Fine scale spatial investigation of multiple insecticide resistance and underlying target-site and metabolic mechanisms in Anopheles gambiae in central Côte d'Ivoire

Genetic surveillance of insecticide resistance in African Anopheles populations to inform malaria vector control

Insecticide resistance in malaria vector populations poses a major threat to malaria control, which relies largely on insecticidal interventions. Contemporary vector-control strategies focus on combatting resistance using multiple insecticides with differing modes of action within the mosquito. However, diverse genetic resistance mechanisms are present in vector populations, and continue to evolve. Knowledge of the spatial distribution of these genetic mechanisms, and how they impact the efficacy of different insecticidal products, is critical to inform intervention deployment decisions. We developed a catalogue of genetic-resistance mechanisms in African malaria vectors that could guide molecular surveillance. We highlight situations where intervention deployment has led to resistance evolution and spread, and identify challenges in understanding and mitigating the epidemiological impacts of resistance.

The impact of agrochemical pollutant mixtures on the selection of insecticide resistance in the malaria vector Anopheles gambiae: insights from experimental evolution and transcriptomics

BACKGROUND

There are several indications that pesticides used in agriculture contribute to the emergence and spread of resistance of mosquitoes to vector control insecticides. However, the impact of such an indirect selection pressure has rarely been quantified and the molecular mechanisms involved are still poorly characterized. In this context, experimental selection with different agrochemical mixtures was conducted in Anopheles gambiae. The multi-generational impact of agrochemicals on insecticide resistance was evaluated by phenotypic and molecular approaches.

METHODS

Mosquito larvae were selected for 30 generations with three different agrochemical mixtures containing (i) insecticides, (ii) non-insecticides compounds, and (iii) both insecticide and non-insecticide compounds. Every five generations, the resistance of adults to deltamethrin and bendiocarb was monitored using bioassays. The frequencies of the kdr (L995F) and ace1 (G119S) target-site mutations were monitored every 10 generations. RNAseq was performed on all lines at generation 30 in order to identify gene transcription level variations and polymorphisms associated with each selection regime.

RESULTS

Larval selection with agrochemical mixtures did not affect bendiocarb resistance and did not select for ace1 mutation. Contrastingly, an increased deltamethrin resistance was observed in the three selected lines. Such increased resistance was not majorly associated with the presence of kdr L995F mutation in selected lines. RNA-seq identified 63 candidate resistance genes over-transcribed in at least one selected line. These include genes coding for detoxification enzymes or cuticular proteins previously associated with insecticide resistance, and other genes potentially associated with chemical stress response. Combining an allele frequency filtering with a Bayesian FST-based genome scan allowed to identify genes under selection across multiple genomic loci, supporting a multigenic adaptive response to agrochemical mixtures.

CONCLUSION

This study supports the role of agrochemical contaminants as a significant larval selection pressure favouring insecticide resistance in malaria vectors. Such selection pressures likely impact kdr mutations and detoxification enzymes, but also more generalist mechanisms such as cuticle resistance, which could potentially lead to cross-tolerance to unrelated insecticide compounds. Such indirect effect of global landscape pollution on mosquito resistance to public health insecticides deserves further attention since it can affect the nature and dynamics of resistance alleles circulating in malaria vectors and impact the efficacy of control vector strategies.

Efficacy of chlorfenapyr-pyrethroid and piperonyl butoxide-pyrethroid long-lasting insecticidal nets (LLINs) compared to pyrethroid-only LLINs for malaria control in Côte d'Ivoire: a three group, cluster randomised trial

BACKGROUND

The massive scale-up of long-lasting insecticidal nets (LLIN) has led to a major reduction in malaria burden in many sub-Saharan African (SSA) countries. The World Health Organization (WHO) has recently issued a strong recommendation for the use of chlorfenapyr-pyrethroid LLINs compared to standard pyrethroid-only LLINs in areas of high insecticide resistance intensity. However, there is still a lack of conclusive evidence on the efficacy of piperonyl butoxide-pyrethroid (PBO-py) LLINs, especially in West Africa, where vector composition and resistance mechanisms may be different from vectors in East Africa.

METHODS

This is a three-arm, superiority, triple-blinded, cluster randomised trial, with village as the unit of randomisation. This study conducted in Côte d'Ivoire will evaluate the efficacy on epidemiological and entomological outcomes of (1) the control arm: MAGNet® LN, which contains the pyrethroid, alpha-cypermethrin, (2) VEERALIN® LN, a net combining the synergist PBO and alpha-cypermethrin, and (3) Interceptor® G2 LN, which incorporates chlorfenapyr and alpha-cypermethrin, two adulticides with different mechanisms of action. A total of 33 villages with an average of 200 households per village will be identified, mapped, and randomised in a ratio of 1:1:1. Nets will be distributed at a central point following national guidelines with 1 net for every 2 people. The primary outcome of the trial will be incidence of malaria cases (confirmed by rapid diagnostic test (RDT)) in a cohort of 50 children aged 6 months to 10 years in each cluster, followed for 12 months (active case detection). Secondary outcomes are cross-sectional community prevalence of malaria infection (confirmed by RDT) in the study population at 6 and 12 months post-intervention (50 randomly selected persons per cluster), vector density, entomological inoculation rate (EIR), and phenotypic and genotypic insecticide resistance at baseline and 12 months post-intervention in 3 sentinel villages in each treatment arm.

DISCUSSION

In addition to generating further evidence for next-generation LLINs, this study will also provide the first evidence for pyrethroid-PBO nets in a West African setting. This could further inform WHO recommendations on the pragmatic use of pyrethroid-PBO nets.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05796193. Registered on April 3, 2023.

EaveTubes for control of vector-borne diseases in Côte d'Ivoire: study protocol for a cluster randomized controlled trial

BACKGROUND

Vector control tools, long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS), have significantly contributed to malaria prevention efforts in sub-Saharan Africa. However, insecticide resistance has seriously hampered their efficacy in recent years and new tools are essential to further progress. In2Care® EaveTubes (ETs) are an inexpensive, new resistance-breaking vector control product under World Health Organization (WHO) evaluation informed by mosquito ecology to efficiently target malaria vectors. By installing ETs in the walls of the house at the eave level that funnel the natural airflow, mosquitoes are drawn in by the same heat and odor cues that typically attract them through open eaves. Once inside an ET, mosquitoes are exposed to insecticide-treated netting placed inside the ET. The aim of this study is to test whether ETs as stand-alone tool have an effect on the epidemiology of malaria in villages where houses have been modified with the ET intervention.

METHODS

A two-armed, cluster randomized controlled trial will be conducted to evaluate the effect of ETs on clinical malaria incidence in children living in Côte d'Ivoire. Thirty-four villages will be selected based on population size and the proportion of houses suitable for modification with ETs (17 treatment arms (ETs + LLINs, 17 control arms (LLINs only)). Based on the population census, 55 households per cluster with eligible children (i.e., between the ages of 6 months to 8 years old at the start of the study) will be randomly selected for recruitment into the active detection cohorts. In the treatment arm, we will enroll eligible children who reside in ET-treated houses. The intervention and control cohorts will be followed for 4 months for baseline covariate measurements and 24 months with intervention. During case detection visits, blood samples will be taken from all febrile children and tested for malaria infection with rapid diagnostic tests (RDTs). All positive clinical malaria infections will be treated. To estimate the impact of the ET on malaria vector densities, entomological measurements (indoor sampling with CDC traps) will be conducted monthly in 20 clusters (10 ET, 10 Control) in 10 randomly selected households per cluster. To estimate the infectiousness of malaria vectors, sporozoite rates will be measured in subsets of the collected mosquito samples.

DISCUSSION

Findings will serve as an efficacy trial of ETs and will be submitted to the WHO Vector Control Advisory Group (VCAG) for assessment of public health value. Entomological outcomes will also be measured as proxies of malaria transmission to help develop guidelines for the evaluation of future In2Care® ETs products.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05736679. Registered on 10 February 2023.

Adverse effects of knocking down chitin synthase A on female reproduction in Culex pipiens pallens (Diptera: Culicidae)

BACKGROUND

Current mosquito-borne disease vector control strategies, largely based on chemical insecticides, are seriously threatened by increasing resistance worldwide. There is also growing concerned about the adverse effects of insecticides on nontarget organisms and the environment, therefore effective and ecologically friendly alternative approaches are urgently needed. Targeting critical steps of reproduction is considered a potential way to control mosquito populations. Herein, we focused on the roles of chitin synthase A (encoded by chsa) in the reproduction of female mosquitoes.

RESULTS

The injection of small interfering RNA targeting Cpchsa in female Culex pipiens pallens (Diptera: Culicidae) had antireproductive effects, including decreased follicle numbers, egg-laying, and hatching rate. Scanning electron microscopy observations showed that Cpchsa silencing caused a defective egg envelope, including absence of the vitelline membrane and cracked chorion layers, which resulted in abnormal permeability. Widely distributed nurse cell apoptosis and follicular epithelial cell autophagy were observed in Cpchsa-silenced ovaries during the vitellogenesis phase. Consistent with the detective egg envelope formation during oogenesis, the exochorionic eggshell structures were also affected in eggs deposited by Cpchsa-silenced mosquitoes.

CONCLUSION

This study provided fundamental evidence for the role of chitin synthase A in the female reproductive process of mosquitoes and might result in a novel alternative strategy for mosquito control. © 2023 Society of Chemical Industry.

Deltamethrin and transfluthrin select for distinct transcriptomic responses in the malaria vector Anopheles gambiae

BACKGROUND

The widespread use of pyrethroid insecticides in Africa has led to the development of strong resistance in Anopheles mosquitoes. Introducing new active ingredients can contribute to overcome this phenomenon and ensure the effectiveness of vector control strategies. Transfluthrin is a polyfluorinated pyrethroid whose structural conformation was thought to prevent its metabolism by cytochrome P450 monooxygenases in malaria vectors, thus representing a potential alternative for managing P450-mediated resistance occurring in the field. In this study, a controlled selection was used to compare the dynamics of resistance between transfluthrin and the widely used pyrethroid deltamethrin in the mosquito Anopheles gambiae. Then, the associated molecular mechanisms were investigated using target-site mutation genotyping and RNA-seq.

METHODS

A field-derived line of An. gambiae carrying resistance alleles at low frequencies was used as starting material for a controlled selection experiment. Adult females were selected across 33 generations with deltamethrin or transfluthrin, resulting in three distinct lines: the Delta-R line (selected with deltamethrin), the Transflu-R line (selected with transfluthrin) and the Tiassale-S line (maintained without selection). Deltamethrin and transfluthrin resistance levels were monitored in each selected line throughout the selection process, as well as the frequency of the L1014F kdr mutation. At generation 17, cross-resistance to other public health insecticides was investigated and transcriptomes were sequenced to compare gene transcription variations and polymorphisms associated with adaptation to each insecticide.

RESULTS

A rapid increase in resistance to deltamethrin and transfluthrin was observed throughout the selection process in each selected line in association with an increased frequency of the L1014F kdr mutation. Transcriptomic data support a broader response to transfluthrin selection as compared to deltamethrin selection. For instance, multiple detoxification enzymes and cuticle proteins were specifically over-transcribed in the Transflu-R line including the known pyrethroid metabolizers CYP6M2, CYP9K1 and CYP6AA1 together with other genes previously associated with resistance in An. gambiae.

CONCLUSION

This study confirms that recurrent exposure of adult mosquitoes to pyrethroids in a public health context can rapidly select for various resistance mechanisms. In particular, it indicates that in addition to target site mutations, the polyfluorinated pyrethroid transfluthrin can select for a broad metabolic response, which includes some P450s previously associated to resistance to classical pyrethroids. This unexpected finding highlights the need for an in-depth study on the adaptive response of mosquitoes to newly introduced active ingredients in order to effectively guide and support decision-making programmes in malaria control.

Efficacy of a 'lethal house lure' against Culex quinquefasciatus from Bouaké city, Côte d'Ivoire

BACKGROUND

Eave tube technology is a novel method of insecticide application that uses an electrostatic coating system to boost insecticide efficacy against resistant mosquitoes. A series of previous experiments showed encouraging insecticidal effects against malaria vectors. This study was undertaken to assess the effects of the eave tube approach on other Culicidae, in particular Culex quinquefasciatus, under laboratory and semi-field conditions.

METHODS

Larvae of Cx. quinquefasciatus from Bouaké were collected and reared to adult stage, and World Health Organization (WHO) cylinder tests were performed to determine their resistance status. WHO standard 3-min cone bioassays were conducted using PermaNet 2.0 netting versus eave tube-treated inserts. To assess the transient exposure effect on Cx. quinquefasciatus, eave tube assay utilizing smelly socks as attractant was performed with exposure time of 30 s, 1 min, and 2 min on 10% beta-cyfluthrin-treated inserts. Residual activity of these treated inserts was then monitored over 9 months. Field tests involving release-recapture of Cx. quinquefasciatus within enclosures around experimental huts fitted with windows and untreated or insecticide-treated eave tubes were conducted to determine house entry preference and the impact of tubes on the survival of this species.

RESULTS

Bouaké Cx. quinquefasciatus displayed high resistance to three out of four classes of insecticides currently used in public health. After 3 min of exposure in cone tests, 10% beta-cyfluthrin-treated inserts induced 100% mortality in Cx. quinquefasciatus, whereas the long-lasting insecticidal net (LLIN) only killed 4.5%. With reduced exposure time on the eave tube insert, mortality was still 100% after 2 min, 88% after 1 min, and 44% after 30 s. Mortality following 1 h exposure on 10% beta-cyfluthrin-treated insert was > 80% continuously up to 7 months post-treatment. Data suggest that Cx. quinquefasciatus have a stronger preference for entering a house through the eaves than through windows. Beta-cyfluthrin-treated inserts were able to kill 51% of resistant Cx. quinquefasciatus released within the enclosure.

CONCLUSIONS

Eave tubes are a novel method for delivery of insecticide to the house. They attract nuisance host-seeking Cx. quinquefasciatus mosquitoes and are as effective in controlling them as they are against pyrethroid-resistant Anopheles gambiae, despite the high level of resistance Cx. quinquefasciatus have developed.

Relationship between insecticide resistance profiles in Anopheles gambiae sensu lato and agricultural practices in Côte d'Ivoire

BACKGROUND

Insecticide-based malaria vector control is increasingly undermined due to the development of insecticide resistance in mosquitoes. Insecticide resistance may partially be related to the use of pesticides in agriculture, while the level and mechanisms of resistance might differ between agricultural practices. The current study aimed to assess whether phenotypic insecticide resistance and associated molecular resistance mechanisms in Anopheles gambiae sensu lato differ between agricultural practices.

METHODS

We collected An. gambiae s.l. larvae in six sites with three different agricultural practices, including rice, vegetable and cocoa cultivation. We then exposed the emerging adult females to discriminating concentrations of bendiocarb (0.1%), deltamethrin (0.05%), DDT (4%) and malathion (5%) using the standard World Health Organization insecticide susceptibility test. To investigate underlying molecular mechanisms of resistance, we used multiplex TaqMan qPCR assays. We determined the frequency of target-site mutations, including Vgsc-L995F/S and Vgsc-N1570Y, and Ace1-G280S. In addition, we measured the expression levels of genes previously associated with insecticide resistance in An. gambiae s.l., including the cytochrome P450-dependent monooxygenases CYP4G16, CYP6M2, CYP6P1, CYP6P3, CYP6P4, CYP6Z1 and CYP9K1, and the glutathione S-transferase GSTe2.

RESULTS

The An. gambiae s.l. populations from all six agricultural sites were resistant to bendiocarb, deltamethrin and DDT, while the populations from the two vegetable cultivation sites were additionally resistant to malathion. Most tested mosquitoes carried at least one mutant Vgsc-L995F allele that is associated with pyrethroid and DDT resistance. In the cocoa cultivation sites, we observed the highest 995F frequencies (80-87%), including a majority of homozygous mutants and several in co-occurrence with the Vgsc-N1570Y mutation. We detected the Ace1 mutation most frequently in vegetable-growing sites (51-60%), at a moderate frequency in rice (20-22%) and rarely in cocoa-growing sites (3-4%). In contrast, CYP6M2, CYP6P3, CYP6P4, CYP6Z1 and CYP9K1, previously associated with metabolic insecticide resistance, showed the highest expression levels in the populations from rice-growing sites compared to the susceptible Kisumu reference strain.

CONCLUSION

In our study, we observed intriguing associations between the type of agricultural practices and certain insecticide resistance profiles in the malaria vector An. gambiae s.l. which might arise from the use of pesticides deployed for protecting crops.

Housing modification for malaria control: impact of a "lethal house lure" intervention on malaria infection prevalence in a cluster randomised control trial in Côte d'Ivoire

BACKGROUND

In recent years, the downward trajectory of malaria transmission has slowed and, in some places, reversed. New tools are needed to further reduce malaria transmission. One approach that has received recent attention is a novel house-based intervention comprising window screening (S) and general house repairs to make the house more mosquito proof, together with EaveTubes (ET) that provide an innovative way of targeting mosquitoes with insecticides as they search for human hosts at night. The combined approach of Screening + EaveTubes (SET) essentially turns the house into a 'lure and kill' device.

METHODS

This study evaluated the impact of SET on malaria infection prevalence in Côte d'Ivoire and compares the result in the primary outcome, malaria case incidence. Malaria infection prevalence was measured in a cross-sectional survey in 40 villages, as part of a cluster-randomised trial evaluating the impact of SET on malaria case incidence.

RESULTS

Infection prevalence, measured by rapid diagnostic test (RDT), was 50.4% and 36.7% in the control arm and intervention arm, respectively, corresponding to an odds ratio of 0.57 (0.45-0.71), p < 0.0001). There was moderate agreement between RDT and microscopy results, with a reduction in odds of infection of 36% recorded when infection was measured by microscopy. Prevalence measured by RDT correlated strongly with incidence at a cluster level.

CONCLUSIONS

In addition to reducing malaria case incidence, house screening and EaveTubes substantially reduced malaria infection prevalence 18 months after installation. Infection prevalence may be a good metric to use for evaluating malaria interventions in areas of similar transmission levels to this setting.

TRIAL REGISTRATION

ISRCTN18145556, registered 1 February 2017.

Escalating pyrethroid resistance in two major malaria vectors Anopheles funestus and Anopheles gambiae (s.l.) in Atatam, Southern Ghana

BACKGROUND

Aggravation of insecticide resistance in malaria vectors is threatening the efforts to control malaria by reducing the efficacy of insecticide-based interventions hence needs to be closely monitored. This study investigated the intensity of insecticide resistance of two major malaria vectors An. funestus sensu stricto (s.s.) and An. gambiae sensu lato (s.l.) collected in southern Ghana and assessed the bio-efficacy of several long-lasting insecticidal nets (LLINs) against these mosquito populations.

METHODS

The insecticide susceptibility profiles of Anopheles funestus s.s. and Anopheles gambiae s.l. populations from Obuasi region (Atatam), southern Ghana were characterized and the bio-efficacy of some LLINs was assessed to determine the impact of insecticide resistance on the effectiveness of these tools. Furthermore, molecular markers associated with insecticide resistance in both species were characterized in the F0 and F1 populations using PCR and qPCR methods.

RESULTS

Anopheles funestus s.s. was the predominant species and was resistant to pyrethroids, organochlorine and carbamate insecticides, but fully susceptible to organophosphates. An. gambiae s.l. was resistant to all four insecticide classes. High intensity of resistance to 5 × and 10 × the discriminating concentration (DC) of pyrethroids was observed in both species inducing a considerable loss of efficacy of long-lasting insecticidal nets (LLINs). Temporal expression analysis revealed a massive 12-fold increase in expression of the CYP6P4a cytochrome P450 gene in An. funestus s.s., initially from a fold change of 41 (2014) to 500 (2021). For both species, the expression of candidate genes did not vary according to discriminating doses. An. gambiae s.l. exhibited high frequencies of target-site resistance including Vgsc-1014F (90%) and Ace-1 (50%) while these mutations were absent in An. funestus s.s.

CONCLUSIONS

The multiple and high intensity of resistance observed in both malaria vectors highlights the need to implement resistance management strategies and the introduction of new insecticide chemistries.

Omitting age-dependent mosquito mortality in malaria models underestimates the effectiveness of insecticide-treated nets

Mathematical models of vector-borne infections, including malaria, often assume age-independent mortality rates of vectors, despite evidence that many insects senesce. In this study we present survival data on insecticide-resistant Anopheles gambiae s.l. from experiments in Côte d’Ivoire. We fit a constant mortality function and two age-dependent functions (logistic and Gompertz) to the data from mosquitoes exposed (treated) and not exposed (control) to insecticide-treated nets (ITNs), to establish biologically realistic survival functions. This enables us to explore the effects of insecticide exposure on mosquito mortality rates, and the extent to which insecticide resistance might impact the effectiveness of ITNs. We investigate this by calculating the expected number of infectious bites a mosquito will take in its lifetime, and by extension the vectorial capacity. Our results show that the predicted vectorial capacity is substantially lower in mosquitoes exposed to ITNs, despite the mosquitoes in the experiment being highly insecticide-resistant. The more realistic age-dependent functions provide a better fit to the experimental data compared to a constant mortality function and, hence, influence the predicted impact of ITNs on malaria transmission potential. In models with age-independent mortality, there is a great reduction for the vectorial capacity under exposure compared to no exposure. However, the two age-dependent functions predicted an even larger reduction due to exposure, highlighting the impact of incorporating age in the mortality rates. These results further show that multiple exposures to ITNs had a considerable effect on the vectorial capacity. Overall, the study highlights the importance of including age dependency in mathematical models of vector-borne disease transmission and in fully understanding the impact of interventions.

Interventions against malaria are most commonly targeted on the adult mosquitoes, which transmit the infection from person to person. One of the most important interventions are bed-nets, treated with insecticides. Unfortunately, extensive exposure of mosquitoes to insecticide has led to widespread evolution of insecticide resistance, which might threaten control strategies. Piecing together the overall impact of resistance on the efficacy of insecticide-treated nets is complex, but can be informed by the use of mathematical models. However, there are some assumptions that the models frequently use which are not realistic in terms of the mosquito biology. In this paper, we formulate a model that includes age-dependent mortality rates, an important parameter in vector control since control strategies most commonly aim to reduce the lifespan of the mosquitoes. By using novel data collected using field-derived insecticide-resistant mosquitoes, we explore the effects that the presence of insecticides on nets have on the mortality rates, as well as the difference incorporating age dependency in the model has on the results. We find that including age-dependent mortality greatly alters the anticipated effects of insecticide-treated nets on mosquito transmission potential, and that ignoring this realism potentially overestimates the negative impact of insecticide resistance.

Evolution of the Ace-1 and Gste2 Mutations and Their Potential Impact on the Use of Carbamate and Organophosphates in IRS for Controlling Anopheles gambiae s.l., the Major Malaria Mosquito in Senegal

Widespread of insecticide resistance amongst the species of the Anopheles gambiae complex continues to threaten vector control in Senegal. In this study, we investigated the presence and evolution of the Ace-1 and Gste2 resistance genes in natural populations of Anopheles gambiae s.l., the main malaria vector in Senegal. Using historical samples collected from ten sentinel health districts, this study focused on three different years (2013, 2017, and 2018) marking the periods of shift between the main public health insecticides families (pyrethroids, carbamates, organophosphates) used in IRS to track back the evolutionary history of the resistance mutations on the Ace-1 and Gste2 loci. The results revealed the presence of four members of the Anopheles gambiae complex, with the predominance of An. arabiensis followed by An. gambiae, An. coluzzii, and An. gambiae–coluzzii hybrids. The Ace-1 mutation was only detected in An. gambiae and An. gambiae–coluzzii hybrids at low frequencies varying between 0.006 and 0.02, while the Gste2 mutation was found in all the species with a frequency ranging between 0.02 and 0.25. The Ace-1 and Gste2 genes were highly diversified with twenty-two and thirty-one different haplotypes, respectively. The neutrality tests on each gene indicated a negative Tajima’s D, suggesting the abundance of rare alleles. The presence and spread of the Ace-1 and Gste2 resistance mutations represent a serious threat to of the effectiveness and the sustainability of IRS-based interventions using carbamates or organophosphates to manage the widespread pyrethroids resistance in Senegal. These data are of the highest importance to support the NMCP for evidence-based vector control interventions selection and targeting.

High-throughput barcoding method for the genetic surveillance of insecticide resistance and species identification in Anopheles gambiae complex malaria vectors

Surveillance of malaria vector species and the monitoring of insecticide resistance are essential to inform malaria control strategies and support the reduction of infections and disease. Genetic barcoding of mosquitoes is a useful tool to assist the high-throughput surveillance of insecticide resistance, discriminate between sibling species and to detect the presence of Plasmodium infections. In this study, we combined multiplex PCR, custom designed dual indexing, and Illumina next generation sequencing for high throughput single nucleotide polymorphism (SNP)-profiling of four species from the Anopheles (An.) gambiae complex (An. gambiae sensu stricto, An. coluzzii, An. arabiensis and An. melas). By amplifying and sequencing only 14 genetic fragments (500 bp each), we were able to simultaneously detect Plasmodium infection; insecticide resistance-conferring SNPs in ace1, gste2, vgsc and rdl genes; the partial sequences of nuclear ribosomal internal transcribed spacers (ITS1 and ITS2) and intergenic spacers (IGS), Short INterspersed Elements (SINE), as well as mitochondrial genes (cox1 and nd4) for species identification and genetic diversity. Using this amplicon sequencing approach with the four selected An. gambiae complex species, we identified a total of 15 non-synonymous mutations in the insecticide target genes, including previously described mutations associated with resistance and two new mutations (F1525L in vgsc and D148E in gste2). Overall, we present a reliable and cost-effective high-throughput panel for surveillance of An. gambiae complex mosquitoes in malaria endemic regions.

Entomological indicators of malaria transmission prior to a cluster-randomized controlled trial of a 'lethal house lure' intervention in central Côte d'Ivoire

Background

A study was conducted prior to implementing a cluster-randomized controlled trial (CRT) of a lethal house lure strategy in central Côte d’Ivoire to provide baseline information on malaria indicators in 40 villages across five health districts.

Methods

Human landing catches (HLC) were performed between November and December 2016, capturing mosquitoes indoors and outdoors between 18.00 and 08.00 h. Mosquitoes were processed for entomological indicators of malaria transmission (human biting, parity, sporozoite, and entomological inoculation rates (EIR)). Species composition and allelic frequencies of kdr-w and ace-1^R mutations were also investigated within the Anopheles gambiae complex.

Results

Overall, 15,632 mosquitoes were captured. Anopheles gambiae sensu lato (s.l.) and Anopheles funestus were the two malaria vectors found during the survey period, with predominance for An. gambiae (66.2%) compared to An. funestus (10.3%). The mean biting rate for An. gambiae was almost five times higher than that for An. funestus (19.8 bites per person per night for An. gambiae vs 4.3 bites per person per night for An. funestus) and this was evident indoors and outdoors. Anopheles funestus was more competent to transmit malaria parasites in the study area, despite relatively lower number tested for sporozoite index (4.14% (63/1521) for An. gambiae vs 8.01% (59/736) for An. funestus; χ² = 12.216; P < 0.0001). There were no significant differences between the proportions infected outdoors and indoors for An. gambiae (4.03 vs 4.13%; χ² = 0.011; P = 0.9197) and for An. funestus (7.89 vs 8.16%; χ² = 2.58^e−29; P = 1). The majority of both infected vectors with malaria parasites harboured Plasmodium falciparum (93.65% for An. gambiae and 98. 31% for An. funestus). Overall, the EIR range for both species in the different districts appeared to be high (0.35–2.20 infected bites per human per night) with the highest value observed in the district of North-Eastern-Bouaké. There were no significant differences between transmission occurring outdoor and indoor for both species. Of the An. gambiae s.l. analysed, only An. gambiae sensu stricto (14.1%) and Anopheles coluzzii (85.9%) were found. The allelic frequencies of kdr and ace-1^R were higher in An. gambiae (0.97 for kdr and 0.19 for ace-1^R) than in An. coluzzii (0.86 for kdr and 0.10 for ace-1^R) (P < 0.001).

Conclusion

Despite universal coverage with long-lasting insecticidal nets (LLINs) in the area, there was an abundance of the malaria vectors (An. gambiae and An. funestus) in the study area in central Côte d’Ivoire. Consistent with high insecticide resistance intensity previously detected in these districts, the current study detected high kdr frequency (> 85%), coupled with high malaria transmission pattern, which could guide the use of Eave tubes in the study areas.

Use of novel lab assays to examine the effect of pyrethroid-treated bed nets on blood-feeding success and longevity of highly insecticide-resistant Anopheles gambiae s.l. mosquitoes

Background

There is a pressing need to improve understanding of how insecticide resistance affects the functional performance of insecticide-treated nets (ITNs). Standard WHO insecticide resistance monitoring assays are designed for resistance surveillance and do not necessarily provide insight into how different frequencies, mechanisms or intensities of resistance affect the ability of ITNs to reduce malaria transmission.

Methods

The current study presents some novel laboratory-based assays that attempt to better simulate realistic exposure of mosquitoes to ITNs and to quantify impact of exposure not only on instantaneous mortality, but also on blood-feeding and longevity, two traits that are central to transmission. The assays evaluated the performance of a standard ITN (Permanet® 2.0; Vestergaard Frandsen), a ‘next generation’ combination ITN with a resistance-breaking synergist (Permanet® 3.0) and an untreated net (UTN), against field-derived Anopheles gambiae sensu lato mosquitoes from Côte d’Ivoire exhibiting a 1500-fold increase in pyrethroid resistance relative to a standard susceptible strain.

Results

The study revealed that the standard ITN induced negligible instantaneous mortality against the resistant mosquitoes, whereas the resistance-breaking net caused high mortality and a reduction in blood-feeding. However, both ITNs still impacted long-term survival relative to the UTN. The impact on longevity depended on feeding status, with blood-fed mosquitoes living longer than unfed mosquitoes following ITN exposure. Exposure to both ITNs also reduced the blood-feeding success, the time spent on the net and blood-feeding duration, relative to the untreated net.

Conclusion

Although a standard ITN did not have as substantial instantaneous impact as the resistance-breaking net, it still had significant impacts on traits important for transmission. These results highlight the benefit of improved bioefficacy assays that allow for realistic exposure and consider sub- or pre-lethal effects to help assess the functional significance of insecticide resistance.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05220-y.

Patterns of Kdr-L995F Allele Emergence Alongside Detoxifying Enzymes Associated with Deltamethrin Resistance in Anopheles gambiae s.l. from North Cameroon

Understanding how multiple insecticide resistance mechanisms occur in malaria vectors is essential for efficient vector control. This study aimed at assessing the evolution of metabolic mechanisms and Kdr L995F/S resistance alleles in Anopheles gambiae s.l. from North Cameroon, following long-lasting insecticidal nets (LLINs) distribution in 2011. Female An. gambiae s.l. emerging from larvae collected in Ouro-Housso/Kanadi, Be-Centre, and Bala in 2011 and 2015, were tested for susceptibility to deltamethrin + piperonyl butoxide (PBO) or SSS-tributyl-phosphoro-thrithioate (DEF) synergists, using the World Health Organization's standard protocol. The Kdr L995F/S alleles were genotyped using Hot Ligation Oligonucleotide Assay. Tested mosquitoes identified using PCR-RFLP were composed of An. arabiensis (68.5%), An. coluzzii (25.5%) and An. gambiae (6%) species. From 2011 to 2015, metabolic resistance increased in Ouro-Housso/Kanadi (up to 89.5% mortality to deltametnrin+synergists in 2015 versus <65% in 2011; p < 0.02), while it decreased in Be-Centre and Bala (>95% mortality in 2011 versus 42-94% in 2015; p < 0.001). Conversely, the Kdr L995F allelic frequencies slightly decreased in Ouro-Housso/Kanadi (from 50% to 46%, p > 0.9), while significantly increasing in Be-Centre and Bala (from 0-13% to 18-36%, p < 0.02). These data revealed two evolutionary trends of deltamethrin resistance mechanisms; non-pyrethroid vector control tools should supplement LLINs in North Cameroon.

Multi-insecticide resistant malaria vectors in the field remain susceptible to malathion, despite the presence of Ace1 point mutations

Insecticide resistance in Anopheles mosquitoes is seriously threatening the success of insecticide-based malaria vector control. Surveillance of insecticide resistance in mosquito populations and identifying the underlying mechanisms enables optimisation of vector control strategies. Here, we investigated the molecular mechanisms of insecticide resistance in three Anopheles coluzzii field populations from southern Côte d’Ivoire, including Agboville, Dabou and Tiassalé. All three populations were resistant to bendiocarb, deltamethrin and DDT, but not or only very weakly resistant to malathion. The absence of malathion resistance is an unexpected result because we found the acetylcholinesterase mutation Ace1-G280S at high frequencies, which would typically confer cross-resistance to carbamates and organophosphates, including malathion. Notably, Tiassalé was the most susceptible population to malathion while being the most resistant one to the pyrethroid deltamethrin. The resistance ratio to deltamethrin between Tiassalé and the laboratory reference colony was 1,800 fold. By sequencing the transcriptome of individual mosquitoes, we found numerous cytochrome P450-dependent monooxygenases – including CYP6M2, CYP6P2, CYP6P3, CYP6P4 and CYP6P5 – overexpressed in all three field populations. This could be an indication for negative cross-resistance caused by overexpression of pyrethroid-detoxifying cytochrome P450s that may activate pro-insecticides, thereby increasing malathion susceptibility. In addition to the P450s, we found several overexpressed carboxylesterases, glutathione S-transferases and other candidates putatively involved in insecticide resistance.

Insecticide-based mosquito control has saved millions of lives from malaria and other vector-borne diseases. However, the emergence and increase of insecticide resistant Anopheles populations seriously threaten to derail malaria control programmes. Surveillance of insecticide resistance and understanding the underlying molecular mechanisms are key for choosing effective vector control strategies. Here, we characterised the degree and mechanisms of resistance in three malaria vector populations from Côte d’Ivoire. Our key finding was that these multi-insecticide resistant malaria vectors largely remained susceptible to malathion, despite the presence of a mutation in the target enzyme of this organophosphate insecticide that would typically confer resistance. Intriguingly, we found overexpression of metabolic P450 enzymes that are known to detoxify insecticides and activate pro-insecticides such as malathion. It is highly probable that, here, we observed P450-mediated negative cross-resistance for the first time in Anopheles field populations. Negative cross-resistance merits further investigation as advantage could be taken of this phenomenon in the fight against multi-resistant malaria vectors.

Reversing insecticide resistance with allelic-drive in Drosophila melanogaster

A recurring target-site mutation identified in various pests and disease vectors alters the voltage gated sodium channel (vgsc) gene (often referred to as knockdown resistance or kdr) to confer resistance to commonly used insecticides, pyrethroids and DDT. The ubiquity of kdr mutations poses a major global threat to the continued use of insecticides as a means for vector control. In this study, we generate common kdr mutations in isogenic laboratory Drosophila strains using CRISPR/Cas9 editing. We identify differential sensitivities to permethrin and DDT versus deltamethrin among these mutants as well as contrasting physiological consequences of two different kdr mutations. Importantly, we apply a CRISPR-based allelic-drive to replace a resistant kdr mutation with a susceptible wild-type counterpart in population cages. This successful proof-of-principle opens-up numerous possibilities including targeted reversion of insecticide-resistant populations to a native susceptible state or replacement of malaria transmitting mosquitoes with those bearing naturally occurring parasite resistant alleles.

Insecticide resistance (IR) poses a major global health challenge. Here, the authors generate common IR mutations in laboratory Drosophila strains and use a CRISPR-based allelic-drive to replace an IR allele with a susceptible wild-type counterpart, providing a potent new tool for vector control.

Short-term metabolic resistance inductive effect of different agrochemical groups on Anopheles gambiae mosquitoes

Background: In order to assess the impact of the different groups of agricultural pesticides used in Côte d'Ivoire on the increase of mosquitoes resistance to insecticides, the expression profiles of 7 P450 cytochromes and one GSTE2 of Anopheles gambiae involved in mosquito resistance to insecticides were studied. The goal of this study was to determine the effect of short exposure of mosquito larvae to different groups of agricultural pesticides on mosquito resistance. Methods: Three groups of pesticides were selected: (i) agricultural insecticide solutions, (ii) non-insecticide pesticide solutions (a mixture of herbicides and fungicides), and (iii) a mixture of the first two. A fourth non-pesticide solution was used as a control. Four groups of each stage 2 larvae (strain Kisumu, male and female) were exposed to 20% concentrated solution for 24 hours. Susceptibility tests for dichlorodiphenyltrichloroethane (DDT) and Deltamethrin were carried out on adults aged 2-5 days. Quantitative reverse transcription polymerase chain reaction (RT-qPCR) was performed to quantify the expression of eight metabolic genes involved in mosquito resistance to insecticides.  Results: Susceptibility to DDT showed a similar increase in the time required to knock down 50% of mosquitoes (kdt50) in l colonies exposed to insecticides and non-insecticides compared to the control colony. As for deltamethrin, kdt50 was higher in the colonies exposed to insecticides and the pesticide mixture compared to the colony exposed to non-insecticides. Of all the genes studied in all colonies, except for CYP6P1 induced only in the colony consisting of the pesticide mixture, no genes were induced. Conclusions: This study confirmed that induction is influenced by the duration, the concentration of the solution and the type of xenobiotic used as an inducer. The overexpression of CYP6P1 confirmed the inductive effect that a short exposure of mosquito larvae to agricultural pesticides could have.

Evaluation of Yahe® and Panda® 2.0 long-lasting insecticidal nets against wild pyrethroid-resistant Anopheles gambiae s.l. from Côte d'Ivoire: an experimental hut trial

BACKGROUND

Long-lasting insecticidal nets (LLINs) have played an important role in reducing the global malaria burden since 2000. They are a core prevention tool used widely by people at risk of malaria. The Vector Control Prequalification mechanism of the Word Health Organization (WHO-Vector Control PQ) established the testing and evaluation guidelines for LLINs before registration for public use. In the present study, two new brands of deltamethrin-impregnated nets (Yahe^® LN and Panda^® Net 2.0) were evaluated in an experimental hut against wild pyrethroid-resistant Anopheles gambiae s.l. in M'Bé nearby Bouaké, central Côte d'Ivoire.

METHODS

The performance of Yahe^® LN and Panda^® Net 2.0 was compared with that of PermaNet 2.0, conventionally treated nets (CTN), and untreated net to assess the blood-feeding inhibition, deterrence, induced exophily, and mortality.

RESULTS

Cone bioassay results showed that Panda^® Net 2.0, PermaNet 2.0 and Yahe^® LN (both unwashed and washed 20 times) induced > 95% knockdown or > 80% mortality of the susceptible Anopheles gambiae Kisumu strain. With the pyrethroid-resistant M'Bé strain, mortality rate for all treated nets did not exceed 70%. There was a significant reduction in entry and blood feeding (p < 0.05) and an increase in exophily and mortality rates (p < 0.05) with all treatments compared to untreated nets, except the CTNs. However, the personal protection induced by these treated nets decreased significantly after 20 washes. The performance of Panda^® Net 2.0 was equal to PermaNet^® 2.0 in terms of inhibiting blood feeding, but better than PermaNet^® 2.0 in terms of mortality.

CONCLUSION

This study showed that Yahe^® LN and Panda^® Net 2.0 met the WHO Pesticide Evaluation Scheme (WHOPES) criteria to undergo phase III trial at the community level. Due to an increasing spread and development of pyrethroid resistance in malaria vectors, control of malaria transmission must evolve into an integrated vector management relying on a large variety of efficient control tools.

An increasing role of pyrethroid-resistant Anopheles funestus in malaria transmission in the Lake Zone, Tanzania

Anopheles funestus is playing an increasing role in malaria transmission in parts of sub-Saharan Africa, where An. gambiae s.s. has been effectively controlled by long-lasting insecticidal nets. We investigated vector population bionomics, insecticide resistance and malaria transmission dynamics in 86 study clusters in North-West Tanzania. An. funestus s.l. represented 94.5% (4740/5016) of all vectors and was responsible for the majority of malaria transmission (96.5%), with a sporozoite rate of 3.4% and average monthly entomological inoculation rate (EIR) of 4.57 per house. Micro-geographical heterogeneity in species composition, abundance and transmission was observed across the study district in relation to key ecological differences between northern and southern clusters, with significantly higher densities, proportions and EIR of An. funestus s.l. collected from the South. An. gambiae s.l. (5.5%) density, principally An. arabiensis (81.1%) and An. gambiae s.s. (18.9%), was much lower and closely correlated with seasonal rainfall. Both An. funestus s.l. and An. gambiae s.l. were similarly resistant to alpha-cypermethrin and permethrin. Overexpression of CYP9K1, CYP6P3, CYP6P4 and CYP6M2 and high L1014S-kdr mutation frequency were detected in An. gambiae s.s. populations. Study findings highlight the urgent need for novel vector control tools to tackle persistent malaria transmission in the Lake Region of Tanzania.

Short-term metabolic resistance inductive effect of different agrochemical groups on Anopheles gambiae mosquitoes

Background: In order to assess the impact of the different groups of agricultural pesticides used in Côte d'Ivoire on the increase of mosquitoes resistance to insecticides, the expression profiles of 7 P450 cytochromes and one GSTE2 of Anopheles gambiae involved in mosquito resistance to insecticides were studied. The goal of this study was to determine the effect of short exposure of mosquito larvae to different groups of agricultural pesticides on mosquito resistance. Methods: Three groups of pesticides were selected: (i) agricultural insecticide solutions, (ii) none-insecticide pesticide solutions (a mixture of herbicides and fungicides), and (iii) a mixture of the first two. A fourth non-pesticide solution was used as a control. Four groups of each stage 2 larvae (strain Kisumu, male and female) were exposed to 20% concentrated solution for 24 hours. Susceptibility tests for dichlorodiphenyltrichloroethane (DDT) and Deltamethrin were carried out on adults aged 2-5 days. Quantitative reverse transcription polymerase chain reaction (RT-qPCR) was performed to quantify the expression of eight metabolic genes involved in mosquito resistance to insecticides.  Results: Susceptibility to DDT showed a similar increase in the time required to knock down 50% of mosquitoes (kdt50) in l colonies exposed to insecticides and none-insecticides compared to the control colony. As for deltamethrin, kdt50 was higher in the colonies exposed to insecticides and the pesticide mixture compared to the colony exposed to none-insecticides. Of all the genes studied in all colonies, except for CYP6P1 induced only in the colony consisting of the pesticide mixture, no genes were induced. Conclusions: This study confirmed that induction is influenced by the duration, the concentration of the solution and the type of xenobiotic used as an inducer. The overexpression of CYP6P1 confirmed the inductive effect that a short exposure of mosquito larvae to agricultural pesticides could have.

Pyrethroid Resistance Aggravation in Ugandan Malaria Vectors Is Reducing Bednet Efficacy

Monitoring cases of insecticide resistance aggravation and the effect on the efficacy of control tools is crucial for successful malaria control. In this study, the resistance intensity of major malaria vectors from Uganda was characterised and its impact on the performance of various insecticide-treated nets elucidated. High intensity of resistance to the discriminating concentration (DC), 5× DC, and 10× DC of pyrethroids was observed in both Anopheles funestus and Anopheles gambiae in Mayuge and Busia leading to significant reduced performance of long-lasting insecticidal nets (LLINs) including the piperonyl butoxide (PBO)-based nets (Olyset Plus). Molecular analysis revealed significant over-expression of cytochrome P450 genes (CYP9K1 and CYP6P9a/b). However, the expression of these genes was not associated with resistance escalation as no difference was observed in the level of expression in mosquitoes resistant to 5× DC and 10× DC compared to 1× DC suggesting that other resistance mechanisms are involved. Such high intensity of pyrethroid resistance in Uganda could have terrible consequences on the effectiveness of insecticide-based interventions and urgent action should be taken to prevent the spread of super-resistance in malaria vectors.