The role of the peridomiciliary area in the elimination of Triatoma infestans from rural Argentine communities

Spatial distribution of Triatoma dimidiata peridomestic colonies modulated by distance between susceptible microhabitat patches

Triatoma dimidiata is a vector of the hemoparasite Trypanosoma cruzi, the causal agent of Chagas disease. It settles reproductive colonies in the peridomicile of the premises. The peridomicile is comprised of a random set of artificial and natural features that overlap and assemble a network of microenvironmental suitable sites (patches) that interact with each other and favor the structure and proliferation of T. dimidiata colonies. The heterogeneity of patch characteristics hinders the understanding and identification of sites susceptible to colonization. In this study, a classification system using a random forest algorithm was used to identify peridomiciles susceptible to colonization to describe the spatial distribution of these sites and their relationship with the colonies of T. dimidiata in ten localities of Yucatan. From 1,000 peridomiciles reviewed, the classification showed that 13.9 % (139) of the patches were highly susceptible (HSP), and 86.1 % (861) were less susceptible (LSP). All localities had at least one HSP. The occupancy by patch type showed that the percentage of total occupancy and by colonies was higher in the HSP, while the occupancy by adult T. dimidiata without evidence of nymphs or exuviae (propagules) was higher in the LSP. A generalized additive model (GAM) revealed that the percentage of occupied patches increases as the abundance of individuals in the localities increases however, the percentage of occupied patches in LSP is lower than occupied in HSP. Distance analyses revealed that colonies and propagules were located significantly closer (approximately 200 m) to a colony in a HSP than any colony in a LSP. The distribution of T. dimidiata in the localities was defined by the distribution of patch type; as the occupancy in these patches increased, a network of peridomestic populations was configured, which may be promoted by a greater abundance of insects inside the localities. These results reveal that the spatial distribution of T. dimidiata individuals and colonies in the peridomicile at the locality scale corresponds to a metapopulation pattern within the localities through a system of patches mediated by distance and level of the vectors' occupancy.

Two sticky traps baited with synthetic host odors to capture Triatoma infestans, an important vector of Chagas disease

Chagas disease is a vector-borne disease caused by Trypanosoma cruzi, which is transmitted by triatomine insects. Triatoma infestans is one of the main vectors. Efforts to eliminate T. infestans have often failed in the Gran Chaco, the largest endemic area of this species. Known methods for assessing triatomine house infestation include timed-manual collections by skilled personnel, bug notifications by householders' and/or non-baited detection devices. However, the detection sensitivity of those methods needs to be improved, especially when the bugs are present at low densities. In this work we design and evaluate the performance of two types of sticky traps (pitfall and climbing traps), when baited with a synthetic host odor lure, to capture T. infestans nymphs within an experimental box under semi-controlled laboratory conditions. Nine assays were conducted for each type of trap using a different experimental box per type of trap design and per treatment. These treatments were: test (T, trap baited with the synthetic lure), positive control (C+, trap baited with a mouse) and negative control (C-, empty trap). One hundred percent of the sticky pitfall and 89% of the climbing traps baited with the synthetic lure captured at least one insect. Moreover, the sticky pitfall trap and the sticky climbing trap, both baited with the synthetic lure, captured 30% and 40% of the insects in a single night, respectively. In both cases, the trap with the synthetic lure captured significantly more insects than the non-baited trap. However, the synthetic lure could be improved, as the traps with this lure captured significantly less insects than the traps with a live host. In summary, the two types of synthetically-baited traps tested were able to capture T. infestans nymphs, indicating that both designs are effective under the laboratory experimental conditions and insect abundance used in this work. These traps deserve to be tested in a field setting.

Effectiveness of Systemic Insecticide Dog Treatment for the Control of Chagas Disease in the Tropics

Chagas disease, caused by Trypanosoma cruzi and transmitted by triatomines, can lead to severe cardiac issues and mortality in many mammals. Recent studies have shown that systemic insecticide treatment of dogs is highly effective in killing triatomines. Here, we assessed the impact of dog treatment on T. cruzi transmission. We developed a mathematical model of T. cruzi transmission among triatomines, dogs, humans, and rodents. We used the model to evaluate the impact of dog treatment regimens on T. cruzi transmission dynamics to determine their effectiveness in reducing T. cruzi infection among hosts. We show that a 3-month treatment regimen may reduce T. cruzi incidence among humans by 59-80% in a high transmission setting, and 26-82% in a low transmission setting. An annual treatment may reduce incidence among humans by 49-74% in a high transmission setting, and by 11-76% in a low transmission setting. However, dog treatment may substantially increase T. cruzi prevalence among dogs if dog consumption of dead triatomines increases. Our model indicates that dog treatment may reduce T. cruzi infections among humans, but it may increase infections in dogs. Therefore, a holistic approach targeting different hosts is necessary for Chagas elimination.

Effects of mating on female reproductive physiology in the insect model, Rhodnius prolixus, a vector of the causative parasite of Chagas disease

The blood-sucking hemipteran Rhodnius prolixus is one of the main vectors of Chagas disease, a neglected tropical disease that affects several million people worldwide. Consuming a blood meal and mating are events with a high epidemiological impact since after each meal, mated females can lay fertile eggs that result in hundreds of offspring. Thus, a better knowledge of the control of R. prolixus reproductive capacity may provide targets for developing novel strategies to control vector populations, thereby reducing vector-host contacts and disease transmission. Here, we have used a combination of gene transcript expression analysis, biochemical assays, hormone measurements and studies of locomotory activity to investigate how mating influences egg development and egg laying rates in R. prolixus females. The results demonstrate that a blood meal increases egg production capacity and leads to earlier egg laying in mated females compared to virgins. Virgin females, however, have increased survival rate over mated females. Circulating juvenile hormone (JH) and ecdysteroid titers are increased in mated females, a process mainly driven through an upregulation of the transcripts for their biosynthetic enzymes in the corpus allatum and ovaries, respectively. Mated females display weaker locomotory activity compared to virgin females, mainly during the photophase. In essence, this study shows how reproductive output and behaviour are profoundly influenced by mating, highlighting molecular, biochemical, endocrine and behavioral features differentially expressed in mated and virgin R. prolixus females.

Triatominae of the semi-arid Chaco in central Argentina

The epidemiological scenario in central Argentinian Chaco region shows persistence of Triatoma infestans domestic populations in endemic areas, with control interventions historically affected by the economic instability of the region. Considering this situation, we aimed to (i) update the information regarding to the diversity of triatomines present in domestic, peridomestic and sylvatic environments in departments historically endemic of the Chaco region, (ii) to report the occurrence of secondary vectors of Chagas disease invading domestic environments and (iii) to discuss the possible sources of dispersal of these sylvatic species towards anthropic habitats. Between November 2017 and March 2020, we visited fourteen rural communities of northwest Córdoba province (central Argentina). Entomological data were collected through community vector surveillance in domiciles, active search in peridomiciles and the use of light and yeast traps in sylvatic environments. Seven Triatominae species were captured invading domiciles (T. guasayana, T. garciabesi, T. platensis, T. delpontei, T. breyeri, Panstrongylus guentheri and T. infestans). T. guasayana and T. garciabesi were the species with the highest number of captures. The 32% of the peridomiciles registered infestation with T. infestans (n ​= 355), mostly in chicken coops and goat pens. In sylvatic environments, T. garciabesi, T. guasayana, T. infestans and P. guentheri were collected. Only one adult specimen of T. infestans was positive for the presence of Trypanosoma cruzi. Our results suggest that the persistence of T. infestans populations in peridomiciles continues to be a serious challenge for control programs, whereas the finding of secondary vectors of Chagas disease actively invading domiciles emphasizes the need to implement new strategies for entomological surveillance.

Rethinking the old hypothesis that new housing construction has an impact on the vector control of Triatoma infestans: A metapopulation analysis

Triatoma infestans (Hemiptera: Reduviidae) is a hematophagous insect, vector of the parasite Trypanosoma cruzi, etiological agent of the Chagas disease. In the south of La Rioja, Argentina, the persistent infestation by triatomines encouraged a program of construction of brand new dwellings in rural areas since 2008, to reduce infestation. According to the metapopulation theory approach, each dwelling was considered to be a patch with different degrees of quality concerning the availability of shelters for T. infestans. Accordingly, brand new dwellings (BNDs) are considered of lower quality for triatomines, compared to traditional dwellings (TDs). The main objective of this study was to analyse the occupancy in patches of different quality and to evaluate the effect of BNDs in the control of T. infestans. 397 patches from three departments of the southern part of La Rioja province were analysed during the period from 2014 to 2017. Six samplings were carried out to estimate the occupancy by T. infestans in patches assigned with different qualities. During the studied period, changes in the occupancy status of the patches among samplings were recorded, and the metapopulation variables which affect the occupancy of T. infestans were analysed. The results showed that all patches, even those considered of lower quality and smaller size, were occupied by T. infestans in a range from 3.8% to 25.5%. In general, the probability of T. infestans occupancy among samplings showed no difference among all studied patches. The occupancy of patches by T. infestans was associated with "department", "number of patches" and "number of positive patches in the radius of 400 m" (analysis GEE models). However, "patch quality", "size", and "distance" showed no effect on T. infestans occupancy. 44.1% of patches with BND were occupied at least once during the study period. Some of these showed "persistent occupancy" by T. infestans. No differences in the T. infestans occupancy between BNDs and TDs were observed. These results allowed us to analyse the impact of the new housing construction as a vector control measure in the metapopulation context of La Rioja province.

High rates of Trypanosoma cruzi infection in goats from Mendoza province, Argentina: Parasite loads in blood and seasonal variation

Mendoza province, in central west Argentina, is considered among the high-risk provinces for vector transmission of Trypanosoma cruzi, the causative agent of Chagas disease. Extensive goat farming is common in large parts of rural Mendoza, and goats may act as a reservoir for this parasite. The objective of this study was to determine infection rates, parasite loads, and seasonal variation of these parameters in T. cruzi infection in goats from rural areas of three departments of Mendoza. A total of 349 peripheral blood samples with EDTA / guanidine were analyzed from goats on 11 farms (three in Lavalle, three in San Carlos, and five in Malargüe department) in spring of 2014, 2015, and 2016; and in fall of 2015 and 2016 (only Malargüe). DNA was extracted using a Phenol: Chloroform: Isoamyl protocol. The detection and quantification of T. cruzi was performed through qPCR amplification using satellite oligonucleotides. Of the 349 blood samples, 267 (77%) were positive, with parasite loads ranging between <0.10 and 10.90 par-eq/mL (median 0.10). In spring, frequencies of infection in the three departments ranged between 86% and 95%, but differences were not significant. Median parasite loads were higher in Lavalle than in the other departments, while those in goats from San Carlos were consistently low. The frequency of infection and parasite loads in Malargüe were significantly higher in spring than in fall. This seasonal variation may have been related to a reduced nutritional status and impaired immune response of goats in spring. In conclusion, the high proportion of positive goats confirms the persistence of T. cruzi in rural Mendoza.

Active dispersal in Triatoma infestans (Klug, 1834) (Hemiptera: Reduviidae: Triatominae): Effects of nutritional status, the presence of a food source and the toxicological phenotype

In Argentina, the main vector of Chagas disease is Triatoma infestans (Klug, 1834). In recent years, the presence of T. infestans was reported in human dwelling after spraying with pyrethroids in several locations in Argentina. The presence of these insects can be attributed, among other factors, to colonization by bugs from sylvatic or peridomestic populations or to the evolution of resistance to insecticides. In both cases, the dispersal of insects is a determining factor. The aim was to determinate the interaction of dispersal with feeding, food resource and resistance to insecticides. Three nutritional states were obtained with the number of feeds (NF) offered (0, 1 or 2). The resistant females were evaluated only with NF1. The experimental arena was 10 m long and contained two shelters. Groups of 30 virgin females of each NF were released in one of the shelters and were able to move during 3 days/nights. Females without possibility of dispersal were the controls. Results showed that the individuals dispersed mainly walking regardless of the number of feeds, the presence of food resource and toxicological phenotype. This type of dispersal presented energy costs in susceptible individuals but not in resistant ones. The numbers of feeds that determined low nutritional states showed greater dispersal and activity associated with the shelter. The presence of a food source had an effect on the dispersal capacity and this depended on the number of feeds. There was a decrease in the dispersal in individuals with one feeding and an absence of response in fasted individuals and with two feeds. On the other hand, a lower dispersal tendency were found in resistant females compared to susceptible females, suggesting that resistance to deltamethrin is associated with adaptive costs.

Effect of reproductive state on active dispersal in Triatoma infestans (Klug, 1834) (Hemiptera: Reduviidae: Triatominae) susceptible and resistant to deltamethrin

Triatoma infestans (Klug, 1834) is the main vector of the Chagas´ disease in Argentina. The presence of insects in the domestic environment after application of pyrethroid insecticides was reported. The colonization and the evolution of insecticide resistance are processes that can explain these control failures. In both processes, the active dispersal of insects is a determining factor. The aim of the present study was to determine whether the reproductive state modulate the dispersal capacity of females of T. infestans susceptible and resistant to deltamethrin. For this, different variables associated with active dispersal were measured in virgin and copulated females both susceptible and resistant to deltamethrin. The experimental arena was 10 m long and contained two shelters. Groups of 12 females of each experimental group were released in one of the shelters and were able to move during 3 days/nights. The results showed that the females mainly dispersed by walking regardless of the reproductive state and the toxicological phenotype. On the other hand, the copula affected the posterior dispersal capacity in females susceptible and resistant to deltamethrin. The copulated females dispersed more times, leaved more times from the shelters and showed a higher proportion of dispersed individuals than the virgin females. Finally, the insecticide resistance reduced the dispersal capacity of females. The resistant insects showed lower number of dispersal events, a lower proportion of dispersed individuals, and lower exit and entry events from/to shelter than susceptible insects. This is the first report of the effect of copula on the capacity of active dispersion in vectors of Chagas disease susceptible and resistant to insecticides.

Effect of nutritional state and dispersal on the reproductive efficiency in Triatoma infestans (Klug, 1834) (Hemiptera: Reduviidae: Triatominae) susceptible and resistant to deltamethrin

The dispersal of insects is determinant in the colonization of new areas and the propagation of insecticide resistance. Nutritional status and reproductive characteristics determine the colonization capacity after the dispersal event. Studies about of the effects of dispersal on reproduction in triatomines are few and none in resistant insects. The aim was to determine the effects of nutritional state and dispersal on the subsequent reproductive potential in Triatoma infestans (Klug, 1834) susceptible and resistant to deltamethrin. Three nutritional states were obtained with the number of feeds (NF) offered (0, 1 or 2). The resistant females were evaluated only with NF1. The experimental arena was 10 m long and contained two shelters. Groups of 30 virgin females of each NF were released in one of the shelters and were able to move during 3 days/nights. Females without possibility of dispersal were the controls. The reproductive parameters were determined on the couples between the experimental females and males from the breeding. The results showed that most of the females dispersed by walking. The dispersal had effects on the reproduction of the deltamethrin-susceptible females and this depended on the number of feeds. Fecundity, fertility and the proportion of females that oviposited were higher in females dispersed with two feeds but was lower in females dispersed with less feeds. In addition, the effect of the dispersal on the reproduction and the life time also depended of the toxicological phenotype. The resistant insect oviposited in higher proportion and showed greater fecundity and more weeks of life when they dispersed that when they did not do it, and was opposite to that observed in susceptible ones. Finally, the resistance to insecticide had an effect on the reproduction and the life span and this effect depended on whether the insects dispersed or not.

Genetic variation and phylogeography of the Triatoma dimidiata complex evidence a potential center of origin and recent divergence of haplogroups having differential Trypanosoma cruzi and DTU infections

The population genetics of Triatoma dimidiata haplogroups was analyzed at landscape and sub-regional scales in Chiapas and regional level across the Mexican Neotropics, and phylogeography of the complex was re-analyzed across its complete geographic range. Two contiguous fragments of the ND4 gene were analyzed due to bias from differential haplogroup specificity using a previously designed sequence. At both landscape (anthropic modification gradient) and regional (demographic, fragmentation, biogeographic, climate) scales, lowest T. dimidiata genetic diversity occurs where there is greatest historical anthropic modification, and where T. cruzi infection prevalence is significantly highest. Trypanosoma cruzi prevalence was significantly higher than expected in haplogroups 1 and 3, while lower than expected in haplogroup 2. There was also a significant difference of DTUI and DTUVI infection frequencies in both haplogroups 1 and 3, while no difference of either in haplogroup 2. All haplogroups from the Mexican Neotropics had moderate to high haplotype diversity, while greatest genetic differentiation was between haplogroups 1 and 3 (above F_ST = 0.868, p < 0.0001). Divergence of the complex from the MRCA was estimated between 0.97 MYA (95% HPD interval = 0.55–1.53 MYA) and 0.85 MYA (95% HPD interval = 0.42–1.5 MYA) for ND4A and both concatenated fragments, respectively, with primary divergence from the MRCA of haplogroups 2 and 3. Effective population size for Mexican haplogroups 1 and 2 increased between 0.02 and 0.03 MYA. This study supports previous ecological niche evidence for the complex´s origin surrounding the Tehuantepec Isthmus, and provides evidence for recent divergence of three primary dimidiata haplogroups, with differential T. cruzi infection frequency and DTU specificity, important components of vector capacity.

Triatoma dimidiata is one of the broadest distributed triatomine species´ complexes transmitting Trypanosoma cruzi. In Mexico, three haplogroups of the T. dimidiata complex have been reported and all are primary vectors of Chagas disease south of the Tehuantepec Isthmus. Given their epidemiological importance, the question arises whether haplogroups have similar genetic diversity in domestic/modified landscapes, as well as infection characteristics and parasite DTU associations, key components of vector capacity. The aim of the present study was to analyze Triatoma dimidiata population genetics across landscapes, sub-regional, regional, and global Neotropical realm scales, using two contiguous fragments of the ND4 gene. Our results support previous evidence for the complex´s origin surrounding the Tehuantepec Isthmus, and provides evidence for recent divergence of three principal dimidiata haplogroups and significant secondary divergence within each. Differential T. cruzi prevalence and Discrete Typing Unit (DTU) specificity for individual haplogroups provide evidence for potential differential vector capacity within the complex in Mexico.

Factors associated with the presence of triatomines in rural areas of south Argentine Chaco

INTRODUCTION

The domestic and peridomestic presence of Triatoma infestans depends on several factors, such as human behavior, vector behavior, ecology, and the environment.

METHODS

This work was conducted in 139 domiciliary units, where triatomines were captured and risk factors in domiciles and peridomicilies were recorded. Household dwellers were interviewed to obtain information about practices regarding this disease and entomological indicators were calculated.

RESULTS

Infestation indices were 59.7% for house compounds, 4.3% for domestic areas and 58.3% for the peridomestic areas. Intradomicile infestation was significantly associated with housing characteristics. The presence of chicken coops in peridomicilies was associated with an increased risk of infestation. Of the respondents, 80% did not recognize the importance of the peridomiciliary structures for triatomine control and had infested peridomicilies.

CONCLUSIONS

The results show the importance of peridomiciles as refuge sites for Triatominae bugs; however, household dwellers do not perceive peridomiciles as areas that favor the presence of vectors. Actions for raising awareness about factors that favor the presence of triatomines are needed to improve the conditions of peridomiciliary environments.

Activity levels of female Triatoma infestans change depending on physiological condition

Background

Triatoma infestans (Hemiptera: Reduviidae) is the main vector of Trypanosoma cruzi, the etiological agent of Chagas disease, in South America. Active dispersal of this vector is the principal cause of recolonization of human dwellings previously treated with insecticides. Due to the persistence of vector populations and their movement between habitats, dispersive behavior studies are important for understanding the epidemiology of Chagas disease. The aim of this study was to analyze the relationship among T. infestans females’ activity levels according to their physiological conditions.

Results

Two groups of insects were used, unfed and fed females. Each was composed of three subgroups in relation to the reproductive state: fifth-stage nymphs, virgin and fertilized females. There was a significant interaction between reproductive and nutritional states among T. infestans female’ activity levels. During the experiments, unfed and fed nymphs remained inactive. Virgin females showed a dual behavior in their movement; fasted insects were more active. Fertilized females, both fed and unfed, were always active.

Conclusion

The reproductive and nutritional conditions of T. infestans females affect their activity levels. When females with different reproductive states remained together, fertilized females showed permanent activity levels, suggesting that this subgroup of females represents the highest epidemiological risk as colonizers of human dwellings.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3117-9) contains supplementary material, which is available to authorized users.

Resistance to deltamethrin by domestic and wild Triatoma infestans populations in the municipality of Toro Toro, Potosi, Bolivia

Background

Chemical control with pyrethroid insecticides has been effective in reducing endemic areas of distribution of Triatoma infestans in the Southern Cone, as well as Bolivia; this had considerably reduced the infestation of households in a large part of the territory. Nowadays, areas such as the Chaco and the Inter-Andean Valleys are regions where the reach of vector control strategies is becoming limited, and infestations of insecticide-treated households are reported more often. The objective of this study was to determine if the persistence of T. infestans stems from changes in the susceptibility of its toxicological profile in four communities in the municipality of Toro Toro, Potosi, Bolivia.

Methods

Susceptibility to deltamethrin of wild and domestic populations of T. infestans was evaluated in two stages (16 populations before and 13 populations after spraying) among DUs (structures in the intra- and peridomicile) and wild ecotopes, in four communities. Serial dilutions of deltamethrin in acetone (0.2 μl) were applied topically on standardized first-stage nymphs. Dose-response results were analyzed with the software PoloPlus and the relationships between lethal doses (LD) and resistance ratios (RR₅₀) were determined.

Results

Different degrees of RR₅₀ were detected among the populations before and after spraying (25.66–54.70 and 21.91–40.67, respectively), as well as in different ecotopes within a DU (DU JC 3, 28.06–36.13, in mixed structures of corrals and chicken coops; and DU JG 3, 46.27–25.70, in kitchen roofs), or in the wild environment of the community JG Sil (29.21–40.67). The mortality of insects undergoing diagnostic dose (DD) was never higher than 34%.

Conclusion

The results obtained in this study showed resistance of T. infestans to deltamethrin in four communities, hence the complexity of this phenomenon is not only limited to the level of communities, but also applies to the microgeographical level, as in different ecotopes present within the DUs. This phenomenon should be considered while planning the activities of control programs.

Wild populations of Triatoma infestans: Compilation of positive sites and comparison of their ecological niche with domestic population niche

BACKGROUND

For several years, the wild populations of Triatoma infestans, main vector of Trypanosoma cruzi causing Chagas disease, have been considered or suspected of being a source of reinfestation of villages. The number of sites reported for the presence of wild T. infestans, often close to human habitats, has greatly increased, but these data are scattered in several publications, and others obtained by our team in Bolivia have not been published yet.

METHODOLOGY/PRINCIPAL FINDINGS

Herein is compiled the largest number of wild sites explored for the presence of T. infestans collected with two methods The standardized methods aimed to determine the relationship between wild T. infestans and the ecoregion, and the directed method help to confirm the presence/absence of triatomines in the ecoregions. Entomological indices were compared between ecoregions and an environmental niche modelling approach, based on bioclimatic variables, was applied. The active search for wild T. infestans in Bolivia suggests a discontinuous distribution from the Andean valleys to the lowlands (Chaco), while the models used suggest a continuous distribution between the two regions and very large areas where wild populations remain to be discovered. The results compile the description of different habitats where these populations were found, and we demonstrate that the environmental niches of wild and domestic populations, defined by climatic variables, are similar but not equivalent, showing that during domestication, T. infestans has conquered new spaces with wider ranges of temperature and precipitation.

CONCLUSIONS/SIGNIFICANCE

The great diversity of wild T. infestans habitats and the comparison of their ecological niches with that of domestic populations confirm the behavioural plasticity of the species that increase the possibility of contact with humans. The result of the geographical distribution model of the wild populations calls for more entomological vigilance in the corresponding areas in the Southern Cone countries and in Bolivia. The current presentation is the most comprehensive inventory of wild T. infestans-positive sites that can be used as a reference for further entomological vigilance in inhabited areas.

Differences in competitive ability for the occupancy of shelters in triatomines

Triatomines (Hemiptera: Reduviidae: Triatominae) are nocturnal blood-sucking insects. During daylight hours they remain in an akinetic state inside their shelters, whereas at dusk they become active and move outside. When they are outside their shelters during the photophase, triatomines are vulnerable to diurnal predators and the period just before dawn is critical to their survival. This work analyses the existence of competitive interactions involved in the occupancy of shelters by triatomines. Behavioural assays were performed in which nymphs of different stages, nutritional status or species were released in an experimental arena containing a space-limited artificial shelter. The proportions of individuals occupying the shelter during the photophase were quantified to estimate the competitive abilities of each stage and species. Intraspecific comparisons showed higher levels of shelter occupancy for fourth over fifth instars and fed over unfed nymphs of Triatoma infestans. Interspecific comparisons showed higher rates of shelter occupancy for Triatoma sordida in comparison with T. infestans, and for T. infestans over Rhodnius prolixus. Arrival order was also relevant to determining shelter occupancy levels: early arrival was advantageous in comparison with later arrival. The study of intra- and interspecific competitive interactions for shelter occupancy provides relevant information about colonization and recolonization processes in the natural environments of triatomines.

House Reinfestation With Triatoma infestans (Hemiptera: Reduviidae) After Community-Wide Spraying With Insecticides in the Argentine Chaco: A Multifactorial Process

We investigated the dynamics and underlying causes of house (re)infestation with Triatoma infestans (Klug 1834) after a community-wide residual spraying with pyrethroids in a well-defined rural section of Pampa del Indio municipality (northeastern Argentina) over a 4-yr period. House infestation was assessed by timed manual searches, during insecticide applications, and by opportunistic householders' bug collections. All reinfested houses were selectively re-sprayed with insecticides. The resident population comprised Qom (66.6%) and Creole (33.4%) households, whose sociodemographic profiles differed substantially. The prevalence of house infestation dropped, less than expected, from 20.5% at baseline to 5.0% at 14 months postspraying (MPS), and then fluctuated between 0.8 and 4.2% over 21-51 MPS. Postspraying house infestation was positively and highly significantly associated with prespraying infestation. Most of the foci detected over 14-21 MPS were considered persistent (residual), some of which were moderately resistant to pyrethroids and were suppressed with malathion. Infestation patterns over 27-51 MPS suggested bug invasion from internal or external foci, but the sources of most findings were unaccounted for. Local spatial analysis identified two hotspots of postspraying house infestation. Using multimodel inference with model averaging, we corroborated that baseline domestic infestation was closely related to refuge availability, housing quality, and occurrence of peridomestic infestation. The diminished effectiveness of single pyrethroid treatments, partly attributable to moderate resistance compounded with rather insensitive vector detection methods and poor housing conditions, contributed to vector persistence. Improved control strategies combined with broad social participation are needed for the sustainable elimination of vector-borne human Chagas disease from the Gran Chaco.

Host-Feeding Sources and Infection With Trypanosoma cruzi of Triatoma infestans and Triatoma eratyrusiformis (Hemiptera: Reduviidae) From the Calchaqui Valleys in Northwestern Argentina

We assessed the prevalence of infection with Trypanosoma cruzi, parasite genotypes (discrete typing units, DTUs), and the host-feeding sources of domestic and peridomestic Triatoma infestans Klug and Triatoma eratyrusiformis Del Ponte in eight rural communities of the subandean Calchaqui valleys in northwestern Argentina. We sought to analyze their epidemiological role in the context of routine vector surveillance and control actions. Infection with T. cruzi was determined by optic microscopy or polymerase chain reaction (PCR) amplification of the hypervariable region of kinetoplast DNA minicircles. Parasite genotypes were identified through a multi PCR-based strategy. Bloodmeal contents were tested with a direct ELISA assay against nine antisera. Human sleeping quarters (domiciles) and peridomestic dry-shrub fences concentrated most of the T. infestans and T. eratyrusiformis infected with T. cruzi, respectively. The most frequent host-feeding sources of T. infestans were chickens (73.1%) in peridomiciles and humans (73.3%) in domiciles, whereas T. eratyrusiformis fed more often on cavid rodents (92.6%), which thrived in the dry-shrub fences. The main T. cruzi DTU identified in both vectors was T. cruzi I (TcI). Triatoma eratyrusiformis was implicated in the local circulation of TcI among cavies and perhaps mice, but infection with other typically domestic DTUs (TcVI and TcII/TcV/TcVI) indicated overlap between (peri)domestic transmission cycles in both vector species. Because dry-shrub fences were not targeted for routine insecticide spraying, they may act as sources of (peri)domestic reinfestation. Triatoma eratyrusiformis is an emergent secondary vector of T. cruzi and plays a significant role in the local transmission of T. cruzi.

Experimental control of Triatoma infestans in poor rural villages of Bolivia through community participation

BACKGROUND

Triatoma infestans is the main vector of Chagas disease in the southern cone countries. Present control strategies based on indoor and outdoor residual insecticide spraying are not sufficient to control disease transmission, particularly in Bolivia. Techniques based on the management of the human environment may be good alternatives or supplements.

METHODS

Social and entomological surveys were carried out in four villages of Bolivia situated in the dry inter-Andean Valleys and the Chaco region. Risk factors for house infestation by T. infestans were identified, and an eco-health intervention based on education and community participation was carried out to reduce the risks of house infestation. It consisted of implementing simple and low cost vector control techniques such as coating of mud walls, cleaning activities and removal of poultry that enter rooms to lay eggs.

RESULTS

The eco-health intervention significantly reduced the number of infested bedrooms, the mean abundance of T. infestans in bedrooms and beds, especially in the Chaco region. Mud wall coating was well accepted and could be proposed as a supplementary tool to the National Program of Chagas Disease Control to enhance the effects of insecticide sprayings.

CONCLUSIONS

Even if cleaning activities were still neglected, community participation proved to be effective in reducing house infestation.

Host-seeking behavior and dispersal of Triatoma infestans, a vector of Chagas disease, under semi-field conditions

Chagas disease affects millions of people in Latin America. The control of this vector-borne disease focuses on halting transmission by reducing or eliminating insect vector populations. Most transmission of Trypanosoma cruzi, the causative agent of Chagas disease, involves insects living within or very close to households and feeding mostly on domestic animals. As animal hosts can be intermittently present it is important to understand how host availability can modify transmission risk to humans and to characterize the host-seeking dispersal of triatomine vectors on a very fine scale. We used a semi-field system with motion-detection cameras to characterize the dispersal of Triatoma infestans, and compare the behavior of vector populations in the constant presence of hosts (guinea pigs), and after the removal of the hosts. The emigration rate – net insect population decline in original refuge – following host removal was on average 19.7% of insects per 10 days compared to 10.2% in constant host populations (p = 0.029). However, dispersal of T. infestans occurred in both directions, towards and away from the initial location of the hosts. The majority of insects that moved towards the original location of guinea pigs remained there for 4 weeks. Oviposition and mortality were observed and analyzed in the context of insect dispersal, but only mortality was higher in the group where animal hosts were removed (p-value <0.01). We discuss different survival strategies associated with the observed behavior and its implications for vector control. Removing domestic animals in infested areas increases vector dispersal from the first day of host removal. The implications of these patterns of vector dispersal in a field setting are not yet known but could result in movement towards human rooms.

Chagas disease is transmitted by triatomine bugs that actively disperse by walking and flying. The control of this vector-borne disease focuses on reducing or eliminating the insect vector populations. Most transmission of Trypanosoma cruzi, the causative agent of Chagas disease, involves insects living within or very close to households and feeding mostly on domestic animals. As animal hosts can be removed due to migration, slaughter, or death, it is important to understand how host availability can modify transmission risk to humans and to characterize the dispersal of triatomine vectors on a very fine scale. We used a semi-field system to characterize the dispersal of Triatoma infestans, and compare the behavior of vector populations in the constant presence of hosts and after the removal of the hosts. The emigration rate – net insect population decline in original refuges – following host removal was on average 19.7% of insects per 10 days compared to 10.2% in constant host populations. Activity of insects was significantly increased when hosts were removed. The removal of domestic animals in infested areas increases vector dispersal, possibly towards nearby human sleeping spaces.

Control of experimental Triatoma infestans populations: effect of pour-on cypermethrin applied to chickens under natural conditions in the Argentinean Chaco region

Among peridomestic structures, chicken coops are sites of major importance for the domestic ecology of Triatoma infestans (Hemiptera: Reduviidae). The aim of this study was to evaluate in an experimental context the effects of a cypermethrin pour-on formulation applied to chickens on blood intake, moulting and mortality in T. infestans, under the natural climatic conditions of a region endemic for Chagas' disease. Experimental chicken huts were made of bricks and covered with plastic mosquito nets. Ninety fourth-instar nymphs were maintained in each hut. The study used a completely random design in which chickens in the experimental group were treated with a cypermethrin pour-on formulation. Five replicates (= huts) of the experimental and control groups were conducted. The number of live T. infestans, blood intake and moults to fifth-instar stage were recorded at 1, 5, 20, 35 and 45 days after the application of cypermethrin. Cumulative mortality was higher in nymphs exposed to treated chickens (> 71%) than in control nymphs (< 50%) (P < 0.01). Blood intake and moulting rate were lower in nymphs fed on treated chickens than in control nymphs (P < 0.05). Pour-on cypermethrin was able to cause significant mortality, although it did not eliminate the experimental population of T. infestans.

Characterization of guinea pig antibody responses to salivary proteins of Triatoma infestans for the development of a triatomine exposure marker

Background

Salivary proteins of Triatoma infestans elicit humoral immune responses in their vertebrate hosts. These immune responses indicate exposure to triatomines and thus can be a useful epidemiological tool to estimate triatomine infestation. In the present study, we analyzed antibody responses of guinea pigs to salivary antigens of different developmental stages of four T. infestans strains originating from domestic and/or peridomestic habitats in Argentina, Bolivia, Chile and Peru. We aimed to identify developmental stage- and strain-specific salivary antigens as potential markers of T. infestans exposure.

Methodology and Principal Findings

In SDS-PAGE analysis of salivary proteins of T. infestans the banding pattern differed between developmental stages and strains of triatomines. Phenograms constructed from the salivary profiles separated nymphal instars, especially the 5^th instar, from adults. To analyze the influence of stage- and strain-specific differences in T. infestans saliva on the antibody response of guinea pigs, twenty-one guinea pigs were exposed to 5^th instar nymphs and/or adults of different T. infestans strains. Western blot analyses using sera of exposed guinea pigs revealed stage- and strain-specific variations in the humoral response of animals. In total, 27 and 17 different salivary proteins reacted with guinea pig sera using IgG and IgM antibodies, respectively. Despite all variations of recognized salivary antigens, an antigen of 35 kDa reacted with sera of almost all challenged guinea pigs.

Conclusion

Salivary antigens are increasingly considered as an epidemiological tool to measure exposure to hematophagous arthropods, but developmental stage- and strain-specific variations in the saliva composition and the respective differences of immunogenicity are often neglected. Thus, the development of a triatomine exposure marker for surveillance studies after triatomine control campaigns requires detailed investigations. Our study resulted in the identification of a potential antigen as useful marker of T. infestans exposure.

Chagas disease is caused by the protozoan parasite Trypanosoma cruzi, and currently affects approximately 8 million people in Latin American countries. Although vector control campaigns against the most effective Chagas disease vector, Triatoma infestans, have been highly successful, T. infestans is re-establishing in once-endemic regions. To monitor re-establishing triatomines, new epidemiological tools are needed. Antibody responses of hosts to triatomine salivary proteins represent a promising tool to detect biting bugs, and highly immunogenic salivary antigens may be used as markers of triatomine exposure. Therefore, we analyzed the antibody response of guinea pigs, common peridomestic hosts of T. infestans, to salivary proteins of nymphs and adults of four different T. infestans strains from Argentina, Bolivia, Chile and Peru. Developmental stage- and strain-specific proteins in the saliva of T. infestans influenced the antibody response of guinea pigs, and different salivary antigens were recognized by guinea pig sera. Despite the variations of immunogenic salivary antigens, a 35 kDa antigen was recognized by almost all guinea pig sera and this antigen may be a useful marker of T. infestans exposure.

Variations in time and space of an Andean wild population of T. infestans at a microgeographic scale

Background

Wild populations of Triatoma infestans are now believed to be the source of reinfestation of dwellings in some Andean areas and could impede the full achievement of vector control campaigns in this region. Given the poor knowledge of these populations in natural conditions, their basic biology traits, such as monthly demographic variations and movements of individuals, were explored.

Methods

A previously identified wild population of T. infestans in a field adjacent to a group of isolated houses in an Andean valley (department of La Paz, Bolivia) was explored using regular capture assays over 13 months in 50 sites selected at the beginning of the study. The capture-mark-recapture method was applied monthly using mouse-baited adhesive traps for captures and fingernail polish of different colors for the marking.

Results

The monthly capture assays did not show significant differences between rainy and dry seasons, showing evidence for a certain stability of the wild T. infestans population with only the nymph population tending to decline during the middle of the rainy season when rain is more intensive. Throughout the study, the monthly average number of bugs was 51.1 ± 25.3 per assay, 91.1% were nymphs, and they were found at 30 of the 50 sites (60%). The number of times a site was positive varied from one to 13. Site infestation was associated with the underground position of the traps, and rocks around and in the surroundings of the traps. The recaptures after marking were successful (138 recaptures over the study). The marking made it possible to detect for 14.5% of the recaptures significant movements of adults (up to 168 m) and nymphs (up to 34 m). Some bugs (nymphs and females) were recaptured after 5 months. For adults, recaptures (46 in total) mostly occurred between September and March. Females were recaptured twice as frequently as males.

Conclusion

The Andean wild populations of T. infestans showed a strong spatial and temporal stability during the year-long study. Dispersal may occur mainly during the rainy season. The capture-mark-recapture method was successful and the longevity of the bugs and the distances covered by nymphs and adults were recorded.

New structures for goat corrals to control peridomestic populations of Triatoma infestans (Hemiptera: Reduviidae) in the Gran Chaco of Argentina

Goat production is an important economic activity for rural communities in the Gran Chaco of Argentina. Goat corrals are important for the survival of peridomestic populations of Triatoma infestans. This study evaluated the impact of modifying the traditional structure of goat corrals on T. infestans populations and goat productivity in the region of Los Llanos (La Rioja). Thirty-nine experimental corrals were constructed and 57 traditional corrals were used as controls. We evaluated the infestations of the control and experimental corrals for five years following construction of the structures. The results showed that the new structures did not prevent the colonization, although it enhanced the detection of infestation at low densities of T. infestans. No significant difference was found in T. infestans population abundance between control and experimental corrals, probably because of the different detectability in the two types of structures, especially among the small nymphs. Although goat productivity average was higher in experimental than in control corrals, no significant difference was found because of high variability. The new structures can be used as a complement to promote the development of rural communities. Acceptability and adoption of the new corrals by the owners was high, as the enclosures offered better protection for the goats, increased growth of kids and facilitated herd handling.

Wild populations of Triatoma infestans are highly connected to intra-peridomestic conspecific populations in the Bolivian Andes

Triatoma infestans, the major vector of Chagas disease south of the Amazon in South America, has a large distribution of wild populations, contrary to what has previously been stated. These populations have been suspected of being the source of reinfestation of human habitats and could impede the full success of vector control campaigns. This study examined gene flow between intra-peridomestic populations and wild populations collected in the surround areas in three Andean localities in Bolivia. The populations were defined according to temporal, ecological, and spatial criteria. After DNA extraction from the legs of each insect, the samples were analyzed using seven microsatellite markers. First, the analysis of molecular variance (AMOVA) detected an absence of differentiation between wild and intra-peridomestic populations, although strong structuring was observed between the populations within each environment. Then for some populations, the Bayesian method of assignment to inferred populations showed very similar assignment patterns of the members of wild or intra-peridomestic populations in each locality. Finally, the detection of the first-generation migrants within the different populations provided evidence of insect displacement from the wild to the intra-peridomestic environment. This result indicates that, after control campaigns in the Andes, controlling this new paradigm of vector transmission risk stemming from the invasion of human habitats by wild populations of T. infestans requires long-term maintenance of public monitoring to keep the risk at a minimal level. Since wild populations of T. infestans have also been detected elsewhere in Argentina, Paraguay, and Chile, there is an urgent need to take these populations into account in future monitoring of Chagas disease transmission.

Potential sources of Triatoma infestans reinfesting peridomiciles identified by morphological characterization in Los Llanos, La Rioja, Argentina

The presence of Triatoma infestans in habitats treated with insecticides constitutes a frequent problem in endemic areas. Basing our study on the hypothesis that descendants of a residual population should be more similar to the pre-treatment population than to any other, we compared the indications of two quantitative morphological approaches. This study seeks to find the origin of 247 T. infestans from three populations found in two chicken coops and a goat corral after treatment with insecticides. The results obtained by quantitative morphology suggest that the T. infestans found between three-34 months after the application of insecticides formed mixed populations with insects derived from residual foci and neighbouring habitats. Our analyses also showed the presence of a phenotype which does not resemble neither the pre-treatment phenotype nor the one from neighbouring populations, suggesting the presence of a particular post-treatment phenotype. The heads size showed some variations in males from different populations and remained unchanged in females, which reinforces the hypothesis of an intraspecific competition for food with priority for females. This article presents, for the first time, the combined analysis of geometric morphometry of heads and antennal phenotypes to identify the composition of reinfesting populations.

Epidemiology of and impact of insecticide spraying on Chagas disease in communities in the Bolivian Chaco

Background

Chagas disease control campaigns relying upon residual insecticide spraying have been successful in many Southern American countries. However, in some areas, rapid reinfestation and recrudescence of transmission have occurred.

Methodology/Principal Findings

We conducted a cross-sectional survey in the Bolivian Chaco to evaluate prevalence of and risk factors for T. cruzi infection 11 years after two rounds of blanket insecticide application. We used a cubic B-spline model to estimate change in force of infection over time based on age-specific seroprevalence data. Overall T. cruzi seroprevalence was 51.7%. The prevalence was 19.8% among children 2–15, 72.7% among those 15–30 and 97.1% among participants older than 30 years. Based on the model, the estimated annual force of infection was 4.3% over the two years before the first blanket spray in 2000 and fell to 0.4% for 2001–2002. The estimated annual force of infection for 2004–2005, the 2 year period following the second blanket spray, was 4.6%. However, the 95% bootstrap confidence intervals overlap for all of these estimates. In a multivariable model, only sleeping in a structure with cracks in the walls (aOR = 2.35; 95% CI = 1.15–4.78), age and village of residence were associated with infection.

Conclusions/Significance

As in other areas in the Chaco, we found an extremely high prevalence of Chagas disease. Despite evidence that blanket insecticide application in 2000 may have decreased the force of infection, active transmission is ongoing. Continued spraying vigilance, infestation surveillance, and systematic household improvements are necessary to disrupt and sustain interruption of infection transmission.

Despite significant gains in the reduction of the burden of Chagas disease in many South American countries, active transmission and significant burden remain in areas such as the Gran Chaco. High initial vector density, poor housing material, peridomestic infestation, insecticide resistance, and a lack of systematic insecticide spraying and vector surveillance have previously been incriminated for failure to interrupt and sustain interruption of transmission. We conducted a census, seroprevalence, and epidemiologic study in a rural area in the Bolivian Chaco. The prevalence of infection was almost 20% in children and over 80% in adults. We estimated the intensity of transmission over time based on infection prevalence by age. We found that after the first spray program, transmission appeared to fall transiently but then increased again quickly. Sleeping in a structure with cracks in the walls, age and village of residence were associated with increased likelihood of infection. These findings suggest that consistently repeated systematic spraying campaigns accompanied by housing improvements are necessary to interrupt and sustain interruption of vector-borne transmission.

The residual efficacy of a cypermethrin pour-on formulation applied on goats on the mortality and blood intake of Triatoma infestans

Triatoma infestans is the main vector of Trypanosoma cruzi, the aetiological agent of Chagas disease in the Gran Chaco region of South America. As a frequent blood meal source for triatomine bugs, domestic goats play a key role in the eco-epidemiology of Chagas disease. The aim of this study was to evaluate the mortality and blood intake of T. infestans fed on goats that had been treated with different doses of pour-on insecticide. Third-instar nymphs were fed on goats that had been treated with 0 cc, 5 cc, 10 cc or 15 cc of a pour-on formulation of cypermethrin. The exposure of T. infestans to animals treated at different post-application intervals revealed a residual activity of the insecticide. The mortality rate in the treated groups was higher than in the control groups until 30 days post-insecticide application (p = 0.03), except in the group treated with 5 cc, in which no mortality was detected after seven days of insecticide application. Rainfall affected the triatomicide effect, reducing the time of residual activity. The cypermethrin pour-on treatment decreased the blood intake of T. infestans. Thirty days after the cypermethrin application, nymph mortality was 16% (± 13) with both doses (10 cc and 15 cc). The 15 cc dose did not result in higher insect mortality or increased persistence compared to the 10 cc dose.

High frequency of human blood in Triatoma dimidiata captured inside dwellings in a rural community in the Yucatan Peninsula, Mexico, but low antibody seroprevalence and electrocardiographic findings compatible with Chagas disease in humans

We studied a small rural community of 411 inhabitants localized in the state of Campeche in the Yucatan Peninsula, Mexico. In 44 collected triatomines captured inside the houses, human feeding source was revealed in 23 of 44 (52%) samples, and chicken feeding source was revealed in 16 of 44 (36%) samples. In a set of 29 triatomines, mouse was the feeding source in 13 (44%) samples, and dog was the feeding source in 7 (24%) samples. Infection index with Trypanosoma cruzi in collected triatomines was 38%, and all parasites belonged to discrete type unit I. Inhabitants referred high contact with triatomine's bite in 60 of 128 (47%) samples, but seroprevalence was 2.3% (3/128). Evidence of electrocardiographic alteration compatible with Chagas disease was observed only in one asymptomatic seropositive subject. In conclusion, Triatoma dimidiata in this region are preferentially infected with T. cruzi I and feed on human beings with relative high frequency, but seroprevalence and Chagas disease in humans is relatively low.

Entomological indices, feeding sources, and molecular identification of Triatoma phyllosoma (Hemiptera: Reduviidae) one of the main vectors of Chagas disease in the Istmo de Tehuantepec, Oaxaca, Mexico

The purpose of this study was to conduct an entomological analysis, determination of feeding sources, and molecular identification of triatomines in five communities of the Istmo de Tehuantepec, Oaxaca. The only found species in two of five searched communities (San Mateo del Mar and Tehuantepec City) was Triatoma phyllosoma. Colonization indices were high in both communities. In San Mateo del Mar, the insects were found indoors and in Tehuantepec City in peridomestic areas. The Trypanosoma cruzi infection indices were 2.1% in San Mateo del Mar and 39.4% in Tehuantepec City. This difference could be related to the high numbers of triatomine feeding on hens in the former community. In contrast, in Tehuantepec, dogs were the principal triatomine feeding sources. All nymphs and adults that were genetically analyzed belonged to the species T. phyllosoma. Low levels of genetic variation were found between vectors from both communities.

Dispersal of Triatoma infestans and other Triatominae species in the arid Chaco of Argentina: flying, walking or passive carriage? The importance of walking females

The aim of this paper was to analyse the active dispersal of Triatoma infestans and the role of chickens as passive carriers of this insect in peridomestic areas of La Rioja, Argentina. To measure active dispersal, monthly catches were made on six consecutive nights for five months (in the warm season) using light traps (for flying insects) and sticky dispersal barriers (for walking insects). The nutritional and reproductive states of adults were evaluated. Over the course of the sampling period, a total of eight flying adults, six walking nymphs and 10 walking adults of the species T. infestans were captured, as well as specimens of Triatoma guasayana, Triatoma eratyrusiformis and Triatoma platensis. Our data demonstrate for the first time that females of T. infestans can disperse by walking. This may be an adaptive strategy because it allows them to move with eggs and/or with good blood reserves, which are not possible when flying. All flying and walking individuals of both genders were of an appropriate physiological state that would allow for colonisation of the target habitat. However, manual inspection of 122 chickens suggests that it is unlikely that these animals passively transport T. infestans. Finally, the dispersal activity of T. infestans was compared with other triatomines using a dispersion index.

Factors affecting infestation by Triatoma infestans in a rural area of the humid Chaco in Argentina: a multi-model inference approach

Background

Transmission of Trypanosoma cruzi by Triatoma infestans remains a major public health problem in the Gran Chaco ecoregion, where understanding of the determinants of house infestation is limited. We conducted a cross-sectional study to model factors affecting bug presence and abundance at sites within house compounds in a well-defined rural area in the humid Argentine Chaco.

Methodology/Principal Findings

Triatoma infestans bugs were found in 45.9% of 327 inhabited house compounds but only in 7.4% of the 2,584 sites inspected systematically on these compounds, even though the last insecticide spraying campaign was conducted 12 years before. Infested sites were significantly aggregated at distances of 0.8–2.5 km. The most frequently infested ecotopes were domiciles, kitchens, storerooms, chicken coops and nests; corrals were rarely infested. Domiciles with mud walls and roofs of thatch or corrugated tarred cardboard were more often infested (32.2%) than domiciles with brick-and-cement walls and corrugated metal-sheet roofs (15.1%). A multi-model inference approach using Akaike's information criterion was applied to assess the relative importance of each variable by running all possible (17,406) models resulting from all combinations of variables. Availability of refuges for bugs, construction with tarred cardboard, and host abundance (humans, dogs, cats, and poultry) per site were positively associated with infestation and abundance, whereas reported insecticide use showed a negative association. Ethnic background (Creole or Toba) adjusted for other factors showed little or no association.

Conclusions/Significance

Promotion and effective implementation of housing improvement (including key peridomestic structures) combined with appropriate insecticide use and host management practices are needed to eliminate infestations. Fewer refuges are likely to result in fewer residual foci after insecticide spraying, and will facilitate community-based vector surveillance. A more integrated perspective that considers simultaneously social, economic and biological processes at local and regional scales is needed to attain effective, sustainable vector and disease control.

Vector-borne transmission of Chagas disease remains a major public health problem in parts of Latin America. Triatoma infestans is the main vector in the countries located in the South American Cone, particularly in the Gran Chaco ecoregion where residual insecticide control has achieved only a moderate, irregular impact. To contribute to improved control strategies, we analyzed the factors associated with the presence and abundance of T. infestans in 327 inhabited houses in a well-defined rural area with no recent vector control interventions in the humid Argentine Chaco. Bugs were found mainly in domiciles, kitchens, storerooms, and chicken coops and nests, particularly where adequate refuge and animal hosts (humans, dogs, cats or poultry) were available. Domiciles constructed from mud were the most often infested, but brick-and-cement domiciles, even in good conditions, were also found infested. Availability of refuge and hosts for T. infestans are key targets for vector control. Ten-fold variations in domestic infestation observed across neighboring villages, and differences in the relevant factors for T. infestans presence with respect to other areas of the Gran Chaco region suggest that host management, building techniques and insecticide use need to be tailored to the local environment, socio-economic characteristics, and climatic conditions.

The infra-red (IR) landscape of Triatoma infestans. An hypothesis about the role of IR radiation as a cue for Triatominae dispersal

This paper presents the infrared (IR) emission spectrum of hosts and habitats of Triatoma infestans in the chaco region of NW Argentina, representing the first attempt to correlate the natural infrared stimulus with the known behaviour of these blood-sucking insect, vectors of Trypanosoma cruzi--causative agent of Chagas disease. The study was carried out in two rural villages of La Rioja Province (Argentina). A FLYR i40 camera was used to obtain IR pictures which were analyzed to determine the thermal range for humans, domestic animals, building materials, and general background emissions. From sunset to the first hours of night, the thermal contrast between hosts and their landscape rises, increasing the likelihood that hosts could be differentiated by the vector. However, some building materials, can retain high temperatures during the night, which might add attractiveness to the presence of hosts. The results suggest that the most attractive habitats for dispersing bugs would be those at short distance, with high CO2 emission and strong IR radiation indicative of host presence. Goats corrals may be the most attractive habitat to disperse, within the domestic habitat. Dispersal would be favoured in periods of low atmospheric water saturation when IR perception is highest. In the IR band, the potential host and habitat discrimination available for the insects fits well with their known sensory capacities and observed dispersive behavior. Research in this area could be of considerable interest in relation to vector surveillance, epidemiology of Chagas disease transmission, and to develop new methods to minimise triatomine colonisation of new habitats.

Influence of the quality and quantity of blood ingested on reproductive parameters and life-span in Triatoma infestans (Klug)

In Triatominae, female fecundity and fertility may be affected by age, adult nutritional status (i.e., blood meal source and amount of blood ingested) and number of matings. Triatoma infestans (Klug) is the main vector of Chagas disease in southern South America and considering that reproductive success is intimately associated with the potential for colonizing or re-colonizing new ecotopes in endemic areas, we studied whether the blood meal source and the amount of blood ingested have influence on reproductive parameters. We constitute two groups: couples feeding regularly on guinea pigs and couples feeding regularly on pigeons. We registered quantity of blood ingested, fecundity, fertility, number of matings, days between the first feeding and mating, copula initiation, oviposition initiation and adult life-span. Results showed that females that fed on guinea pigs exhibited high fecundity and fertility, higher number of matings and they needed a lower amount of blood to form an egg. The number of matings and fecundity increased linearly and significantly with the quantity of blood ingested for both meal sources. Results from lineal regression between life-span and fecundity showed a positive and significant relation for both meal sources. The number of matings showed a positive relation with fecundity for both meal sources but significant only for guinea pigs. In T. infestans, the quantity of blood ingested could be a determinant of their reproductive efficiency. This species is mainly adapted to human dwelling and peridomestic structures where there is low host diversity. Considering that this species is in contact with mammals over other food sources, a greater reproductive success may result from an adaptation to this environment.

Community participation in Chagas disease vector surveillance: systematic review

BACKGROUND

Vector control has substantially reduced Chagas disease (ChD) incidence. However, transmission by household-reinfesting triatomines persists, suggesting that entomological surveillance should play a crucial role in the long-term interruption of transmission. Yet, infestation foci become smaller and harder to detect as vector control proceeds, and highly sensitive surveillance methods are needed. Community participation (CP) and vector-detection devices (VDDs) are both thought to enhance surveillance, but this remains to be thoroughly assessed.

METHODOLOGY/PRINCIPAL FINDINGS

We searched Medline, Web of Knowledge, Scopus, LILACS, SciELO, the bibliographies of retrieved studies, and our own records. Data from studies describing vector control and/or surveillance interventions were extracted by two reviewers. Outcomes of primary interest included changes in infestation rates and the detection of infestation/reinfestation foci. Most results likely depended on study- and site-specific conditions, precluding meta-analysis, but we re-analysed data from studies comparing vector control and detection methods whenever possible. Results confirm that professional, insecticide-based vector control is highly effective, but also show that reinfestation by native triatomines is common and widespread across Latin America. Bug notification by householders (the simplest CP-based strategy) significantly boosts vector detection probabilities; in comparison, both active searches and VDDs perform poorly, although they might in some cases complement each other.

CONCLUSIONS/SIGNIFICANCE

CP should become a strategic component of ChD surveillance, but only professional insecticide spraying seems consistently effective at eliminating infestation foci. Involvement of stakeholders at all process stages, from planning to evaluation, would probably enhance such CP-based strategies.

Phenotypic variability and population structure of peridomestic Triatoma infestans in rural areas of the arid Chaco (Western Argentina): spatial influence of macro- and microhabitats

Triatoma infestans, one of the main vectors of Trypanosoma cruzi, is strongly associated with rural human ecotopes. Infested peridomestic places could act as a source of house infestation as a result of the movement of vectors among habitats. The quantitative study of phenotypic traits and nutritional variables could be informative about the population structure and the relative mobility of vectors. This is the first approach toward analyzing the phenetic characteristics of T. infestans and the relationship with their potential role to invade and colonize other habitats in the arid Chaco region. This article compares the phenotypic structure of T. infestans populations living in chicken coops and goat corrals, analyzing the geometric morphometry of wings and heads, antennal phenotypes, and the nutritional status of 257 adults and nymphs. The study area remained untreated with insecticides during the 15 years before the present study. The insects collected in peridomestic structures located in two zones 80 km apart exhibited significant differences in their morphotypes, suggesting an environmental effect and/or the absence of individual exchange between zones. The population structure was clear in T. infestans from goat corrals and chicken coops in one zone and less pronounced in the other. These results suggest the existence of a different rate of individual exchange among ecotopes within each zone. Morphometric variables of heads and wings were significantly correlated in insects collected in goat corrals but not in chicken coops, suggesting a habitat effect and supporting the hypothesis of different canalization forces that affect the two organs. The nutritional status of adults indicated a low dispersal probability with zonal differences. This article provides the first combined morphometric analysis of the head of adults and fifth instar nymphs in triatomines and the first combined analysis of head and wing in T. infestans.

Characterization of the dispersal of non-domiciliated Triatoma dimidiata through the selection of spatially explicit models

Background

Chagas disease is a major parasitic disease in Latin America, prevented in part by vector control programs that reduce domestic populations of triatomines. However, the design of control strategies adapted to non-domiciliated vectors, such as Triatoma dimidiata, remains a challenge because it requires an accurate description of their spatio-temporal distributions, and a proper understanding of the underlying dispersal processes.

Methodology/Principal Findings

We combined extensive spatio-temporal data sets describing house infestation dynamics by T. dimidiata within a village, and spatially explicit population dynamics models in a selection model approach. Several models were implemented to provide theoretical predictions under different hypotheses on the origin of the dispersers and their dispersal characteristics, which we compared with the spatio-temporal pattern of infestation observed in the field. The best models fitted the dynamic of infestation described by a one year time-series, and also predicted with a very good accuracy the infestation process observed during a second replicate one year time-series. The parameterized models gave key insights into the dispersal of these vectors. i) About 55% of the triatomines infesting houses came from the peridomestic habitat, the rest corresponding to immigration from the sylvatic habitat, ii) dispersing triatomines were 5–15 times more attracted by houses than by peridomestic area, and iii) the moving individuals spread on average over rather small distances, typically 40–60 m/15 days.

Conclusion/Significance

Since these dispersal characteristics are associated with much higher abundance of insects in the periphery of the village, we discuss the possibility that spatially targeted interventions allow for optimizing the efficacy of vector control activities within villages. Such optimization could prove very useful in the context of limited resources devoted to vector control.

Chagas disease is one of the most important neglected diseases in Latin America. Although insecticides have been successfully sprayed to control domiciliated vector populations, this strategy has proven to be ineffective in areas where non-domiciliated vectors immigrating from peridomestic or sylvatic ecotopes can (re-)infest houses. The development of strategies for the control of non-domiciliated vectors has thus been identified by the World Health Organization as a major challenge. Such development primarily requires a description of the spatio-temporal dynamics of infestation by these vectors, and a good understanding of their dispersal. We combined for the first time extensive spatio-temporal data sets describing house infestation dynamics by Triatoma dimidiata inside one village, and spatially explicit population dynamics models. The models fitted and predicted remarkably the observed infestation dynamics. They thus provided both key insights into the dispersal of T. dimidiata in this area, and a suitable mathematical background to evaluate the efficacy of various control strategies. Interestingly, the observed and modelled patterns of infestation suggest that interventions could focus on the periphery of the village, where there is the highest risk of transmission. Such spatial optimization may allow for reducing the cost of control, compensating for repeated interventions necessary for non-domiciliated vectors.

Spatial re-establishment dynamics of local populations of vectors of Chagas disease

Background

Prevention of Chagas disease depends mainly on control of the insect vectors that transmit infection. Unfortunately, the vectors have been resurgent in some areas. It is important to understand the dynamics of reinfestation where it occurs. Here we show how continuous- and discrete-time models fitted to patch-level infestation states can elucidate different aspects of re-establishment. Triatoma infestans, the main vector of Chagas disease, reinfested sites in three villages in northwest Argentina after community-wide insecticide spraying in October 1992.

Methodology/Principal Findings

Different methods of estimating the probabilities of bug establishment on each site were compared. The results confirmed previous results showing a 6-month time lag between detection of a new infestation and dispersal events. The analysis showed that more new bug populations become established from May to November than from November to May. This seasonal increase in bug establishment coincides with a seasonal increase in dispersal distance. In the fitted models, the probability of new bug establishment increases with increasing time since last detected infestation.

Conclusions/Significance

These effects of season and previous infestation on bug establishment challenge our current understanding of T. infestans ecology and highlight important gaps in knowledge. Experiments necessary to close these gaps are discussed.

Chagas disease is transmitted by blood-sucking bugs (vectors) and presents a severe public health threat in the Americas. Worldwide there are approximately 10 million people infected with Chagas disease, a disease for which there is currently no effective cure. Vector suppression is the main strategy to control the spread of this disease. Unfortunately, the vectors have been resurgent in some areas. It is important to understand the dynamics of reinfestation where it occurs. Here we show how different models fitted to patch-level bug infestation data can elucidate different aspects of re-establishment dynamics. Our results demonstrated a 6-month time lag between detection of a new infestation and dispersal events, seasonality in dispersal rates and effects of previous vector infestation on subsequent vector establishment rates. In addition we provide estimates of dispersal distances and the effect of insecticide spraying on rates of vector re-establishment. While some of our results confirm previous findings, the effects of season and previous infestation on bug establishment challenge our current understanding of T. infestans ecology and highlight important gaps in our knowledge of T. infestans dispersal.

A new method for forensic DNA analysis of the blood meal in chagas disease vectors demonstrated using Triatoma infestans from Chuquisaca, Bolivia

Background

Feeding patterns of the vector are important in the epidemiology of Chagas disease, the leading cause of heart disease in Latin America. Chagas disease is caused by the parasite, Trypanasoma cruzi, which is transmitted by blood feeding insects. Historically, feeding behaviours of haematophagous insects have been investigated using serological reactions, which have detection limits in terms of both taxonomic resolution, and quantity and quality of the blood meal. They are labor intensive, require technical expertise, need fresh or frozen samples and antibodies often are either not available commercially or the resources for synthesis and purification are not available. We describe an assay to identify vertebrate blood meal sources, and the parasite T. cruzi using species-specific PCR assays from insect vectors and use the method to provide information regarding three questions: (1) Do domestic and peri-domestic (chicken coop and animal corral) habitats vary in the blood meals detected in the vectors? (2) What is the pattern of multiple blood meals? (3) Does the rate of T. cruzi infection vary among habitats and is it associated with specific blood meal types?

Methodology/Principal Findings

Assays based on the polymerase chain reaction were evaluated for identification of the blood meal source in the heamatophagous Chagas disease vector Triatoma infestans. We evaluate a technique to identify 11 potential vertebrate food sources from the complex mixture extracted from the vector's abdomen. We tested the assay on 81 T. infestans specimens collected from the Andean highlands in the department of Chuquisaca, located in central Bolivia, one of the regions in South America where sylvatic T. infestans have been reported. This area is suggested to be the geographic origin of T. infestans and has very high human infection rates that may be related to sylvatic vector populations.

Conclusion/Significance

The results of the assays revealed that a high percentage of insects collected in human dwellings had fed on peri-domestic animals. In contrast, one insect from a chicken coop but no bugs from corrals tested positive for human blood. Forty-eight percent of insects tested positive for more than one vertebrate species. T. cruzi infection was detected in 42% of the specimens. From the epidemiological point of view, the results reveal an overall pattern of movement from peri-domestic structures to human habitations for T. infestans in this region of Bolivia as well as the important role of pigs, dogs, chickens and guinea pigs in the dynamics of T. cruzi infection.

Infestation of rural houses by Triatoma infestans in the region of Los Llanos (La Rioja, Argentina)

Vectorial transmission of Chagas disease has been strongly reduced in most parts of the Southern Cone countries of South America, except in the Gran Chaco region of Argentina, Bolivia, and Paraguay. Given periodical interruptions of the vector control programmes in the endemic region of the Gran Chaco of Argentina, the vectorial transmission of the disease has been increasing during the last years. From the beginning of 2004, the provincial Ministry of Health of La Rioja, Argentina, started a vector control programme to cover the rural houses of the Los Llanos area in the southwestern area of the Gran Chaco region. This article reports the result of a standardized entomological survey and insecticide application against Chagas disease vectors in the intra and peridomestic structures of the rural houses of Los Llanos. A total of 4062 houses were inspected, of which 46.8% were found to be infested by Triatoma infestans. Infestation by vector species other than T. infestans was less than 0.5%(T. eratyrusiformis and T. platensis). Intradomestic infestation was found in 27.2%, whereas peridomestic infestation was found in 39.3% of the houses. The lowest figure of intradomestic infestation was 6.6% (Department F Varela), and the highest value of intradomestic infestation was 45.1% (Department Independencia). In spite of the demonstrated success of vector control elsewhere, this study shows that the vector populations are susceptible to pyrethroid insecticides in the southern area of the Gran Chaco of Argentina, that there still are regions where rural houses show heavy infestation by T. infestans associated with big peridomestic structures and that the vectorial transmission of the Chagas disease will continue, unless a sustained and well organized vector control effort is installed in the region.

Periurban Trypanosoma cruzi-infected Triatoma infestans, Arequipa, Peru

Simple interventions may facilitate vector control and prevent periurban transmission of Chagas disease.

In Arequipa, Peru, vectorborne transmission of Chagas disease by Triatoma infestans has become an urban problem. We conducted an entomologic survey in a periurban community of Arequipa to identify risk factors for triatomine infestation and determinants of vector population densities. Of 374 households surveyed, triatomines were collected from 194 (52%), and Trypanosoma cruzi–carrying triatomines were collected from 72 (19.3%). Guinea pig pens were more likely than other animal enclosures to be infested and harbored 2.38× as many triatomines. Stacked brick and adobe enclosures were more likely to have triatomines, while wire mesh enclosures were protected against infestation. In human dwellings, only fully stuccoed rooms were protected against infestation. Spatially, households with triatomines were scattered, while households with T. cruzi–infected triatomines were clustered. Keeping small animals in wire mesh cages could facilitate control of T. infestans in this densely populated urban environment.

Re-establishment of local populations of vectors of Chagas disease after insecticide spraying

1. Prevention of Chagas disease is mainly dependent on control of the insect vectors that transmit infection. Unfortunately, this control is not wholly successful and the vectors have been resurgent in some areas. Where re-infestation has occurred, it is important to understand the dynamics of the process. We investigated how a metapopulation framework can elucidate key aspects of re-infestation and thereby contribute to more efficient disease control.2.Triatoma infestans, the main vector of Chagas disease, re-infested sites in three villages in north-west Argentina after community-wide insecticide spraying in October 1992. Ten surveys were carried out at 6-monthly intervals from November 1994 to May 1999.3. Comparisons were made of different methods of estimating the sources of dispersal and the number of sites in which bug infestations became established.4. The results indicated that (i) the number of dispersing Triatoma infestans from a given site was proportional to the number of bugs found at the site; (ii) there was a 6-month time lag between detection of a new infestation and dispersal events; (iii) the relationship between infestations and new establishments varied by season.5. Three of 156 sites at which bugs were found were estimated to be the source of more than 50% of establishment events. These three sites were the only ones with large, persistent bug populations.6.Synthesis and applications. To reduce the risk of human Chagas disease, identifying those few sites infested with large, persistent bug populations and targeting control measures at those sites should greatly improve the efficiency of vector control. The appropriate seasonal timing of vector control could also greatly increase its efficiency. Specific recommendations for the timing of insecticide spraying require further research to establish how the observed temporal pattern of bug establishment is associated with the seasonality of bug dispersal.

Reinfestation sources for Chagas disease vector, Triatoma infestans, Argentina

Reinfestation by Triatoma infestans after insecticide spraying has caused elimination efforts in the dry Chaco region to fail repeatedly. The sources and spatial extent that need to be considered to understand the reinfestation pattern and to plan a comprehensive control program were studied in 2 adjacent rural communities in northwestern Argentina from 1993 to 1997. The effects of external, residual, and primary sources on the reinfestation pattern were evaluated by using geographic information systems, satellite imagery, spatial statistics, and 5-year retrospective data for 1,881 sites. The reinfestation process depended on primary internal sources and on surrounding infested communities. In the dry Chaco, successfully reducing the risk for reinfestation in a community depends on treating all communities and isolated sites within 1,500 m of the target community. In addition, during the surveillance phase, spraying all sites within 500 m of new foci will delay reinfestation.

Infestation of rural houses by Triatoma infestans (Hemiptera: Reduviidae) in southern area of Gran Chaco in Argentina

The impact of control activities against Triatoma infestans (Klug) (Hemiptera: Reduviidae) in South America has a marked contrast within and outside the Gran Chaco region. Development of a geographic information system, as part of an improvement in control program activities, allowed analysis of the spatial pattern of house infestations by T. infestans before and after house spraying with deltamethrin in the San Martin Department (an arid Chaco region of central Argentina). The overall peridomestic infestation index decreased from 48.2 to 28.2% after insecticide application. House infestation was spatially clustered in regions with low or high infestation levels that were located east and southwest of the department, respectively. This pattern was detected both before and after the insecticide application. Three environmental variables calculated from a temporal series of MODIS imagery (average of night temperature, maximum of day temperature, and temporal variation of vegetation index) were capable of correctly discriminating 96% of the places belonging to either high or low house infestation observed after the insecticide application.

Seasonal variations in active dispersal of natural populations of Triatoma infestans in rural north-western Argentina

The flight dispersal of Triatoma infestans Klug (Hemiptera: Reduviidae) is one of the main mechanisms determining community re-infestation after control interventions. An empirical model of flight initiation coupled with data from a longitudinal study predicted that the flight dispersal of T. infestans would peak in summer. To test this prediction, longitudinal light trap collections were conducted during 3-8 nights in March (late summer), July (winter) and November (spring) 2003, and in March 2004 in a rural community in north-west Argentina. Following each light-trapping collection date, all peridomestic sites around light traps were inspected to assess the relative abundance and nutritional status of T. infestans at each site. A total of 21 adult and five nymph T. infestans, six Triatoma guasayana Wygodzinsky & Abalos, and nine Triatoma garciabesi Carcavallo et al. were collected in 96 light-trapping nights, whereas 696 T. infestans were collected from the peridomestic sites that surrounded the light traps. The arrival of T. infestans in the light traps occurred in 64% of catch stations and peaked in the summer surveys (10-14 bugs) compared with spring and winter surveys. When winds were < 5 km/h, the arrival of adult T. infestans at the light traps was significantly associated with maximum temperature and relative humidity. This is the first field report of seasonal variations in the flight dispersal activity of T. infestans.

Comparative trial of effectiveness of pyrethroid insecticides against peridomestic populations of Triatoma infestans in northwestern Argentina

The effects of different pyrethroid insecticides, formulations, and doses on peridomestic populations of Triatoma infestans (Klug) were evaluated in 128 houses with 148 identified infested peridomestic sites in northwestern Argentina between October 2003 and March 2005. Four treatments were randomly assigned within each community: two doses of 5% suspension concentrate beta-cypermethrin in water applied with manual compression sprayers, the standard dose (S) at 50 mg and a double dose (2S) at 100 mg active ingredient (AI)/m2; and two emulsifiable concentrates diluted in diesel fuel and applied with power sprayers, 25% cypermethrin (100 mg [AI] /m2) (CF) and 10% permethrin (170 mg [AI]/m2) (DF). Infestation was assessed by timed manual collections with a dislodging agent at baseline, 5, 12, and 17 mo postspraying, and the sites found to be reinfested at 5 mo postspraying were selectively resprayed. Only 2S eliminated T. infestans from all peridomestic sites up to 12 mo postspraying, and it was significantly more effective than all other treatments. At 5 mo postspraying, more sites treated with CF or DF rather than S had bug colonies that probably represented residual foci, which they also failed in eliminating after a second spray. At 17 mo postspraying, the prevalence of reinfested peridomestic sites was 5% for 2S, 29% for S, 43% for CF, and 54% for DF. The application of suspension concentrate pyrethroids in dose twice as large as that currently in use in the attack phase produces a greater initial impact and may eliminate peridomestic populations of T. infestans.