Pyrethroid resistance distribution in Triatoma infestans and environmental association along the Argentine endemic zone.

Insecticide resistance is considered a barrier to chemical control of Triatoma infestans, the main vector of Chagas disease in the Southern Cone of South America. Although initiatives to reduce the incidence of the disease in the region have integrated different strategies, they have mainly relied on vector elimination using pyrethroid insecticides such as deltamethrin. Reports of pyrethroid resistance in connection with T. infestans control failures first emerged in northern Argentina and southern Bolivia. Recently, a mosaic pyrethroid-resistant focus has been described in the center of the Argentine Gran Chaco (Department of General Güemes, Chaco Province), characterized by the presence of susceptible and very highly resistant populations in the same area. The involvement of different resistance mechanisms has been proposed, together with the contribution of environmental variables that promote the toxicological heterogeneity described. In the endemic zone of Argentina, however, new questions arise: Are there any other clusters of resistance? Is there a relationship between the distribution of resistance and environmental variables (as has been observed at smaller scale)? We studied toxicological data from insects collected and analyzed at 224 localities between 2010 and 2020 as part of the resistance monitoring conducted by the Chagas National Program. The sites were classified according to the survival rate of insects exposed to a discriminant dose of deltamethrin: 0-0.19 were considered susceptible, 0.2-0.79 low-resistance, and 0.8-1 high-resistance. Localities were georeferenced to describe the spatial distribution of resistance and to identify environmental variables (demographics, land use, urbanization, connectivity, and climate) potentially associated with resistance. We used Generalized Linear Models (GLMs) to examine the association between resistance and environmental predictors, selecting error distributions based on the response variable definition. For the entire period, 197 susceptible localities were distributed across the endemic zone. Localities with different survival rates were found throughout the area; 9 high-resistance localities circled the two previously identified resistant foci, and 18 low-resistance in 6 provinces, highlighting their relevance for control planning. Precipitation variables were linked to resistance in all the GLMs evaluated. Presence/absence models were the most accurate, with precipitation, distance from the capital city, and land use contributing to the distribution of resistance. This information could be valuable for improving T. infestans control strategies in future scenarios characterized by unpredictable changes in land use and precipitation.

Transcriptomic modulation in response to an intoxication with deltamethrin in a population of Triatoma infestans with low resistance to pyrethroids.

BACKGROUND: Triatoma infestans is the main vector of Chagas disease in the Southern Cone. The resistance to pyrethroid insecticides developed by populations of this species impairs the effectiveness of vector control campaigns in wide regions of Argentina. The study of the global transcriptomic response to pyrethroid insecticides is important to deepen the knowledge about detoxification in triatomines. METHODOLOGY AND FINDINGS: We used RNA-Seq to explore the early transcriptomic response after intoxication with deltamethrin in a population of T. infestans which presents low resistance to pyrethroids. We were able to assemble a complete transcriptome of this vector and found evidence of differentially expressed genes belonging to diverse families such as chemosensory and odorant-binding proteins, ABC transporters and heat-shock proteins. Moreover, genes related to transcription and translation, energetic metabolism and cuticle rearrangements were also modulated. Finally, we characterized the repertoire of previously uncharacterized detoxification-related gene families in T. infestans and Rhodnius prolixus. CONCLUSIONS AND SIGNIFICANCE: Our work contributes to the understanding of the detoxification response in vectors of Chagas disease. Given the absence of an annotated genome from T. infestans, the analysis presented here constitutes a resource for molecular and physiological studies in this species. The results increase the knowledge on detoxification processes in vectors of Chagas disease, and provide relevant information to explore undescribed potential insecticide resistance mechanisms in populations of these insects.

Genetic structure of deltamethrin-resistant populations of Triatoma infestans (Hemiptera: Reduviidae) in the Gran Chaco.

The genetic structure of natural populations offers insight into the complexities of their dynamics, information that can be relevant to vector control strategies. Microsatellites are useful neutral markers to investigate the genetic structure and gene flow in Triatoma infestans, one of the main vectors of Chagas disease in South America. Recently, a heterogeneous pyrethroid-resistant hotspot was found in the Argentine Gran Chaco, characterized by the highest levels of deltamethrin resistance found at the present time. We applied population genetics analyses to microsatellite and village data and search for associations between the genetic variability and the heterogeneous toxicological pattern previously found. We genotyped 10 microsatellite loci in 67 T. infestans from 6 villages with no, low, and high pyrethroid resistance. The most genetically diverse populations were those susceptible or with low values of resistance. In contrast, high-resistance populations had lower herozygosity and some monomorphic loci. A negative association was found between variability and resistant ratios. Global and pairwise FSTs indicated significant differentiation between populations. The only susceptible population was discriminated in all the performed studies. Low-resistance populations were also differentiated by a discriminant analysis of principal components (DAPC) and were composed mostly by the same two genetic clusters according to STRUCTURE Bayesian algorithm. Individuals from the high-resistance populations were overlapped in the DAPC and shared significant proportions of a genetic cluster. These observations suggest that the resistant populations might have a common origin, although more genetic markers and samples are required to test this hypothesis more rigorously.

First evidence of the mutations associated with pyrethroid resistance in head lice (Phthiraptera: Pediculidae) from Honduras.

BACKGROUND: The human head louse, Pediculus humanus capitis, is a cosmopolitan blood-sucking ectoparasite affecting mostly schoolchildren in both developed and developing countries. In Honduras, chemical pediculicides are the first line of treatment, with permethrin as their main active ingredient. Despite the extended use of these products, there is currently no research investigating insecticide resistance in Honduran head lice. In head lice, the most common mechanism is knockdown resistance (kdr), which is the result of two point mutations and the associated amino acid substitutions, T917I and L920F, within the voltage-sensitive sodium channel (VSSC). METHODS: Genomic DNA was extracted from 83 head lice collected in the localities of San Buenaventura and La Hicaca, Honduras. Polymerase chain reaction (PCR) was used to amplify a 332-bp fragment of the VSSC gene that contains a site affected by C/T mutation which results in a T917I amino acid substitution on each human head louse genomic DNA fragments. RESULTS: The C/T non-synonymous mutation which results in the T917I kdr amino acid substitution was detected in both head lice populations at frequencies ranging between 0.45-0.5. Globally, the frequency of this substitution was 0.47. Of these, 5 (6.1%) were homozygous susceptible and 78 (93.9%) were heterozygotes. The kdr-resistant homozygote (RR) was not detected in the studied populations. Thus, 93.9% of the head lice collected in Honduras harbored only one T917I allele. Exact test for the Hardy-Weinberg equilibrium for both localities showed that genotype frequencies differed significantly from expectation. In addition, San Buenaventura and La Hicaca populations had an inbreeding coefficient (Fis) < 0, suggesting an excess of heterozygotes. CONCLUSIONS: To our knowledge, this is the first study showing the presence of the C/T mutation responsible of the T917I kdr allele associated with pyrethroid resistance in P. h. capitis from Honduras. The PCR-restriction fragment length polymorphism (RFLP) employed here has demonstrated to be a reliable, economic, and reproducible assay that can be used to accurately genotype individual head lice for the mutation encoding the resistance-conferring T917I amino acid substitution. This highlights the necessity of proactive resistance management programmes designed to detect pyrethroid mutations before they become established within populations of head lice.

Resistance to deltamethrin in Triatoma infestans: microgeographical distribution, validation of a rapid detection bioassay and evaluation of a fumigant canister as control alternative strategy.

Triatoma infestans (Klug) (Hemiptera: Reduviidae) is the main vector of Chagas disease in the Southern Cone of America and resistance to pyrethroid insecticides has been detected in several areas from its geographical distribution. Pyrethroid resistance presents a complex geographical pattern at different spatial scales. However, it is still unknown if the toxicological variability is a common feature within villages of the Gran Chaco were high resistance was descripted. The objectives of this study were to determine: (a) the microgeographical distribution of the deltamethrin-resistance in insects from Pampa Argentina village, (b) the performance of the insecticide impregnated paper bioassay to evaluate deltamethrin-resistance in field collected insects and (c) the lethal activity of the fumigant canister containing DDVP against insects resistant to deltamethrin. High survival of T. infestans exposed to discriminant dose was observed in the samples of all the evaluated dwellings, suggesting that the resistance to deltamethrin is homogeneous at the microgeographical level. Resistance determination by impregnated paper bioassay was similar to traditional topical determination, highlighting the use of this rapid methodology in field large-scale monitoring. The fumigant canister was not effective against resistant insects, remarking the need to develop suitable formulations that ensure minimal toxicological risk and high effectivity.

Insecticide Resistance Mechanisms in Triatoma infestans (Reduviidae: Triatominae): The Putative Role of Enhanced Detoxification and Knockdown Resistance (kdr) Allele in a Resistant Hotspot From the Argentine Chaco.

Chagas disease affects around 6 million people in the world, and in Latin America, it is mainly transmitted by the kissing bug. Chemical control of the vector with pyrethroid insecticides has been the most frequently used tool to reduce the disease incidence. Failures of field control have been detected in areas of the Argentinian Gran Chaco that correlate with high levels of insecticide resistance. Here, we provide evidence of the mechanisms involved in the resistance to insecticides of field populations of T. infestans from General Güemes Department (Chaco Province, Argentina). The biochemical analysis suggests the increase in the activity of the degradative enzymes P450 oxidases and esterases as a minor contributive mechanism in low-resistance populations. The molecular study revealed high frequencies of the kdr L925I mutation at the voltage-gated sodium channel as responsible for the high resistance ratios detected. This knowledge contributes to the generation of comprehensive vector control strategies that reduce the incidence of the disease.

High effectiveness of an adulticide-larvicide formulation for field control of sandflies (Diptera: Psychodidae) in the city of Clorinda, Argentina.

In Argentina, Leishmania infantum (syn. L. chagasi) is the etiologic agent of human visceral leishmaniosis (HVL), and Lutzomyia longipalpis (Diptera: Psychodidae: Phlebotominae) is the main vector. The objective of this study was to evaluate the effectiveness and residual effect of two commercial insecticide formulations, one with permethrin and pyriproxyfen as active ingredients (Dragon Max®) and the other with only permethrin (Flop®) for the control of sandflies. Both formulations were applied in chicken coops and other surroundings structures of the peridomicile of urban houses in Clorinda, Formosa (Argentina). Entomological monitoring was carried out weekly for 44 weeks after the intervention. The results showed great effectiveness and residual effect up to 21 weeks post-intervention for Dragon Max®. This result could be explained by the excellent larvicidal activity of the Insect Growth Regulator (IGR) pyriproxyfen against the immature forms of phlebotomines and by the delay on the restoration of the natural threshold of the vector population in treated sites.

Kdr mutations in Triatoma infestans from the Gran Chaco are distributed in two differentiated foci: Implications for pyrethroid resistance management.

Point mutations in the voltage-gated sodium channel, the primary target of pyrethroid insecticides, have been associated with the resistance in Triatoma infestans, an important vector of Chagas' disease. Hence, the sustainability of vector control programs requires the implementation of resistance management strategies. We determined the sensitivity of the molecular assays previously designed for early resistance detection to be used in pooled samples from a wide area of the endemic region, and validated them for their routine use in control campaigns for the monitoring of insecticide resistance in T. infestans. Consequently, we used these methods to examine the distribution of resistance-associated mutations in the sodium channel gene in populations of T. infestans from the Argentinean and Bolivian Gran Chaco. The PASA and REA assays tested proved sensitive enough to detect kdr SNPs in pooled samples, indicating these assays are suitable for routine screening in insecticide resistance surveillance. Two geographically differentiated foci were detected in T. infestans populations from the Argentinean and Bolivian Gran Chaco, with populations on the Bolivian-Argentinean border carrying L1014F mutation, and those from the Argentinean Chaco carrying L925I mutation. In all highly resistant populations analyzed, one of both kdr mutations was present, and toxicological assays determined that all pyrethroid resistant populations analyzed herein were sensitive to fenitrothion. The principal cause of pyrethroid resistance in T. infestans from the Gran Chaco ecoregion is kdr mutations in the sodium channel. Different levels of resistance occur in different populations carrying identical mutation, suggesting the existence of contributory mechanisms.