Spatial pattern of reinfestation by Triatoma infestans in Chancanif, Argentina.

We examined the environmental correlates and the spatial pattern of infestation by Triatoma infestans, a vector of Chagas disease, in a rural area of Argentina five years following an insecticidal campaign. Patterns of infestation were identified in an entomological survey, as mapped with high-resolution satellite imagery and analyzed in a geographic information system. Logistic regression was used to relate infestation to observed household characteristics as well as the location and density of households. Location was the most significant predictor of infestation for domiciles. For peridomestic structures surrounding the domiciles, the combination of location and the presence/absence of goat pens was most significant. In considering any infestation, whether domiciliary or peridomestic, the combination of location, presence/absence of animal pens, and the type of household construction were found to be most significant. Using these statistical relationships to back-classify the field data resulted in accuracies between 85% and 87%. A map of infestation probability for the town of Chancaní was developed from the logistic regression.     

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.     

The biting rate of Triatoma infestans in Argentina

1. The daily proportion of fed individuals in a population of the reduviid bug Triatoma infestans (Klug), maintained under natural climatic conditions in experimental chicken houses in central Argentina, was estimated from the proportion of bugs retaining colourless urine in the rectum. 2. From the estimates of feeding frequency throughout a 1 year period, it was shown that temperature has a dominant effect on biting rate, but density-dependent effects became apparent during the warmest months. 3. These and other data on the determinants of blood consumption by T. infestans, were incorporated into a detailed hypothesis of density regulation in this species.     

Triatoma infestans in Greater Buenos Aires, Argentina

The Health Administration Agencies of many municipalities in Greater Buenos Aires (GBA) receive frequent reports on triatomines in houses. The aim of this work was to identify and describe the dispersal foci of Triatoma infestans in an urban neighborhood of GBA, and contribute to the knowledge of the epidemiological situation in the region. In June 1998, potentially infested places were entomologically evaluated. T. infestans was only detected in a hen building for egg production, which housed approximately 6,000 birds. A total of 2,930 insects were collected. Density was about 9 triatomines/m(2). The proportions of fifth instar nymphs and adults were significantly higher than those of the other stages (p<0.001). The number of triatomines collected largely exceeded the highest domestic infestation found in one house from rural endemic areas of Argentina. Though triatomines were negative for Trypanosoma cruzi, they could acquire the parasite by coming in contact with infected people living in GBA. Besides, the numerous and widely distributed places housing hens and chickens, would favor the settlement of the vector. Together, both facts may constitute a risk of parasitic vectorial transmission. It is recommended to intensify systematic activities of vector search and case detection in GBA.     

Dispersive flight by Triatoma infestans under natural climatic conditions in Argentina

Male and female T. infestans were released on two consecutive nights in the salinas of Cordoba Province, Argentina, when air temperatures during the flying period averaged 28.5 degrees C. 136 males (43% of the total released) and 170 females (57%) flew on the first night, and 6 males (18%) and 7 females (27%) on the second. Of these, we recovered 23 males and 14 females within 100 m, and a further female within 200 m, all from the first night's release. The remainder appear to have flown more than 100 m and possibly more than 200 m. In support of this conclusion 4 male and 5 female bugs were recovered in an abandoned brick house 550 m from the release point. Since the proportion of bugs apparently flying more than 200 m is considerably greater than that deduced from previous experiments at lower temperatures (Lehane & Schofield, 1981) it is possible that temperature influences not only the proportion of bugs flying but also the distance flown. Flight appears to be affected by wind speed. On the first night, when there was negligible wind, 43% of male and 57% of female bugs flew. On the second night, with winds gusting at 4-5 m/s, only 18% of the male and 27% of the female bugs flew.     

Key Source Habitats and Potential Dispersal of Triatoma infestans Populations in Northwestern Argentina: Implications for Vector Control

Background

Triatoma infestans —the principal vector of the infection that causes Chagas disease— defies elimination efforts in the Gran Chaco region. This study identifies the types of human-made or -used structures that are key sources of these bugs in the initial stages of house reinfestation after an insecticide spraying campaign.

Methodology and Principal Findings

We measured demographic and blood-feeding parameters at two geographic scales in 11 rural communities in Figueroa, northwest Argentina. Of 1,297 sites searched in spring, 279 (21.5%) were infested. Bug abundance per site and female fecundity differed significantly among habitat types (ecotopes) and were highly aggregated. Domiciles (human sleeping quarters) had maximum infestation prevalence (38.7%), human-feeding bugs and total egg production, with submaximal values for other demographic and blood-feeding attributes. Taken collectively peridomestic sites were three times more often infested than domiciles. Chicken coops had greater bug abundance, blood-feeding rates, engorgement status, and female fecundity than pig and goat corrals. The host-feeding patterns were spatially structured yet there was strong evidence of active dispersal of late-stage bugs between ecotopes. Two flight indices predicted that female fliers were more likely to originate from kitchens and domiciles, rejecting our initial hypothesis that goat and pig corrals would dominate.

Conclusions and Significance

Chicken coops and domiciles were key source habitats fueling rapid house reinfestation. Focusing control efforts on ecotopes with human-fed bugs (domiciles, storerooms, goat corrals) would neither eliminate the substantial contributions to bug population growth from kitchens, chicken coops, and pig corrals nor stop dispersal of adult female bugs from kitchens. Rather, comprehensive control of the linked network of ecotopes is required to prevent feeding on humans, bug population growth, and bug dispersal simultaneously. Our study illustrates a demographic approach that may be applied to other regions and triatomine species for the design of innovative, improved vector control strategies.     

Ecological and Sociodemographic Determinants of House Infestation by Triatoma infestans in Indigenous Communities of the Argentine Chaco

BackgroundThe Gran Chaco ecoregion, a hotspot for Chagas and other neglected tropical diseases, is home to >20 indigenous peoples. Our objective was to identify the main ecological and sociodemographic determinants of house infestation and abundance of Triatoma infestans in traditional Qom populations including a Creole minority in Pampa del Indio, northeastern Argentina.MethodsA cross-sectional survey determined house infestation by timed-manual searches with a dislodging aerosol in 386 inhabited houses and administered questionnaires on selected variables before full-coverage insecticide spraying and annual vector surveillance. We fitted generalized linear models to two global models of domestic infestation and bug abundance, and estimated coefficients via multimodel inference with model averaging.

Principal Findings

Most Qom households were larger and lived in small-sized, recently-built, precarious houses with fewer peridomestic structures, and fewer livestock and poultry than Creoles’. Qom households had lower educational level and unexpectedly high residential mobility. House infestation (31.9%) was much lower than expected from lack of recent insecticide spraying campaigns and was spatially aggregated. Nearly half of the infested houses examined had infected vectors. Qom households had higher prevalence of domestic infestation (29.2%) than Creoles’ (10.0%), although there is large uncertainty around the adjusted OR. Factors with high relative importance for domestic infestation and/or bug abundance were refuge availability, distance to the nearest infested house, domestic insecticide use, indoor presence of poultry, residential overcrowding, and household educational level.

Conclusions and Significance

Our study highlights the importance of sociodemographic determinants of domestic infestation such as overcrowding, education and proximity to the nearest infested house, and corroborates the role of refuge availability, domestic use of insecticides and household size. These factors may be used for designing improved interventions for sustainable disease control and risk stratification. Housing instability, household mobility and migration patterns are key to understanding the process of house (re)infestation in the Gran Chaco.     

Molecular phylogeography of the Chagas' disease vector Triatoma infestans in Argentina

Triatoma infestans is the main vector of Chagas' disease in South America between latitudes 10°S and 46°S. A multilocus microsatellite data set of 836 individuals from 27 populations of T. infestans, from all its range of distribution in Argentina, was analyzed. Our results favor the hypothesis of two independent migration events of colonization in Argentina and secondary contacts. The majority of the populations of the western provinces of Catamarca, La Rioja, San Juan and the west of Cordoba province, had almost no shared ancestry with the rest of the populations analyzed. Probably those populations, belonging to localities close to the Andean region, could have been established by the dispersal line of T. infestans that would have arrived to Argentina through the Andes, whereas most of the rest of the populations analyzed may have derived from the dispersal line of T. infestans in non-Andean lowlands. Among them, those from the provinces of Formosa, Chaco, Santiago del Estero and Santa Fe shared different percentages of ancestry and presented lower degree of genetic differentiation. The migratory movement linked to regional economies and possibly associated with passive dispersal, would allow a higher genetic exchange among these populations of T. infestans. This study, using microsatellite markers, provides a new approach for evaluating the validity of the different hypotheses concerning the evolutionary history of this species. Two major lineages of T. infestans, an Andean and non-Andean, are suggested.     

Mitochondrial 16S DNA variation in populations of Triatoma infestans from Argentina

Abstract.  Variation in the mtDNA 16S ribosomal RNA gene in populations of Triatoma infestans (Klug) was surveyed. DNA sequence comparisons yielded 18 haplotypes among 130 individuals from 16 localities that represent a large proportion of the range of T. infestans in Argentina. The most common genotype in all populations was found in 76.9% of individuals and two other haplotypes were shared among different populations. The remaining 15 haplotypes were present exclusively in one of the populations, suggesting currently low levels of genetic exchange. Analysis of mtDNA 16S sequences uncovered substantial genetic variation among T. infestans populations. Haplotype and nucleotide diversities varied among populations, from 0% to 0.84% and 0% to 0.29%, respectively. It appears that this locus has a low mutation rate. Uncorrected pairwise differences of T. infestans haplotypes ranged from 0% to 1.2%. The molecular phylogeny supported the monophyly of T. infestans haplotypes and clustered two different pairs of haplotypes with a moderate degree of bootstrap support (∼ 60%). Mitochondrial DNA phylogeographic differentiation was not evident, suggesting a recent rapid spread of the species. Analysis of molecular variance showed hierarchical structure in the data. Considerably less variation was found among T. infestans populations from the northwest and northeast regions than among those belonging to the central area. Such a lack of variation may be indicative of one or more past population bottlenecks.     

Blastocrithidia triatomae n. sp. Found in Triatoma infestans from Argentina

SYNOPSIS. Blastocrithidia triatomae n. sp. found in the digestive tract of T. infestans in Argentina is described. Cysts attached to the flagellum, a conspicuous characteristic of the new species, are described and shown in photomicrographs. Since T. infestans may harbor both T. cruzi and B. triatomae, or either one independently, it is essential that they be distinguished in xenodiagnosis.     

Comparison of two sampling methods for domestic populations of Triatoma infestans in north-west Argentina

Abstract. In mud-and-thatch houses of Santiago del Estero Province, northwest Argentina, where no insecticides had been sprayed officially to control domestic infestations of the Chagas disease vector Triatoma infestans (Klug) (Hemiptera: Reduviidae), two knockdown (KD) insecticidal collection procedures were compared with the standard flush-out (FO) method for sampling T. infestans. Bugs were collected by FO using 0.2% tetramethrin in bedrooms of (1) 41 houses of Amama village employing 1 man-hour of capture effort per house, and (2) 19 houses of Trinidad and Mercedes villages employing 4 man-hours/house. From the same houses, 2-5 days after the manual FO collection, bugs were collected by KD indoor-spraying of deltamethrin 25mga.i./m² in Amama, or burning of one γ-HCH (=γ-BHC) fumigant tablet 3.1g of γ-isomer) per bedroom in Trinidad and Mercedes. The majority of infestations were detected by both methods, the proportion of positive houses being 81% at Amama and 95% at the other villages. Although the FO method was more sensitive, at least because it was applied first, the KD method detected infestations in 25% of houses where bugs were not found by FO. Bug densities estimated by FO or by subsequent KD in each house were significantly correlated: r = 0.795 for deltamethrin; r= 0.882 for y-HCH. Compared with FO collections of T.infestans large stages, i.e. adults plus fourth and fifth instar nymphs, the KD catch averaged 0.88x with deltamethrin and 0.57× with γ-HCH, regardless of the apparent population density of bugs per house. However, the KD method has practical advantages of speed and standardization.     

Distribution and pathogenicity of Trypanosoma cruzi isolated from peridomestic populations of Triatoma infestans and Triatoma guasayana from rural Western Argentina

We assessed the distribution of Trypanosoma cruzi infection in peridomestic triatomines collected manually at a district-wide scale in rural villages around Olta, Western Argentina, and typed the isolated strains according to their pathogenicity to laboratory mice. Of 1623 triatomines examined, only 14 (0.9%) were infected with T. cruzi based on microscopical examination of feces. The prevalence of T. cruzi infection was 0.8% in Triatoma infestans, 2.3% in T. guasayana, and nil in T. garciabesi, T. platensis, and T. eratyrusiformis. Local transmission occurred in kitchens, store-rooms and goat corrals or nearby, though at very low levels. T. cruzi was detected by at least one parasitological method in 11 (79%) of 14 microscope-positive bugs. Hemoculture was the most sensitive method (67%) followed by culture of organ homogenates, histopathology or xenodiagnosis of inoculated suckling mice (55-58%), and culture of microscope-positive bug feces (46%). The evidence suggests that most of the isolated T. cruzi strains would be myotropic type III. Our study establishes for the first time that peridomestic, microscope-positive T. guasayana nymphs were actually infected with T. cruzi, and may be implicated as a putative secondary vector of T. cruzi in domestic or peridomestic sites.     

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.     

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.     

Domestic Animal Hosts Strongly Influence Human-Feeding Rates of the Chagas Disease Vector Triatoma infestans in Argentina

Background

The host species composition in a household and their relative availability affect the host-feeding choices of blood-sucking insects and parasite transmission risks. We investigated four hypotheses regarding factors that affect blood-feeding rates, proportion of human-fed bugs (human blood index), and daily human-feeding rates of Triatoma infestans, the main vector of Chagas disease.

Methods

A cross-sectional survey collected triatomines in human sleeping quarters (domiciles) of 49 of 270 rural houses in northwestern Argentina. We developed an improved way of estimating the human-feeding rate of domestic T. infestans populations. We fitted generalized linear mixed-effects models to a global model with six explanatory variables (chicken blood index, dog blood index, bug stage, numbers of human residents, bug abundance, and maximum temperature during the night preceding bug catch) and three response variables (daily blood-feeding rate, human blood index, and daily human-feeding rate). Coefficients were estimated via multimodel inference with model averaging.

Findings

Median blood-feeding intervals per late-stage bug were 4.1 days, with large variations among households. The main bloodmeal sources were humans (68%), chickens (22%), and dogs (9%). Blood-feeding rates decreased with increases in the chicken blood index. Both the human blood index and daily human-feeding rate decreased substantially with increasing proportions of chicken- or dog-fed bugs, or the presence of chickens indoors. Improved calculations estimated the mean daily human-feeding rate per late-stage bug at 0.231 (95% confidence interval, 0.157–0.305).

Conclusions and Significance

Based on the changing availability of chickens in domiciles during spring-summer and the much larger infectivity of dogs compared with humans, we infer that the net effects of chickens in the presence of transmission-competent hosts may be more adequately described by zoopotentiation than by zooprophylaxis. Domestic animals in domiciles profoundly affect the host-feeding choices, human-vector contact rates and parasite transmission predicted by a model based on these estimates.     

Population dynamics and control of Triatoma infestans

Analysis of field populations of Triatoma infestans (Hemiptera: Reduviidae), after a 3-year study, shows that population growth rate is affected by both density-dependent and density-independent mortality. Although an equilibrium exists, apparently as a consequence of a density dependent-mechanism, population density fluctuates throughout the year because of the effect of monthly mean minimum temperature as a density-independent source of mortality. Simulation studies based on Moran curves shows that high population densities have an approximately constant extinction probability (around 0.20), independently of the season the population starts growing. However, at very low population densities, the extinction probability depends strongly on the season when the population begins to grow. Very low density populations beginning in winter or autumn have the highest extinction probability. The outcome of the simulation studies coincides with results observed in field populations affected by insecticide application at different seasons.