Improved vector control of Triatoma infestans limited by emerging pyrethroid resistance across an urban-to-rural gradient in the Argentine Chaco

Morphofunctional characteristics of flight-related traits in deltamethrin-resistant and susceptible Triatoma infestans (Klug, 1834) of the Argentinean Chaco

BACKGROUND

Chagas disease, transmitted by triatomine bugs, is a major vector-borne parasitic disease in Latin America. Triatoma infestans, the principal vector in the Southern Cone, is primarily controlled through residual insecticide spraying. However, resistance to pyrethroids, especially in Northern Argentina and Southern Bolivia, has emerged. Resistant T. infestans populations exhibit reduced fitness, including impacts on reproductive success and dispersal capacity. This study investigates the flight potential and morphological changes in T. infestans populations with varying levels of insecticide resistance, hypothesizing that resistance may induce morphological changes in wing and head structures related to dispersal.

METHODS

We analyzed three resistance profiles of T. infestans-susceptible (S), moderately resistant (MR), and highly resistant (HR)-collected from ten domestic or peridomestic sites in two municipalities from Chaco province, Argentina. We registered flight muscle development and measured flight-related traits (wings, heads, and the stiff and membranous portions of the wing) using a landmark-based methodology. We also assessed morphological disparity and covariation of these traits across toxicological groups.

RESULTS

Significant morphological differences were found between resistant and susceptible populations. The frequency of insects with and without muscle varied across toxicological groups only for females, exhibiting the highest proportion of HR insects with fight muscle (86.21%). MR and HR males exhibited smaller stiff portions of the wing and heads than S males. Shape variation analysis showed that S females had wider forewings than resistant females, while HR females had narrower wings with a wider stiff portion. Susceptible males had wider and longer wings compared with resistant groups. Additionally, resistant populations showed greater morphological disparity and reduced covariation between flight-related traits.

CONCLUSIONS

Our study shows that pyrethroid resistance in T. infestans is linked to morphological changes in flight-related traits. These changes suggest a tradeoff between resistance and flight capacity, with energy allocated to resistance mechanisms potentially limiting flight. The reduced covariation between flight traits in resistant individuals supports the idea of pleiotropic effects. While resistant individuals may perform better in insecticide treated areas, their reduced flight capacity could limit long-distance dispersal, affecting population dynamics and vector control efforts.

Odor masking to decrease the attraction of Triatoma infestans (Hemiptera: Reduviidae, Triatominae), a Chagas disease vector, towards a host

Chagas disease, vectored by kissing bugs, poses a public health problem across the Americas. The best way for reducing disease transmission is through vector control, which is currently based on the use of insecticides. However, insecticide resistance, and environmental and health issues, stress the need for new, environmentally-friendly methods for reducing vector-host contacts. Kissing bugs use the odor blend emitted by hosts to locate them. The proportions of the constituents of that blend are important to determine the attraction evoked by it. We aimed at assessing whether artificial alterations of those proportions could render an attractive host, unattractive for the kissing bug Triatoma infestans. Thus, the behavior of the bugs after adding a source of nonanal, a component of vertebrate odor detected by the bugs, to an attractive mouse was studied. For this, nymphs were tested using a dual-choice trap-olfactometer and different doses of nonanal. Addition of nonanal resulted in a dose- dependent reduction in the attraction of the bugs towards the mouse although it did not prevent the bugs from leaving their release zone nor it affected their (reduced) host preference near the host. Nonanal per se did not evoke any behavior at the doses tested. We suggest that the increase in the proportion of nonanal resulted in masking of the mouse odor. This approach could help reach a "push" effect within an effective push- pull strategy for kissing bugs. That strategy would help diminish the vector-host contacts and, therefore, Chagas disease incidence, in an environmentally-friendly manner. It remains to be established whether this strategy proves effective in the field when adequate odor dispensers and socially acceptable odors are used.

Unusual frequency of Trypanosoma cruzi DTU TcI and predominance of hybrid lineages in Triatoma infestans before and after control interventions in the Argentinian Chaco

Transmission of Trypanosoma cruzi involves diverse hosts, vectors and parasitic genotypes, in different environments. In recent decades, the distribution of T. cruzi has altered due to urbanization of affected people and vectors. We implemented a longitudinal intervention program between 2015 and 2022 which aimed to suppress (peri)domestic Triatoma infestans in the municipality of Avia Terai (Chaco Province, Argentina), and found a marginal risk of domestic vector-borne transmission across the rural-to-urban gradient after interventions. Here, we identified the parasite Discrete Typing Units (DTUs) in infected T. infestans collected throughout the intervention program (37 insects pre-intervention and 7 post-intervention). Identification of DTUs was conducted by two methodologies, using DNA extracted from T. infestans rectal ampoules. We also assessed the association between blood-feeding sources and DTUs. Complete DTU identification was achieved in 48 % of samples. The hybrid lineages TcV or TcVI and their combinations predominated (72 %), followed by TcI (16 %) and mixed infections of TcI and hybrid lineages (14 %). Half (50 %) of the houses harbored TcI infected bugs either alone or mixed with TcII/TcV/TcVI. Humans predominated as the bloodmeal sources in all insects with identified DTU. All DTUs (TcI, TcV and TcII/TcV/TcVI) were recorded in both rural and peri-urban environments, with 62 % of the houses having more than one DTU. These results confirm the predominance of hybrid lineages in domestic transmission cycles of the Argentine Chaco. However, the finding of several triatomines infected with TcI both pre- and post-intervention raises the question of which host(s) are involved in its transmission.

What happens when the insecticide does not kill? A review of sublethal toxicology and insecticide resistance in triatomines

Chagas disease is considered one of the most important human parasitosis in the United States. This disease is mainly transmitted by insects of the subfamily Triatominae. The chemical vector control is the main tool for reducing the incidence of the disease. However, the presence of triatomines after pyrethroids spraying has been reported in some regions, as in the case of Triatoma infestans in Argentina and Bolivia. The presence of insects can be explained by the colonization from neighbouring areas, the reduction of insecticide dose to sublethal levels due to environmental factors, and/or by the evolution of insecticide resistance. In the last two scenarios, a proportion of the insects is not killed by insecticide and gives rise to residual populations. This article focuses on the toxicological processes associated with these scenarios in triatomines. Sublethal doses may have different effects on insect biology, that is, sublethal effects, which may contribute to the control. In addition, for insect disease vectors, sublethal doses could have negative effects on disease transmission. The study of sublethal effects in triatomines has focused primarily on the sequence of symptoms associated with nervous intoxication. However, the effects of sublethal doses on excretion, reproduction and morphology have also been studied. Rhodnius prolixus and T. infestans and pyrethroids insecticides were the triatomine species and insecticides, respectively, mainly studied. Insecticide resistance is an evolutionary phenomenon in which the insecticide acts as a selective force, concentrating on the insect population's pre-existing traits that confer resistance. This leads to a reduction in the susceptibility to the insecticide, which was previously effective in controlling this species. The evolution of resistance in triatomines received little attention before the 2000s, but after the detection of the first focus of resistance associated with chemical control failures in T. infestans from Argentina in 2002, the study of resistance increased remarkably. A significant number of works have studied the geographical distribution, the resistance mechanisms, the biological modifications associated with resistance, the environmental influences and the genetic of T. infestans resistant to pyrethroid insecticides. Currently, studies of insecticide resistance are gradually being extended to other areas and other species. The aim of this article was to review the knowledge on both phenomena (sublethal effects and insecticide resistance) in triatomines. For a better understanding of this article, some concepts and processes related to insect-insecticide interactions, individual and population toxicology and evolutionary biology are briefly reviewed. Finally, possible future lines of research in triatomine toxicology are discussed.

Humans seropositive for Trypanosoma cruzi co-infected with intestinal helminths have higher infectiousness, parasitaemia and Th2-type response in the Argentine Chaco

BACKGROUND

The Gran Chaco ecoregion is a well-known hotspot of several neglected tropical diseases (NTDs) including Chagas disease, soil-transmitted helminthiasis and multiparasitic infections. Interspecific interactions between parasite species can modify host susceptibility, pathogenesis and transmissibility through immunomodulation. Our objective was to test the association between human co-infection with intestinal parasites and host parasitaemia, infectiousness to the vector and immunological profiles in Trypanosoma cruzi-seropositive individuals residing in an endemic region of the Argentine Chaco.

METHODS

We conducted a cross-sectional serological survey for T. cruzi infection along with an intestinal parasite survey in two adjacent rural villages. Each participant was tested for T. cruzi and Strongyloides stercoralis infection by serodiagnosis, and by coprological tests for intestinal parasite detection. Trypanosoma cruzi bloodstream parasite load was determined by quantitative PCR (qPCR), host infectiousness by artificial xenodiagnosis and serum human cytokine levels by flow cytometry.

RESULTS

The seroprevalence for T. cruzi was 16.1% and for S. stercoralis 11.5% (n = 87). We found 25.3% of patients with Enterobius vermicularis. The most frequent protozoan parasites were Blastocystis spp. (39.1%), Giardia lamblia (6.9%) and Cryptosporidium spp. (3.4%). Multiparasitism occurred in 36.8% of the examined patients. Co-infection ranged from 6.9% to 8.1% for T. cruzi-seropositive humans simultaneously infected with at least one protozoan or helminth species, respectively. The relative odds of being positive by qPCR or xenodiagnosis (i.e. infectious) of 28 T. cruzi-seropositive patients was eight times higher in people co-infected with at least one helminth species than in patients with no such co-infection. Trypanosoma cruzi parasite load and host infectiousness were positively associated with helminth co-infection in a multiple regression analysis. Interferon-gamma (IFN-γ) response, measured in relation to interleukin (IL)-4 among humans infected with T. cruzi only, was 1.5-fold higher than for T. cruzi-seropositive patients co-infected with helminths. The median concentration of IL-4 was significantly higher in T. cruzi-seropositive patients with a positive qPCR test than in qPCR-negative patients.

CONCLUSIONS

Our results show a high level of multiparasitism and suggest that co-infection with intestinal helminths increased T. cruzi parasitaemia and upregulated the Th2-type response in the study patients.

Slow recovery rates and spatial aggregation of Triatoma infestans populations in an area with high pyrethroid resistance in the Argentine Chaco

BACKGROUND

The emergence of pyrethroid resistance has threatened the elimination of Triatoma infestans from the Gran Chaco ecoregion. We investigated the status and spatial distribution of house infestation with T. infestans and its main determinants in Castelli, a municipality of the Argentine Chaco with record levels of triatomine pyrethroid resistance, persistent infestation over 2005-2014, and limited or no control actions over 2015-2020.

METHODS

We conducted a 2-year longitudinal survey to assess triatomine infestation by timed manual searches in a well-defined rural section of Castelli including 14 villages and 234 inhabited houses in 2018 (baseline) and 2020, collected housing and sociodemographic data by on-site inspection and a tailored questionnaire, and synthetized these data into three indices generated by multiple correspondence analysis.

RESULTS

The overall prevalence of house infestation in 2018 (33.8%) and 2020 (31.6%) virtually matched the historical estimates for the period 2005-2014 (33.7%) under recurrent pyrethroid sprays. While mean peridomestic infestation remained the same (26.4-26.7%) between 2018 and 2020, domestic infestation slightly decreased from 12.2 to 8.3%. Key triatomine habitats were storerooms, domiciles, kitchens, and structures occupied by chickens. Local spatial analysis showed significant aggregation of infestation and bug abundance in five villages, four of which had very high pyrethroid resistance approximately over 2010-2013, suggesting persistent infestations over space-time. House bug abundance within the hotspots consistently exceeded the estimates recorded in other villages. Multiple regression analysis revealed that the presence and relative abundance of T. infestans in domiciles were strongly and negatively associated with indices for household preventive practices (pesticide use) and housing quality. Questionnaire-derived information showed extensive use of pyrethroids associated with livestock raising and concomitant spillover treatment of dogs and (peri) domestic premises.

CONCLUSIONS

Triatoma infestans populations in an area with high pyrethroid resistance showed slow recovery and propagation rates despite limited or marginal control actions over a 5-year period. Consistent with these patterns, independent experiments confirmed the lower fitness of pyrethroid-resistant triatomines in Castelli compared with susceptible conspecifics. Targeting hotspots and pyrethroid-resistant foci with appropriate house modification measures and judicious application of alternative insecticides with adequate toxicity profiles are needed to suppress resistant triatomine populations and prevent their eventual regional spread.

Extended stage duration and diminished fecundity in deltamethrin-resistant Triatoma infestans (Klug, 1834) of the Argentinean Chaco

Pyrethroid-resistance is an emergent trait in populations of various insect species. For Triatoma infestans (Klug, 1834) (Heteroptera: Reduviidae), the major vector of Chagas disease in the southern part of South America, hotspot areas of pyrethroid-resistance have recently been found in the Chaco Province of Argentina. Resistant condition can reduce fitness of individuals in the absence of insecticide exposure, that is, fitness costs. We evaluated the existence of developmental and/or reproductive costs in T. infestans collected from two areas of pyrethroid-resistance in Chaco Province, Argentina. Three toxicological groups were defined from field-collected insects: susceptible (survival <20%), moderately resistant (survival between 20% and 80%) and highly resistant (survival >80%). Cohorts of the three toxicological groups were followed-up to study life cycle and reproductive parameters. Additionally, we parameterized matrix population growth models. First and IV nymphal stages of the resistant groups exhibited a longer stage duration than susceptible ones. The reproductive days and hatching success showed significant lower values revealing reproductive costs for the resistant groups. Matrix analysis showed lower population growth rates for the resistant groups. Our results support developmental and reproductive costs for pyrethroid-resistant individuals. This trait could be interpreted as lower population recovery ability for pyrethroid-resistant individuals compared to susceptible insects after alternative vector control actions.

Highlights, challenges, and perspectives in basic and applied chemical ecology of triatomines

Triatomines are vectors of Chagas disease. Due to failures in their control, there is an urgent need for more efficient and environmentally friendly monitoring and control tools. These hematophagous insects rely heavily on chemical information from the environment to detect hosts and cues/signals from conspecifics. Chemical ecology includes the elucidation of the functional role of chemicals mediating interactions between organisms. Studies on the chemical ecology of triatomines are leading to novel methods for their monitor and control. Thus, laboratory tests to develop chemical attractants and repellents are promissory and have led to the design of, for example, efficient baited traps. However, the monitoring and control tools proposed until now have not been as effective in the field.

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.

Marginal risk of domestic vector-borne Trypanosoma cruzi transmission after improved vector control of Triatoma infestans across a rural-to-urban gradient in the Argentine Chaco

The interruption of domestic vector-borne transmission of Trypanosoma cruzi in the Americas remains one of the main goals of the World Health Organization 2021-2030 road map for neglected tropical diseases. We implemented a longitudinal intervention program over 2015-2022 to suppress (peri)domestic Triatoma infestans in the municipality of Avia Terai, Chaco Province, Argentina and found that house infestation (3851 houses inspected) and triatomine abundance decreased over the first 2 years post-intervention (YPI), and remained stable thereafter associated to moderate pyrethroid resistant foci. Here we assessed selected components of transmission risk after interventions across the rural-to-urban gradient. We used multistage random sampling to select a municipality-wide sample of T. infestans. We examined 356 insects collected in 87 houses for T. cruzi infection using kDNA-PCR and identified their bloodmeal sources using an indirect ELISA. The overall prevalence of T. cruzi infection post-intervention was 1.7% (95% CI 0.7-3.6). Few houses (5.7%) (95% CI 2.5-12.8) harbored infected triatomines across the gradient. Infected triatomines were found in 5 peri-urban or rural dwellings over 1-4 years post-intervention. No infected insect was found in the urban area. The human blood index decreased from 66.2 at baseline to 42.8 at 1YPI and then increased to 92.9 at 4-5 YPI in the few infested domiciles detected. The percentage of houses with human-fed bugs displayed a similar temporal trend. Our results indicate marginal risks of domestic vector-borne transmission across the district after implementation of the intervention program. Ensuring sustainable vector surveillance coupled with human etiological diagnosis and treatment in hiperendemic areas like the Gran Chaco region, is urgently needed. 252 words.

Urban vectors of Chagas disease in the American continent: A systematic review of epidemiological surveys

BACKGROUND

Chagas is a complex and multidimensional socio-environmental health phenomenon, in which different components converge and interact. Historically, this disease was associated with insect vectors found in the rural environment. However, in the Americas, we are currently facing a new paradigm, in which different scenarios allow maintaining the vectorial transmission of the parasite through triatomine populations that either occasionally enter the dwellings or colonize urban environments.

METHODOLOGY/PRINCIPAL FINDINGS

Records of scientific reports available in the PubMed and LILACS search engines were retrieved, using three criteria according to the main triatomine genera of epidemiological importance and to the general scientific production on Chagas disease in urban contexts. Results showed that records on the occurrence of vectors in urban dwellings began to increase in the last three decades. Results also showed that the main species of triatomines collected inside dwellings (18 in total) belong mainly to the genera Triatoma and Panstrongylus, with most species (16/18, 88.8%) infected with the parasite, and that infestation of triatomine species occurs in all types of cities (small, medium and large, including megalopolises), from Argentina to the USA.

CONCLUSIONS/SIGNIFICANCE

Urban Chagas represents a new challenge that adds a different dimension to the problem of Chagas disease due to the particular characteristics of the lifestyle in urban agglomerates. The new scenario will require adaptations of the programs of control of vector to this shift from rural to urban settlements.

New Cell Lines Derived from Laboratory Colony Triatoma infestans and Rhodnius prolixus, Vectors of Trypanosoma cruzi, Do Not Harbour Triatoma Virus

Triatomine bugs of the genera Triatoma and Rhodnius are vectors of Chagas disease, a neglected tropical disease of humans in South America caused by Trypanosoma cruzi. Triatoma virus (TrV), a natural pathogen of Triatoma infestans, has been proposed as a possible tool for the bio-control of triatomine bugs, but research into this virus has been hampered by a lack of suitable host cells for in vitro propagation. Here we report establishment and partial characterisation of continuous cell lines from embryos of T. infestans (TIE/LULS54) and Rhodnius prolixus (RPE/LULS53 and RPE/LULS57). RNAseq screening by a sequence-independent, single primer amplification approach confirmed the absence of TrV and other RNA viruses known to infect R. prolixus, indicating that these new cell lines could be used for propagation of TrV.

Towards environmental detection of Chagas disease vectors and pathogen

Chagas disease vector control relies on prompt, accurate identification of houses infested with triatomine bugs for targeted insecticide spraying. However, most current detection methods are laborious, lack standardization, have substantial operational costs and limited sensitivity, especially when triatomine bug densities are low or highly focal. We evaluated the use of FTA cards or cotton-tipped swabs to develop a low-technology, non-invasive method of detecting environmental DNA (eDNA) from both triatomine bugs and Trypanosoma cruzi for use in household surveillance in eastern Colombia, an endemic region for Chagas disease. Study findings demonstrated that Rhodnius prolixus eDNA, collected on FTA cards, can be detected at temperatures between 21 and 32 °C, when deposited by individual, recently blood-fed nymphs. Additionally, cotton-tipped swabs are a feasible tool for field sampling of both T. cruzi and R. prolixus eDNA in infested households and may be preferable due to their lower cost. eDNA detection should not yet replace current surveillance tools, but instead be evaluated in parallel as a more sensitive, higher-throughput, lower cost alternative. eDNA collection requires virtually no skills or resources in situ and therefore has the potential to be implemented in endemic communities as part of citizen science initiatives to control Chagas disease transmission.