Increased pyrethroid resistance in malaria vectors and decreased bed net effectiveness, Burkina Faso

Physical and insecticidal durability of Interceptor®, Interceptor® G2, and PermaNet® 3.0 insecticide-treated nets in Burkina Faso: results of durability monitoring in three sites from 2019 to 2022

BACKGROUND

National Malaria Programmes (NMPs) monitor the durability of insecticide-treated nets (ITNs) to inform procurement and replacement decisions. This is crucial for new dual active ingredients (AI) ITNs, for which less data is available. Pyrethroid-only ITN (Interceptor®) and dual AI (Interceptor® G2, and PermaNet® 3.0) ITNs were assessed across three health districts over 36 months in southern Burkina Faso to estimate median ITN survival, insecticidal efficacy, and to identify factors contributing to field ITN longevity.

METHODS

Durability was monitored through a prospective study of a cohort of nets distributed during the 2019 mass campaign. Three health districts were selected for their similar pyrethroid-resistance, environmental, epidemiological, and population profiles. Households were recruited after the mass campaign, with annual household questionnaire follow-ups over three years. Each round, ITNs were withdrawn for bioassays and chemical residue testing. Key measures were the percentage of cohort ITNs in serviceable condition, insecticidal effectiveness, and chemical residue content against target dose. Cox proportional hazard models were used to identify determinants influencing ITN survival.

RESULTS

At endline, the median useful life was 3.2 (95% CI 2.5-4.0) years for PermaNet® 3.0 ITNs in Orodara, 2.6 (95% CI 1.9-3.2) years for Interceptor® G2 ITNs in Banfora and 2.4 (95% CI 1.9-2.9) years for Interceptor® ITNs in Gaoua. Factors associated with ITN survival included cohort ITNs from Orodara (adjusted hazard ratio (aHR) = 0.58, p = 0.026), households seeing less rodents (aHR = 0.66, p = 0.005), female-headed households (aHR = 0.66, p = 0.044), exposure to social behavior change (SBC) messages (aHR = 0.52, ≤ 0.001) and folding nets when not in use (aHR = 0.47, p < 0.001). At endline, PermaNet® 3.0 ITN recorded 24-h mortality of 26% against resistant mosquitos on roof panels, with an 84% reduction in PBO content. Interceptor® G2 ITN 72-h mortality was 51%, with a 67% reduction in chlorfenapyr content. Interceptor® ITN 24-h mortality was 71%, with an 84% reduction in alpha-cypermethrin content.

CONCLUSION

Only PermaNet® 3.0 ITNs surpassed the standard three-year survival threshold. Identified protective factors should inform SBC messaging. Significant decreases in chemical content and resulting impact on bioefficacy warrant more research in other countries to better understand dual AI ITN insecticidal performance.

Level of awareness and utilization of insecticide-treated bed nets among medical students as measures for reducing malaria episodes

This study examined the level of awareness and utilization of insecticide-treated bed nets among medical students as measures for reducing malaria episodes in Delta State University, Abraka. It was a descriptive study with objectives and research questions formulated to achieve the study design. A sample size of 200 male and female students resident in the campus hostels were selected using random sampling technique. A self-structured questionnaire was designed and administered to the study participants, however, only 148 copies of the questionnaires were successfully retrieved and used for the study. Data generated were subjected to quantitative statistical analysis for frequencies, percentages, average mean and Chi-square testing. Findings revealed that the level of awareness was significantly associated with the role of health workers in the distribution of insecticide-treated bed nets in Delta State University, Abraka, although, factors hindering health workers from distributing insecticide-treated bed nets were identified. There was significant difference between perception of medical students and the utilization of insecticide-treated bed nets on risk of malaria spread. In addition, there was significant difference between the benefits of using insecticide-treated bed nets and the prevention and control of malaria. We therefore conclude that regular utilization of insecticide-treated bed nets due to adequate awareness eliminates contact with mosquitoes and prevents transmitting vectors of malaria from having contact with the users of insecticide-treated bed net. Massive health education campaign is recommended to further scale up the awareness and effective utilization of insecticide-treated bed nets towards prevention and control of malaria bites among students in Delta State University, Abraka.

Video-tracked Anopheles arabiensis entry and exit behaviour at washed and damaged pyrethroid-treated bednets

Insecticide-treated nets (ITNs) are the most effective method for malaria prevention in Africa. Using near-infrared video tracking in a laboratory environment, we recorded and assessed bednet entry and exit by a northern Tanzanian population of Anopheles arabiensis at a human-occupied untreated net and a PermaNet® 2.0 ITN. Both had 12 holes, each 10 cm in diameter, punctured at specific locations, and the ITN was washed 20 times to further simulate the wear and tear of ageing. Washing reduced the insecticide content of ITNs by 61%, which then showed similar rates to the untreated nets for net entry (39% entered untreated net and 41% entered ITN; p = 0.84) and exit (37% and 43%, respectively; p = 0.67). Regardless of treatment, approximately 40% of mosquitoes entered nets within 20 s of first appearing in the field of view and reached the volunteer's skin within 5 s of entering the net. Mortality rates post-exposure were significantly higher (p = 0.048) at ITNs (26.6%; 95% CI 13.4%-39.7%) than at untreated controls (6.4%; 95% CI 1.8%-14.6%). The washed and aged ITN provided little additional personal protection for the sleeper over an untreated net. Simple adjustments to materials and design that could extend the effective lifespan of ITNs are discussed.

Anopheles gambiae s.s. resistance to pyrethroids and DDT in semi-urban and rural areas of the Moyen-Ogooué Province, Gabon

BACKGROUND

Pyrethroids are the main insecticides used in vector control for malaria. However, their extensive use in the impregnation of long-lasting insecticidal nets (LLINs) and indoor residual spraying has led to the development of resistance, threatening its success as a tool for malaria control. Baseline data prior to large scale distribution of LLINs are important for the implementation of efficient strategies. However, no data on the susceptibility of malaria vectors is available in the Moyen-Ogooué Province in Gabon. The aim of this study was to assess the susceptibility to pyrethroids and organochlorides of malaria vectors from a semi-urban and rural areas of the province and to determine the frequency of insecticide resistance genes.

METHODS

Larvae were collected from breeding sites in Lambaréné and Zilé and reared to adults. Three to five-day old female Anopheles gambiae sensu lato mosquitoes were used in cone tube assays following the WHO susceptibility tests protocol for adult mosquitoes. A subsample was molecularly identified using the SINE200 protocol and the frequency of Vgsc-1014 F and - 1014 S mutations were determined.

RESULTS

Anopheles gambiae sensu stricto (s.s.) was the sole species present in both Lambaréné and Zilé. Mosquito populations from the two areas were resistant to pyrethroids and organochlorides. Resistance was more pronounced for permethrin and DDT with mortality lower than 7% for both insecticides in the two study areas. Mosquitoes were statistically more resistant (P < 0.0001) to deltamethrin in Lambaréné (51%) compared to Zilé (76%). All the mosquitoes tested were heterozygous or homozygous for the knockdown resistance (Kdr) mutations Vgsc-L1014F and Vgsc-L1014S with a higher proportion of Vgsc-L1014F homozygous in Lambaréné (76.7%) compared to Zilé (57.1%).

CONCLUSION

This study provides evidence of widespread resistance to pyrethroids in An. gambiae s.s., the main malaria vector in the Moyen-Ogooué Province. Further investigation of the mechanisms underlining the resistance of An. gambiae s.s. to pyrethroids is needed to implement appropriate insecticide resistance management strategies.

Impact of Pyrethroid Plus Piperonyl Butoxide Synergist-Treated Nets on Malaria Incidence 24 Months after a National Distribution Campaign in Rwanda

Malaria remains a public health priority in Rwanda. The use of insecticide-treated nets (ITNs) is a key malaria prevention tool. However, expanding pyrethroid resistance threatens the gains made in malaria control. In 2018, the Rwandan malaria program strategic approach included the use of newer types of ITNs such as pyrethroid plus piperonyl butoxide (PBO) synergist-treated nets to counter pyrethroid resistance. In February 2020, 5,892,280 ITNs were distributed countrywide; 1,085,517 of these were PBO nets distributed in five districts. This study was a pragmatic observational study that leveraged the 2020 net distribution and routinely collected confirmed malaria cases to determine the impact of PBO nets 1 and 2 years after ITN distribution. No differences were observed in the average net coverage between the PBO and standard net districts. A significant reduction in malaria incidence was reported in both the PBO (P = 0.019) and two control districts that received standard nets (P = 0.008) 1 year after ITN distribution. However, 2 years after, this reduction was sustained only in the PBO (P = 0.013) and not in the standard net districts (P = 0.685). One year after net distribution, all districts had a significant reduction in malaria incidence rate (incidence rate ratio < 1). In the second year, incidence in districts with PBO nets continued to decrease, whereas in districts with standard nets, incidences were similar to predistribution levels. The results indicate that PBO nets are a promising tool to combat pyrethroid resistance in Rwanda, with protective effects of up to 2 years post distribution.

PermaNet Dual, a new deltamethrin-chlorfenapyr mixture net, shows improved efficacy against pyrethroid-resistant Anopheles gambiae sensu lato in southern Benin

Pyrethroid-chlorfenapyr nets have demonstrated improved entomological and epidemiological impact in trials across Africa. This is driving increased demand for this novel net class in malaria-endemic countries. PermaNet Dual is a new deltamethrin-chlorfenapyr net developed by Vestergaard Sàrl to provide more options to malaria control programmes. We performed an experimental hut trial to evaluate the efficacy of PermaNet Dual against wild, free-flying pyrethroid-resistant Anopheles gambiae sensu lato in Covè, Benin. PermaNet Dual induced superior levels of mosquito mortality compared to a pyrethroid-only net and a pyrethroid-piperonyl butoxide net both when unwashed (77% with PermaNet Dual vs. 23% with PermaNet 2.0 and 56% with PermaNet 3.0, p < 0.001) and after 20 standardised washes (75% with PermaNet Dual vs. 14% with PermaNet 2.0 and 30% with PermaNet 3.0, p < 0.001). Using a provisional non-inferiority margin defined by the World Health Organisation, PermaNet Dual was also non-inferior to a pyrethroid-chlorfenapyr net that has demonstrated improved public health value (Interceptor G2), for vector mortality (79% vs. 76%, OR = 0.878, 95% CIs 0.719-1.073) but not for blood-feeding protection (35% vs. 26%, OR = 1.424, 95% CIs 1.177-1.723). PermaNet Dual presents an additional option of this highly effective net class for improved control of malaria transmitted by pyrethroid-resistant mosquitoes.

Overcoming insecticide resistance in Anopheles mosquitoes by using faster-acting solid forms of deltamethrin

BACKGROUND

Controlling malaria-transmitting Anopheles mosquitoes with pyrethroid insecticides is becoming increasingly challenging because of widespread resistance amongst vector populations. The development of new insecticides and insecticidal formulations is time consuming and costly, however. A more active crystalline form of deltamethrin, prepared by heating the commercial crystalline form, previously was reported to be 12-times faster acting against susceptible North American Anopheles quadrimaculatus mosquitoes. Herein the potential for heat-activated deltamethrin dispersed on chalk to overcome various resistance mechanisms amongst five West African Anopheles strains is investigated, and its long-term sustained lethality evaluated.

METHODS

The more active deltamethrin form was generated in a commercial dust containing deltamethrin by heating the material as purchased. Tarsal contact bioassays were conducted to investigate its efficacy, potency, and speed of action against resistant Anopheles populations compared to the commercially available form of deltamethrin dust.

RESULTS

In all cases, D-Fense Dust heated to generate the more active form of deltamethrin was substantially more effective than the commercially available formulation. 100% of both Banfora M and Kisumu populations were knocked down 10 min post-exposure with no recovery afterwards. Gaoua-ara and Tiefora strains exhibited 100% knockdown within 15 min, and the VK7 2014 strain exhibited 100% knockdown within 20 min. In all cases, 100% mortality was observed 24 h post-exposure. Conversely, the commercial formulation (unheated) resulted in less than 4% mortality amongst VK7 2014, Banfora, and Gaoua-ara populations by 24 h, and Tiefora and Kisumu mosquitoes experienced 14 and 47% mortality by 24 h, respectively. The heat-activated dust maintained comparable efficacy 13 months after heating.

CONCLUSIONS

The heat-activated form of commercial deltamethrin D-Fense Dust outperformed the material as purchased, dramatically increasing efficacy against all tested pyrethroid-resistant strains. This increase in lethality was retained for 13 months of storage under ambient conditions in the laboratory. Higher energy forms of commonly used insecticides may be employed to overcome various resistance mechanisms seen in African Anopheles mosquitoes through more rapid uptake of insecticide molecules from their respective solid surfaces. That is, resistant mosquitoes can be killed with an insecticide to which they are resistant without altering the molecular composition of the insecticide.

Impact of MiraNet® long-lasting insecticidal net against Anopheles arabiensis wild population of Northern Tanzania

Despite high levels of pyrethroid resistance reported in malaria vectors, long-lasting insecticidal nets (LNs) still play a key role in controlling malaria transmission. This study tested the efficacy of MiraNet®, a pyrethroid-based LN against a wild population of Anopheles arabiensis in northern Tanzania. DuraNet® was used as a positive control in this evaluation. Standard WHO laboratory bioefficacy evaluations of MiraNet and DuraNet that were unwashed or had been washed 20 times indicated optimal knockdown and mortality for both net types against a susceptible strain of Anopheles gambiae s.s. Standard experimental hut evaluations were conducted to evaluate the efficacy of both nets against a wild population of An. arabiensis. The killing effect of MiraNet was 54.5% for unwashed and 50% for 20 times washed while DuraNet achieved 44.4% mortality for unwashed and 47.4% for 20 times washed against wild An. arabiensis. Both DuraNet and MiraNet exhibited significantly higher killing effects (> 44.4%). There was no significant difference in deterrence or induced exophily detected between the treatment arms for either net. Additionally, there were no adverse effects reported among hut sleepers. The results of this study indicate that the pyrethroid net MiraNet can be used effectively against wild populations of An. gambiae s.l. of low to moderate resistant levels from Northern Tanzania.

Repeat Ivermectin Mass Drug Administrations for Malaria Control II: Protocol for a Double-blind, Cluster-Randomized, Placebo-Controlled Trial for the Integrated Control of Malaria

BACKGROUND

The gains made against malaria have stagnated since 2015, threatened further by increasing resistance to insecticides and antimalarials. Improvement in malaria control necessitates a multipronged strategy, which includes the development of novel tools. One such tool is mass drug administration (MDA) with endectocides, primarily ivermectin, which has shown promise in reducing malaria transmission through lethal and sublethal impacts on the mosquito vector.

OBJECTIVE

The primary objective of the study is to assess the impact of repeated ivermectin MDA on malaria incidence in children aged ≤10 years.

METHODS

Repeat Ivermectin MDA for Malaria Control II is a double-blind, placebo-controlled, cluster-randomized, and parallel-group trial conducted in a setting with intense seasonal malaria transmission in Southwest Burkina Faso. The study included 14 discrete villages: 7 (50%) randomized to receive standard measures (seasonal malaria chemoprevention [SMC] and bed net use for children aged 3 to 59 months) and placebo, and 7 (50%) randomized to receive standard measures and monthly ivermectin MDA at 300 μg/kg for 3 consecutive days, provided under supervision to all eligible village inhabitants, over 2 successive rainy seasons. Nonpregnant individuals >90 cm in height were eligible for ivermectin MDA, and cotreatment with ivermectin and SMC was not permitted. The primary outcome is malaria incidence in children aged ≤10 years, as assessed by active case surveillance. The secondary safety outcome of repeated ivermectin MDA was assessed through active and passive adverse event monitoring.

RESULTS

The trial intervention was conducted from July to November in 2019 and 2020, with additional sampling of humans and mosquitoes occurring through February 2022 to assess postintervention changes in transmission patterns. Additional human and entomological assessments were performed over the 2 years in a subset of households from 6 cross-sectional villages. A subset of individuals underwent additional sampling in 2020 to characterize ivermectin pharmacokinetics and pharmacodynamics. Analysis and unblinding will commence once the database has been completed, cleaned, and locked.

CONCLUSIONS

Our trial represents the first study to directly assess the impact of a novel approach for malaria control, ivermectin MDA as a mosquitocidal agent, layered into existing standard-of-care interventions. The study was designed to leverage the current SMC deployment infrastructure and will provide evidence regarding the additional benefit of ivermectin MDA in reducing malaria incidence in children.

TRIAL REGISTRATIONS

ClinicalTrials.gov NCT03967054; https://clinicaltrials.gov/ct2/show/NCT03967054 and Pan African Clinical Trials Registry PACT201907479787308; https://pactr.samrc.ac.za/TrialDisplay.aspx?TrialID=8219.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

DERR1-10.2196/41197.

Countrywide insecticide resistance monitoring and first report of the presence of the L1014S knock down resistance in Niger, West Africa

BACKGROUND

Mass distribution of insecticide-treated nets (ITNs) is the principal malaria vector control strategy adopted by Niger. To better inform on the most appropriate ITN to distribute, the National Malaria Control Programme (NMCP) of Niger and its partners, conducted insecticide resistance monitoring in selected sites across the country.

METHODS

The susceptibility of Anopheles gambiae sensu lato (s.l.) to chlorfenapyr and pyrethroid insecticides was investigated in a total of sixteen sites in 2019 and 2020, using 2-5-day-old adults reared from wild collected larvae per site. The susceptibility status, pyrethroid resistance intensity at 5 and 10 times the diagnostic concentrations, and piperonyl butoxide (PBO) synergism with diagnostic concentrations of deltamethrin, permethrin and alpha-cypermethrin were assessed using WHO bioassays. Two doses (100 and 200 µg/bottle) of chlorfenapyr were tested using the CDC bottle assay method. Species composition and allele frequencies for knock-down resistance (kdr-L1014F and L1014S) and acetylcholinesterase (ace-1 G119S) mutations were further characterized using polymerase chain reaction (PCR).

RESULTS

High resistance intensity to all pyrethroids tested was observed in all sites except for alpha-cypermethrin in Gaya and Tessaoua and permethrin in Gaya in 2019 recording moderate resistance intensity. Similarly, Balleyara, Keita and Tillabery yielded moderate resistance intensity for alpha-cypermethrin and deltamethrin, and Niamey V low resistance intensity against deltamethrin and permethrin in 2020. Pre-exposure to PBO substantially increased susceptibility with average increases in mortality between 0 and 70% for tested pyrethroids. Susceptibility to chlorfenapyr (100 µg/bottle) was recorded in all sites except in Tessaoua and Magaria where susceptibility was recorded at the dose of 200 µg/bottle. Anopheles coluzzii was the predominant malaria vector species in most of the sites followed by An. gambiae sensu stricto (s.s.) and Anopheles arabiensis. The kdr-L1014S allele, investigated for the first time, was detected in the country. Both kdr-L1014F (frequencies [0.46-0.81]) and L1014S (frequencies [0.41-0.87]) were present in all sites while the ace-1 G119S was between 0.08 and 0.20.

CONCLUSION

The data collected will guide the NMCP in making evidence-based decisions to better adapt vector control strategies and insecticide resistance management in Niger, starting with mass distribution of new generation ITNs such as interceptor G2 and PBO ITNs.

East Africa International Center of Excellence for Malaria Research: Impact on Malaria Policy in Uganda

Malaria is the leading cause of disease burden in sub-Saharan Africa. In 2010, the East Africa International Center of Excellence for Malaria Research, also known as the Program for Resistance, Immunology, Surveillance, and Modeling of Malaria (PRISM), was established to provide a comprehensive approach to malaria surveillance in Uganda. We instituted cohort studies and a robust malaria and entomological surveillance network at selected public health facilities that have provided a platform for monitoring trends in malaria morbidity and mortality, tracking the impact of malaria control interventions (indoor residual spraying of insecticide [IRS], use of long-lasting insecticidal nets [LLINs], and case management with artemisinin-based combination therapies [ACTs]), as well as monitoring of antimalarial drug and insecticide resistance. PRISM studies have informed Uganda's malaria treatment policies, guided selection of LLINs for national distribution campaigns, and revealed widespread pyrethroid resistance, which led to changes in insecticides delivered through IRS. Our continuous engagement and interaction with policy makers at the Ugandan Ministry of Health have enabled PRISM to share evidence, best practices, and lessons learned with key malaria stakeholders, participate in malaria control program reviews, and contribute to malaria policy and national guidelines. Here, we present an overview of interactions between PRISM team members and Ugandan policy makers to demonstrate how PRISM's research has influenced malaria policy and control in Uganda.

Multiple insecticide resistance and first evidence of V410L kdr mutation in Aedes (Stegomyia) aegypti (Linnaeus) from Burkina Faso

The response to recent dengue outbreaks in Burkina Faso was insecticide-based, despite poor knowledge of the vector population's susceptibility to the insecticides used. Here, we report on the susceptibility to the main insecticide classes and identify important underlying mechanisms in Aedes aegypti populations in Ouagadougou and Banfora, in 2019 and 2020. Wild Ae. aegypti were tested as adults in WHO bioassays and then screened in real time melting curve qPCR analyses to genotype the F1534C, V1016I, and V410L Aedes kdr mutations. Ae. aegypti showed moderate resistance to 0.1% bendiocarb (80-95% survival post-exposure), 0.8% Malathion (60-100%), 0.21% pirimiphos-methyl (75% - 97%), and high resistance to 0.03% deltamethrin (20-70%). PBO pre-exposure partially restored pyrethroid susceptibility. Genotyping detected high frequency of 1534C allele (0.92) and moderate 1016I (0.1-0.32). The V410L mutation was detected in Burkina Faso for the first time (frequency 0.1-0.36). Mosquitoes surviving 4 h exposure to 0.03% deltamethrin had significantly higher frequencies of the F1534C mutation than dead mosquitoes (0.70 vs. 0.96, p < 0.0001) and mosquitoes surviving 2 - 4 h exposure had a significantly reduced life span. Ae. aegypti from Burkina Faso are resistant to multiple insecticide classes with multiple mechanisms involved, demonstrating the essential role of insecticide resistance monitoring within national dengue control programmes.

Vector control: agents of selection on malaria parasites?

Insect vectors are responsible for spreading many infectious diseases, yet interactions between pathogens/parasites and insect vectors remain poorly understood. Filling this knowledge gap matters because vectors are evolving in response to the deployment of vector control tools (VCTs). Yet, whilst the evolutionary responses of vectors to VCTs are being carefully monitored, the knock-on consequences for parasite evolution have been overlooked. By examining how mosquito responses to VCTs impact upon malaria parasite ecology, we derive a framework for predicting parasite responses. Understanding how VCTs affect the selection pressures imposed on parasites could help to mitigate against parasite evolution that leads to unfavourable epidemiological outcomes. Furthermore, anticipating parasite evolution will inform monitoring strategies for VCT programmes as well as uncovering novel VCT strategies.

Laboratory and experimental hut trial evaluation of VECTRON™ T500 for indoor residual spraying (IRS) against insecticide resistant malaria vectors in Burkina Faso

Background: Malaria cases in some areas could be attributed to vector resistant to the insecticide. World Health Organization recommended insecticides for vector control are limited in number. It is essential to find rotational partners for existing Indoor Residual Spraying (IRS) products. VECTRON™ T500 is a novel insecticide with broflanilide as active ingredient. It has a mode of action on mosquitoes completely different to usually used. The aim of this study was to determine the optimum effective dose and efficacy of VECTRONTM T500 against susceptible and resistant strains of Anopheles in Burkina Faso. Methods: VECTRON™T500 was sprayed at 50, 100 and 200 mg/m² doses onto mud and concrete blocks using Potter Spray Tower. The residual activity of broflanilide was assessed through cone bioassays 1 week and then monthly up to 14 months post spraying. Its efficacy was evaluated at 100 and 150 mg/m² against wild free-flying mosquitoes in experimental huts on both substrates. Actellic 300CS was applied at 1000 mg/m² as reference product. Cone assays were conducted monthly, using susceptible and resistant mosquito strains. Results: In the laboratory, VECTRON™ T500 showed residual efficacy (≥80% mortality) on An. gambiae Kisumu up to 12 and 14 months, respectively, on concrete and mud blocks. Similar results were found with 100 and 200 mg/m² using An. coluzzii pyrethroid resistant strain. In experimental huts, a total of 19,552 An. gambiae s.l. were collected. Deterrence, blood-feeding inhibition and exophily with VECTRON™ treated huts were very low. At 100 and 150 mg/m², mortality of wild An. gambiae s.l. ranged between 55% and 73%. Monthly cone bioassay mortality remained >80% up to 9 months. Conclusions: VECTRON™ T500 shows great potential as IRS formulation for malaria vector control. It can be added to the arsenal of IRS products for use in rotations to control malaria and manage mosquito insecticide resistance.

Molecular Drivers of Multiple and Elevated Resistance to Insecticides in a Population of the Malaria Vector Anopheles gambiae in Agriculture Hotspot of West Cameroon

(1) Background: Malaria remains a global public health problem. Unfortunately, the resistance of malaria vectors to commonly used insecticides threatens disease control and elimination efforts. Field mosquitoes have been shown to survive upon exposure to high insecticide concentrations. The molecular mechanisms driving this pronounced resistance remain poorly understood. Here, we elucidated the pattern of resistance escalation in the main malaria vector Anopheles gambiae in a pesticide-driven agricultural hotspot in Cameroon and its impact on vector control tools; (2) Methods: Larval stages and indoor blood-fed female mosquitoes (F0) were collected in Mangoum in May and November and forced to lay eggs; the emerged mosquitoes were used for WHO tube, synergist and cone tests. Molecular identification was performed using SINE PCR, whereas TaqMan-based PCR was used for genotyping of L1014F/S and N1575Y kdr and the G119S-ACE1 resistance markers. The transcription profile of candidate resistance genes was performed using qRT-PCR methods. Characterization of the breeding water and soil from Mangoum was achieved using the HPLC technique; (3) Results: An. gambiae s.s. was the only species in Mangoum with 4.10% infection with Plasmodium. These mosquitoes were resistant to all the four classes of insecticides with mortality rates <7% for pyrethroids and DDT and <54% for carbamates and organophophates. This population also exhibited high resistance intensity to pyrethroids (permethrin, alpha-cypermethrin and deltamethrin) after exposure to 5× and 10× discriminating doses. Synergist assays with PBO revealed only a partial recovery of susceptibility to permethrin, alpha-cypermethrin and deltamethrin. Only PBO-based nets (Olyset plus and permaNet 3.0) and Royal Guard showed an optimal efficacy. A high amount of alpha-cypermethrin was detected in breeding sites (5.16-fold LOD) suggesting ongoing selection from agricultural pesticides. The 1014F-kdr allele was fixed (100%) whereas the 1575Y-kdr (37.5%) and the 119S Ace-1R (51.1%) were moderately present. Elevated expression of P450s, respectively, in permethrin and deltamethrin resistant mosquitoes [CYP6M2 (10 and 34-fold), CYP6Z1(17 and 29-fold), CYP6Z2 (13 and 65-fold), CYP9K1 (13 and 87-fold)] supports their role in the observed resistance besides other mechanisms including chemosensory genes as SAP1 (28 and 13-fold), SAP2 (5 and 5-fold), SAP3 (24 and 8-fold) and cuticular genes as CYP4G16 (6 and 8-fold) and CYP4G17 (5 and 27-fold). However, these candidate genes were not associated with resistance escalation as the expression levels did not differ significantly between 1×, 5× and 10× surviving mosquitoes; (4) Conclusions: Intensive and multiple resistance is being selected in malaria vectors from a pesticide-based agricultural hotspot of Cameroon leading to loss in the efficacy of pyrethroid-only nets. Further studies are needed to decipher the molecular basis underlying such resistance escalation to better assess its impact on control interventions.

A closer look at the WHO cone bioassay: video analysis of the hidden effects of a human host on mosquito behaviour and insecticide contact

Background

The WHO cone test is one of three tests currently used to evaluate the efficacy of insecticide-treated bed nets (ITNs). It generates two test outputs, knockdown and 24-h mortality, both indicative of immediate toxicity but that reveal little about the nature of mosquito and ITN interaction or how results translate to real-world settings.

Methods

A human arm held 5 mm behind the net surface acted as a host attractant during cone tests and a smartphone was used to capture mosquito behaviour in the cone. Post-exposure blood feeding and survival for nine days were recorded; ingested blood meal size was determined by measuring excreted haematin. Four strains of Anopheles gambiae (insecticide susceptible: Kisumu and N’gousso; insecticide resistant: Banfora and VK7) were tested with and without the host attractant using untreated, Permanet 2.0 and Olyset nets. Video recordings were scan sampled every five seconds to record mosquito positions on either the net, in flight or in contact with the cone. Generalized estimating equations were used to analyse all data except survival within nine days which was analysed using Weighted Cox Regression.

Results

Net contact was the most frequently recorded behaviour in all Anopheles spp. strains on all nets. Adding the human host as attractant triggered excitatory behaviours: in all strains, the magnitude of net contact was significantly decreased compared to tests without a host. ITN exposure altered the observed behaviour of the two susceptible strains, which exhibited a decreased response to the host during ITN tests. The resistant strains did not alter their behaviour during ITN tests. Significantly less net contact was observed during Olyset Net tests compared to Permanet 2.0. The host presence affected survival after exposure: Banfora and VK7 mosquitoes exposed to Permanet 2.0 with a host lived longer compared to tests performed without a host. However, mosquitoes that blood-fed and survived long enough to digest the blood meal did not exhibit significantly reduced longevity regardless of the presence of the host attractant.

Conclusions

Simple modifications to the WHO cone test and extension of post-test monitoring beyond the current 24 h enable detailed behavioural characterizations of individual ITNs to be compiled. The effects observed from testing with a host and including blood feeding suggest that more representative estimates of true of ITN efficacy are gained with these modifications than when using the current testing protocol.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04232-4.

Outdoor biting and pyrethroid resistance as potential drivers of persistent malaria transmission in Zanzibar

BACKGROUND

Low-level of malaria transmission persist in Zanzibar despite high coverage of core vector control interventions. This study was carried out in hot-spot sites to better understand entomological factors that may contribute to residual malaria transmission in Zanzibar.

METHODS

A total of 135 households were randomly selected from six sites and consented to participate with 20-25 households per site. Mosquito vector surveillance was carried out indoors and outdoors from 6:00 pm-7:00 am using miniaturized double net trap (DN-Mini™). Additional collections were done indoors using mouth aspirators to retrieve resting mosquitoes from wall and ceiling surfaces, and outdoors using resting bucket and pit traps. All collected mosquitoes were morphologically and genetically (PCR) analysed in the laboratory. All collected anopheline and blood-fed mosquitoes were analysed for sporozoite infection and blood meal host preferences by Circumsporozoite Protein ELISA and blood meal ELISA, respectively. The differences between indoor and outdoor mosquito biting rates were analysed using generalized linear mixed models. Levels of resistance to commonly used insecticides were quantified by WHO susceptibility tests.

RESULTS

Out of 704 malaria vectors collected across 135 households, PCR analysis shows that 98.60% were Anopheles arabiensis, 0.6% Anopheles merus and 0.6% Anopheles gambiae sensu stricto. Sporozoite ELISA analysis indicates that all mosquitoes were negative for the malaria parasite. The results show that more An. arabiensis were collected outdoor (~ 85%) compared to indoor (~ 15%). Furthermore, large numbers of An. arabiensis were caught in outdoor resting sites, where the pit trap (67.2%) collected more mosquitoes compared to the outdoor DN-Mini trap (32.8%). Nearly two-thirds (60.7%) of blood-fed mosquitoes had obtained blood meals from non-human hosts. Mosquitoes displayed non-uniform susceptibility status and resistance intensity among the tested insecticides across the study sites to all WHO recommended insecticides across the study sites.

CONCLUSION

This study suggests that in contexts such as Zanzibar, testing of novel techniques to complement indoor protection and targeting outdoor biting and/or resting mosquitoes, may be warranted to complement existing interventions and contribute to malaria elimination efforts. The study highlights the need to implement novel interventions and/or adaptations of strategies that can target outdoors biting mosquitoes.

Using an antimalarial in mosquitoes overcomes Anopheles and Plasmodium resistance to malaria control strategies

The spread of insecticide resistance in Anopheles mosquitoes and drug resistance in Plasmodium parasites is contributing to a global resurgence of malaria, making the generation of control tools that can overcome these roadblocks an urgent public health priority. We recently showed that the transmission of Plasmodium falciparum parasites can be efficiently blocked when exposing Anopheles gambiae females to antimalarials deposited on a treated surface, with no negative consequences on major components of mosquito fitness. Here, we demonstrate this approach can overcome the hurdles of insecticide resistance in mosquitoes and drug resistant in parasites. We show that the transmission-blocking efficacy of mosquito-targeted antimalarials is maintained when field-derived, insecticide resistant Anopheles are exposed to the potent cytochrome b inhibitor atovaquone, demonstrating that this drug escapes insecticide resistance mechanisms that could potentially interfere with its function. Moreover, this approach prevents transmission of field-derived, artemisinin resistant P. falciparum parasites (Kelch13 C580Y mutant), proving that this strategy could be used to prevent the spread of parasite mutations that induce resistance to front-line antimalarials. Atovaquone is also highly effective at limiting parasite development when ingested by mosquitoes in sugar solutions, including in ongoing infections. These data support the use of mosquito-targeted antimalarials as a promising tool to complement and extend the efficacy of current malaria control interventions.

Effective control of malaria is hampered by resistance to vector-targeted insecticides and parasite-targeted drugs. This situation is exacerbated by a critical lack of chemical diversity in both interventions and, as such, new interventions are urgently needed. Recent laboratory studies have shown that an alternative approach based on treating Anopheles mosquitoes directly with antimalarial compounds can make mosquitoes incapable of transmitting the Plasmodium parasites that cause malaria. While promising, showing that mosquito-targeted antimalarials remain effective against wild parasites and mosquitoes, including drug- and insecticide-resistant populations in malaria-endemic countries, is crucial to the future viability of this approach. In this study, carried out in the US and Burkina Faso, we show that insecticide-resistance mechanisms found in highly resistant, natural Anopheles mosquito populations do not interfere with the transmission blocking activity of tarsal exposure to the antimalarial atovaquone, and that mosquito-targeted antimalarial exposure can block transmission of parasites resistant to the main therapeutic antimalarial drug artemisinin. By combining lab, and field-based studies in this way we have demonstrated that this novel approach can be effective in areas where conventional control measures are no longer as effective.

Laboratory and experimental hut trial evaluation of VECTRON™ T500 for indoor residual spraying (IRS) against insecticide resistant malaria vectors in Burkina Faso

Background: Malaria cases in some areas could be attributed to vector resistant to the insecticide. World Health Organization recommended insecticides for vector control are limited in number. It is essential to find rotational partners for existing Indoor Residual Spraying (IRS) products. VECTRON™ T500 is a novel insecticide with broflanilide as active ingredient. It has a mode of action on mosquitoes completely different to usually used. The aim of this study was to determine the optimum effective dose and efficacy of VECTRONTM against susceptible and resistant strains of Anopheles in Burkina Faso. Methods: VECTRON™ was sprayed at 50, 100 and 200 mg/m² doses onto mud and concrete blocks using Potter Spray Tower. The residual activity of broflanilide was assessed through cone bioassays 1 week and then monthly up to 14 months post spraying. Its efficacy was evaluated at 100 and 150 mg/m² against wild free-flying mosquitoes in experimental huts on both substrates. Actellic 300CS was applied at 1000 mg/m² as reference product. Cone assays were conducted monthly, using susceptible and resistant mosquito strains. Results: In the laboratory, VECTRON™ showed residual efficacy (≥80% mortality) on An. gambiae Kisumu up to 12 and 14 months, respectively, on concrete and mud blocks. Similar results were found with 100 and 200 mg/m² using An. coluzzii pyrethroid resistant strain. In experimental huts, a total of 19,552 An. gambiae s.l. were collected. Deterrence, blood-feeding inhibition and exophily with VECTRON™ treated huts were very low. At 100 and 150 mg/m², mortality of wild An. gambiae s.l. ranged between 55% and 73%. Monthly cone bioassay mortality remained >80% up to 9 months. Conclusions: VECTRON™ shows great potential as IRS formulation for malaria vector control. It can be added to the arsenal of IRS products for use in rotations to control malaria and manage mosquito insecticide resistance.

Evaluation of an accelerometer-based monitor for detecting bed net use and human entry/exit using a machine learning algorithm

Background

Distribution of long-lasting insecticidal bed nets (LLINs) is one of the main control strategies for malaria. Improving malaria prevention programmes requires understanding usage patterns in households receiving LLINs, but there are limits to what standard cross-sectional surveys of self-reported LLIN use can provide. This study was designed to assess the performance of an accelerometer-based approach for measuring a range of LLIN use behaviours as a proof of concept for more granular LLIN-use monitoring over longer time periods.

Methods

This study was carried out under controlled conditions from May to July 2018 in Liverpool, UK. A single accelerometer was affixed to the side panel of an LLIN and participants carried out five LLIN use behaviours: (1) unfurling a net; (2) entering an unfurled net; (3) lying still as if sleeping; (4) exiting from under a net; and, (5) folding up a net. The randomForest package in R, a supervised non-linear classification algorithm, was used to train models on 20-s epochs of tagged accelerometer data. Models were compared in a validation dataset using overall accuracy, sensitivity and specificity, receiver operating curves and the area under the curve (AUC).

Results

The five-category model had overall accuracy of 82.9% in the validation dataset, a sensitivity of 0.681 for entering a net, 0.632 for exiting, 0.733 for net down, and 0.800 for net up. A simplified four-category model, combining entering/exiting a net into one category had accuracy of 94.8%, and increased sensitivity for net down (0.756) and net up (0.829). A further simplified three-category model, identifying sleeping, net up, and a combined net down/enter/exit category had accuracy of 96.2% (483/502), with an AUC of 0.997 for net down and 0.987 for net up. Models for detecting entering/exiting by adults were significantly more accurate than for children (87.8% vs 70.0%; p < 0.001) and had a higher AUC (p = 0.03).

Conclusions

Understanding how LLINs are used is crucial for planning malaria prevention programmes. Accelerometer-based systems provide a promising new methodology for studying LLIN use. Further work exploring accelerometer placement, frequency of measurements and other machine learning approaches could make these methods even more accurate in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04102-z.

The effect of blood feeding on insecticide resistance intensity and adult longevity in the major malaria vector Anopheles funestus (Diptera: Culicidae)

Insecticide-based vector control is key to the reduction and elimination of malaria. Although insecticide resistance is common in malaria vector populations, the operational implications are often unclear. High intensity pyrethroid resistance in the major malaria vector Anopheles funestus has been linked to control failure in Southern Africa. The aim of this study was to assess linkages between mosquito age, blood feeding and the intensity of pyrethroid resistance in two An. funestus laboratory strains that originate from southern Mozambique, namely the moderately pyrethroid resistant FUMOZ and the highly resistant FUMOZ-R. Resistance tended to decline with age. This effect was significantly mitigated by blood feeding and was most apparent in cohorts that received multiple blood meals. In the absence of insecticide exposure, blood feeding tended to increase longevity of An. funestus females and, following insecticide exposure, enhanced their levels of deltamethrin resistance, even in older age groups. These effects were more marked in FUMOZ-R compared to FUMOZ. In terms of programmatic decision-making, these data suggest that it would be useful to assess the level and intensity of resistance in older female cohorts wherever possible, notwithstanding the standard protocols for resistance testing using age-standardised samples.

Sympatric Populations of the Anopheles gambiae Complex in Southwest Burkina Faso Evolve Multiple Diverse Resistance Mechanisms in Response to Intense Selection Pressure with Pyrethroids

Simple Summary

Targeting mosquitoes with insecticides is one of the most effective methods to prevent malaria transmission. Although numbers of malaria cases have declined substantially this century, this pattern is not universal and Burkina Faso has one of the highest burdens of malaria; it is also a hotspot for the evolution of insecticide resistance in malaria vectors. We have established laboratory colonies from multiple species within the An. gambiae complex, the most efficient group of malaria vectors in the world, from larval collections in southwest Burkina Faso. Using bioassays with different insecticides widely used to control public health pests, we provide a profile of insecticide resistance in each of these colonies and, using molecular tools, reveal the genetic changes underpinning this resistance. We show that, whilst many resistance mechanisms are shared between species, there are some important differences which may affect resistance to current and future insecticide classes. The complexity, and diversity of resistance mechanisms highlights the importance of screening any potential new insecticide intended for use in malaria control against a wide range of populations. These stable laboratory colonies provide a valuable resource for insecticide discovery, and for further studies on the evolution and dispersal of insecticide resistance within and between species.

Abstract

Pyrethroid resistance in the Anopheles vectors of malaria is driving an urgent search for new insecticides that can be used in proven vector control tools such as insecticide treated nets (ITNs). Screening for potential new insecticides requires access to stable colonies of the predominant vector species that contain the major pyrethroid resistance mechanisms circulating in wild populations. Southwest Burkina Faso is an apparent hotspot for the emergence of pyrethroid resistance in species of the Anopheles gambiae complex. We established stable colonies from larval collections across this region and characterised the resistance phenotype and underpinning genetic mechanisms. Three additional colonies were successfully established (1 An. coluzzii, 1 An. gambiae and 1 An. arabiensis) to add to the 2 An. coluzzii colonies already established from this region; all 5 strains are highly resistant to pyrethroids. Synergism assays found that piperonyl butoxide (PBO) exposure was unable to fully restore susceptibility although exposure to a commercial ITN containing PBO resulted in 100% mortality. All colonies contained resistant alleles of the voltage gated sodium channel but with differing proportions of alternative resistant haplotypes. RNAseq data confirmed the role of P450s, with CYP6P3 and CYP6Z2 elevated in all 5 strains, and identified many other resistance mechanisms, some found across strains, others unique to a particular species. These strains represent an important resource for insecticide discovery and provide further insights into the complex genetic changes driving pyrethroid resistance.

Treatment-seeking and uptake of malaria prevention strategies among pregnant women and caregivers of children under-five years during COVID-19 pandemic in rural communities in South West Uganda: a qualitative study

Background

Despite efforts to avert the negative effects of malaria, there remain barriers to the uptake of prevention measures, and these have hindered its eradication. This study explored the factors that influence uptake of malaria prevention strategies among pregnant women and children under-five years and the impact of COVID-19 in a malaria endemic rural district in Uganda.

Methods

This was a qualitative case study that used focus group discussions, in-depth interviews, and key informant interviews involving pregnant women, caregivers of children under-five years, traditional birth attendants, village health teams, local leaders, and healthcare providers to explore malaria prevention uptake among pregnant women and children under-five years. The interviews were audio-recorded, transcribed and data were analyzed using thematic content approach.

Results

Seventy-two participants were enrolled in the Focus Group Discussions, 12 in the in-depth interviews, and 2 as key informants. Pregnant women and caregivers of children under-five years were able to recognize causes of malaria, transmission, and symptoms. All participants viewed malaria prevention as a high priority, and the use of insecticide-treated mosquito bed nets (ITNs) was upheld. Participants' own experiences indicated adverse effects of malaria to both pregnant women and children under-five. Home medication and the use of local herbs were a common practice. Some participants didn’t use any of the malaria prevention methods due to deliberate refusal, perceived negative effects of the ITNs, and family disparity. The Corona Virus Disease-2019 (COVID-19) control measures did not abate the risk of malaria infection but these were deleterious to healthcare access and the focus of malaria prevention.

Conclusions

Although pregnant women and caregivers of children under-five years recognized symptoms of malaria infection, healthcare-seeking was not apt as some respondents used alternative approaches and delayed seeking formal healthcare. It is imperative to focus on the promotion of malaria prevention strategies and address drawbacks associated with misconceptions about these interventions, and promotion of health-seeking behaviors. As COVID-19 exacerbated the effect of malaria prevention uptake and healthcare seeking, it’s critical to recommit and integrate COVID-19 prevention measures in normative living and restrict future barriers to healthcare access.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12889-022-12771-3.

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

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

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

Risk of Plasmodium falciparum infection in south-west Burkina Faso: potential impact of expanding eligibility for seasonal malaria chemoprevention

Burkina Faso has one of the highest malaria burdens in sub-Saharan Africa despite the mass deployment of insecticide-treated nets (ITNs) and use of seasonal malaria chemoprevention (SMC) in children aged up to 5 years. Identification of risk factors for Plasmodium falciparum infection in rural Burkina Faso could help to identify and target malaria control measures. A cross-sectional survey of 1,199 children and adults was conducted during the peak malaria transmission season in the Cascades Region of south-west Burkina Faso in 2017. Logistic regression was used to identify risk factors for microscopically confirmed P. falciparum infection. A malaria transmission dynamic model was used to determine the impact on malaria cases averted of administering SMC to children aged 5-15 year old. P. falciparum prevalence was 32.8% in the study population. Children aged 5 to < 10 years old were at 3.74 times the odds (95% CI = 2.68-5.22, P < 0.001) and children aged 10 to 15 years old at 3.14 times the odds (95% CI = 1.20-8.21, P = 0.02) of P. falciparum infection compared to children aged less than 5 years old. Administration of SMC to children aged up to 10 years is predicted to avert an additional 57 malaria cases per 1000 population per year (9.4% reduction) and administration to children aged up to 15 years would avert an additional 89 malaria cases per 1000 population per year (14.6% reduction) in the Cascades Region, assuming current coverage of pyrethroid-piperonyl butoxide ITNs. Malaria infections were high in all age strata, although highest in children aged 5 to 15 years, despite roll out of core malaria control interventions. Given the burden of infection in school-age children, extension of the eligibility criteria for SMC could help reduce the burden of malaria in Burkina Faso and other countries in the region.

Understanding the role of serological and clinical data on assessing the dynamic of malaria transmission: a case study of Bagamoyo district, Tanzania

Introduction

naturally acquired blood-stage malaria antibodies and malaria clinical data have been reported to be useful in monitoring malaria change over time and as a marker of malaria exposure. This study assessed the total immunoglobulin G (IgG) levels to Plasmodium falciparum schizont among infants (5-17 months), estimated malaria incidence using routine health facility-based surveillance data and predicted trend relation between anti-schizont antibodies and malaria incidence in Bagamoyo.

Methods

252 serum samples were used for assessment of total IgG by enzyme-linked immunosorbent assay and results were expressed in arbitrary units (AU). 147/252 samples were collected in 2021 during a blood-stage malaria vaccine trial [ClinicalTrials.gov NCT04318002], and 105/252 were archived samples of malaria vaccine trial conducted in 2012 [ClinicalTrials.gov NCT00866619]. Malaria incidence was calculated from outpatient clinic data of malaria rapid test or blood smear positive results retrieved from District-Health-Information-Software-2 (DHIS2) between 2013 and 2020. Cross-sectional data from both studies were analysed using STATA version 14.

Results

this study demonstrated a decline in total anti-schizont IgG levels from 490.21AU in 2012 to 97.07AU in 2021 which was related to a fall in incidence from 58.25 cases/1000 person-year in 2013 to 14.28 cases/1000 person-year in 2020. We also observed a significant difference in incidence when comparing high and low malaria transmission areas and by gender. However, we did not observe differences when comparing total anti-schizont antibodies by gender and study year.

Conclusion

total anti-schizont antibody levels appear to be an important serological marker of exposure for assessing the dynamic of malaria transmission in infants living in malaria-endemic regions.

Bioefficacy and durability of Olyset® Plus, a permethrin and piperonyl butoxide-treated insecticidal net in a 3-year long trial in Kenya

BACKGROUND

Long-lasting insecticide nets (LLINs) are a core malaria intervention. LLINs should retain efficacy against mosquito vectors for a minimum of three years. Efficacy and durability of Olyset^® Plus, a permethrin and piperonyl butoxide (PBO) treated LLIN, was evaluated versus permethrin treated Olyset^® Net. In the absence of WHO guidelines of how to evaluate PBO nets, and considering the manufacturer's product claim, Olyset^® Plus was evaluated as a pyrethroid LLIN.

METHODS

This was a household randomized controlled trial in a malaria endemic rice cultivation zone of Kirinyaga County, Kenya between 2014 and 2017. Cone bioassays and tunnel tests were done against Anopheles gambiae Kisumu. The chemical content, fabric integrity and LLIN survivorship were monitored. Comparisons between nets were tested for significance using the Chi-square test. Exact binomial distribution with 95% confidence intervals (95% CI) was used for percentages. The WHO efficacy criteria used were ≥ 95% knockdown and/or ≥ 80% mortality rate in cone bioassays and ≥ 80% mortality and/or ≥ 90% blood-feeding inhibition in tunnel tests.

RESULTS

At 36 months, Olyset^® Plus lost 52% permethrin and 87% PBO content; Olyset^® Net lost 24% permethrin. Over 80% of Olyset^® Plus and Olyset^® Net passed the WHO efficacy criteria for LLINs up to 18 and 12 months, respectively. At month 36, 91.2% Olyset^® Plus and 86.4% Olyset^® Net survived, while 72% and 63% developed at least one hole. The proportionate Hole Index (pHI) values representing nets in good, serviceable and torn condition were 49.6%, 27.1% and 23.2%, respectively for Olyset^® Plus, and 44.9%, 32.8% and 22.2%, respectively for Olyset^® Net but were not significantly different.

CONCLUSIONS

Olyset^® Plus retained efficacy above or close to the WHO efficacy criteria for about 2 years than Olyset^® Net (1-1.5 years). Both nets did not meet the 3-year WHO efficacy criteria, and showed little attrition, comparable physical durability and survivorship, with 50% of Olyset^® Plus having good and serviceable condition after 3 years. Better community education on appropriate use and upkeep of LLINs is essential to ensure effectiveness of LLIN based malaria interventions.

The genetic architecture of target-site resistance to pyrethroid insecticides in the African malaria vectors Anopheles gambiae and Anopheles coluzzii

Resistance to pyrethroid insecticides is a major concern for malaria vector control. Pyrethroids target the voltage-gated sodium channel (VGSC), an essential component of the mosquito nervous system. Substitutions in the amino acid sequence can induce a resistance phenotype. We use whole-genome sequence data from phase 2 of the Anopheles gambiae 1000 Genomes Project (Ag1000G) to provide a comprehensive account of genetic variation in the Vgsc gene across 13 African countries. In addition to known resistance alleles, we describe 20 other non-synonymous nucleotide substitutions at appreciable population frequency and map these variants onto a protein model to investigate the likelihood of pyrethroid resistance phenotypes. Thirteen of these novel alleles were found to occur almost exclusively on haplotypes carrying the known L995F kdr (knock-down resistance) allele and may enhance or compensate for the L995F resistance genotype. A novel mutation I1527T, adjacent to a predicted pyrethroid-binding site, was found in tight linkage with V402L substitutions, similar to allele combinations associated with resistance in other insect species. We also analysed genetic backgrounds carrying resistance alleles, to determine which alleles have experienced recent positive selection, and describe ten distinct haplotype groups carrying known kdr alleles. Five of these groups are observed in more than one country, in one case separated by over 3000 km, providing new information about the potential for the geographical spread of resistance. Our results demonstrate that the molecular basis of target-site pyrethroid resistance in malaria vectors is more complex than previously appreciated, and provide a foundation for the development of new genetic tools for insecticide resistance management.

Evidence supporting deployment of next generation insecticide treated nets in Burkina Faso: bioassays with either chlorfenapyr or piperonyl butoxide increase mortality of pyrethroid-resistant Anopheles gambiae

BACKGROUND

Pyrethroid resistance poses a major threat to the efficacy of insecticide-treated nets (ITNs) in Burkina Faso and throughout sub-Saharan Africa, particularly where resistance is present at high intensity. For such areas, there are alternative ITNs available, including the synergist piperonyl butoxide (PBO)-based ITNs and dual active ingredient ITNs such as Interceptor G2 (treated with chlorfenapyr and alpha-cypermethrin). Before deploying alternative ITNs on a large scale it is crucial to characterize the resistance profiles of primary malaria vector species for evidence-based decision making.

METHODS

Larvae from the predominant vector, Anopheles gambiae sensu lato (s.l.) were collected from 15 sites located throughout Burkina Faso and reared to adults for bioassays to assess insecticide resistance status. Resistance intensity assays were conducted using WHO tube tests to determine the level of resistance to pyrethroids commonly used on ITNs at 1×, 5 × and 10 × times the diagnostic dose. WHO tube tests were also used for PBO synergist bioassays with deltamethrin and permethrin. Bottle bioassays were conducted to determine susceptibility to chlorfenapyr at a dose of 100 µg/bottle.

RESULTS

WHO tube tests revealed high intensity resistance in An. gambiae s.l. to deltamethrin and alpha-cypermethrin in all sites tested. Resistance intensity to permethrin was either moderate or high in 13 sites. PBO pre-exposure followed by deltamethrin restored full susceptibility in one site and partially restored susceptibility in all but one of the remaining sites (often reaching mortality greater than 80%). PBO pre-exposure followed by permethrin partially restored susceptibility in 12 sites. There was no significant increase in permethrin mortality after PBO pre-exposure in Kampti, Karangasso-Vigué or Mangodara; while in Seguenega, Orodara and Bobo-Dioulasso there was a significant increase in mortality, but rates remained below 50%. Susceptibility to chlorfenapyr was confirmed in 14 sites.

CONCLUSION

High pyrethroid resistance intensity in An. gambiae s.l. is widespread across Burkina Faso and may be a predictor of reduced pyrethroid ITN effectiveness. PBO + deltamethrin ITNs would likely provide greater control than pyrethroid nets. However, since susceptibility in bioassays was not restored in most sites following pre-exposure to PBO, Interceptor G2 may be a better long-term solution as susceptibility was recorded to chlorfenapyr in nearly all sites. This study provides evidence supporting the introduction of both Interceptor G2 nets and PBO nets, which were distributed in Burkina Faso in 2019 as part of a mass campaign.

Small-scale release of non-gene drive mosquitoes in Burkina Faso: from engagement implementation to assessment, a learning journey

Background

Innovative tools are needed to complement the existing approach for malaria elimination. Gene drive mosquitoes are one potential new technology in the control of malaria vectors. Target Malaria is one of the research projects developing this technology, and in July 2019, the project proceeded to an important step for this evaluation pathway: the small-scale release of non-gene drive sterile male mosquitoes in a village in Burkina Faso. In addition to the entomological and laboratory work to prepare for this important milestone, significant community and stakeholder engagement work was done. The existing guidelines on gene drive mosquito provide an overall framework for such engagement work. However, they do not provide a road map on how to proceed or what benchmarks should be used to assess this work.

Methods

This study provides a review of engagement activities relevant to field trials on non-gene drive genetically-modified mosquitoes as well as an assessment framework—using both qualitative and quantitative studies as well as an audit procedure. The latter was implemented to evaluate whether the release activities could proceed with the appropriate level of agreement from the community.

Results

This paper shows the importance of this first phase of work to innovate and learn about engagement processes for responsible research in the field of genetic approaches for malaria vector control. The function of these assessments is crucial for the learning agenda. The assessments demonstrated ways to increase understanding and ensure effective progress with field studies and, therefore, the pathway for responsible research.

Conclusion

Gene drive technology is increasingly considered as a promising approach to control vector borne diseases, in particular malaria. Stakeholders’ involvement in this research process is one of the recurring requirements in international guidance documents. With this paper Target Malaria offers an opportunity to explore the practical achievements and challenges of stakeholder engagement during early phases of a technology evaluation, and in particular how it implemented an assessment framework to learn from its experience.

Review and Meta-Analysis of the Evidence for Choosing between Specific Pyrethroids for Programmatic Purposes

Simple Summary

A group of insecticides, called pyrethroids, are the main strategy for controlling the mosquito vectors of malaria. Pyrethroids are used in all insecticide-treated bednets, and many indoor residual spray programmes (in which insecticides are sprayed on the interior walls of houses). There are different types of pyrethroids within the class (e.g., deltamethrin and permethrin). Across the world, mosquitoes are showing signs of resistance to the pyrethroids, such as reduced mortality following contact. However, it is unclear if this resistance is uniform across the pyrethroid class (i.e., if a mosquito is resistant to deltamethrin, whether it is resistant to permethrin at the same level). In addition, it is not known if switching between different pyrethroids can be used to effectively maintain mosquito control when resistance to a single pyrethroid has been detected. This review examined the evidence from molecular studies, resistance testing from laboratory and field data, and mosquito behavioural assays to answer these questions. The evidence suggested that in areas where pyrethroid resistance exists, different mortality seen between the pyrethroids is not necessarily indicative of an operationally relevant difference in control performance, and there is no reason to rotate between common pyrethroids (i.e., deltamethrin, permethrin, and alpha-cypermethrin) as an insecticide resistance management strategy.

Abstract

Pyrethroid resistance is widespread in malaria vectors. However, differential mortality in discriminating dose assays to different pyrethroids is often observed in wild populations. When this occurs, it is unclear if this differential mortality should be interpreted as an indication of differential levels of susceptibility within the pyrethroid class, and if so, if countries should consider selecting one specific pyrethroid for programmatic use over another. A review of evidence from molecular studies, resistance testing with laboratory colonies and wild populations, and mosquito behavioural assays were conducted to answer these questions. Evidence suggested that in areas where pyrethroid resistance exists, different results in insecticide susceptibility assays with specific pyrethroids currently in common use (deltamethrin, permethrin, α-cypermethrin, and λ-cyhalothrin) are not necessarily indicative of an operationally relevant difference in potential performance. Consequently, it is not advisable to use rotation between these pyrethroids as an insecticide-resistance management strategy. Less commonly used pyrethroids (bifenthrin and etofenprox) may have sufficiently different modes of action, though further work is needed to examine how this may apply to insecticide resistance management.

Partial indoor residual spraying with pirimiphos-methyl as an effective and cost-saving measure for the control of Anopheles gambiae s.l. in northern Ghana

The scale up of indoor residual spraying (IRS) and insecticide treated nets have contributed significantly to global reductions in malaria prevalence over the last two decades. However, widespread pyrethroid resistance has necessitated the use of new and more expensive insecticides for IRS. Partial IRS with pirimiphos-methyl in experimental huts and houses in a village-wide trial was evaluated against Anopheles gambiae s.l. in northern Ghana. Four different scenarios in which either only the top or bottom half of the walls of experimental huts were sprayed, with or without also spraying the ceiling were compared. Mortality of An. gambiae s.l. on partially sprayed walls was compared with the standard procedures in which all walls and ceiling surfaces are sprayed. A small-scale trial was then conducted to assess the effectiveness, feasibility, and cost of spraying only the upper walls and ceiling as compared to full IRS and no spraying in northern Ghana. Human landing catches were conducted to estimate entomological indices and determine the effectiveness of partial IRS. An established transmission dynamics model was parameterized by an analysis of the experimental hut data and used to predict the epidemiological impact and cost effectiveness of partial IRS for malaria control in northern Ghana. In the experimental huts, partial IRS of the top (IRR 0.89, p = 0.13) or bottom (IRR 0.90, p = 0.15) half of walls and the ceiling was not significantly less effective than full IRS in terms of mosquito mortality. In the village trial, the annual entomological inoculation rate was higher for the unsprayed control (217 infective bites/person/year (ib/p/yr)) compared with the fully and partially sprayed sites, with 28 and 38 ib/p/yr, respectively. The transmission model predicts that the efficacy of partial IRS against all-age prevalence of malaria after six months would be broadly equivalent to a full IRS campaign in which 40% reduction is expected relative to no spray campaign. At scale, partial IRS in northern Ghana would have resulted in a 33% cost savings ($496,426) that would enable spraying of 36,000 additional rooms. These findings suggest that partial IRS is an effective, feasible, and cost saving approach to IRS that could be adopted to sustain and expand implementation of this key malaria control intervention.

Risk factors for Plasmodium falciparum infection in pregnant women in Burkina Faso: a community-based cross-sectional survey

Background

Malaria in pregnancy remains a public health problem in sub-Saharan Africa. Identifying risk factors for malaria in pregnancy could assist in developing interventions to reduce the risk of malaria in Burkina Faso and other countries in the region.

Methods

Two cross-sectional surveys were carried out to measure Plasmodium falciparum infection using microscopy in pregnant women in Saponé Health District, central Burkina Faso. Data were collected on individual, household and environmental variables and their association with P. falciparum infection assessed using multivariable analysis.

Results

A total of 356 pregnant women were enrolled in the surveys, 174 during the dry season and 182 during the wet season. The mean number of doses of sulfadoxine–pyrimethamine for Intermittent Preventive Treatment in pregnancy (IPTp-SP) was 0.4 doses during the first trimester, 1.1 doses at the second and 2.3 doses at the third. Overall prevalence of P. falciparum infection by microscopy was 15.7%; 17.8% in the dry season and 13.7% in the wet season. 88.2% of pregnant women reported sleeping under an insecticide-treated net (ITN) on the previous night. The odds of P. falciparum infection was 65% lower in women who reported using an ITN compared to those that did not use an ITN (Odds ratio, OR = 0.35, 95% CI 0.14–0.86, p = 0.02). IPTp-SP was also associated with reduced P. falciparum infection, with each additional dose of IPTp-SP reducing the odds of infection by 44% (OR = 0.56, 95% CI 0.39–0.79, p = 0.001). Literate women had a 2.54 times higher odds of P. falciparum infection compared to illiterate women (95% CI 1.31–4.91, p = 0.006).

Conclusions

The prevalence of P. falciparum infection among pregnant women remains high in Burkina Faso, although use of IPTp-SP and ITNs were found to reduce the odds of infection. Despite this, compliance with IPTp-SP remains far from that recommended by the National Malaria Control Programme and World Health Organization. Behaviour change communication should be strengthened to encourage compliance with protective malaria control tools during pregnancy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03896-8.

Insecticide resistance and behavioural adaptation as a response to long-lasting insecticidal net deployment in malaria vectors in the Cascades region of Burkina Faso

The decline in malaria across Africa has been largely attributed to vector control using long-lasting insecticidal nets (LLINs). However, this intervention has prompted widespread insecticide resistance (IR) and been associated with changes in mosquito behaviour that reduce their contact with LLINs. The relative importance and rate at which IR and behavioural adaptations emerge are poorly understood. We conducted surveillance of mosquito behaviour and IR at 12 sites in Burkina Faso to assess the magnitude and temporal dynamics of insecticide, biting and resting behaviours in vectors in the 2-year period following mass LLIN distribution. Insecticide resistance was present in all vector populations and increased rapidly over the study period. In contrast, no longitudinal shifts in LLIN-avoidance behaviours (earlier or outdoor biting and resting) were detected. There was a moderate but statistically significant shift in vector species composition from Anopheles coluzzii to Anopheles gambiae which coincided with a reduction in the proportion of bites preventable by LLINs; possibly driven by between-species variation in behaviour. These findings indicate that adaptations based on insecticide resistance arise and intensify more rapidly than behavioural shifts within mosquito vectors. However, longitudinal shifts in mosquito vector species composition were evident within 2 years following a mass LLIN distribution. This ecological shift was characterized by a significant increase in the exophagic species (An. gambiae) and coincided with a predicted decline in the degree of protection expected from LLINs. Although human exposure fell through the study period due to reducing vector densities and infection rates, such ecological shifts in vector species along with insecticide resistance were likely to have eroded the efficacy of LLINs. While both adaptations impact malaria control, the rapid increase of the former indicates this strategy develops more quickly in response to selection from LLINS. However, interventions targeting both resistance strategies will be needed.

Colonization and Authentication of the Pyrethroid-Resistant Anopheles gambiae s.s. Muleba-Kis Strain; an Important Test System for Laboratory Screening of New Insecticides

Simple Summary

Malaria control and prevention have traditionally relied on the use of insecticides in the form of treated bed nets or residual spraying in households. However, scaling up of these interventions—based on few available insecticide classes—resulted in the development and spread of insecticide resistance in malaria-transmitting mosquitoes. There is therefore an urgent need for introducing and applying new insecticides that are effective against these mosquitoes. Laboratories tasked with evaluating the efficacy of novel insecticides need to establish a large colony of resistant mosquitoes. In this study, we report the procedures used and challenges faced during the establishment and maintenance of a resistant mosquito strain in the laboratory which reflects the characteristics of the wild-resistant mosquito populations found in East Africa.

Abstract

Background: The emergence and spread of insecticide resistance in malaria vectors to major classes of insecticides call for urgent innovation and application of insecticides with novel modes of action. When evaluating new insecticides for public health, potential candidates need to be screened against both susceptible and resistant mosquitoes to determine efficacy and to identify potential cross-resistance to insecticides currently used for mosquito control. The challenges and lessons learned from establishing, maintaining, and authenticating the pyrethroid-resistant An. gambiae s.s. Muleba-Kis strain at the KCMUCo-PAMVERC Test Facility are described in this paper. Methods: Male mosquitoes from the F₁ generation of wild-pyrethroid resistant mosquitoes were cross-bred with susceptible female An. gambiae s.s. Kisumu laboratory strain followed by larval selection using a pyrethroid insecticide solution. Periodic screening for phenotypic and genotypic resistance was done. WHO susceptibility tests and bottle bioassays were used to assess the phenotypic resistance, while Taqman™ assays were used to screen for known target-site resistance alleles (kdr and ace-1). Additionally, the strains were periodically assessed for quality control by monitoring adult weight and wing length. Results: By out-crossing the wild mosquitoes with an established lab strain, a successful resistant insectary colony was established. Intermittent selection pressure using alphacypermethrin has maintained high kdr mutation (leucine-serine) frequencies in the selected colony. There was consistency in the wing length and weight measurements from the year 2016 to 2020, with the exception that one out of four years was significantly different. Mean annual wing length varied between 0.0142–0.0028 mm compared to values obtained in 2016, except in 2019 where it varied by 0.0901 mm. Weight only varied by approximately 0.001 g across four years, except in 2017 where it differed by 0.005 g. Routine phenotypic characterization on Muleba-Kis against pyrethroids using the WHO susceptibility test indicated high susceptibility when type I pyrethroids were used compared to type II pyrethroids. Dynamics on susceptibility status also depended on the lapse time when the selection was last done. Conclusions: This study described the procedure for introducing, colonizing, and maintaining a resistant An. gambiae s.s. strain in the laboratory with leucine to serine substitution kdr allele which reflects the features of the wild-resistant population in East Africa. Challenges in colonizing a wild-resistant mosquito strain were overcome by out-crossing between mosquito strains of desired traits followed by intermittent insecticide selection at the larval stage to select for the resistant phenotype.

Cuticular hydrocarbons are associated with mating success and insecticide resistance in malaria vectors

Anopheles coluzzii females, important malaria vectors in Africa, mate only once in their lifetime. Mating occurs in aerial swarms with a high male-to-female ratio, where traits underlying male mating success are largely unknown. Here, we investigated whether cuticular hydrocarbons (CHCs) influence mating success in natural mating swarms in Burkina Faso. As insecticides are widely used in this area for malaria control, we also determined whether CHCs affect insecticide resistance levels. We find that mated males have higher CHC abundance than unmated controls, suggesting CHCs could be determinants of mating success. Additionally, mated males have higher insecticide resistance under pyrethroid challenge, and we show a link between resistance intensity and CHC abundance. Taken together, our results suggest that CHC abundance may be subject to sexual selection in addition to selection by insecticide pressure. This has implications for insecticide resistance management, as these traits may be sustained in the population due to their benefits in mating even in the absence of insecticides.

In this study, Adams et al. investigate the effect of cuticular hydrocarbons on mating success in natural mosquito mating swarms. These hydrocarbons confer both higher mating success and increased resistance to pyrethroid, suggesting sexual selection for insecticide resistance in this population secondary to mating success.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Threats to the effectiveness of insecticide-treated bednets for malaria control: thinking beyond insecticide resistance

From 2004 to 2019, insecticide-treated bednets (ITNs) have been the most effective tool for reducing malaria morbidity and mortality in sub-Saharan Africa. Recently, however, the decline in malaria cases and deaths has stalled. Some suggest that this inertia is due to increasing resistance in malaria vectors to the pyrethroid insecticides used for treating ITNs. However, there is presently little evidence to reach this conclusion and we therefore recommend that a broader perspective to evaluate ITN effectiveness in terms of access to nets, use of nets, bioefficacy, and durability should be taken. We argue that a single focus on insecticide resistance misses the bigger picture. To improve the effects of ITNs, net coverage should increase by increasing funding for programmes, adopting improved strategies for increasing ITN uptake, and enhancing the longevity of the active ingredients and the physical integrity of nets, while simultaneously accelerating the development and evaluation of novel vector control tools.

Pyrethroid Resistance Aggravation in Ugandan Malaria Vectors Is Reducing Bednet Efficacy

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

Comparative proteomics reveals mechanisms that underlie insecticide resistance in Culex pipiens pallens Coquillett

Mosquito control based on chemical insecticides is considered as an important element of the current global strategies for the control of mosquito-borne diseases. Unfortunately, the development of insecticide resistance of important vector mosquito species jeopardizes the effectiveness of insecticide-based mosquito control. In contrast to target site resistance, other mechanisms are far from being fully understood. Global protein profiles among cypermethrin-resistant, propoxur-resistant, dimethyl-dichloro-vinyl-phosphate-resistant and susceptible strain of Culex pipiens pallens were obtained and proteomic differences were evaluated by using isobaric tags for relative and absolute quantification labeling coupled with liquid chromatography/tandem mass spectrometric analysis. A susceptible strain of Culex pipiens pallens showed elevated resistance levels after 25 generations of insecticide selection, through iTRAQ data analysis detected 2,502 proteins, of which 1,513 were differentially expressed in insecticide-selected strains compared to the susceptible strain. Finally, midgut differential protein expression profiles were analyzed, and 62 proteins were selected for verification of differential expression using iTRAQ and parallel reaction monitoring strategy, respectively. iTRAQ profiles of adaptation selection to three insecticide strains combined with midgut profiles revealed that multiple insecticide resistance mechanisms operate simultaneously in resistant insects of Culex pipiens pallens. Significant molecular resources were developed for Culex pipiens pallens, potential candidates were involved in metabolic resistance and reducing penetration or sequestering insecticide. Future research that is targeted towards RNA interference of the identified metabolic targets, such as cuticular proteins, cytochrome P450s, glutathione S-transferases and ribosomal proteins proteins and biological pathways (drug metabolism—cytochrome P450, metabolism of xenobiotics by cytochrome P450, oxidative phosphorylation, ribosome) could lay the foundation for a better understanding of the genetic basis of insecticide resistance in Culex pipiens pallens.

Global protein profiles were compared among a susceptible strain of Cx. pipiens pallens and strains that were cypermethrin-resistant, propoxur-resistant, and dimethyl-dichloro-vinyl-phosphate-resistant after 25 generations of selection by distinct chemical insecticide families, multiple mechanisms were found to operate simultaneously in resistant mosquitoes of Cx. pipiens pallens, including mechanisms to lower penetration of or sequester the insecticide or to increase biodegradation of the insecticide via subtle alterations in either the cuticular protein levels or the activities of detoxification enzymes (P450s and glutathione S-transferases).

Sub-lethal aquatic doses of pyriproxyfen may increase pyrethroid resistance in malaria mosquitoes

BACKGROUND

Pyriproxyfen (PPF), an insect growth hormone mimic is widely used as a larvicide and in some second-generation bed nets, where it is combined with pyrethroids to improve impact. It has also been evaluated as a candidate for auto-dissemination by adult mosquitoes to control Aedes and Anopheles species. We examined whether PPF added to larval habitats of pyrethroid-resistant malaria vectors can modulate levels of resistance among emergent adult mosquitoes.

METHODOLOGY

Third-instar larvae of pyrethroid-resistant Anopheles arabiensis (both laboratory-reared and field-collected) were reared in different PPF concentrations, between 1×10-9 milligrams active ingredient per litre of water (mgAI/L) and 1×10-4 mgAI/L, or no PPF at all. Emergent adults escaping these sub-lethal exposures were tested using WHO-standard susceptibility assays on pyrethroids (0.75% permethrin and 0.05% deltamethrin), carbamates (0.1% bendiocarb) and organochlorides (4% DDT). Biochemical basis of pyrethroid resistance was investigated by pre-exposure to 4% PBO. Bio-efficacies of long-lasting insecticide-treated nets, Olyset® and PermaNet 2.0 were also examined against adult mosquitoes with or without previous aquatic exposure to PPF.

RESULTS

Addition of sub-lethal doses of PPF to larval habitats of pyrethroid-resistant An. arabiensis, consistently resulted in significantly reduced mortalities of emergent adults when exposed to pyrethroids, but not to bendiocarb or DDT. Mortality rates after exposure to Olyset® nets, but not PermaNet 2.0 were also reduced following aquatic exposures to PPF. Pre-exposure to PBO followed by permethrin or deltamethrin resulted in significant increases in mortality, compared to either insecticide alone.

CONCLUSIONS

Partially-resistant mosquitoes exposed to sub-lethal aquatic concentrations of PPF may become more resistant to pyrethroids than they already are without such pre-exposures. Studies should be conducted to examine whether field applications of PPF, either by larviciding or other means actually exacerbates pyrethroid-resistance in areas where signs of such resistance already exist in wild the vector populations. The studies should also investigate mechanisms underlying such magnification of resistance, and how this may impact the potential of PPF-based interventions in areas with pyrethroid resistance.

Structural and biophysical correlation of anti-NANP antibodies with in vivo protection against P. falciparum

The most advanced P. falciparum circumsporozoite protein-based malaria vaccine, RTS,S/AS01 (RTS,S), confers partial protection but with antibody titers that wane relatively rapidly, highlighting the need to elicit more potent and durable antibody responses. Here, we elucidate crystal structures, binding affinities and kinetics, and in vivo protection of eight anti-NANP antibodies derived from an RTS,S phase 2a trial and encoded by three different heavy-chain germline genes. The structures reinforce the importance of homotypic Fab-Fab interactions in protective antibodies and the overwhelmingly dominant preference for a germline-encoded aromatic residue for recognition of the NANP motif. In this study, antibody apparent affinity correlates best with protection in an in vivo mouse model, with the more potent antibodies also recognizing epitopes with repeating secondary structural motifs of type I β- and Asn pseudo 310 turns; such insights can be incorporated into design of more effective immunogens and antibodies for passive immunization.

Quantifying and characterizing hourly human exposure to malaria vectors bites to address residual malaria transmission during dry and rainy seasons in rural Southwest Burkina Faso

Background

To sustain the efficacy of malaria vector control, the World Health Organization (WHO) recommends the combination of effective tools. Before designing and implementing additional strategies in any setting, it is critical to monitor or predict when and where transmission occurs. However, to date, very few studies have quantified the behavioural interactions between humans and Anopheles vectors in Africa. Here, we characterized residual transmission in a rural area of Burkina Faso where long lasting insecticidal nets (LLIN) are widely used.

Methods

We analysed data on both human and malaria vectors behaviours from 27 villages to measure hourly human exposure to vector bites in dry and rainy seasons using a mathematical model. We estimated the protective efficacy of LLINs and characterised where (indoors vs. outdoors) and when both LLIN users and non-users were exposed to vector bites.

Results

The percentage of the population who declared sleeping under a LLIN the previous night was very high regardless of the season, with an average LLIN use ranging from 92.43 to 99.89%. The use of LLIN provided > 80% protection against exposure to vector bites. The proportion of exposure for LLIN users was 29–57% after 05:00 and 0.05–12% before 20:00. More than 80% of exposure occurred indoors for LLIN users and the estimate reached 90% for children under 5 years old in the dry cold season.

Conclusions

LLINs are predicted to provide considerable protection against exposure to malaria vector bites in the rural area of Diébougou. Nevertheless, LLIN users are still exposed to vector bites which occurred mostly indoors in late morning. Therefore, complementary strategies targeting indoor biting vectors in combination with LLIN are expected to be the most efficient to control residual malaria transmission in this area.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12889-021-10304-y.

Quantifying individual variability in exposure risk to mosquito bites in the Cascades region, Burkina Faso

Background

The Cascades region, Burkina Faso, has a high malaria burden despite reported high insecticide-treated mosquito net (ITN) use. Human and vector activities outside the hours when indoor interventions offer direct protection from infectious bites potentially increase exposure risk to bites from malaria-transmitting Anopheles mosquitoes. This work investigated the degree of variation in human behaviour both between individuals and through time (season) to quantify how it impacts exposure to malaria vectors.

Methods

Patterns in human overnight activity (18:00–06:00) to quantify time spent using an ITN across 7 successive nights in two rural communities, Niakore (N = 24 participants) and Toma (71 participants), were observed in the dry and rainy seasons, between 2017 and 2018. Hourly human landing Anopheles mosquito catches were conducted in Niakore specifically, and Cascades region generally, between 2016 and 2017. Data were statistically combined to estimate seasonal variation in time spent outdoors and Anopheles bites received per person per night (bpppn).

Results

Substantial variability in exposure to outdoor Anopheles bites was detected within and between communities across seasons. In October, when Anopheles densities are highest, an individual’s risk of Anopheles bites ranged from 2.2 to 52.2 bites per person per night (bpppn) within the same week with variable risk dependent on hours spent indoors. Comparably higher outdoor human activity was observed in April and July but, due to lower Anopheles densities estimated, bpppn were 0.2–4.7 and 0.5–32.0, respectively. Males and people aged over 21 years were predicted to receive more bites in both sentinel villages.

Conclusion

This work presents one of the first clear descriptions of the degree of heterogeneity in time spent outdoors between people and across the year. Appreciation of sociodemographic, cultural and entomological activities will help refine approaches to vector control.

Repurposing the orphan drug nitisinone to control the transmission of African trypanosomiasis

Tsetse transmit African trypanosomiasis, which is a disease fatal to both humans and animals. A vaccine to protect against this disease does not exist so transmission control relies on eliminating tsetse populations. Although neurotoxic insecticides are the gold standard for insect control, they negatively impact the environment and reduce populations of insect pollinator species. Here we present a promising, environment-friendly alternative to current insecticides that targets the insect tyrosine metabolism pathway. A bloodmeal contains high levels of tyrosine, which is toxic to haematophagous insects if it is not degraded and eliminated. RNA interference (RNAi) of either the first two enzymes in the tyrosine degradation pathway (tyrosine aminotransferase (TAT) and 4-hydroxyphenylpyruvate dioxygenase (HPPD)) was lethal to tsetse. Furthermore, nitisinone (NTBC), an FDA-approved tyrosine catabolism inhibitor, killed tsetse regardless if the drug was orally or topically applied. However, oral administration of NTBC to bumblebees did not affect their survival. Using a novel mathematical model, we show that NTBC could reduce the transmission of African trypanosomiasis in sub-Saharan Africa, thus accelerating current disease elimination programmes.

Comparative assessment of insecticide resistance phenotypes in two major malaria vectors, Anopheles funestus and Anopheles arabiensis in south-eastern Tanzania

BACKGROUND

Long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS) have greatly reduced malaria transmission in sub-Saharan Africa, but are threatened by insecticide resistance. In south-eastern Tanzania, pyrethroid-resistant Anopheles funestus are now implicated in > 80% of malaria infections, even in villages where the species occurs at lower densities than the other vector, Anopheles arabiensis. This study compared the insecticide resistance phenotypes between the two malaria vectors in an area where pyrethroid-LLINs are widely used.

METHODS

The study used the World Health Organization (WHO) assays with 1×, 5× and 10× insecticide doses to assess levels of resistance, followed by synergist bioassays to understand possible mechanisms of the observed resistance phenotypes. The tests involved adult mosquitoes collected from three villages across two districts in south-eastern Tanzania and included four insecticide classes.

FINDINGS

At baseline doses (1×), both species were resistant to the two candidate pyrethroids (permethrin and deltamethrin), but susceptible to the organophosphate (pirimiphos-methyl). Anopheles funestus, but not An. arabiensis was also resistant to the carbamate (bendiocarb). Both species were resistant to DDT in all villages except in one village where An. arabiensis was susceptible. Anopheles funestus showed strong resistance to pyrethroids, surviving the 5× and 10× doses, while An. arabiensis reverted to susceptibility at the 5× dose. Pre-exposure to the synergist, piperonyl butoxide (PBO), enhanced the potency of the pyrethroids against both species and resulted in full susceptibility of An. arabiensis (> 98% mortality). However, for An. funestus from two villages, permethrin-associated mortalities after pre-exposure to PBO only exceeded 90% but not 98%.

CONCLUSIONS

In south-eastern Tanzania, where An. funestus dominates malaria transmission, the species also has much stronger resistance to pyrethroids than its counterpart, An. arabiensis, and can survive more classes of insecticides. The pyrethroid resistance in both species appears to be mostly metabolic and may be partially addressed using synergists, e.g. PBO. These findings may explain the continued persistence and dominance of An. funestus despite widespread use of pyrethroid-treated LLINs, and inform new intervention choices for such settings. In short and medium-term, these may include PBO-based LLINs or improved IRS with compounds to which the vectors are still susceptible.

Reduced long-lasting insecticidal net efficacy and pyrethroid insecticide resistance are associated with over-expression of CYP6P4, CYP6P3 and CYP6Z1 in populations of Anopheles coluzzii from South-East Côte d'Ivoire

Background

Resistance to major public health insecticides in Côte d’Ivoire has intensified and now threatens the long-term effectiveness of malaria vector control interventions.

Methods

This study evaluated the bioefficacy of conventional and next-generation long-lasting insecticidal nets (LLINs), determined resistance profiles, and characterized molecular and metabolic mechanisms in wild Anopheles coluzzii from Southeast Côte d’Ivoire in 2019.

Results

Phenotypic resistance was intense: >25% of mosquitoes survived exposure to 10 times the doses of pyrethroids required to kill susceptible populations. Similarly, the 24-hour mortality rate with deltamethrin-only LLINs was very low and not significantly different from that with an untreated net. Sublethal pyrethroid exposure did not induce significant delayed vector mortality effects 72 hours later. In contrast, LLINs containing the synergist piperonyl butoxide, or new insecticides clothianidin and chlorfenapyr, were highly toxic to A. coluzzii. Pyrethroid-susceptible A. coluzzii were significantly more likely to be infected with malaria, compared with those that survived insecticidal exposure. Pyrethroid resistance was associated with significant overexpression of CYP6P4, CYP6P3, and CYP6Z1.

Conclusions

Study findings raise concerns regarding the operational failure of standard LLINs and support the urgent deployment of vector control interventions incorporating piperonyl butoxide, chlorfenapyr, or clothianidin in areas of high resistance intensity in Côte d’Ivoire.

Cytochrome P450-based metabolic insecticide resistance in Anopheles and Aedes mosquito vectors: Muddying the waters

Despite the substantial progress achieved in the characterization of cytochrome P450 (CYP) -based resistance mechanisms in mosquitoes, a number of questions remain unanswered. These include: (i) the regulation and physiology of resistance conferring CYPs; (ii) the actual contribution of CYPs in resistance alone or in combination with other detoxification partners or other resistance mechanisms; (iii) the association between overexpression levels and allelic variation, with the catalytic activity and the intensity of resistance and (iv) the true value of molecular diagnostics targeting CYP markers, for driving decision making in the frame of Insecticide Resistance Management applications. Furthermore, the translation of CYP - based insecticide resistance research in mosquitoes into practical applications, is being developed, but it is not fully exploited, as yet. Examples include the production of high throughput platforms for screening the liability (stability) or inhibition potential of novel insecticidal leads and synergists (add-ons), as well as the exploration of the negative cross resistance concept (i.e. detoxification of certain insecticides, but activation of others pro-insecticides). The goal of this review is to critically summarise the current knowledge and the gaps of the CYP-based metabolic insecticide resistance in Anopheles and Aedes mosquito vectors. The progress and limitations of the protein and the reverse/forward genetic approaches, the understanding and importance of molecular and physiological aspects, as well as the current and future exploitation routes of CYP research are discussed.

A deltamethrin crystal polymorph for more effective malaria control

Pyrethroid contact insecticides are mainstays of malaria control, but their efficacies are declining due to widespread insecticide resistance in Anopheles mosquito populations, a major public health challenge. Several strategies have been proposed to overcome this challenge, including insecticides with new modes of action. New insecticides, however, can be expensive to implement in low-income countries. Here, we report a simple and inexpensive method to improve the efficacy of deltamethrin, the most active and most commonly used pyrethroid, by more than 10 times against Anopheles mosquitoes. Upon heating for only a few minutes, the commercially available deltamethrin crystals, form I, melt and crystallize upon cooling into a polymorph, form II, which is much faster acting against fruit flies and mosquitoes. Epidemiological modeling suggests that the use of form II in indoor residual spraying in place of form I would significantly suppress malaria transmission, even in the presence of high levels of resistance. The simple preparation of form II, coupled with its kinetic stability and markedly higher efficacy, argues that form II can provide a powerful, timely, and affordable malaria control solution for low-income countries that are losing protection in the face of worldwide pyrethroid resistance.

Development of an Insecticide-Free Trapping Bednet to Control Mosquitoes and Manage Resistance in Malaria Vector Control: A New Way of Thinking

Simple Summary

Insecticide resistance in mosquitoes reduces the effectiveness of malaria control interventions and has reversed the gains made in reducing malaria morbidity. Hence, new strategies are needed to mitigate the spread of resistance, preserve the efficacy of available insecticides, and restore the effectiveness of control. To combat resistance to insecticides in malaria mosquitoes, WHO recommends using long-lasting insecticide-impregnated mosquito bednets (LLINs) as well as the synergist piperonyl-butoxide (PBO). PBO enhances the insecticidal effect of the treated bednet. Unfortunately, decreases in performance of PBO-LLINs are now reported in some regions of Africa where mosquitoes are resistant to insecticides. Our objective was to develop an insecticide-free, mechanical solution that kills mosquitoes regardless of their insecticide resistance status, ultimately overcoming the problem of insecticide resistance. We designed and developed an insecticide-free mosquito trapping bednet for mass mosquito trapping and killing, the “T-Net”, and we show its efficacy compared to a conventional LLIN in Africa. Mathematical models were also developed to predict T-Net efficacy in individual homes and at the community level.

Abstract

Mosquito-borne malaria kills 429,000 people each year with the problem being acute in sub-Saharan Africa. The successes gained with long-lasting pyrethroid-treated bednets are now in jeopardy because of wide-spread, pyrethroid resistance in mosquitoes. Using crowd modeling theory normalized for standard bednet architecture, we were able to design an attract–trap–kill technology for mosquitoes that does not require insecticides. Using three-dimensional polyester knitting and heat fixation, trap funnels were developed with high capture efficacy with no egression under worst-case laboratory conditions. Field testing in Africa in WHO huts with Gen1-3 T (trap)-Nets validated our model, and as predicted, Gen3 had the highest efficacy with a 4.3-fold greater trap–kill rate with no deterrence or repellency compared to Permanet 2.0, the most common bednet in Africa. A T-Net population model was developed based on field data to predict community-level mosquito control compared to a pyrethroid bednet. This model showed the Gen3 non-insecticidal T-Net under field conditions in Africa against pyrethroid resistant mosquitoes was 12.7-fold more efficacious than single chemical, pyrethroid-treated nets.