Mapping trends in insecticide resistance phenotypes in African malaria vectors

Assessing insecticide susceptibility status of Anopheles mosquitoes in Gondar zuria district, Northwest Ethiopia

Insecticide-based vector control, which comprises the use of insecticide-treated bed nets (ITNs) and indoor residual spraying (IRS), is the key method of malaria control in Ethiopia. However, its effectiveness is threatened as malaria vectors become resistant to insecticides. Thus, the aim of this study was to monitor the insecticide susceptibility status of malaria vectors. WHO susceptibility tests were used to detect knock-down and mortality rate in the wild malaria vectors collected in Gondar zuria woreda, Northwest Ethiopia. The WHO diagnostic doses of 0.75% permethrin, 0.05% deltamethrin, 0.05% alpha-cypermethrin, 0.1% propoxur and 0.25% pirimiphos-methyl were used. The major malaria vectors in Ethiopia, Anopheles gambiae s.l Anopheles funestus group, and Anopheles Pharoensis, were susceptible, to pirimiphos-methyl and propoxur. However, resistant to permethrin (mortality rate of 88.8%), alphacypermethrin (mortality rate of 67.5%), and deltamethrin (mortality rate of 73.8%). Although permethrin restoration was only 96% in permethrin resistant Anopheles mosquitoes, the efficacy of alphacypermethrin and deltamethrin was totally restored by pre-exposure to PBO. The susceptibility of malaria vectors to pirimiphos-methyl, propoxur, and PBO + pyrethroid insecticides is encouraging for successful malaria control. Further investigations are needed to better understand the molecular basis of pyrethroids insecticide resistant-malaria vectors.

Does washing insecticide-treated nets 20 times for experimental hut evaluations provide a suitable proxy for their end-of-life performance under household conditions?

BACKGROUND

Insecticide-treated nets (ITNs) are washed 20 times as part of experimental hut trials to simulate the loss of active ingredient (AI) occurring over their intended 3-year lifespan and estimate insecticidal durability. The ability of the 20-wash method to predict the end-of-life performance of ITNs has not been empirically validated.

METHODS

We performed an experimental hut trial to compare the efficacy of new ITNs unwashed and washed 20 times to field-aged ITNs withdrawn from households 3 years post-distribution against a pyrethroid-resistant vector population in Covè, Benin. Four products from pyrethroid-only (Interceptor®), pyrethroid-piperonyl butoxide (PermaNet® 3.0), pyrethroid-pyriproxyfen (Royal Guard®) and pyrethroid-chlorfenapyr (Interceptor® G2) ITN types were tested. Net pieces were tested in bioassays and sent for chemical analysis to assess differences in surface AI bioavailability and total chemical content between washed and field-aged nets. Susceptibility bioassays were also performed to assess insecticide resistance in the Covè vector population.

RESULTS

Mosquito mortality in experimental huts was similar or slightly higher with field-aged nets than washed nets with Interceptor® (11% vs. 10%, p = 0.339, OR = 1.19, 95% CIs [0.84, 1.69]), PermaNet® 3.0 (12% vs. 18%, p < 0.001, OR = 1.78, 95% CIs [1.34, 2.38]) and Royal Guard® (9% vs. 14%, p = 0.076, OR = 1.33, 95% CIs: [0.97, 1.83]). Likewise, field-aged Royal Guard® induced a similar reduction in fertility to washed Royal Guard® (22% vs. 29%, p = 0.066). In contrast, mortality was significantly lower with field-aged nets Interceptor® G2 compared to washed nets (54% vs. 19%, p < 0.001, OR = 0.18, 95% CIs [0.14, 0.24]). Blood-feeding inhibition was higher with field-aged nets than washed nets across all ITN types. Retention of non-pyrethroid AIs was lower than for the pyrethroid, particularly with field-aged nets (PermaNet® 3.0 (roof): 25% vs. 68%, p < 0.001, Royal Guard®: 27% vs. 53%, p < 0.001, Interceptor® G2: 14% vs. 39%, p < 0.001).

CONCLUSIONS

In this setting, the 20-wash method provided a suitable proxy for the end-of-life killing and sterilising performance of Interceptor®, PermaNet® 3.0 and Royal Guard® in experimental huts. In contrast, washing overestimated the end-of-life performance of Interceptor® G2 for mortality and underestimated the personal protection of all field-aged ITNs.

High pyrethroid resistance is associated with high frequencies of 1014F and 1014S kdr mutations in Anopheles arabiensis (Diptera: Culicidae) from Ouagadougou, Burkina Faso

Malaria remains a major public health threat in Burkina Faso, as in most sub-Saharan Africa countries. Malaria control relies mainly on long-lasting insecticide-treated nets (LLINs) and indoor residual spraying. In Burkina Faso, an escalating of insecticide resistance has been observed over the last decades. This study aimed to investigate insecticide resistance and the underlying mechanisms in Anopheles gambiae complex in Ouagadougou. Anopheles gambiae s.l. larvae were collected from gutters and ponds, in Zogona, Tampouy and Tanghin, 3 localities in Ouagadougou from July to October 2018. The larvae were reared in the laboratory to adults stage and susceptibility profile to pyrethroid, carbamate, and organophosphate insecticides was assessed using WHO tube assays. Mosquito species and mutations linked with insecticide resistance, were identified through PCR. More than 95% of the collected An. gambiae s.l. were An. arabiensis. An. arabiensis displayed high resistance to permethrin and deltamethrin, with mortalities below 30%, but was fully susceptible to bendiocarb, fenitrothion, and malathion. A high-frequency of the pyrethroid resistance-associated kdr mutation 1014F (0.81) was recorded, while the frequency of 1014S mutation (0.18) was lower. However, the carbamate and organophosphate-associated Ace-1 119S mutation was not detected. Localities and breeding site type appear to influence pyrethroid resistance in the An. arabiensis population of Ouagadougou. The high resistance to pyrethroids in An. arabiensis of urban Ouagadougou is underpinned, at least in part by high-frequency kdr mutations. This result supports the switch to next-generation LLINs, in well-established pyrethroid resistance zones of Burkina Faso including Ouagadougout.

LLIN Evaluation in Uganda Project (LLINEUP2) - Effect of long-lasting insecticidal nets (LLINs) treated with pyrethroid plus pyriproxyfen vs LLINs treated with pyrethroid plus piperonyl butoxide in Uganda: A cluster-randomised trial

Long-lasting insecticidal nets (LLINs) are the cornerstone of malaria control, but their effectiveness is threatened by pyrethroid resistance. We embedded a pragmatic, cluster-randomised trial into Uganda's national LLIN distribution campaign in 2020-2021, comparing pyrethroid-piperonyl butoxide (PBO) LLINs to pyrethroid-pyriproxyfen LLINs. Target communities surrounding public health facilities (clusters, n=64), covering 32 districts were included. Clusters were randomised 1:1 in blocks of two by district to receive: (1) pyrethroid-PBO LLINs (PermaNet 3.0, n=32) or (2) pyrethroid-pyriproxyfen LLINs (Royal Guard, n=32). LLINs were delivered from 7 November 2020 to 26 March 2021. Malaria surveillance data were collected from health facilities from 1 November 2019 until 31 March 2023. Cluster-level estimates of malaria incidence in residents of all ages (primary outcome) were generated from enhanced health facility surveillance data. Cross-sectional community surveys were conducted in randomly selected households (at least 50 per cluster) at 12-months (24 November 2021 to 1 April 2022) and 24-months (23 November 2022 to 21 March 2023) post-LLIN distribution. Overall, 186,364 clinical malaria episodes were diagnosed in cluster residents during 398,931 person-years of follow-up. At 24-months, malaria incidence was lower than baseline in both arms (pyrethroid-PBO: 465 vs 676 episodes per 1000 person-years; pyrethroid-pyriproxyfen: 469 vs 674 episodes per 1000 person-years); but there was no evidence of a difference between the arms (incidence rate ratio 1.06, 95% confidence interval [CI] 0.91-1.22, p=0.47). Two years post-distribution, ownership of at least one LLIN for every two household residents was low in both arms (41.1% pyrethroid-PBO vs 38.6% pyrethroid-pyriproxyfen). Parasite prevalence in children aged 2-10 years was no different between the arms in either survey (24-months: 26.1% pyrethroid-PBO; 29.5% pyrethroid-pyriproxyfen; odds ratio 1.29 [95% CI: 0.81-2.05], p=0.29). The effectiveness of pyrethroid-PBO LLINs and pyrethroid-pyriproxyfen LLINs was no different in Uganda, but two years after mass distribution, LLIN coverage was inadequate. Trial registration: NCT04566510. Registered 28 September 2020, https://clinicaltrials.gov/ct2/show/NCT04566510.

Elevating larval source management as a key strategy for controlling malaria and other vector-borne diseases in Africa

Larval source management (LSM) has a long history of advocacy and successes but is rarely adopted where funds are limited. The World Health Organization (WHO) guidelines on malaria prevention recommend the use of LSM as a supplementary intervention to the core vector control methods (insecticide-treated nets and indoor residual spraying), arguing that its feasibility in many settings can be limited by larval habitats being numerous, transient, and difficult to find or treat. Another key argument is that there is insufficient high-quality evidence for its effectiveness to support wide-scale implementation. However, the stagnation of progress towards malaria elimination demands that we consider additional options to the current emphasis on insecticidal commodities targeting adult mosquitoes inside homes. This letter is the result of a global, crossdisciplinary collaboration comprising: (a) detailed online expert discussions, (b) a narrative review of countries that have eliminated local malaria transmission, and (c) a mathematical modeling exercise using two different approaches. Together, these efforts culminated in seven key recommendations for elevating larval source management as a strategy for controlling malaria and other mosquito-borne diseases in Africa (Box 1). LSM encompasses the use of larvicide (a commodity) as well as various environmental sanitation measures. Together, these efforts lead to the long-term reduction of mosquito populations, which benefits the entire community by controlling both disease vector and nuisance mosquitoes. In this paper, we argue that the heavy reliance on large-scale cluster-randomized controlled trials (CRTs) to generate evidence on epidemiological endpoints restricts the recommendation of approaches to only those interventions that can be measured by functional units and deliver relatively uniform impact and, therefore, are more likely to receive financial support for conducting these trials. The explicit impacts of LSM may be better captured by using alternative evaluation approaches, especially high-quality operational data and a recognition of locally distinct outcomes and tailored strategies. LSM contributions are also evidenced by the widespread use of LSM strategies in nearly all countries that have successfully achieved malaria elimination. Two modelling approaches demonstrate that a multifaceted strategy, which incorporates LSM as a central intervention alongside other vector control methods, can effectively mitigate key biological threats such as insecticide resistance and outdoor biting, leading to substantial reductions in malaria cases in representative African settings. This argument is extended to show that the available evidence is sufficient to establish the link between LSM approaches and reduced disease transmission of mosquito-borne illnesses. What is needed now is a significant boost in the financial resources and public health administration structures necessary to train, employ and deploy local-level workforces tasked with suppressing mosquito populations in scientifically driven and ecologically sensitive ways. In conclusion, having WHO guidelines that recognize LSM as a key intervention to be delivered in multiple contextualized forms would open the door to increased flexibility for funding and aid countries in implementing the strategies that they deem appropriate. Financially supporting the scale-up of LSM with high-quality operations monitoring for vector control in combination with other core tools can facilitate better health. The global health community should reconsider how evidence and funding are used to support LSM initiatives.

Quantifying the potential value of entomological data collection for programmatic decision-making on malaria control in sub-Saharan African settings

BACKGROUND

The availability of many tools for malaria control leads to complex decisions regarding the most cost-effective intervention package based on local epidemiology. Mosquito characteristics influence the impact of vector control, but entomological surveillance is often limited due to a lack of resources in national malaria programmes.

METHODS

This study quantified the monetary value of information provided by entomological data collection for programmatic decision-making using a mathematical model of Plasmodium falciparum transmission. The 3-year impact and cost of various intervention packages was simulated in different sub-Saharan African settings, including combinations of scaling-up insecticide-treated nets (ITN), switching to next-generation ITNs, and a treatment and prevention package. The DALYs averted and their net monetary benefit were compared at different cost-effectiveness thresholds and the value of resolving uncertainty in entomological model parameters was calculated.

RESULTS

Across transmission settings and at cost-effectiveness thresholds over US$170 per DALY averted, the most cost-effective intervention package was switching to and scaling up pyrethroid-pyrrole ITNs combined with the treatment and prevention package. The median expected value of perfect information on the entomological indicators was US$0.05 (range 0.02-0.23) and US$0.17 (range 0.09-1.43) per person at risk at thresholds of US$75 and US$1000 per DALY averted, respectively. This represented less than 2% of the net monetary benefit of implementing the most cost-effective intervention package. Value of information estimates at cost-effectiveness thresholds over US$250 were higher than current investments into entomological monitoring by the US President's Malaria Initiative.

CONCLUSIONS

These results suggest that entomological data collection should not delay implementation of interventions with demonstrated efficacy in most settings, but that sustained investments into and use of entomological surveillance are nevertheless worthwhile and have broad value to national malaria programmes.

Modelling the seasonal dynamics of Aedes albopictus populations using a spatio-temporal stacked machine learning model

Various modelling techniques are available to understand the temporal and spatial variations of the phenology of species. Scientists often rely on correlative models, which establish a statistical relationship between a response variable (such as species abundance or presence-absence) and a set of predominantly abiotic covariates. The choice of the modeling approach, i.e., the algorithm, is itself a significant source of variability, as different algorithms applied to the same dataset can yield disparate outcomes. This inter-model variability has led to the adoption of ensemble modelling techniques, among which stacked generalisation, which has recently demonstrated its capacity to produce robust results. Stacked ensemble modelling incorporates predictions from multiple base learners or models as inputs for a meta-learner. The meta-learner, in turn, assimilates these predictions and generates a final prediction by combining the information from all the base learners. In our study, we utilized a recently published dataset documenting egg abundance observations of Aedes albopictus collected using ovitraps. and a set of environmental predictors to forecast the weekly median number of mosquito eggs using a stacked machine learning model. This approach enabled us to (i) unearth the seasonal egg-laying dynamics of Ae. albopictus for 12 years; (ii) generate spatio-temporal explicit forecasts of mosquito egg abundance in regions not covered by conventional monitoring initiatives. Our work establishes a robust methodological foundation for forecasting the spatio-temporal abundance of Ae. albopictus, offering a flexible framework that can be tailored to meet specific public health needs related to this species.

Temporal evolution of insecticide resistance and bionomics in Anopheles funestus, a key malaria vector in Uganda

Insecticide resistance escalation is decreasing the efficacy of vector control tools. Monitoring vector resistance is paramount in order to understand its evolution and devise effective counter-solutions. In this study, we monitored insecticide resistance patterns, vector population bionomics and genetic variants associated with resistance over 3 years from 2021 to 2023 in Uganda. Anopheles funestus s.s was the predominant species in Mayuge but with evidence of hybridization with other species of the An. funestus group. Sporozoite infection rates were relatively very high with a peak of 20.41% in March 2022. Intense pyrethroid resistance was seen against pyrethroids up to 10-times the diagnostic concentration but partial recovery of susceptibility in PBO synergistic assays. Among bednets, only PBO-based nets (PermaNet 3.0 Top and Olyset Plus) and chlorfenapyr-based net (Interceptor G2) had high mortality rates. Mosquitoes were fully susceptible to chlorfenapyr and organophosphates, moderately resistant to clothianidin and resistant to carbamates. The allele frequency of key P450, CYP9K1, resistance marker was constantly very high but that for CYP6P9A/b were very low. Interestingly, we report the first detection of resistance alleles for Ace1 gene (RS =  ~ 13%) and Rdl gene (RS =  ~ 21%, RR =  ~ 4%) in Uganda. The qRT-PCR revealed that Cytochrome P450s CYP9K1, CYP6P9A, CYP6P9b, CYP6P5 and CYP6M7 were consistently upregulated while a glutathione-S-transferase gene (GSTE2) showed low expression. Our study shows the complexity of insecticide resistance patterns and underlying mechanisms, hence constant and consistent spatial and temporal monitoring is crucial to rapidly detect changing resistance profiles which is key in informing deployment of counter interventions.

The experimental hut efficacy of next-generation insecticide-treated nets against pyrethroid-resistant malaria vectors after 12, 24 and 36 months of household use in Benin

BACKGROUND

Cluster-randomized controlled trials (cluster-RCTs) have demonstrated variation in the epidemiological efficacy of different next-generation insecticide-treated net (ITN) types, with some providing shorter-lived impact than others. Further studies are needed to assess changes in the insecticidal durability of these ITNs over time to complement cluster-RCT results.

METHODS

A series of experimental hut trials were performed to evaluate the bioefficacy of new and field-aged next-generation ITNs (PermaNet® 3.0, Royal Guard®, Interceptor® G2) compared to a pyrethroid-only net (Interceptor®) against pyrethroid-resistant malaria vectors in Covè, southern Benin. Field-aged nets were withdrawn from households at 12, 24 and 36 months. Net pieces cut from whole ITNs were analysed for chemical content, and susceptibility bioassays were performed during each trial to assess changes in insecticide resistance in the Covè vector population.

RESULTS

Interceptor® G2 induced superior mosquito mortality than the other ITNs across all time points. The improved mortality with Interceptor® G2 compared to Interceptor® was evident across all time points but was greater with new nets (odds ratio (OR) = 8.6, 95% CI [7.4, 10.1]) than field-aged nets (OR = 2.5, 95% CI [1.8, 3.5] at 12 months, OR = 2.4, 95% CI [1.6, 3.7] at 24 months and OR = 2.9, 95% CI [1.6, 5.1] at 36 months). New Royal Guard® reduced mosquito fertility compared to the other ITNs, but this improvement fell after field-ageing, particularly at 24 months when it was similar to Interceptor® (11% vs 3%, p = 0.08). When new, mortality was significantly higher with PermaNet® 3.0 compared to Interceptor® (OR = 3.6, 95% CI [3.0, 4.2]); however, this benefit was lost with field-aged nets at 12 months (OR = 1.1, 95% CI [0.8, 1.5]) and 24 months (OR = 0.6, 95% CI [0.4, 0.9]). Retention of the non-pyrethroid compound in next-generation nets was low after 36 months (27% for PermaNet® 3.0, 26% for Royal Guard® and 15% for Interceptor® G2).

CONCLUSIONS

Interceptor® G2 outperformed the other ITNs, confirming the superiority of pyrethroid-chlorfenapyr nets over other net types. When new, all next-generation ITNs showed superior bioefficacy compared to Interceptor®; however, the size of this improvement fell after field-ageing due to poor durability of the non-pyrethroid compound. These findings emphasize the need to enhance the insecticidal durability of next-generation ITNs.

The epidemiological benefit of pyrethroid-pyrrole insecticide treated nets against malaria: an individual-based malaria transmission dynamics modelling study

BACKGROUND

Insecticide treated nets (ITNs) are the most important malaria prevention tool in Africa but the rise of pyrethroid resistance in mosquitoes is likely impeding control. WHO has recommended a novel pyrethroid-pyrrole ITN following evidence of epidemiological benefit in two cluster-randomised, controlled trials (CRTs). It remains unclear how effective more costly pyrethroid-pyrrole ITNs are compared with other tools, or whether they should be deployed when budgets are limited. We aimed to compare the epidemiological impact and cost-effectiveness of the mass distribution of pyrethroid-pyrrole ITNs relative to other ITNs over 3 years in different African settings.

METHODS

In this individual-based malaria transmission dynamics modelling study we characterise the entomological impact of ITNs using data from a systematic review of experimental hut trials from across Africa. This African entomological data was used to inform an individual-based malaria transmission dynamics model, which was validated against CRT results from Benin and Tanzania. The full impact of new ITNs was quantified for trial sites and simulation was used to project impact in different settings which were included within an accessible interface (the Malaria Intervention Tool) to support National Malaria Programmes to explore how vector control tools and budgets could be allocated across regions to avert the most cases.

FINDINGS

The model projects that distributing pyrethroid-pyrrole ITNs averted 65% (95% credible interval 48-74) of cases over 3 years in Tanzania, and 75% (28-93) in Benin. The model indicates that trials might have underestimated the benefits of switching ITNs by 12-16% over 3 years because participants stopped using trial-allocated nets. In moderate endemicity non-trial settings, pyrethroid-pyrrole ITNs are projected to reduce malaria prevalence by 25-60% and switching from pyrethroid-only ITNs is probably highly cost-effective in most locations given current prices, averting an additional 10-30% of cases.

INTERPRETATION

The benefit of pyrethroid-pyrrole ITNs varies by setting but is generally the most cost-effective indoor vector control intervention in Africa. National Malaria Programmes can strategise deployment to maximise impact. Entomological data could broadly predict epidemiological impact, although there are some inconsistencies, illustrating the challenge in capturing the dynamics across diverse settings.

FUNDING

Unitaid, Bill & Melinda Gates Foundation, the UK Medical Research Council, Wellcome Trust, and the UK Foreign, Commonwealth & Development Office.

Managing vector-borne diseases in a geoAI-enabled society. Malaria as an example

More than 17 % of all infectious diseases are caused by vector-borne diseases resulting in more than 1 billion cases and over 1 million deaths each year. Of these malaria continues to be a global burden in over eighty countries. As societies become more digitalised, the availability of geospatially enabled health and disease information will become more abundant. With this, the ability to assess health and disease risks in real-time will become a reality. The purpose of this study was to examine how geographic information, geospatial technologies and spatial data science are being used to reduce the burden of vector-borne diseases such as malaria and explore the opportunities that lie ahead with GeoAI and other geospatial technology advancements. Malaria is a dynamic and complex system and as such a range of data and approaches are needed to tackle different parts of the malaria cycle at different local and global scales. Geospatial technologies provide an integrated framework vital for monitoring, analysing and managing vector-borne diseases. GeoAI and technological advancements are useful for enhancing real-time assessments, accelerating the decision making process and spatial targeting of interventions. Training is needed to enhance the use of geospatial information for the management of vector-borne diseases.

Copy number variants underlie major selective sweeps in insecticide resistance genes in Anopheles arabiensis

To keep ahead of the evolution of resistance to insecticides in mosquitoes, national malaria control programmes must make use of a range of insecticides, both old and new, while monitoring resistance mechanisms. The outdoor-biting malaria vector Anopheles arabiensis is of increasing concern for malaria transmission because it is apparently less susceptible to many indoor control interventions, yet knowledge of its mechanisms of resistance remains limited. Furthermore, comparatively little is known in general about resistance to non-pyrethroid insecticides such as pirimiphos-methyl (PM), which are crucial for effective control in the context of globally high resistance to pyrethroids. We performed a genome-wide association study to determine the molecular mechanisms of resistance to the pyrethroid deltamethrin (commonly used in bednets) and PM (widespread use for indoor spraying), in An. arabiensis from 2 regions in Tanzania. Genomic regions of positive selection in these populations were largely driven by copy number variants (CNVs) in gene families involved in metabolic resistance. We found evidence of a new gene cluster involved in resistance to PM, identifying a strong selective sweep tied to a CNV in the carboxylesterase genes Coeae2g - Coeae6g. Using complementary data from another malaria vector, An. coluzzii, in Ghana, we show that copy number at this locus is significantly associated with PM resistance. Similarly, for deltamethrin, resistance was strongly associated with a novel CNV allele in the Cyp6aa / Cyp6p cluster (Cyp6aap_Dup33). Against this background of metabolic resistance, resistance caused by mutations in the insecticide target sites was very rare or absent. Mutations in the pyrethroid target site Vgsc were at very low frequency in Tanzania, yet combining these samples with 3 An. arabiensis individuals from West Africa revealed a startling evolutionary diversity, with up to 5 independent origins of Vgsc-995 mutations found within just 8 haplotypes. Thus, despite having been first recorded over 10 years ago, Vgsc resistance mutations in Tanzanian An. arabiensis have remained at stable low frequencies. Overall, our results provide a new copy number marker for monitoring resistance to PM in malaria mosquitoes, and reveal the complex picture of resistance patterns in An. arabiensis.

Empowering rural communities for effective larval source management: A small-scale field evaluation of a community-led larviciding approach to control malaria in south-eastern Tanzania

Introduction

Larval source management, particularly larviciding, is mainly implemented in urban settings to control malaria and other mosquito-borne diseases. In Tanzania, the government has recently expanded larviciding to rural settings across the country, but implementation faces multiple challenges, notably inadequate resources and limited know-how by technical staff. This study evaluated the potential of training community members to identify, characterize and target larval habitats of Anopheles funestus mosquitoes, the dominant vector of malaria transmission in south-eastern Tanzania.

Methods

A mixed-methods study was used. First, interviewer-administered questionnaires were employed to assess knowledge, awareness, and perceptions of community members towards larviciding (N = 300). Secondly community-based volunteers were trained to identify and characterize aquatic habitats of dominant malaria vector species, after which they treated the most productive habitats with a locally-manufactured formulation of the biolarvicide, Bacillus thuringiensis var. israelensis. Longitudinal surveys of mosquito adults and larvae were used to assess impacts of the community-led larviciding programme in two villages in rural south-eastern Tanzania.

Results

At the beginning of the program, the majority of village residents were unaware of larviciding as a potential malaria prevention method, and about 20 % thought that larvicides could be harmful to the environment and other insects. The trained community volunteers identified and characterized 360 aquatic habitats, of which 45.6 % had Anopheles funestus, the dominant malaria vector in the area. The preferred larval habitats for An. funestus were deep and had either slow- or fast-moving waters. Application of biolarvicides reduced the abundance of adult An. funestus and Culex spp. species inside human houses in the same villages, by 46.3 % and 35.4 % respectively. Abundance of late-stage instar larvae of the same taxa was also reduced by 74 % and 42 %, respectively.

Conclusion

This study demonstrates that training community members to identify, characterize, and target larval habitats of the dominant malaria vectors can be effective for larval source management in rural Tanzania. Community-led larviciding reduced the densities of adult and late-stage instar larvae of An. funestus and Culex spp. inside houses, suggesting that this approach may have potential for malaria control in rural settings. However, efforts are still needed to increase awareness of larviciding in the relevant communities.

Analysis of the 24-h biting patterns and human exposures to malaria vectors in south-eastern Tanzania

BACKGROUND

Afrotropical malaria vectors are generally believed to bite nocturnally, leading to the predominant use of insecticide-treated nets (ITNs), which target indoor, nighttime-biting mosquitoes. This focus is reinforced by biases in entomological surveys, which largely overlook daytime mosquito activity. However, recent evidence challenges this paradigm, showing that Anopheles biting can extend way into the daytime, coinciding with human activities at dawn, daytime and evenings, suggesting a broader risk spectrum and potential protection gaps. We have therefore investigated the diurnal and nocturnal biting patterns of the malaria vectors Anopheles arabiensis and Anopheles funestus in south-eastern Tanzania, to better understand the scope of residual transmission and inform strategies for improved control.

METHODS

Host-seeking mosquitoes were collected hourly using miniaturized double net traps, both indoors and outdoors over 24-h periods between June 2023 and February 2024. Concurrently, human activities indoors and outdoors were monitored half-hourly to correlate with mosquito collections. A structured questionnaire was used to assess household members' knowledge, perceptions and experiences regarding exposure to mosquito bites during both nighttime and daytime.

RESULTS

Nocturnal biting by An. arabiensis peaked between 7 p.m. and 11 p.m. while that of An. funestus peaked later, between 1 a.m. and 3 a.m. Daytime biting accounted for 15.03% of An. arabiensis catches, with peaks around 7-11 a.m. and after 4 p.m., and for 14.15% of An. funestus catches, peaking around mid-mornings, from 10 a.m. to 12 p.m. Nighttime exposure to An. arabiensis was greater outdoors (54.5%), while daytime exposure was greater indoors (80.4%). For An. funestus, higher exposure was observed indoors, both at nighttime (57.1%) and daytime (69%). Plasmodium falciparum sporozoites were detected in both day-biting and night-biting An. arabiensis. Common daytime activities potentially exposing residents during peak biting hours included household chores, eating, sleeping (including due to sickness), resting in the shade or under verandas and playing (children). From evenings onwards, exposures coincided with resting, socializing before bedtime and playtime (children). Nearly all survey respondents (95.6%) reported experiencing daytime mosquito bites, but only 28% believed malaria was transmissible diurnally.

CONCLUSIONS

This study updates our understanding of malaria vector biting patterns in south-eastern Tanzania, revealing considerable additional risk in the mornings, daytime and evenings. Consequently, there may be more gaps in the protection provided by ITNs, which primarily target nocturnal mosquitoes, than previously thought. Complementary strategies are needed to holistically suppress vectors regardless of biting patterns (e.g. using larval source management) and to extend personal protection limits (e.g. using repellents). Additionally, community engagement and education on mosquito activity and protective measures could significantly reduce malaria transmission risk.

The potential of gene drives in malaria vector species to control malaria in African environments

Gene drives are a promising means of malaria control with the potential to cause sustained reductions in transmission. In real environments, however, their impacts will depend on local ecological and epidemiological factors. We develop a data-driven model to investigate the impacts of gene drives that causes vector population suppression. We simulate gene drive releases in sixteen ~ 12,000 km2 areas of west Africa that span variation in vector ecology and malaria prevalence, and estimate reductions in vector abundance, malaria prevalence and clinical cases. Average reductions in vector abundance ranged from 71.6-98.4% across areas, while impacts on malaria depended strongly on which vector species were targeted. When other new interventions including RTS,S vaccination and pyrethroid-PBO bednets were in place, at least 60% more clinical cases were averted when gene drives were added, demonstrating the benefits of integrated interventions. Our results show that different strategies for gene drive implementation may be required across different African settings.

Longitudinal trials illustrate interactive effects between declining Bt efficacy against Helicoverpa zea (Lepidoptera: Noctuidae) and planting dates of corn

Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) has evolved resistance to insecticidal toxins from Bacillus thuringiensis (Bt) Berliner (Bacillales: Bacillaceae) expressed in genetically engineered corn, Zea mays L. This study provides an overview of field trials from Georgia, North Carolina, and South Carolina evaluating Bt and non-Bt corn hybrids from 2009 to 2022 to show changes in susceptibility in H. zea to Bt corn. The reduction in kernel injury relative to a non-Bt hybrid averaged across planting dates generally declined over time for Cry1A.105 + Cry2Ab2 corn. In addition, there was a significant interaction with planting date used as a covariate. The reduction in kernel injury remained above 80% and did not vary with planting date from 2009 to 2014, whereas a significant decline with planting date was found in this reduction from 2015 to 2022. For Cry1Ab + Cry1F corn, the reduction in kernel injury relative to a non-Bt hybrid averaged across planting dates did not vary among years. The reduction in kernel injury significantly declined with planting date from 2012 to 2022. Kernel injury as a proxy for H. zea pressure was greater in late-planted trials in non-Bt corn hybrids. Our study showed that Bt hybrids expressing Cry1A.105 + Cry2Ab2 are now less effective in later planted trials in reducing H. zea injury; however, this was not the case during the earlier years of adoption of corn expressing these 2 toxins when resistance alleles were likely less frequent in H. zea populations. The implications for management of H. zea and for insect resistance management are discussed.

Power calculation for mosquito bioassays: Quantifying variability in the WHO tube bioassay and developing sample size guidance for the PBO synergism assay using a Shiny application

Background

The WHO tube bioassay is a method for exposing mosquitos to determine susceptibility to insecticides, with mortality to discriminating doses <98% indicating possible resistance and <90% confirming resistance. This bioassay is also used for synergism testing to assess if susceptibility is restored by pre-exposure to the synergist piperonyl butoxide.

Methods

Here we perform testing with pyrethroid-susceptible and pyrethroid-resistant An. gambiae to quantify the variability of the WHO tube bioassay and identify its sources. These estimates of within and between day variability are then used to evaluate the power of the bioassay to detect a mortality difference between pyrethroid-only and pyrethroid-PBO.

Results

We show that approximately two-thirds of variation occurs between days, with the pyrethroid-susceptible strain twice as variable as the pyrethroid-resistant strain. The total number of mosquitoes in the tube and their bodyweight contributes to approximately 10% of this variability. Changes in temperature and humidity, within a climate-controlled insectary, didn't impact mortality. Using a simulation-based framework, we show that the current synergism guidelines, using a 4x4 design, can reliably detect a difference between 90% and 100% mortality (>90% power). However, as the mortality of either group gets closer to 50%, a 10% difference between groups is more difficult to reliably detect. In the worst-case scenario where the mortality of either group is 50%, the mortality difference must be >22.5% to be detected with 80% power. We provide an R shiny application to assess power for other comparisons.

Conclusions

Our findings indicate that detecting synergism with the WHO tube assay is more difficult than assumed by the current WHO guidelines. Additionally, we demonstrate the value of using a Shiny application to make the outputs of simulation-based power analysis readily available to end-users, allowing them to determine the number of tubes needed to detect a given mortality difference.

Longitudinal survey of insecticide resistance in a village of central region of Burkina Faso reveals co-occurrence of 1014F, 1014S and 402L mutations in Anopheles coluzzii and Anopheles arabiensis

BACKGROUND

Pyrethroid resistance is one of the major threats for effectiveness of insecticide-treated bed nets (ITNs) in malaria vector control. Genotyping of mutations in the voltage gated sodium channel (VGSC) gene is widely used to easily assess the evolution and spread of pyrethroid target-site resistance among malaria vectors. L1014F and L1014S substitutions are the most common and best characterized VGSC mutations in major African malaria vector species of the Anopheles gambiae complex. Recently, an additional substitution involved in pyrethroid resistance, i.e. V402L, has been detected in Anopheles coluzzii from West Africa lacking any other resistance alleles at locus 1014. The evolution of target-site resistance mutations L1014F/S and V402L was monitored in An. coluzzii and Anopheles arabiensis specimens from a Burkina Faso village over a 10-year range after the massive ITN scale-up started in 2010.

METHODS

Anopheles coluzzii (N = 300) and An. arabiensis (N = 362) specimens collected both indoors and outdoors by different methods (pyrethrum spray catch, sticky resting box and human landing collections) in 2011, 2015 and 2020 at Goden village were genotyped by TaqMan assays and sequencing for the three target site resistance mutations; allele frequencies were statistically investigated over the years.

RESULTS

A divergent trend in resistant allele frequencies was observed in the two species: 1014F decreased in An. coluzzii (from 0.76 to 0.52) but increased in An. arabiensis (from 0.18 to 0.70); 1014S occurred only in An. arabiensis and slightly decreased over time (from 0.33 to 0.23); 402L increased in An. coluzzii (from 0.15 to 0.48) and was found for the first time in one An. arabiensis specimen. In 2020 the co-occurrence of different resistance alleles reached 43% in An. coluzzii (alleles 410L and 1014F) and 32% in An. arabiensis (alleles 1014F and 1014S).

CONCLUSIONS

Overall, an increasing level of target-site resistance was observed among the populations with only 1% of the two malaria vector species being wild type at both loci, 1014 and 402, in 2020. This, together with the co-occurrence of different mutations in the same specimens, calls for future investigations on the possible synergism between resistance alleles and their phenotype to implement local tailored intervention strategies.

Mosquito Shield™, a transfluthrin passive emanator, protects against pyrethroid-resistant Anopheles gambiae sensu lato in central Benin

BACKGROUND

Spatial repellents can provide personal and household protection against biting vector mosquitoes by volatizing repellents into the air within a given area. Mosquito Shield™ is a transfluthrin passive emanator undergoing evaluation for malaria control. Studies evaluating its entomological impact against different local malaria vector populations would help guide its deployment in endemic countries.

METHODS

A two-arm single-blinded small-scale household randomised entomological trial was conducted to assess the impact of Mosquito Shield™ on the human landing rate of wild pyrethroid-resistant Anopheles gambiae sensu lato (s.l.) vector mosquitoes in houses in the Ganhoua village of the Zakpota District of central Benin. From a total of 30 houses, 15 were randomly allocated to receive Mosquito Shield™, while the remainder received a placebo product. The trial lasted through the life of the Mosquito Shield™ product (32 days). Mosquito sampling was performed by human landing catches at baseline and at 6 timepoints post-intervention (days 0-1, 7-8, 14-15, 21-22, 28-29 and 31-32). Collections were performed for 2 nights at each sampling time point. WHO cylinder bioassays were conducted during the trial with F1 An. gambiae s.l. mosquitoes that emerged from larvae from the study area to assess the intensity of resistance to pyrethroids in the wild vector population.

RESULTS

The vector population in the study area showed a high intensity of resistance to pyrethroids. Baseline An. gambiae s.l. human landing rates were similar in houses in both study arms before product application (11.53/person/night vs 11.67/person/night, p > 0.05). A total of 5736 mosquitoes were collected in the placebo control arm and 3862 in the Mosquito Shield™ arm post-intervention. Overall An. gambiae s.l. post-intervention human landing rates were significantly lower in houses in the Mosquito Shield™ arm (18.13/person/night) compared to the houses in the placebo control arm (26.84/person/night, IRR = 0.658, p < 0.001). Over the lifespan of the product, Mosquito Shield™ provided a significant protective efficacy of 34.2% (22.1-44.4%, p < 0.001) against wild pyrethroid-resistant An. gambiae s.l. vectors compared to the placebo. Human landing rates of other nuisance vector mosquito species (Culex and Mansonia) were also reduced in houses treated with Mosquito Shield™ compared to the placebo.

CONCLUSION

Mosquito Shield™, a transfluthrin passive emanator, provided significant protection against pyrethroid-resistant malaria vectors to households in Benin. The spatial repellent shows potential to reduce malaria transmission by pyrethroid-resistant An. gambiae s.l. vector mosquitoes and cover gaps in malaria control when deployed to complement existing vector control interventions.

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.

Entomological survey of sibling species in the Anopheles funestus group in Tanzania confirms the role of Anopheles parensis as a secondary malaria vector

BACKGROUND

Malaria transmission in Tanzania is driven by mosquitoes of the Anopheles gambiae complex and Anopheles funestus group. The latter includes An. funestus s.s., an anthropophilic vector, which is now strongly resistant to public health insecticides, and several sibling species, which remain largely understudied despite their potential as secondary vectors. This paper provides the initial results of a cross-country study of the species composition, distribution and malaria transmission potential of members of the Anopheles funestus group in Tanzania.

METHODS

Mosquitoes were collected inside homes in 12 regions across Tanzania between 2018 and 2022 using Centres for Disease Control and Prevention (CDC) light traps and Prokopack aspirators. Polymerase chain reaction (PCR) assays targeting the noncoding internal transcribed spacer 2 (ITS2) and 18S ribosomal DNA (18S rDNA) were used to identify sibling species in the An. funestus group and presence of Plasmodium infections, respectively. Where DNA fragments failed to amplify during PCR, we sequenced the ITS2 region to identify any polymorphisms.

RESULTS

The following sibling species of the An. funestus group were found across Tanzania: An. funestus s.s. (50.3%), An. parensis (11.4%), An. rivulorum (1.1%), An. leesoni (0.3%). Sequencing of the ITS2 region in the nonamplified samples showed that polymorphisms at the priming sites of standard species-specific primers obstructed PCR amplification, although the ITS2 sequences closely matched those of An. funestus s.s., barring these polymorphisms. Of the 914 samples tested for Plasmodium infections, 11 An. funestus s.s. (1.2%), and 2 An. parensis (0.2%) individuals were confirmed positive for P. falciparum. The highest malaria transmission intensities [entomological inoculation rate (EIR)] contributed by the Funestus group were in the north-western region [108.3 infectious bites/person/year (ib/p/y)] and the south-eastern region (72.2 ib/p/y).

CONCLUSIONS

Whereas An. funestus s.s. is the dominant malaria vector in the Funestus group in Tanzania, this survey confirms the occurrence of Plasmodium-infected An. parensis, an observation previously made in at least two other occasions in the country. The findings indicate the need to better understand the ecology and vectorial capacity of this and other secondary malaria vectors in the region to improve malaria control.

LLIN Evaluation in Uganda Project (LLINEUP): modelling the impact of COVID-19-related disruptions on delivery of long-lasting insecticidal nets on malaria indicators in Uganda

BACKGROUND

Disruptions in malaria control due to COVID-19 mitigation measures were predicted to increase malaria morbidity and mortality in Africa substantially. In Uganda, long-lasting insecticidal nets (LLINs) are distributed nationwide every 3-4 years, but the 2020-2021 campaign was altered because of COVID-19 restrictions so that the timing of delivery of new nets was different from the original plans made by the National Malaria Control Programme.

METHODS

A transmission dynamics modelling exercise was conducted to explore how the altered delivery of LLINs in 2020-2021 impacted malaria burden in Uganda. Data were available on the planned LLIN distribution schedule for 2020-2021, and the actual delivery. The transmission model was used to simulate 100 health sub-districts, and parameterized to match understanding of local mosquito bionomics, net use estimates, and seasonal patterns based on data collected in 2017-2019 during a cluster-randomized trial (LLINEUP). Two scenarios were compared; simulated LLIN distributions matching the actual delivery schedule, and a comparable scenario simulating LLIN distributions as originally planned. Model parameters were otherwise matched between simulations.

RESULTS

Approximately 70% of the study population received LLINs later than scheduled in 2020-2021, although some areas received LLINs earlier than planned. The model indicates that malaria incidence in 2020 was substantially higher in areas that received LLINs late. In some areas, early distribution of LLINs appeared less effective than the original distribution schedule, possibly due to attrition of LLINs prior to transmission peaks, and waning LLIN efficacy after distribution. On average, the model simulations predicted broadly similar overall mean malaria incidence in 2021 and 2022. After accounting for differences in cluster population size and LLIN distribution dates, no substantial increase in malaria burden was detected.

CONCLUSIONS

The model results suggest that the disruptions in the 2020-2021 LLIN distribution campaign in Uganda did not substantially increase malaria burden in the study areas.

Efficacy of pyrethroid-pyriproxyfen and pyrethroid-chlorfenapyr nets on entomological indicators of malaria transmission: third year of a randomised controlled trial in Benin

The present cluster-randomised control trial aims to assess the entomological efficacy of pyrethroid-pyriproxyfen and pyrethroid-chlorfenapyr LLINs compared to the standard pyrethroid-only LLINs, in their third year of community usage. Adult mosquito collections were performed every 3 months, in 4 randomly selected houses in each of the 60 trial clusters, using human landing catches. Adult mosquitoes were morphologically identified and Anopheles vectors were molecularly speciated and screened for the presence of the L1014F kdr mutation using PCR. Plasmodium falciparum sporozoite infection was assessed using ELISA. A subset of An. gambiae s.l. was also dissected to examine parity and fertility rates across study arms. There was no evidence of a significant reduction in indoor vector density and entomological inoculation rate by the pyrethroid-pyriproxyfen [DR 0.94 (95% CI 0.46-1.88), p = 0.8527; and RR 1.10 (95% CI 0.44-2.72), p = 0.8380], and pyrethroid-chlorfenapyr [DR 0.74 (95% CI 0.37-1.48), p = 0.3946; and RR 1.00 (95% CI 0.40-2.50), p = 0.9957] LLINs, respectively. The same trend was observed outdoors. Frequencies of the L1014F kdr mutation, as well as parous and fertility rates, were similar between study arms. In the third year after net distribution, entomological indicators show that the two dual active-ingredients nets performed similarly to the standard pyrethroid-only LLIN. To maintain malaria gains, it is crucial that net distribution cycles fit with their operational lifespan.

Distribution and insecticide resistance profile of the major malaria vector Anopheles funestus group across the African continent

There has been significant progress in malaria control in the last 2 decades, with a decline in mortality and morbidity. However, these gains are jeopardised by insecticide resistance, which negatively impacts the core interventions, such as insecticide-treated nets (ITN) and indoor residual spraying (IRS). While most malaria control and research efforts are still focused on Anopheles gambiae complex mosquitoes, Anopheles funestus remains an important vector in many countries and, in some cases, contributes to most of the local transmission. As countries move towards malaria elimination, it is important to ensure that all dominant vector species, including An. funestus, an important vector in some countries, are targeted. The objective of this review is to compile and discuss information related to A. funestus populations' resistance to insecticides and the mechanisms involved across Africa, emphasising the sibling species and their resistance profiles in relation to malaria elimination goals. Data on insecticide resistance in An. funestus malaria vectors in Africa were extracted from published studies. Online bibliographic databases, including Google Scholar and PubMed, were used to search for relevant studies. Articles published between 2000 and May 2023 reporting resistance of An. funestus to insecticides and associated mechanisms were included. Those reporting only bionomics were excluded. Spatial variation in species distribution and resistance to insecticides was recorded from 174 articles that met the selection criteria. It was found that An. funestus was increasingly resistant to the four classes of insecticides recommended by the World Health Organisation for malaria vector control; however, this varied by country. Insecticide resistance appears to reduce the effectiveness of vector control methods, particularly IRS and ITN. Biochemical resistance due to detoxification enzymes (P450s and glutathione-S-transferases [GSTs]) in An. funestus was widely recorded. However, An. funestus in Africa remains susceptible to other insecticide classes, such as organophosphates and neonicotinoids. This review highlights the increasing insecticide resistance of An. funestus mosquitoes, which are important malaria vectors in Africa, posing a significant challenge to malaria control efforts. While An. funestus has shown resistance to the recommended insecticide classes, notably pyrethroids and, in some cases, organochlorides and carbamates, it remains susceptible to other classes of insecticides such as organophosphates and neonicotinoids, providing potential alternative options for vector control strategies. The study underscores the need for targeted interventions that consider the population structure and geographical distribution of An. funestus, including its sibling species and their insecticide resistance profiles, to effectively achieve malaria elimination goals.

Exposure to low-concentration fipronil impairs survival, behavior, midgut morphology and physiology of Aedes aegypti larvae

The Aedes aegypti mosquito is a vector for various arboviruses, including dengue and yellow fever. Insecticides, such as pyrethroids and organophosphates, are widely used to manage and control these insects. However, mosquitoes have developed resistance to these chemicals. Therefore, this study aimed to investigate the effects of the commercial formulation of fipronil (Tuit® Florestal; 80% purity) on the survival, behavior, morphology, and proteins related to signaling pathways of the midgut in A. aegypti larvae under controlled laboratory conditions. Significant reductions in immature survival were observed in all concentrations of fipronil tested. Low insecticide concentration (0.5 ppb) led to decreased locomotor activity in the larvae and caused disorganization of the epithelial tissue in the midgut. Moreover, exposure to the insecticide decreased the activity of detoxifying enzymes such as catalase, superoxide dismutase, and glutathione-S-transferase. On the other hand, the insecticide increased protein oxidation and nitric oxide levels. The detection of LC3, caspase-3, and JNK proteins, related to autophagy and apoptosis, increased after exposure. However, there was a decrease in the positive cells for ERK 1/2. Furthermore, the treatment with fipronil decreased the number of positive cells for the proteins FMRF, Prospero, PH3, Wg, Armadillo, Notch, and Delta, which are related to cell proliferation and differentiation. These findings demonstrate that even at low concentrations, fipronil exerts larvicidal effects on A. aegypti by affecting behavior and enzymatic detoxification, inducing protein oxidation, free radical generation, midgut damage and cell death, and inhibiting cell proliferation and differentiation. Thus, this insecticide may represent a viable alternative for controlling the spread of this vector.

Lingering challenges in malaria elimination efforts in sub-Saharan Africa: Insights and potential solutions

Introduction

Between 2000 and 2015, significant gains were recorded in reducing the global burden of malaria due to enhanced global collaboration and increased funding. However, progress has stagnated post-2015, and the COVID-19 pandemic seems to have reversed some of these gains, necessitating a critical reevaluation of interventions. This paper aims to analyze the setbacks and offer recommendations for advancement in malaria control and prevention in sub-Saharan Africa.

Methods

We conducted searches on Google Scholar, PubMed, and relevant organization websites to identify relevant studies on malaria control and prevention and associated challenges in sub-Saharan Africa from 2015 to the present. Additionally, studies on individual sub-Saharan African countries were reviewed to ensure comprehensiveness. Data from selected studies were extracted and analyzed using a narrative synthesis approach to offer a concise overview of the evidence.

Findings

We observe that the halt in progress of malaria control in sub-Saharan Africa has deep roots in socioeconomic, political, and environmental factors. These challenges are exacerbated by the population explosion in the region, low coverage of interventions due to funding deficits and incessant crises, and the degradation of the efficacy of existing malaria commodities.

Conclusion

Sub-Saharan Africa is at a crossroads in its fight against malaria. Promising new frontiers such as malaria vaccines, preventive monoclonal antibodies, new-generation insecticide-treated nets, and potentially artificial intelligence-driven technologies offer hope in advancing malaria control and prevention in the region. Through commitment and collaboration, leveraging these opportunities can help surmount challenges and ultimately eliminate malaria in sub-Saharan Africa.

Complete series method (CSM): a convenient method to reduce daily heterogeneity when evaluating the regeneration time (RT) of insecticide-treated nets (ITNs)

BACKGROUND

"Regeneration time" (RT) denotes the time required to obtain a stable mortality rate for mosquitoes exposed to insecticide-treated nets (ITNs) after three consecutive washes of a net in a day. The RT informs the wash interval used to artificially age ITNs to simulate their lifetime performance under user conditions (20 washes). RT was estimated following World Health Organization (WHO) longitudinal method (LM) procedures. Longitudinal evaluation may introduce heterogeneity due to mosquito batch variability, complicating RT determination. To overcome this, nets at each stage of regeneration (i.e., 1, 2, 3, 5 and 7 days post wash) were prepared in advance and refrigerated; then, a complete regeneration series was tested with a single mosquito batch on 1 testing day, completing four series over 4 days. This study compared the complete series method (CSM) against the LM.

METHODS

The overall heterogeneity in the methods for estimating RT of one incorporated alpha-cypermethrin and piperonyl butoxide (PBO) and one incorporated permethrin with PBO ITNs was determined using laboratory-reared resistant Anopheles arabiensis under standard laboratory conditions. LM methods and CSM were compared in two experiments with refrigerated nets acclimated for (i) 2 h (test 1) and (ii) 3 h (test 2). Four regeneration replicates per day were tested per ITN product with 50 mosquitoes exposed per replicate (equivalent sample size to LM). The heterogeneity from these methods was compared descriptively.

RESULTS

The intra-method variability for unwashed pieces was minimal, with variance of 1.26 for CSM and 1.18 for LM. For unwashed nets, LM had substantially greater variance and ratio of LM:CSM was 2.66 in test 1 and 2.49 in test 2. The magnitude of mortality measured in bioassays depended on sample acclimation after refrigeration.

CONCLUSIONS

The CSM is a convenient method for determining the regeneration times. ITNs are prepared in advance, reducing pressure to prepare all samples to start on a single day. A complete regeneration series of samples is removed from the refrigerator, defrosted and evaluated on a single day with one mosquito batch reducing the influence of mosquito batch heterogeneity on results. Replicates can be conducted over several days but do not have to be conducted on consecutive days, allowing easy facility scheduling.

Spatio-temporal characterization of phenotypic resistance in malaria vector species

BACKGROUND

Malaria, a deadly disease caused by Plasmodium protozoa parasite and transmitted through bites of infected female Anopheles mosquitoes, remains a significant public health challenge in sub-Saharan Africa. Efforts to eliminate malaria have increasingly focused on vector control using insecticides. However, the emergence of insecticide resistance (IR) in malaria vectors pose a formidable obstacle, and the current IR mapping models remain static, relying on fixed coefficients. This study introduces a dynamic spatio-temporal approach to characterize phenotypic resistance in Anopheles gambiae complex and Anopheles arabiensis. We developed a cellular automata (CA) model and applied it to data collected from Ethiopia, Nigeria, Cameroon, Chad, and Burkina Faso. The data encompasses georeferenced records detailing IR levels in mosquito vector populations across various classes of insecticides. In characterizing the dynamic patterns of confirmed resistance, we identified key driving factors through correlation analysis, chi-square tests, and extensive literature review.

RESULTS

The CA model demonstrated robustness in capturing the spatio-temporal dynamics of confirmed IR states in the vector populations. In our model, the key driving factors included insecticide usage, agricultural activities, human population density, Land Use and Land Cover (LULC) characteristics, and environmental variables.

CONCLUSIONS

The CA model developed offers a robust tool for countries that have limited data on confirmed IR in malaria vectors. The embrace of a dynamical modeling approach and accounting for evolving conditions and influences, contribute to deeper understanding of IR dynamics, and can inform effective strategies for malaria vector control, and prevention in regions facing this critical health challenge.

Subcutaneous Administration of a Monoclonal Antibody to Prevent Malaria

BACKGROUND

Subcutaneous administration of the monoclonal antibody L9LS protected adults against controlled Plasmodium falciparum infection in a phase 1 trial. Whether a monoclonal antibody administered subcutaneously can protect children from P. falciparum infection in a region where this organism is endemic is unclear.

METHODS

We conducted a phase 2 trial in Mali to assess the safety and efficacy of subcutaneous administration of L9LS in children 6 to 10 years of age over a 6-month malaria season. In part A of the trial, safety was assessed at three dose levels in adults, followed by assessment at two dose levels in children. In part B of the trial, children were randomly assigned, in a 1:1:1 ratio, to receive 150 mg of L9LS, 300 mg of L9LS, or placebo. The primary efficacy end point, assessed in a time-to-event analysis, was the first P. falciparum infection, as detected on blood smear performed at least every 2 weeks for 24 weeks. A secondary efficacy end point was the first episode of clinical malaria, as assessed in a time-to-event analysis.

RESULTS

No safety concerns were identified in the dose-escalation part of the trial (part A). In part B, 225 children underwent randomization, with 75 children assigned to each group. No safety concerns were identified in part B. P. falciparum infection occurred in 36 participants (48%) in the 150-mg group, in 30 (40%) in the 300-mg group, and in 61 (81%) in the placebo group. The efficacy of L9LS against P. falciparum infection, as compared with placebo, was 66% (adjusted confidence interval [95% CI], 45 to 79) with the 150-mg dose and 70% (adjusted 95% CI, 50 to 82) with the 300-mg dose (P<0.001 for both comparisons). Efficacy against clinical malaria was 67% (adjusted 95% CI, 39 to 82) with the 150-mg dose and 77% (adjusted 95% CI, 55 to 89) with the 300-mg dose (P<0.001 for both comparisons).

CONCLUSIONS

Subcutaneous administration of L9LS to children was protective against P. falciparum infection and clinical malaria over a period of 6 months. (Funded by the National Institute of Allergy and Infectious Diseases; ClinicalTrials.gov number, NCT05304611.).

A mathematical model for mapping the insecticide resistance trend in the Anopheles gambiae mosquito population under climate variability in Africa

The control of arthropod disease vectors using chemical insecticides is vital in combating malaria, however the increasing insecticide resistance (IR) poses a challenge. Furthermore, climate variability affects mosquito population dynamics and subsequently IR propagation. We present a mathematical model to decipher the relationship between IR in Anopheles gambiae populations and climate variability. By adapting the susceptible-infected-resistant (SIR) framework and integrating temperature and rainfall data, our model examines the connection between mosquito dynamics, IR, and climate. Model validation using field data achieved 92% accuracy, and the sensitivity of model parameters on the transmission potential of IR was elucidated (e.g. μPRCC = 0.85958, p-value < 0.001). In this study, the integration of high-resolution covariates with the SIR model had a significant impact on the spatial and temporal variation of IR among mosquito populations across Africa. Importantly, we demonstrated a clear association between climatic variability and increased IR (width = [0-3.78], α = 0.05). Regions with high IR variability, such as western Africa, also had high malaria incidences thereby corroborating the World Health Organization Malaria Report 2021. More importantly, this study seeks to bolster global malaria combat strategies by highlighting potential IR 'hotspots' for targeted intervention by National malria control programmes.

Pyrethroid resistance and gene expression profile of a new resistant An. gambiae colony from Uganda reveals multiple resistance mechanisms and overexpression of Glutathione-S-Transferases linked to survival of PBO-pyrethroid combination

Background

The effectiveness of long-lasting insecticidal nets (LLINs) are being threatened by growing resistance to pyrethroids. To restore their efficacy, a synergist, piperonyl butoxide (PBO) which inhibits cytochrome P450s has been incorporated into pyrethroid treated nets. A trial of PBO-LLINs was conducted in Uganda from 2017 and we attempted to characterize mechanisms of resistance that could impact intervention efficacy.

Methods

We established an Anopheles gambiae s.s colony in 2018 using female mosquitoes collected from Busia district in eastern Uganda. We first assessed the phenotypic resistance profile of this colony using WHO tube and net assays using a deltamethrin dose-response approach. The Busia colony was screened for known resistance markers and RT-qPCR targeting 15 genes previously associated with insecticide resistance was performed.

Results

The Busia colony had very high resistance to deltamethrin, permethrin and DDT. In addition, the colony had moderate resistance to alpha-cypermethrin and lambda-cyhalothrin but were fully susceptible to bendiocarb and fenitrothion. Exposure to PBO in combination with permethrin and deltamethrin resulted in higher mortality rates in both net and tube assays, with a higher mortality observed in net assays than tube assays. The kdr marker, Vgsc-995S was at very high frequency (91.7-98.9%) whilst the metabolic markers Coeae1d and Cyp4j5-L43F were at very low (1.3% - 11.5%) and moderate (39.5% - 44.7%) frequencies respectively. Our analysis showed that gene expression pattern in mosquitoes exposed to deltamethrin, permethrin or DDT only were similar in comparison to the susceptible strain and there was significant overexpression of cytochrome P450s, glutathione-s-transferases (GSTs) and carboxyl esterases (COEs). However, mosquitoes exposed to both PBO and pyrethroid strikingly and significantly only overexpressed closely related GSTs compared to unexposed mosquitoes while major cytochrome P450s were underexpressed.

Conclusions

The high levels of pyrethroid resistance observed in Busia appears associated with a wide range of metabolic gene families.

Transcriptomic analysis of Anopheles gambiae from Benin reveals overexpression of salivary and cuticular proteins associated with cross-resistance to pyrethroids and organophosphates

BACKGROUND

Insecticide resistance (IR) is one of the major threats to malaria vector control programs in endemic countries. However, the mechanisms underlying IR are poorly understood. Thus, investigating gene expression patterns related to IR can offer important insights into the molecular basis of IR in mosquitoes. In this study, RNA-Seq was used to characterize gene expression in Anopheles gambiae surviving exposure to pyrethroids (deltamethrin, alphacypermethrin) and an organophosphate (pirimiphos-methyl).

RESULTS

Larvae of An. gambiae s.s. collected from Bassila and Djougou in Benin were reared to adulthood and phenotyped for IR using a modified CDC intensity bottle bioassay. The results showed that mosquitoes from Djougou were more resistant to pyrethroids (5X deltamethrin: 51.7% mortality; 2X alphacypermethrin: 47.4%) than Bassila (1X deltamethrin: 70.7%; 1X alphacypermethrin: 77.7%), while the latter were more resistant to pirimiphos-methyl (1.5X: 48.3% in Bassila and 1X: 21.5% in Djougou). RNA-seq was then conducted on resistant mosquitoes, non-exposed mosquitoes from the same locations and the laboratory-susceptible An. gambiae s.s. Kisumu strain. The results showed overexpression of detoxification genes, including cytochrome P450s (CYP12F2, CYP12F3, CYP4H15, CYP4H17, CYP6Z3, CYP9K1, CYP4G16, and CYP4D17), carboxylesterase genes (COEJHE5E, COE22933) and glutathione S-transferases (GSTE2 and GSTMS3) in all three resistant mosquito groups analyzed. Genes encoding cuticular proteins (CPR130, CPR10, CPR15, CPR16, CPR127, CPAP3-C, CPAP3-B, and CPR76) were also overexpressed in all the resistant groups, indicating their potential role in cross resistance in An. gambiae. Salivary gland protein genes related to 'salivary cysteine-rich peptide' and 'salivary secreted mucin 3' were also over-expressed and shared across all resistant groups.

CONCLUSION

Our results suggest that in addition to metabolic enzymes, cuticular and salivary gland proteins could play an important role in cross-resistance to multiple classes of insecticides in Benin. These genes warrant further investigation to validate their functional role in An. gambiae resistance to insecticides.

Insights and challenges of insecticide resistance modelling in malaria vectors: a review

BACKGROUND

Malaria is one of the most devastating tropical diseases, resulting in loss of lives each year, especially in children under the age of 5 years. Malaria burden, related deaths and stall in the progress against malaria transmission is evident, particularly in countries that have moderate or high malaria transmission. Hence, mitigating malaria spread requires information on the distribution of vectors and the drivers of insecticide resistance (IR). However, owing to the impracticality in establishing the critical need for real-world information at every location, modelling provides an informed best guess for such information. Therefore, this review examines the various methodologies used to model spatial, temporal and spatio-temporal patterns of IR within populations of malaria vectors, incorporating pest-biology parameters, adopted ecological principles, and the associated modelling challenges.

METHODS

The review focused on the period ending March 2023 without imposing restrictions on the initial year of publication, and included articles sourced from PubMed, Web of Science, and Scopus. It was also limited to publications that deal with modelling of IR distribution across spatial and temporal dimensions and excluded articles solely focusing on insecticide susceptibility tests or articles not published in English. After rigorous selection, 33 articles met the review's elibility criteria and were subjected to full-text screening.

RESULTS

Results show the popularity of Bayesian geostatistical approaches, and logistic and static models, with limited adoption of dynamic modelling approaches for spatial and temporal IR modelling. Furthermore, our review identifies the availability of surveillance data and scarcity of comprehensive information on the potential drivers of IR as major impediments to developing holistic models of IR evolution.

CONCLUSIONS

The review notes that incorporating pest-biology parameters, and ecological principles into IR models, in tandem with fundamental ecological concepts, potentially offers crucial insights into the evolution of IR. The results extend our knowledge of IR models that provide potentially accurate results, which can be translated into policy recommendations to combat the challenge of IR in malaria control.

Copy number variants underlie the major selective sweeps in insecticide resistance genes in Anopheles arabiensis from Tanzania

To keep ahead of the evolution of resistance to insecticides in mosquitoes, national malaria control programmes must make use of a range of insecticides, both old and new, while monitoring resistance mechanisms. Knowledge of the mechanisms of resistance remains limited in Anopheles arabiensis, which in many parts of Africa is of increasing importance because it is apparently less susceptible to many indoor control interventions. Furthermore, comparatively little is known in general about resistance to non-pyrethroid insecticides such as pirimiphos-methyl (PM), which are crucial for effective control in the context of resistance to pyrethroids. We performed a genome-wide association study to determine the molecular mechanisms of resistance to deltamethrin (commonly used in bednets) and PM, in An. arabiensis from two regions in Tanzania. Genomic regions of positive selection in these populations were largely driven by copy number variants (CNVs) in gene families involved in resistance to these two insecticides. We found evidence of a new gene cluster involved in resistance to PM, identifying a strong selective sweep tied to a CNV in the Coeae2g-Coeae6g cluster of carboxylesterase genes. Using complementary data from An. coluzzii in Ghana, we show that copy number at this locus is significantly associated with PM resistance. Similarly, for deltamethrin, resistance was strongly associated with a novel CNV allele in the Cyp6aa / Cyp6p cluster. Against this background of metabolic resistance, target site resistance was very rare or absent for both insecticides. Mutations in the pyrethroid target site Vgsc were at very low frequency in Tanzania, yet combining these samples with three An. arabiensis individuals from West Africa revealed a startling diversity of evolutionary origins of target site resistance, with up to 5 independent origins of Vgsc-995 mutations found within just 8 haplotypes. Thus, despite having been first recorded over 10 years ago, Vgsc resistance mutations in Tanzanian An. arabiensis have remained at stable low frequencies. Overall, our results provide a new copy number marker for monitoring resistance to PM in malaria mosquitoes, and reveal the complex picture of resistance patterns in An. arabiensis.

Video augmentation of the WHO cone assay to quantify mosquito behavioural responses to insecticide-treated nets

BACKGROUND

Insecticide-treated nets (ITNs) using pyrethroids have been the main vector control tools deployed in malaria endemic countries and are responsible for the dramatic reduction in African malaria cases in the early 2000s. The World Health Organization (WHO) cone test was designed to assess the rapid toxicity effects of pyrethroid exposure on mosquito vectors but has yielded no insights beyond 60-min knockdown and 24-h mortality. As dual-active-ingredient (AI) ITNs become more widespread, bioassays that can provide realistic assessment of single- and dual-treated ITNs (i.e. nets with more than one active ingredient) are urgently needed.

METHODS

We present an augmentation of the cone test that enables accurate quantification of vector behavioural responses (specifically movement, spatial and temporal occupancy) to ITNs using video recording and bespoke software that uses background segmentation methods to detect spatial changes in the movement of mosquitoes within the cone. Four strains of Anopheles gambiae sensu lato (s.l.) were exposed to four ITNs (PermaNet 2.0, PermaNet 3.0, Olyset Net, Interceptor G2) and untreated nets in these modified cone tests. Life history data (post-exposure blood-feeding, blood meal weight, longevity) for individual mosquitoes were recorded.

RESULTS

All mosquitoes responded to the presence of ITNs, spending from 1.48 to 3.67 times more time in the upper region of the cone, depending on the ITN type. Of all ITNs, PermaNet 2.0 provoked the smallest change in behavioural response. Activity in the cone influenced observed post-exposure longevity, and in resistant strains exposed to Interceptor G2, the higher the activity, the greater the risk of dying, as long as the proportion of activity at the net surface was less than 50%. All ITNs inhibited blood-feeding, and smaller blood meals were taken when mosquitoes fed.

CONCLUSIONS

The additional mosquito behaviour data obtained by using this modification to the WHO cone test provides unique insight into the innate responses of different mosquito strains on untreated nets and the entomological mode of action of ITNs, important evidence when evaluating ITN characteristics.

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.

Current and future opportunities of autodissemination of pyriproxyfen approach for malaria vector control in urban and rural Africa

Despite the progress made in reducing malaria burden, new ways to address the increasing challenges of insecticide resistance and the invasion and spread of exotic malaria vectors such as Anopheles stephensi in Africa are urgently needed. While African countries are adopting larviciding as a complementary intervention for malaria vector control, the autodissemination technology has the potential to overcome barriers associated with the identification and treatment of prolific habitats that impede conventional larviciding approaches in rural settings. The autodissemination technology as a "lure and release" strategy works by exploiting the resting behavior of gravid mosquitoes to transfer lethal concentration of biological or chemical insecticide such as pyriproxyfen (PPF), an insect growth regulator (IGRs) to their oviposition sites and result in adult emergence inhibition. Despite the evidence of the autodissemination approach to control other mosquito-borne diseases, there is growing and promising evidence for its use in controlling malaria vectors in Africa, which highlights the momentous research that needs to be sustained. This article reviews the evidence for efficacy of the autodissemination approach using PPF and discusses its potential as efficient and affordable complementary malaria vector control intervention in Africa. In the previous studies that were done in controlled semi-field environments, autodissemination with PPF demonstrated its potential in reducing densities of captive population of malaria vectors such as Anopheles gambiae and Anopheles arabiensis. Of importance, empirical evidence and biology-informed mathematical models to demonstrate the utility of the autodissemination approach to control wild populations of malaria vectors under field environment either alone or in combination with other tools are underway. Among others, the key determining factors for future introduction of this approach at scale is having scalable autodissemination devices, optimized PPF formulations, assess its integration/complementarity to existing conventional larviciding, and community perception and acceptance of the autodissemination approach.

Genome-wide association studies reveal novel loci associated with pyrethroid and organophosphate resistance in Anopheles gambiae and Anopheles coluzzii

Resistance to insecticides in Anopheles mosquitoes threatens the effectiveness of malaria control, but the genetics of resistance are only partially understood. We performed a large scale multi-country genome-wide association study of resistance to two widely used insecticides: deltamethrin and pirimiphos-methyl, using sequencing data from An. gambiae and An. coluzzii from ten locations in West Africa. Resistance was highly multi-genic, multi-allelic and variable between populations. While the strongest and most consistent association with deltamethrin resistance came from Cyp6aa1, this was based on several independent copy number variants (CNVs) in An. coluzzii, and on a non-CNV haplotype in An. gambiae. For pirimiphos-methyl, signals included Ace1, cytochrome P450s, glutathione S-transferases and the nAChR target site of neonicotinoid insecticides. The regions around Cyp9k1 and the Tep family of immune genes showed evidence of cross-resistance to both insecticides. These locally-varying, multi-allelic patterns highlight the challenges involved in genomic monitoring of resistance, and may form the basis for improved surveillance methods.

Effectiveness of dual active ingredient insecticide-treated nets in preventing malaria: A systematic review and meta-analysis

Malaria vectors have demonstrated resistance to pyrethroid-based insecticides used in insecticide-treated nets, diminishing their effectiveness. This systematic review and meta-analysis investigated two forms of dual active-ingredient (DAI) insecticide-treated nets (ITN(s)) for malaria prevention. A comprehensive search was conducted on July 6th 2022. The databases searched included PubMed, Embase, CINAHL, amongst others. Trials were eligible if they were conducted in a region with ongoing malaria transmission. The first DAI ITN investigated were those that combined a pyrethroid with a non-pyrethroid insecticides. The second DAI ITN investigated were that combined a pyrethroid with an insect growth regulator. These interventions were compared against either a pyrethroid-only ITN, or ITNs treated with pyrethroid and piperonyl-butoxide. Assessment of risk of bias was conducted in duplicate using the Cochrane risk of bias 2 tool for cluster-randomised trials. Summary data was extracted using a custom data-extraction instrument. This was conducted by authors THB, JCS and SH. Malaria case incidence was the primary outcome and has been meta-analysed, adverse events were narratively synthesised. The review protocol is registered on PROSPERO (CRD42022333044). From 9494 records, 48 reports were screened and 13 reports for three studies were included. These studies contained data from 186 clusters and all reported a low risk of bias. Compared to pyrethroid-only ITNs, clusters that received pyrethroid-non-pyrethroid DAI ITNs were associated with 305 fewer cases per 1000-person years (from 380 fewer cases to 216 fewer cases) (IRR = 0.55, 95%CI: 0.44-0.68). However, this trend was not observed in clusters that received pyrethroid-insect growth regulator ITNs compared to pyrethroid-only ITNs (from 280 fewer cases to 135 more) (IRR = 0.90, 95%CI: 0.73-1.13). Pyrethroid-non-pyrethroid DAI ITNs demonstrated consistent reductions in malaria case incidence and other outcomes across multiple comparisons. Pyrethroid-non-pyrethroid DAI ITNs may present a novel intervention for the control of malaria.

Modeling the Impact of a Highly Potent Plasmodium falciparum Transmission-Blocking Monoclonal Antibody in Areas of Seasonal Malaria Transmission

Transmission-blocking interventions can play an important role in combating malaria worldwide. Recently, a highly potent Plasmodium falciparum transmission-blocking monoclonal antibody (TB31F) was demonstrated to be safe and efficacious in malaria-naive volunteers. Here we predict the potential public health impact of large-scale implementation of TB31F alongside existing interventions. We developed a pharmaco-epidemiological model, tailored to 2 settings of differing transmission intensity with already established insecticide-treated nets and seasonal malaria chemoprevention interventions. Community-wide annual administration (at 80% coverage) of TB31F over a 3-year period was predicted to reduce clinical incidence by 54% (381 cases averted per 1000 people per year) in a high-transmission seasonal setting, and 74% (157 cases averted per 1000 people per year) in a low-transmission seasonal setting. Targeting school-aged children gave the largest reduction in terms of cases averted per dose. An annual administration of the transmission-blocking monoclonal antibody TB31F may be an effective intervention against malaria in seasonal malaria settings.

Recombinant expression and characterization of GSTd3 from a resistant population of Anopheles arabiensis and comparison of DDTase activity with GSTe2

The development of insecticide resistance in malaria vectors is a challenge for the global effort to control and eradicate malaria. Glutathione S-transferases (GSTs) are multifunctional enzymes involved in the detoxification of many classes of insecticides. For mosquitoes, it is known that overexpression of an epsilon GST, GSTe2, confers resistance towards DDT and pyrethroids. In addition to GSTe2, consistent overexpression of a delta class GST, GSTd3, has been observed in insecticide resistant populations of different malaria vector species. However, the functional role of GSTd3 towards DDT resistance has not yet been investigated. Here, we recombinantly expressed both GSTe2 and GSTd3 from Anopheles arabiensis and compared their metabolic activities against DDT. Both AaGSTd3 and AaGSTe2 exhibited CDNB-conjugating and glutathione peroxidase activity and DDT metabolism was observed for both GSTs. However, the DDT dehydrochlorinase activity exhibited by AaGSTe2 was much higher than for AaGSTd3, and AaGSTe2 was also able to eliminate DDE although the metabolite could not be identified. Molecular modeling revealed subtle differences in the binding pocket of both enzymes and a better fit of DDT within the H-site of AaGSTe2. The overexpression but much lower DDT metabolic activity of AaGSTd3, might suggest that AaGSTd3 sequesters DDT. These findings highlight the complexity of insecticide resistance in the major malaria vectors and the difficulties associated with control of the vectors using DDT, which is still used for indoor residual spraying.

Application of mathematical modelling to inform national malaria intervention planning in Nigeria

BACKGROUND

For their 2021-2025 National Malaria Strategic Plan (NMSP), Nigeria's National Malaria Elimination Programme (NMEP), in partnership with the World Health Organization (WHO), developed a targeted approach to intervention deployment at the local government area (LGA) level as part of the High Burden to High Impact response. Mathematical models of malaria transmission were used to predict the impact of proposed intervention strategies on malaria burden.

METHODS

An agent-based model of Plasmodium falciparum transmission was used to simulate malaria morbidity and mortality in Nigeria's 774 LGAs under four possible intervention strategies from 2020 to 2030. The scenarios represented the previously implemented plan (business-as-usual), the NMSP at an 80% or higher coverage level and two prioritized plans according to the resources available to Nigeria. LGAs were clustered into 22 epidemiological archetypes using monthly rainfall, temperature suitability index, vector abundance, pre-2010 parasite prevalence, and pre-2010 vector control coverage. Routine incidence data were used to parameterize seasonality in each archetype. Each LGA's baseline malaria transmission intensity was calibrated to parasite prevalence in children under the age of five years measured in the 2010 Malaria Indicator Survey (MIS). Intervention coverage in the 2010-2019 period was obtained from the Demographic and Health Survey, MIS, the NMEP, and post-campaign surveys.

RESULTS

Pursuing a business-as-usual strategy was projected to result in a 5% and 9% increase in malaria incidence in 2025 and 2030 compared with 2020, while deaths were projected to remain unchanged by 2030. The greatest intervention impact was associated with the NMSP scenario with 80% or greater coverage of standard interventions coupled with intermittent preventive treatment in infants and extension of seasonal malaria chemoprevention (SMC) to 404 LGAs, compared to 80 LGAs in 2019. The budget-prioritized scenario with SMC expansion to 310 LGAs, high bed net coverage with new formulations, and increase in effective case management rate at the same pace as historical levels was adopted as an adequate alternative for the resources available.

CONCLUSIONS

Dynamical models can be applied for relative assessment of the impact of intervention scenarios but improved subnational data collection systems are required to allow increased confidence in predictions at sub-national level.

Impacts of dual active-ingredient bed nets on the behavioural responses of pyrethroid resistant Anopheles gambiae determined by room-scale infrared video tracking

BACKGROUND

The success of insecticide treated bed nets (ITNs) for malaria vector control in Africa relies on the behaviour of various species of Anopheles. Previous research has described mosquito behavioural alterations resulting from widespread ITN coverage, which could result in a decrease in net efficacy. Here, behaviours were compared including timings of net contact, willingness to refeed and longevity post-exposure to two next-generation nets, PermaNet^® 3.0 (P3 net) and Interceptor^® G2 (IG2 net) in comparison with a standard pyrethroid-only net (Olyset Net™ (OL net)) and an untreated net.

METHODS

Susceptible and resistant Anopheles gambiae mosquitoes were exposed to the nets with a human volunteer host in a room-scale assay. Mosquito movements were tracked for 2 h using an infrared video system, collecting flight trajectory, spatial position and net contact data. Post-assay, mosquitoes were monitored for a range of sublethal insecticide effects.

RESULTS

Mosquito net contact was focused predominantly on the roof for all four bed nets. A steep decay in activity was observed for both susceptible strains when P3 net and OL net were present and with IG2 net for one of the two susceptible strains. Total mosquito activity was higher around untreated nets than ITNs. There was no difference in total activity, the number, or duration, of net contact, between any mosquito strain, with similar behaviours recorded in susceptible and resistant strains at all ITNs. OL net, P3 net and IG2 net all killed over 90% of susceptible mosquitoes 24 h after exposure, but this effect was not seen with resistant mosquitoes where mortality ranged from 16 to 72%. All treated nets reduced the willingness of resistant strains to re-feed when offered blood 1-h post-exposure, with a more pronounced effect seen with P3 net and OL net than IG2 net.

CONCLUSION

These are the first results to provide an in-depth description of the behaviour of susceptible and resistant Anopheles gambiae strains around next-generation bed nets using a room-scale tracking system to capture multiple behaviours. These results indicate that there is no major difference in behavioural responses between mosquito strains of differing pyrethroid susceptibility when exposed to these new ITNs under the experimental conditions used.

Insecticide resistance management strategies for public health control of mosquitoes exhibiting polygenic resistance: A comparison of sequences, rotations, and mixtures

Malaria control uses insecticides to kill Anopheles mosquitoes. Recent successes in malaria control are threatened by increasing levels of insecticide resistance (IR), requiring insecticide resistance management (IRM) strategies to mitigate this problem. Field trials of IRM strategies are usually prohibitively expensive with long timeframes, and mathematical modeling is often used to evaluate alternative options. Previous IRM models in the context of malaria control assumed IR to have a simple (monogenic) basis, whereas in natural populations, IR will often be a complex polygenic trait determined by multiple genetic variants. A quantitative genetics model was developed to model IR as a polygenic trait. The model allows insecticides to be deployed as sequences (continuous deployment until a defined withdrawal threshold, termed "insecticide lifespan", as indicated by resistance diagnosis in bioassays), rotations (periodic switching of insecticides), or full-dose mixtures (two insecticides in one formulation). These IRM strategies were compared based on their "strategy lifespan" (capped at 500 generations). Partial rank correlation and generalized linear modeling was used to identify and quantify parameters driving the evolution of resistance. Random forest models were used to identify parameters offering predictive value for decision-making. Deploying single insecticides as sequences or rotations usually made little overall difference to their "strategy lifespan", though rotations displayed lower mean and peak resistances. Deploying two insecticides in a full-dose mixture formulation was found to extend the "strategy lifespan" when compared to deploying each in sequence or rotation. This pattern was observed regardless of the level of cross resistance between the insecticides or the starting level of resistance. Statistical analysis highlighted the importance of insecticide coverage, cross resistance, heritability, and fitness costs for selecting an appropriate IRM strategy. Full-dose mixtures appear the most promising of the strategies evaluated, with the longest "strategy lifespans". These conclusions broadly corroborate previous results from monogenic models.

Current and future opportunities of autodissemination of pyriproxyfen approach for malaria vector control in urban and rural Africa

Despite the progress made in reducing malaria burden, new ways to address the increasing challenges of insecticide resistance and the invasion and spread of exotic malaria vectors such as Anopheles stephensi in Africa are urgently needed. While African countries are adopting larviciding as a complementary intervention for malaria vector control, the autodissemination technology has the potential to overcome barriers associated with the identification and treatment of prolific habitats that impede conventional larviciding approaches in rural settings. The autodissemination technology as a “lure and release” strategy works by exploiting the resting behavior of gravid mosquitoes to transfer lethal concentration of biological or chemical insecticide such as pyriproxyfen (PPF), an insect growth regulator (IGRs) to their oviposition sites and result in adult emergence. Despite the evidence of the autodissemination approach to control other mosquito-borne diseases, there is growing and promising evidence for its use in controlling malaria vectors in Africa, which highlights the momentous research that needs to be sustained. This article reviews the evidence for efficacy of the autodissemination approach using PPF and discusses its potential as efficient and affordable complementary malaria vector control intervention in Africa. In the previous studies that were done in controlled semi-field environments, autodissemination with PPF demonstrated its potential in reducing densities of captive population of malaria vectors such as Anopheles gambiae and Anopheles arabiensis. Of importance, empirical evidence and biology-informed mathematical models to demonstrate the utility of the autodissemination approach to control wild populations of malaria vectors under field environment either alone or in combination with other tools are underway. Among others, the key determining factors for future introduction of this approach at scale is having scalable autodissemination devices, optimized PPF formulations, assess its integration/complementarity to existing conventional larviciding, and community perception and acceptance of the autodissemination approach.

Fine-scale spatial distribution of deltamethrin resistance and population structure of Anopheles funestus and Anopheles arabiensis populations in Southern Mozambique

BACKGROUND

Insecticide resistance in malaria vectors can be spatially highly heterogeneous, yet population structure analyses frequently find relatively high levels of gene flow among mosquito populations. Few studies have contemporaneously assessed phenotypic, genotypic and population structure analysis on mosquito populations and none at fine geographical scales. In this study, genetic diversity, population structure, and insecticide resistance profiles of Anopheles funestus and Anopheles arabiensis were examined across mosquito populations from and within neighbouring villages.

METHODS

Mosquitoes were collected from 11 towns in southern Mozambique, as well as from different neighbourhoods within the town of Palmeira, during the peak malaria transmission season in 2016. CDC bottle bioassay and PCR assays were performed with Anopheles mosquitoes at each site to determine phenotypic and molecular insecticide resistance profiles, respectively. Microsatellite analysis was conducted on a subsample of mosquitoes to estimate genetic diversity and population structure.

RESULTS

Phenotypic insecticide resistance to deltamethrin was observed in An. funestus sensu stricto (s.s.) throughout the area, though a high level of mortality variation was seen. However, 98% of An. funestus s.s. were CYP6P9a homozygous resistant. An. arabiensis was phenotypically susceptible to deltamethrin and 99% were kdr homozygous susceptible. Both Anopheles species exhibited high allelic richness and heterozygosity. Significant deviations from Hardy-Weinberg equilibrium were observed, and high linkage disequilibrium was seen for An. funestus s.s., supporting population subdivision. However, the FST values were low for both anophelines (- 0.00457 to 0.04213), Nm values were high (9.4-71.8 migrants per generation), AMOVA results showed almost 100% genetic variation among and within individuals, and Structure analysis showed no clustering of An. funestus s.s. and An. arabiensis populations. These results suggest high gene flow among mosquito populations.

CONCLUSION

Despite a relatively high level of phenotypic variation in the An. funestus population, molecular analysis shows the population is admixed. These data indicate that CYP6P9a resistance markers do not capture all phenotypic variation in the area, but also that resistance genes of high impact are likely to easily spread in the area. Conversely, other strategies, such as transgenic mosquito release programmes will likely not face challenges in this locality.

Widespread occurrence of copy number variants and fixation of pyrethroid target site resistance in Anopheles gambiae (s.l.) from southern Côte d'Ivoire

Resistance to pyrethroid and organophosphate insecticides in the malaria vector Anopheles gambiae (s.l.) is conferred by a variety of genetic mutations, including single nucleotide polymorphisms (SNPs) and copy number variants (CNVs). Knowledge of the distribution of these mutations in mosquito populations is a prerequisite for establishing better strategies for their management. In this study, a total of 755 Anopheles gambiae (s.l.) from southern Côte d'Ivoire were exposed to deltamethrin or pirimiphos-methyl insecticides and were screened to assess the distribution of SNPs and CNVs known or believed to confer resistance to one or other of the insecticide classes. Most individuals from the An. gambiae (s.l.) complex were identified by molecular tests as Anopheles coluzzii. Survival to deltamethrin (from 94% to 97%) was higher than to pirimiphos-methyl (from 10% to 49%). In An. gambiae (s.s.), the SNP in the Voltage Gated Sodium Channel (Vgsc) at the 995F locus (Vgsc-995F) was fixed, while other target site mutations were rare or absent (Vgsc-402L: 0%; Vgsc-1570Y: 0%, Acetylcholinesterase Acel-280S: 14%). In An. coluzzii, Vgsc-995F was the target site SNP found at highest frequency (65%) followed by other target site mutations (Vgsc-402L: 36%; Vgsc-1570Y: 0.33%; Acel-280S: 45%). The Vgsc-995S SNP was not present. The presence of the Ace1-280S SNP was found to be significantly linked to the presence of the Ace1-CNV, Ace1_AgDup. Significant association was found between the presence of the Ace1_AgDup and pirimiphos-methyl resistance in An. gambiae (s.s.) but not in An. coluzzii. The deletion Ace1_Del97 was found in one specimen of An. gambiae (s.s.). Four CNVs in the Cyp6aa/Cyp6p gene cluster, which contains genes of known importance for resistance, were detected in An. coluzzii, the most frequent being Dup 7 (42%) and Dup 14 (26%). While none of these individual CNV alleles were significantly associated with resistance, copy number in the Cyp6aa gene region in general was associated with increased resistance to deltamethrin. Elevated expression of Cyp6p3 was nearly associated with deltamethrin resistance, although there was no association of resistance with copy number. Use of alternative insecticides and control methods to arrest resistance spread in An. coluzzii populations is merited.

Quantifying the direct and indirect protection provided by insecticide treated bed nets against malaria

Long lasting insecticidal nets (LLINs) provide both direct and indirect protection against malaria. As pyrethroid resistance evolves in mosquito vectors, it will be useful to understand how the specific benefits LLINs afford individuals and communities may be affected. Here we use modelling to show that there is no minimum LLIN usage needed for users and non-users to benefit from community protection. Modelling results also indicate that pyrethroid resistance in local mosquitoes will likely diminish the direct and indirect benefits from insecticides, leaving the barrier effects intact, but LLINs are still expected to provide enhanced benefit over untreated nets even at high levels of pyrethroid resistance.

Optimisation of laboratory-rearing parameters for Anopheles funestus larvae and adults

Anopheles funestus is one of the major malaria vectors in Africa. As with the other main vectors, insecticide resistance in this species threatens existing vector control strategies. Unfortunately, scientific investigations, which could improve understanding of this vector species or lead to the development of new control strategies, are currently limited by difficulties in laboratory rearing of the species. In an attempt to optimise laboratory-rearing conditions for An. funestus, the effect of an artificial blood-feeding system for adults, different larval diet doses, and a range of other rearing conditions on the life history traits of an existing colony were investigated. Firstly, fecundity and fertility in An. funestus adult females fed on either live guinea pigs or bovine blood supplied through an artificial membrane feeding system were assessed. Secondly, a life-table approach was used to assess the impact of larval food dose (mg/larvae), larval density (larvae/cm²), and the depth of water used for larval rearing on life history traits. Fecundity was significantly higher when females were blood-fed on live anaesthetised guinea pigs than when fed on defibrinated bovine blood. However, the fertility of these eggs did not differ significantly between the two feeding methods or blood meal sources. Mosquitoes fed on defibrinated bovine blood using the artificial membrane feeding system showed an increase in egg production when the blood-feeding frequency was increased, but this difference was not statistically significant. The quantity of larval food influenced both time-to-pupation and pupal production. Increasing the larval densities resulted in reduced both time-to-pupation and pupal productivity. An optimal larval density of 0.48 larvae/cm² was vital in preventing overcrowding. Increased water depth in the larval trays, was associated with significantly lower pupal production and reduced pupal weight. In conclusion, these results show that An. funestus can be reared using defibrinated bovine blood delivered via an artificial membrane feeding system. The quantity of larval food, optimal larval density, and depth of water used for larval rearing are critical factors influencing colony productivity. These findings can be used to improve current guidelines for rearing An. funestus under insectary conditions.

Measuring entomological parameters before implementing a study on asymptomatic carriers of Plasmodium falciparum in the Zè District in southern Benin

BACKGROUND

The objective of this study was to estimate malaria transmission and insecticide resistance status in malaria vectors in Adjrako village from Zè District in Southern Benin. The present study was carried out prior to investigations on infectivity of blood from asymptomatic carriers of Plasmodium falciparum to malaria vector mosquitoes.

METHODS

Human landing collections (HLCs) were performed in Adjrako village during the rainy season (September-November 2021). In this village, host-seeking mosquitoes were collected during three nights per survey from 22:00 to 06:00 in six randomly selected houses. Malaria vectors were dissected in orders to determinate their parity. Plasmodium falciparum infection in malaria vectors was determined by qPCR and the entomological inoculation rate (EIR) was calculated. The World Health Organization (WHO) insecticide susceptibility test-kits were used to evaluate the susceptibility of Anopheles gambiae sensu lato (s.l.) to deltamethrin at 0.05% and bendiocarb at 0.1%.

RESULTS

A total of 3260 females of mosquitoes belonging to 4 genera (Anopheles, Culex, Aedes and Mansonia) were collected. Most of the mosquitoes collected were An. gambiae sensu lato (s.l.). The entomological inoculation rate (EIR) for the three collection months was 8.7 infective bites per person and the parity rate was 84%. Mortality rates of An. gambiae s.l. exposed to 0.05% deltamethrin and 0.1% bendiocarb were 18% and 96%, respectively, indicating that this vector population was resistant to deltamethrin and possibly resistant to bendiocarb in the study area.

CONCLUSION

This study showed that malaria transmission is effective in the study area and that An. gambiae s.l. is the main malaria vector. The entomological parameters indicate this study area is potentially favourable for investigations on P. falciparum asymptomatic carriers.