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.

Global malaria predictors at a localized scale.

Malaria is a life-threatening disease caused by Plasmodium parasites transmitted by Anopheles mosquitoes. In 2021, more than 247 million cases of malaria were reported worldwide, with an estimated 619,000 deaths. While malaria incidence has decreased globally in recent decades, some public health gains have plateaued, and many endemic hotspots still face high transmission rates. Understanding local drivers of malaria transmission is crucial but challenging due to the complex interactions between climate, entomological and human variables, and land use. This study focuses on highly climatically suitable and endemic areas in Côte d'Ivoire to assess the explanatory power of coarse climatic predictors of malaria transmission at a fine scale. Using data from 40 villages participating in a randomized controlled trial of a household malaria intervention, the study examines the effects of climate variation over time on malaria transmission. Through panel regressions and statistical modeling, the study investigates which variable (temperature, precipitation, or entomological inoculation rate) and its form (linear or unimodal) best explains seasonal malaria transmission and the factors predicting spatial variation in transmission. The results highlight the importance of temperature and rainfall, with quadratic temperature and all precipitation models performing well, but the causal influence of each driver remains unclear due to their strong correlation. Further, an independent, mechanistic temperature-dependent R 0 model based on laboratory data aligns well with observed malaria incidence rates, emphasizing the significance and predictability of temperature suitability across scales. By contrast, entomological variables, such as entomological inoculation rate, were not strong predictors of human incidence in this context. Finally, the study explores the predictors of spatial variation in malaria, considering land use, intervention, and entomological variables. The findings contribute to a better understanding of malaria transmission dynamics at local scales, aiding in the development of effective control strategies in endemic regions.

Rice farmers' knowledge, attitudes and practices towards mosquitoes in irrigation schemes in Côte d'Ivoire: a qualitative study.

BACKGROUND: Irrigated rice cultivation in sub-Saharan Africa not only brings more malaria vectors to nearby communities, but also greater malaria risk. To aid the implementation of mosquito control in rice-growing communities, it is necessary to understand how farmers understand, view and manage their responsibility in mosquito generation and whether they are interested in coordinating to minimize it. METHODS: Qualitative methods (observation grids, semi-structured in-depth interviews and focus group discussions) were used to reveal the perceptions of mosquitoes and their control in two irrigated rice farming communities in central Côte d'Ivoire near the M'bé and Lokapli irrigation schemes. RESULTS: All rice farmers viewed mosquitoes as severe nuisances, and most acknowledged that they caused djèkouadjo (malaria) and were less numerous during harmattan (dry season). Many study participants believed that mosquitoes originated from grasses and stagnant water around villages. Only those living closer in proximity (~ 1 km) to the paddies believed that mosquitoes came from the bas-fonds (irrigated lowlands). However, they did not associate mosquito production with rice cultivation. Some farmers believed that there were more mosquitoes in recent years than historically because of the dam construction, but remarked on the importance of the dam (and bas-fonds) for their livelihood. Many farmers were not convinced that mosquito control could occur at farm-level. CONCLUSIONS: To enhance accountability amongst rice farmers, there is a need for greater awareness on the rice-mosquito link, and emphasis that the link does not imply a trade-off between food production and health. Training should not only be directed towards farming communities, but also agricultural and health extension workers. Future riceland mosquito control methods must focus on improving crop productivity and address collective action problems that may occur.

Attractive targeted sugar bait: the pyrrole insecticide chlorfenapyr and the anti-malarial pharmaceutical artemether-lumefantrine arrest Plasmodium falciparum development inside wild pyrethroid-resistant Anopheles gambiae s.s. mosquitoes.

BACKGROUND: Attractive targeted sugar bait (ATSB) is a novel approach to vector control, offering an alternative mode of insecticide delivery via the insect alimentary canal, with potential to deliver a variety of compounds new to medical entomology and malaria control. Its potential to control mosquitoes was recently demonstrated in major field trials in Africa. The pyrrole chlorfenapyr is an insecticide new to malaria vector control, and through its unique mode of action-disruption of ATP mediated energy transfer in mitochondria-it may have direct action on energy transfer in the flight muscle cells of mosquitoes. It may also have potential to disrupt mitochondrial function in malarial parasites co-existing within the infected mosquito. However, little is known about the impact of such compounds on vector competence in mosquitoes responsible for malaria transmission. METHODS: In this study, ATSBs containing chlorfenapyr insecticide and, as a positive control, the anti-malarial drugs artemether/lumefantrine (A/L) were compared for their effect on Plasmodium falciparum development in wild pyrethroid-resistant Anopheles gambiae sensu stricto (s.s.) and for their capacity to reduce vector competence. Female mosquitoes were exposed to ATSB containing either sublethal dose of chlorfenapyr (CFP: 0.025%) or concentrations of A/L ranging from 0.4/2.4 mg/ml to 2.4/14.4 mg/ml, either shortly before or after taking infective blood meals. The impact of their component compounds on the prevalence and intensity of P. falciparum infection were compared between treatments. RESULTS: Both the prevalence and intensity of infection were significantly reduced in mosquitoes exposed to either A/L or chlorfenapyr, compared to unexposed negative control mosquitoes. The A/L dose (2.4/14.4 mg/ml) totally erased P. falciparum parasites: 0% prevalence of infection in female mosquitoes exposed compared to 62% of infection in negative controls (df = 1, χ2 = 31.23 p < 0.001). The dose of chlorfenapyr (0.025%) that killed < 20% females in ATSB showed a reduction in oocyte density of 95% per midgut (0.18/3.43 per midgut). CONCLUSION: These results are evidence that chlorfenapyr, in addition to its direct killing effect on the vector, has the capacity to block Plasmodium transmission by interfering with oocyte development inside pyrethroid-resistant mosquitoes, and through this dual action may potentiate its impact under field conditions.

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.

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.

Anopheles vector distribution and malaria transmission dynamics in Gbêkê region, central Côte d'Ivoire.

BACKGROUND: A better understanding of vector distribution and malaria transmission dynamics at a local scale is essential for implementing and evaluating effectiveness of vector control strategies. Through the data gathered in the framework of a cluster randomized controlled trial (CRT) evaluating the In2Care (Wageningen, Netherlands) Eave Tubes strategy, the distribution of the Anopheles vector, their biting behaviour and malaria transmission dynamics were investigated in Gbêkê region, central Côte d'Ivoire. METHODS: From May 2017 to April 2019, adult mosquitoes were collected monthly using human landing catches (HLC) in twenty villages in Gbêkê region. Mosquito species wereidentified morphologically. Monthly entomological inoculation rates (EIR) were estimated by combining the HLC data with mosquito sporozoite infection rates measured in a subset of Anopheles vectors using PCR. Finally, biting rate and EIR fluctuations were fit to local rainfall data to investigate the seasonal determinants of mosquito abundance and malaria transmission in this region. RESULTS: Overall, Anopheles gambiae, Anopheles funestus, and Anopheles nili were the three vector complexes found infected in the Gbêkê region, but there was a variation in Anopheles vector composition between villages. Anopheles gambiae was the predominant malaria vector responsible for 84.8% of Plasmodium parasite transmission in the area. An unprotected individual living in Gbêkê region received an average of 260 [222-298], 43.5 [35.8-51.29] and 3.02 [1.96-4] infected bites per year from An. gambiae, An. funestus and An. nili, respectively. Vector abundance and malaria transmission dynamics varied significantly between seasons and the highest biting rate and EIRs occurred in the months of heavy rainfall. However, mosquitoes infected with malaria parasites remained present in the dry season, despite the low density of mosquito populations. CONCLUSION: These results demonstrate that the intensity of malaria transmission is extremely high in Gbêkê region, especially during the rainy season. The study highlights the risk factors of transmission that could negatively impact current interventions that target indoor control, as well as the urgent need for additional vector control tools to target the population of malaria vectors in Gbêkê region and reduce the burden of the disease.

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.

Exploring alternative insecticide delivery options in a "lethal house lure" for malaria vector control.

The In2Care EaveTube is a house modification designed to block and kill malaria mosquitoes using an electrostatic netting treated with insecticide powder. A previous study demonstrated prolonged duration of effective action of insecticide-treated electrostatic netting in a semi-field setting. As part of a cluster randomized controlled trial (CRT) of the EaveTube intervention in Côte d'Ivoire, we investigated the residual efficacy of a pyrethroid insecticide deployed in EaveTubes under village conditions of use. We also explored the scope of using existing malaria control technologies including LLINs and IRS as alternative methods to deliver insecticides in the lethal house lure. The efficacy of beta-cyfluthrin was monitored over time using the "eave tube bioassay" method. Mortality of beta-cyfluthrin exposed pyrethroid resistant Anopheles gambiae mosquitoes was > 80% after 4 months. The impact (mosquito mortality) of PVC tubes coated with pirimiphos methyl was similar to that of beta-cyfluthrin treated insert (66.8 vs. 62.8%) in release-recapture experiments in experimental huts. Efficacy was significantly lower with all the LLINs tested; however, the roof of PermaNet 3.0 induced significantly higher mosquito mortality (50.4%) compared to Olyset Plus (25.9%) and Interceptor G2 (21.6%) LLINs. The efficacy of the alternative delivery methods was short-lived with mortality decreasing below 50% within 2 months in residual activity bioassays. None of the products tested appeared superior to the powder treatments. Further research is therefore required to identify suitable insecticide delivery options in EaveTube for malaria vector control.

Assessing the variability in experimental hut trials evaluating insecticide-treated nets against malaria vectors.

Experimental hut trials (EHTs) are used to evaluate indoor vector control interventions against malaria vectors in a controlled setting. The level of variability present in the assay will influence whether a given study is well powered to answer the research question being considered. We utilised disaggregated data from 15 previous EHTs to gain insight into the behaviour typically observed. Using simulations from generalised linear mixed models to obtain power estimates for EHTs, we show how factors such as the number of mosquitoes entering the huts each night and the magnitude of included random effects can influence study power. A wide variation in behaviour is observed in both the mean number of mosquitoes collected per hut per night (ranging from 1.6 to 32.5) and overdispersion in mosquito mortality. This variability in mortality is substantially greater than would be expected by chance and should be included in all statistical analyses to prevent false precision of results. We utilise both superiority and non-inferiority trials to illustrate our methodology, using mosquito mortality as the outcome of interest. The framework allows the measurement error of the assay to be reliably assessed and enables the identification of outlier results which could warrant further investigation. EHTs are increasingly playing an important role in the evaluation and regulation of indoor vector control interventions so it is important to ensure that these studies are adequately powered.

A new WHO bottle bioassay method to assess the susceptibility of mosquito vectors to public health insecticides: results from a WHO-coordinated multi-centre study.

BACKGROUND: The continued spread of insecticide resistance in mosquito vectors of malaria and arboviral diseases may lead to operational failure of insecticide-based interventions if resistance is not monitored and managed efficiently. This study aimed to develop and validate a new WHO glass bottle bioassay method as an alternative to the WHO standard insecticide tube test to monitor mosquito susceptibility to new public health insecticides with particular modes of action, physical properties or both. METHODS: A multi-centre study involving 21 laboratories worldwide generated data on the susceptibility of seven mosquito species (Aedes aegypti, Aedes albopictus, Anopheles gambiae sensu stricto [An. gambiae s.s.], Anopheles funestus, Anopheles stephensi, Anopheles minimus and Anopheles albimanus) to seven public health insecticides in five classes, including pyrethroids (metofluthrin, prallethrin and transfluthrin), neonicotinoids (clothianidin), pyrroles (chlorfenapyr), juvenile hormone mimics (pyriproxyfen) and butenolides (flupyradifurone), in glass bottle assays. The data were analysed using a Bayesian binomial model to determine the concentration-response curves for each insecticide-species combination and to assess the within-bioassay variability in the susceptibility endpoints, namely the concentration that kills 50% and 99% of the test population (LC50 and LC99, respectively) and the concentration that inhibits oviposition of the test population by 50% and 99% (OI50 and OI99), to measure mortality and the sterilizing effect, respectively. RESULTS: Overall, about 200,000 mosquitoes were tested with the new bottle bioassay, and LC50/LC99 or OI50/OI99 values were determined for all insecticides. Variation was seen between laboratories in estimates for some mosquito species-insecticide combinations, while other test results were consistent. The variation was generally greater with transfluthrin and flupyradifurone than with the other compounds tested, especially against Anopheles species. Overall, the mean within-bioassay variability in mortality and oviposition inhibition were < 10% for most mosquito species-insecticide combinations. CONCLUSION: Our findings, based on the largest susceptibility dataset ever produced on mosquitoes, showed that the new WHO bottle bioassay is adequate for evaluating mosquito susceptibility to new and promising public health insecticides currently deployed for vector control. The datasets presented in this study have been used recently by the WHO to establish 17 new insecticide discriminating concentrations (DCs) for either Aedes spp. or Anopheles spp. The bottle bioassay and DCs can now be widely used to monitor baseline insecticide susceptibility of wild populations of vectors of malaria and Aedes-borne diseases worldwide.

Inferring the epidemiological benefit of indoor vector control interventions against malaria from mosquito data.

The cause of malaria transmission has been known for over a century but it is still unclear whether entomological measures are sufficiently reliable to inform policy decisions in human health. Decision-making on the effectiveness of new insecticide-treated nets (ITNs) and the indoor residual spraying of insecticide (IRS) have been based on epidemiological data, typically collected in cluster-randomised control trials. The number of these trials that can be conducted is limited. Here we use a systematic review to highlight that efficacy estimates of the same intervention may vary substantially between trials. Analyses indicate that mosquito data collected in experimental hut trials can be used to parameterize mechanistic models for Plasmodium falciparum malaria and reliably predict the epidemiological efficacy of quick-acting, neuro-acting ITNs and IRS. Results suggest that for certain types of ITNs and IRS using this framework instead of clinical endpoints could support policy and expedite the widespread use of novel technologies.

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-1R 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 = 2.58e-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-1R were higher in An. gambiae (0.97 for kdr and 0.19 for ace-1R) than in An. coluzzii (0.86 for kdr and 0.10 for ace-1R) (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.

Optimising the deployment of vector control tools against malaria: a data-informed modelling study.

BACKGROUND: Concern that insecticide resistant mosquitoes are threatening malaria control has driven the development of new types of insecticide treated nets (ITNs) and indoor residual spraying (IRS) of insecticide. Malaria control programmes have a choice of vector control interventions although it is unclear which controls should be used to combat the disease. The study aimed at producing a framework to easily compare the public health impact and cost-effectiveness of different malaria prevention measures currently in widespread use. METHODS: We used published data from experimental hut trials conducted across Africa to characterise the entomological effect of pyrethroid-only ITNs versus ITNs combining a pyrethroid insecticide with the synergist piperonyl butoxide (PBO). We use these estimates to parameterise a dynamic mathematical model of Plasmodium falciparum malaria which is validated for two sites by comparing simulated results to empirical data from randomised control trials (RCTs) in Tanzania and Uganda. We extrapolated model simulations for a series of potential scenarios likely across the sub-Saharan African region and include results in an online tool (Malaria INtervention Tool [MINT]) that aims to identify optimum vector control intervention packages for scenarios with varying budget, price, entomological and epidemiological factors. FINDINGS: Our model indicates that switching from pyrethroid-only to pyrethroid-PBO ITNs could averted up to twice as many cases, although the additional benefit is highly variable and depends on the setting conditions. We project that annual delivery of long-lasting, non-pyrethroid IRS would prevent substantially more cases over 3-years, while pyrethroid-PBO ITNs tend to be the most cost-effective intervention per case averted. The model was able to predict prevalence and efficacy against prevalence in both RCTs for the intervention types tested. MINT is applicable to regions of sub-Saharan Africa with endemic malaria and provides users with a method of designing intervention packages given their setting and budget. INTERPRETATION: The most cost-effective vector control package will vary locally. Models able to recreate results of RCTs can be used to extrapolate outcomes elsewhere to support evidence-based decision making for investment in vector control. FUNDING: Medical Research Council, IVCC, Wellcome Trust. TRANSLATION: For the French translation of the abstract see Supplementary Materials section.

Technical Workflow Development for Integrating Drone Surveys and Entomological Sampling to Characterise Aquatic Larval Habitats of Anopheles funestus in Agricultural Landscapes in Côte d'Ivoire.

Land-use practices such as agriculture can impact mosquito vector breeding ecology, resulting in changes in disease transmission. The typical breeding habitats of Africa's second most important malaria vector Anopheles funestus are large, semipermanent water bodies, which make them potential candidates for targeted larval source management. This is a technical workflow for the integration of drone surveys and mosquito larval sampling, designed for a case study aiming to characterise An. funestus breeding sites near two villages in an agricultural setting in Côte d'Ivoire. Using satellite remote sensing data, we developed an environmentally and spatially representative sampling frame and conducted paired mosquito larvae and drone mapping surveys from June to August 2021. To categorise the drone imagery, we also developed a land cover classification scheme with classes relative to An. funestus breeding ecology. We sampled 189 potential breeding habitats, of which 119 (63%) were positive for the Anopheles genus and nine (4.8%) were positive for An. funestus. We mapped 30.42 km2 of the region of interest including all water bodies which were sampled for larvae. These data can be used to inform targeted vector control efforts, although its generalisability over a large region is limited by the fine-scale nature of this study area. This paper develops protocols for integrating drone surveys and statistically rigorous entomological sampling, which can be adjusted to collect data on vector breeding habitats in other ecological contexts. Further research using data collected in this study can enable the development of deep-learning algorithms for identifying An. funestus breeding habitats across rural agricultural landscapes in Côte d'Ivoire and the analysis of risk factors for these sites.

Insecticide Resistance Patterns and Mechanisms in Aedes aegypti (Diptera: Culicidae) Populations Across Abidjan, Côte d'Ivoire Reveal Emergent Pyrethroid Resistance.

From 2008 to 2017, the city of Abidjan, Côte d'Ivoire experienced several Aedes-borne disease epidemics which required control of the vector mosquito population based on the reduction of larval habitats and insecticidal sprays for adult mosquitoes. This study was undertaken to assess the insecticide susceptibility status of Aedes aegypti (Linnaeus) in the city of Abidjan. Immature Ae. aegypti were sampled from several larval habitats within seven communes of Abidjan and reared to adults. Three to five days old F1 emerged adults were tested for susceptibility using insecticide-impregnated papers and the synergist piperonyl butoxide (PBO) following WHO bioassay guidelines. The results showed that Ae. aegypti populations from Abidjan were resistant to 0.1% propoxur, and 1% fenitrothion, with mortality rates ranging from 0% to 54.2%. Reduced susceptibility (93.4-97.5% mortality) was observed to 0.05% deltamethrin, 0.75% permethrin, 0.05% lambda-cyhalothrin, 5% malathion, and 0.8% chlorpyrifos-methyl. This reduced susceptibility varied depending on the insecticide and the collection site. The restoration of mortality when the mosquitoes were pre-exposed to the synergist PBO suggests that increased activity of oxidases could be contributing to resistance. Three kdr mutations (V410L, V1016I, and F1534C) were present in populations tested, with low frequencies for the Leu410 (0.28) and Ile1016 (0.32) alleles and high frequencies for the Cys1534 allele (0.96). These findings will be used to inform future arbovirus vector control activities in Abidjan.

Impact and cost-effectiveness of a lethal house lure against malaria transmission in central Côte d'Ivoire: a two-arm, cluster-randomised controlled trial.

BACKGROUND: New vector control tools are required to sustain the fight against malaria. Lethal house lures, which target mosquitoes as they attempt to enter houses to blood feed, are one approach. Here we evaluated lethal house lures consisting of In2Care (Wageningen, Netherlands) Eave Tubes, which provide point-source insecticide treatments against host-seeking mosquitoes, in combination with house screening, which aims to reduce mosquito entry. METHODS: We did a two-arm, cluster-randomised controlled trial with 40 village-level clusters in central Côte d'Ivoire between Sept 26, 2016, and April 10, 2019. All households received new insecticide-treated nets at universal coverage (one bednet per two people). Suitable households within the clusters assigned to the treatment group were offered screening plus Eave Tubes, with Eave Tubes treated using a 10% wettable powder formulation of the pyrethroid ß-cyfluthrin. Because of the nature of the intervention, treatment could not be masked for households and field teams, but all analyses were blinded. The primary endpoint was clinical malaria incidence recorded by active case detection over 2 years in cohorts of children aged 6 months to 10 years. This trial is registered with ISRCTN, ISRCTN18145556. FINDINGS: 3022 houses received screening plus Eave Tubes, with an average coverage of 70% across the intervention clusters. 1300 eligible children were recruited for active case detection in the control group and 1260 in the intervention group. During the 2-year follow-up period, malaria case incidence was 2·29 per child-year (95% CI 1·97-2·61) in the control group and 1·43 per child-year (1·21-1·65) in the intervention group (hazard ratio 0·62, 95% CI 0·51-0·76; p<0·0001). Cost-effectiveness simulations suggested that screening plus Eave Tubes has a 74·0% chance of representing a cost-effective intervention, compared with existing healthcare activities in Côte d'Ivoire, and is similarly cost-effective to other core vector control interventions across sub-Saharan Africa. No serious adverse events associated with the intervention were reported during follow-up. INTERPRETATION: Screening plus Eave Tubes can provide protection against malaria in addition to the effects of insecticide-treated nets, offering potential for a new, cost-effective strategy to supplement existing vector control tools. Additional trials are needed to confirm these initial results and further optimise Eave Tubes and the lethal house lure concept to facilitate adoption. FUNDING: The Bill & Melinda Gates Foundation.

The role of human and mosquito behaviour in the efficacy of a house-based intervention.

Housing improvement such as blocking eaves and screening windows can help in reducing exposure to indoor biting mosquitoes. The impacts of physical barriers could potentially be boosted by the addition of a mechanism that kills mosquitoes as they attempt to enter the house. One example is to combine household screening with EaveTubes, which are insecticide-treated tubes inserted into closed eaves that attract and kill host-searching mosquitoes. The epidemiological impact of screening + EaveTubes is being evaluated in a large cluster randomized trial in Cote d'Ivoire. The study presented here is designed as a complement to this trial to help better understand the functional roles of screening and EaveTubes. We began by evaluating householder behaviour and household condition in the study villages. This work revealed that doors (and to some extent windows) were left open for large parts of the evening and morning, and that even houses modified to make them more 'mosquito proof' often had possible entry points for mosquitoes. We next built two realistic experimental houses in a village to enable us to explore how these aspects of behaviour and household quality affected the impact of screening and EaveTubes. We found that screening could have a substantial impact on indoor mosquito densities, even with realistic household condition and behaviour. By contrast, EaveTubes had no significant impact on indoor mosquito density, either as a stand-alone intervention or in combination with screening. However, there was evidence that mosquitoes recruited to the EaveTubes, and the resulting mortality could create a community benefit. These complementary modes of action of screening and EaveTubes support the rationale of combining the technologies to create a 'Lethal House Lure'. This article is part of the theme issue 'Novel control strategies for mosquito-borne diseases'.

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

Routine monitoring of occurrence, levels and mechanisms of insecticide resistance informs effective management strategies, and should be used to assess the effect of new tools on resistance. As part of a cluster randomised controlled trial evaluating a novel insecticide-based intervention in central Côte d'Ivoire, we assessed resistance and its underlying mechanisms in Anopheles gambiae populations from a subset of trial villages. Resistance to multiple insecticides in An. gambiae s.s. and An. coluzzii was detected across villages, with dose-response assays demonstrating extremely high resistance intensity to the pyrethroid deltamethrin (> 1,500-fold), and mortality following exposure to pyrethroid-treated bednets was low (< 30% mortality in cone bioassays). The 1014F kdr mutation was almost fixed (≥ 90%) in all villages but the 1575Y kdr-amplifying mutation was relatively rare (< 15%). The carbamate and organophosphate resistance-associated Ace-1 G119S mutation was also detected at moderate frequencies (22-43%). Transcriptome analysis identified overexpression of P450 genes known to confer pyrethroid resistance (Cyp9K1, Cyp6P3, and Cyp6M2), and also a carboxylesterase (COEAE1F) as major candidates. Cyp6P3 expression was high but variable (up to 33-fold) and correlated positively with deltamethrin resistance intensity across villages (r2 = 0.78, P = 0.02). Tools and strategies to mitigate the extreme and multiple resistance provided by these mechanisms are required in this area to avoid future control failures.