteads is confounded by the possible re-infestation from surrounding sylvatic areas.
Our study region is an ecologically diverse but geographically small valley–mountain environment in the department of Chuquisaca in Southern Bolivia. This region has high levels of house infestation and vector and human T. cruzi infection –. The use of microsatellite loci, now routine in many insect population genetic studies because they are inherently more polymorphic than allozyme loci and generally not targets of selection, allows us to detect population structure with more statistical power .