Assessing the efficacy of two dual-active ingredients long-lasting insecticidal nets for the control of malaria transmitted by pyrethroid-resistant vectors in Benin: study protocol for a three-arm, single-blinded, parallel, cluster-randomized controlled trial

Effectiveness of pyriproxyfen-pyrethroid and chlorfenapyr-pyrethroid long-lasting insecticidal nets (LLINs) compared with pyrethroid-only LLINs for malaria control in the third year post-distribution: a secondary analysis of a cluster-randomised controlled trial in Benin

BACKGROUND

Malaria continues to kill approximately 650 000 people each year. There is evidence that some second-generation insecticide-treated nets, which combine insecticide formulations with different modes of action, are protective against malaria while the nets are new; however, evidence for their impact over 3 years is scarce. In this study, we report the third-year results of a cluster-randomised controlled trial assessing the long-term effectiveness of dual-active ingredient long-lasting insecticidal nets (LLINs).

METHODS

This is a secondary analysis of a cluster-randomised controlled trial, carried out between May 23, 2019, and April 30, 2023, in southern Benin. Restricted randomisation was used to assign 60 clusters (villages or groups of villages with a minimum of 100 households) to the three study groups (1:1:1) to evaluate the efficacy of pyriproxyfen-pyrethroid LLINs and chlorfenapyr-pyrethroid LLINs compared with pyrethroid-only LLINs (reference) against malaria transmission. The study staff and communities were masked to the group allocation. The primary outcome was malaria incidence measured over the third year after LLIN distribution, in a cohort of children aged 6 months to 9 years at the time of enrolment, in the intention-to-treat population. Here, we present the data of the third year post-LLIN distribution. The trial was registered with ClinicalTrials.gov, NCT03931473.

FINDINGS

Study net use declined over the 3 years and was consistently lowest in the pyriproxyfen-pyrethroid LLIN group (at 36 months: 889 [39·4%] of 2257 participants vs 1278 [52·2%] of 2450 participants for the chlorfenapyr-pyrethroid LLIN group and 1400 [57·6%] of 2430 participants for the pyrethroid-only LLIN group). The cohort of children for the third year of follow-up (600 per group) were enrolled between April 9 and 30, 2022. Mean malaria incidence during the third year after distribution was 1·19 cases per child-year (95% CI 1·09-1·29) in the pyrethroid-only LLIN reference group, 1·21 cases per child-year (1·12-1·31) in the pyriproxyfen-pyrethroid LLIN group (hazard ratio [HR] 1·02, 95% CI 0·71-1·44; p=0·92), and 0·96 cases per child-year (0·88-1·05) in the chlorfenapyr-pyrethroid LLIN group (HR 0·80, 0·56-1·17; p=0·25). No adverse events related to study nets were reported by participants.

INTERPRETATION

During the third year, as was also observed during the first 2 years, the pyriproxyfen-pyrethroid LLIN group did not have superior protection against malaria cases compared with the standard LLIN group. In the third year, people living in the chlorfenapyr-pyrethroid LLIN group no longer benefited from greater protection against malaria cases and infections than those living in the pyrethroid-only LLIN group. This was probably influenced by lower study net use than previous years and the declining concentration of partner insecticides in the nets.

FUNDING

UNITAID, The Global Fund.

TRANSLATION

For the French translation of the abstract see Supplementary Materials section.

Insecticide resistance of Miami-Dade Culex quinquefasciatus populations and initial field efficacy of a new resistance-breaking adulticide formulation

Sporadic outbreaks of human cases of West Nile virus (WNV), primarily vectored by Culex quinquefasciatus Say in suburban and urban areas, have been reported since introduction of the virus into Florida in 2001. Miami-Dade County, Florida is part of one of the largest metropolitan areas in the United States, supports Cx. quinquefasciatus year-round, and recently experienced over 60 human cases of WNV during one outbreak. To facilitate more effective integrated vector management and public health protection, we used the Centers for Disease Control and Prevention (CDC) bottle bioassay method to evaluate the susceptibility of adult Cx. quinquefasciatus collected from 29 locations throughout Miami-Dade County to pyrethroid and organophosphate adulticide active ingredients (AIs) used by Miami-Dade County Mosquito Control. We also determined the frequency of the 1014 knockdown resistance (kdr) mutation for Cx. quinquefasciatus from a subset of 17 locations. We detected resistance to two pyrethroid AIs in all tested locations (permethrin: 27 locations, deltamethrin: 28 locations). The 1014F allele was widely distributed throughout all 17 locations sampled; however, 29.4% of these locations lacked 1014F homozygotes even though phenotypic pyrethroid resistance was present. Organophosphate resistance was more variable; 20.7% of the locations tested were susceptible to malathion, and 33.3% of the populations were susceptible to naled. We subsequently conducted a field trial of ReMoa Tri, a recently approved multiple AI adulticide formulation labelled for resistant mosquitoes, against a mixed location field population of Miami-Dade Cx. quinquefasciatus. Average 24-hr mortality was 65.1 ± 7.2% and 48-hr mortality increased to 85.3 ± 9.1%, indicating good control of these resistant Cx. quinquefasciatus. This current study shows that insecticide resistance is common in local Cx. quinquefasciatus but effective options are available to maintain control during active disease transmission in Miami-Dade County.

The impact of pyrethroid-pyriproxyfen and pyrethroid-chlorfenapyr long-lasting insecticidal nets on density of primary malaria vectors Anopheles gambiae s.s. and Anopheles coluzzii in Benin: a secondary analysis of a cluster randomised controlled trial

BACKGROUND

Long-lasting insecticidal nets (LLINs) may have different impacts on distinct mosquito vector species. We assessed the efficacy of pyrethroid-pyriproxyfen and pyrethroid-chlorfenapyr LLINs on the density of Anopheles gambiae s.s. and An. coluzzii compared to pyrethroid-only nets in a three-arm cluster randomised control trial in Benin.

METHODS

Indoor and outdoor collections of adult mosquitoes took place in 60 clusters using human landing catches at baseline and every 3 months for 2 years. After morphological identification, around 15% of randomly selected samples of An. gambiae s.l. were dissected to determine parity, species (using PCR).

RESULTS

Overall, a total of 46,613 mosquito specimens were collected at baseline and 259,250 in the eight quarterly collections post-net distribution. Post-net distribution, approximately 70% of the specimens of An. gambiae s.l. speciated were An. coluzzii, while the rest were mostly composed of An. gambiae s.s. with a small proportion (< 1%) of hybrids (An. gambiae/coluzzii). There was no evidence of a significant reduction in vector density indoors in either primary vector species [An. coluzzii: DR (density ratio) = 0.62 (95% CI 0.21-1.77), p = 0.3683 for the pyrethroid-pyriproxyfen LLIN and DR = 0.56 (95% CI 0.19-1.62), p = 0.2866 for the pyrethroid-chlorfenapyr LLIN, An. gambiae s.s.: DR = 0.52 (95% CI 0.18-1.46), p = 0.2192 for the pyrethroid-pyriproxyfen LLIN and DR = 0.53 (95% CI 0.19-1.46), p = 0.2222 for the pyrethroid-chlorfenapyr]. The same trend was observed outdoors. Parity rates of An. gambiae s.l. were also similar across study arms.

CONCLUSIONS

Compared with pyrethroid-only LLINs, pyrethroid-chlorfenapyr LLINs and pyrethroid-pyriproxyfen LLINs performed similarly against the two primary mosquito species An. gambiae s.s. and An. coluzzii in Benin.

Effectiveness of piperonyl butoxide and pyrethroid-treated long-lasting insecticidal nets (LLINs) versus pyrethroid-only LLINs with and without indoor residual spray against malaria infection: third year results of a cluster, randomised controlled, two-by-two factorial design trial in Tanzania

BACKGROUND

After decades of success in reducing malaria through the scale-up of pyrethroid long-lasting insecticidal nets (LLINs), the decline in the malaria burden has stalled, coinciding with the rapid spread of pyrethroid resistance. In a previously reported study, nets treated with a pyrethroid and a synergist, piperonyl butoxide (PBO), demonstrated superior efficacy compared to standard pyrethroid LLINs (std-LLINs) against malaria. Evidence was used to support the public health recommendation of PBO-Pyrethroid-LLIN by the World Health Organization in 2018. This study looks at the third year of rollout of these nets in Muleba district, Tanzania to inform whether policy guidelines need to be updated.

METHODS

A four-group cluster randomized trial (CRT) using a two-by-two factorial design was carried out between January 2014 and December 2017. A total of 48 clusters, were randomized in a 1:1:1:1 ratio to the following treatment groups, each intervention being provided once in 2015: 1/std-LLIN; 2/PBO-pyrethroid LLIN; 3/std-LLIN + Indoor Residual Spraying (IRS) and 4/PBO-Pyrethroid-LLIN + IRS. During the third year follow-up, malaria infection prevalence in 80 children per cluster, aged 6 months to 14 years, was measured at 28- and 33-months post-intervention and analysed as intention-to-treat (ITT) and per protocol (PP). Mosquito collections were performed monthly in all clusters, using CDC light traps in 7 randomly selected houses per cluster.

RESULTS

At 28 and 33 months, study net usage among household participants was only 47% and 31%, respectively. In ITT analysis, after 28 months malaria infection prevalence among 7471 children was 80.9% in the two std-LLIN groups compared to 69.3% in the two PBO-Pyrethroid-LLIN (Odds Ratio: 0.45, 95% Confidence Interval: 0.21-0.95, p-value: 0.0364). After 33 months the effect was weaker in the ITT analysis (prevalence 59.6% versus 49.9%, OR: 0.60, 95%CI:0.32-1.13, p-value: 0.1131) but still evident in the PP analysis (57.2% versus 44.2%, OR: 0.34, 95%CI: 0.16-0.71, p-value: 0.0051). Mean number of Anopheles per night collected per house was similar between PBO-Pyrethroid-LLIN groups (5.48) and std-LLIN groups (5.24) during the third year.

CONCLUSIONS

Despite low usage of PBO- Pyrethroid LLIN, a small impact of those nets on malaria infection prevalence was still observed in the 3rd year with the most protection offered to children still using them. To maximize impact, it is essential that net re-distribution cycles are aligned with this LLIN lifespan to maintain maximum coverage.

TRIAL REGISTRATION

The trial was registered with ClinicalTrials.gov (registration number NCT02288637).

Evaluating the attrition, fabric integrity and insecticidal durability of two dual active ingredient nets (Interceptor® G2 and Royal® Guard): methodology for a prospective study embedded in a cluster randomized controlled trial in Benin

BACKGROUND

Following the World Health Organization (WHO) endorsement of dual active ingredient (AI) nets, an increased uptake of pyrethroid-chlorfenapyr and pyrethroid-pyriproxyfen nets is expected. Studies evaluating their physical and insecticidal durability are essential for making programmatic and procurement decisions. This paper describes the methodology for a prospective study to evaluate the attrition, fabric integrity, insecticidal durability of Interceptor® G2 (alpha-cypermethrin-chlorfenapyr) and Royal Guard® (alpha-cypermethrin-pyriproxyfen), compared to Interceptor® (alpha-cypermethrin), embedded in a 3-arm cluster randomized controlled trial (cRCT) in the Zou Department of Benin.

METHODS

Ten clusters randomly selected from each arm of the cRCT will be used for the study. A total of 750 ITNs per type will be followed in 5 study clusters per arm to assess ITN attrition and fabric integrity at 6-, 12-, 24- and 36-months post distribution, using standard WHO procedures. A second cohort of 1800 nets per type will be withdrawn every 6 months from all 10 clusters per arm and assessed for chemical content and biological activity in laboratory bioassays at each time point. Alpha-cypermethrin bioefficacy in Interceptor® and Royal Guard® will be monitored in WHO cone bioassays and tunnel tests using the susceptible Anopheles gambiae Kisumu strain. The bioefficacy of the non-pyrethroid insecticides (chlorfenapyr in Interceptor® G2 and pyriproxyfen in Royal Guard®) will be monitored using the pyrethroid-resistant Anopheles coluzzii Akron strain. Chlorfenapyr activity will be assessed in tunnel tests while pyriproxyfen activity will be assessed in cone bioassays in terms of the reduction in fertility of blood-fed survivors observed by dissecting mosquito ovaries. Nets withdrawn at 12, 24 and 36 months will be tested in experimental hut trials within the cRCT study area against wild free-flying pyrethroid resistant An. gambiae sensu lato to investigate their superiority to Interceptor® and to compare them to ITNs washed 20 times for experimental hut evaluation studies. Mechanistic models will also be used to investigate whether entomological outcomes with each dual ITN type in experimental hut trials can predict their epidemiological performance in the cRCT.

CONCLUSION

This study will provide information on the durability of two dual AI nets (Interceptor® G2 and Royal Guard®) in Benin and will help identify suitable methods for monitoring the durability of their insecticidal activity under operational conditions. The modelling component will determine the capacity of experimental hut trials to predict the epidemiological performance of dual AI nets across their lifespan.

Ivermectin as a novel malaria control tool: Getting ahead of the resistance curse

Reduction in malaria clinical cases is strongly dependent on the ability to prevent Anopheles infectious bites. Vector control strategies using long-lasting insecticidal nets and indoor residual spraying with insecticides have contributed to significantly reduce the incidence of malaria in many endemic countries, especially in the Sub-Saharan region. However, global progress in reducing malaria cases has plateaued since 2015 mostly due to the increased insecticide resistance and behavioral changes in Anopheles vectors. Additional control strategies are thus required to further reduce the burden of malaria and contain the spread of resistant and invasive Anopheles vectors. The use of endectocides such as ivermectin as an additional malaria control tool is now receiving increased attention, driven by its different mode of action compared to insecticides used so far and its excellent safety record for humans. In this opinion article, we discuss the advantages and disadvantages of using ivermectin for malaria control with a focus on the risk of selecting ivermectin resistance in malaria vectors. We also highlight the importance of understanding how ivermectin resistance could develop in mosquitoes and what its underlying mechanisms and associated molecular markers are, and propose a research agenda to manage this phenomenon.

High efficacy of chlorfenapyr-based net Interceptor® G2 against pyrethroid-resistant malaria vectors from Cameroon

BACKGROUND

The increasing reports of resistance to pyrethroid insecticides associated with reduced efficacy of pyrethroid-only interventions highlight the urgency of introducing new non-pyrethroid-only control tools. Here, we investigated the performance of piperonyl-butoxide (PBO)-pyrethroid [Permanet 3.0 (P3.0)] and dual active ingredients (AI) nets [Interceptor G2 (IG2): containing pyrethroids and chlorfenapyr and Royal Guard (RG): containing pyrethroids and pyriproxyfen] compared to pyrethroid-only net Royal Sentry (RS) against pyrethroid-resistant malaria vectors in Cameroon.

METHODS

The efficacy of these tools was firstly evaluated on Anopheles gambiae s.l. and Anopheles funestus s.l. from Gounougou, Mibellon, Mangoum, Nkolondom, and Elende using cone/tunnel assays. In addition, experimental hut trials (EHT) were performed to evaluate the performance of unwashed and 20 times washed nets in semi-field conditions. Furthermore, pyrethroid-resistant markers were genotyped in dead vs alive, blood-fed vs unfed mosquitoes after exposure to the nets to evaluate the impact of these markers on net performance. The XLSTAT software was used to calculate the various entomological outcomes and the Chi-square test was used to compare the efficacy of various nets. The odds ratio and Fisher exact test were then used to establish the statistical significance of any association between insecticide resistance markers and bed net efficacy.

RESULTS

Interceptor G2 was the most effective net against wild pyrethroid-resistant An. funestus followed by Permanet 3.0. In EHT, this net induced up to 87.8% mortality [95% confidence interval (CI): 83.5-92.1%) and 55.6% (95% CI: 48.5-62.7%) after 20 washes whilst unwashed pyrethroid-only net (Royal Sentry) killed just 18.2% (95% CI: 13.4-22.9%) of host-seeking An. funestus. The unwashed Permanet 3.0 killed up to 53.8% (95% CI: 44.3-63.4%) of field-resistant mosquitoes and 47.2% (95% CI: 37.7-56.7%) when washed 20 times, and the Royal Guard 13.2% (95% CI: 9.0-17.3%) for unwashed net and 8.5% (95% CI: 5.7-11.4%) for the 20 washed net. Interceptor G2, Permanet 3.0, and Royal Guard provided better personal protection (blood-feeding inhibition 66.2%, 77.8%, and 92.8%, respectively) compared to pyrethroid-only net Royal Sentry (8.4%). Interestingly, a negative association was found between kdrw and the chlorfenapyr-based net Interceptor G2 (χ2 = 138; P < 0.0001) with homozygote-resistant mosquitoes predominantly found in the dead ones.

CONCLUSIONS

The high mortality recorded with Interceptor G2 against pyrethroid-resistant malaria vectors in this study provides first semi-field evidence of high efficacy against these major malaria vectors in Cameroon encouraging the implementation of this novel net for malaria control in the country. However, the performance of this net should be established in other locations and on other major malaria vectors before implementation at a large scale.

Assessing the susceptibility and efficacy of traditional neurotoxic (pyrethroid) and new-generation insecticides (chlorfenapyr, clothianidin, and pyriproxyfen), on wild pyrethroid-resistant populations of Anopheles gambiae from southern Benin

BACKGROUND

The objective of this study was to determine the susceptibility of wild Anopheles gambiae sensu lato (s.l.) from southern Benin to the new insecticides (chlorfenapyr (CFP), pyriproxyfen (PPF), and clothianidin (CTD)) and assess the efficacy of insecticide-treated bed nets (ITNs) that contain these new products.

METHODS

Wild An. gambiae from the Benin communes of Allada, Ifangni, Akpro-Missérété, and Porto-Novo were tested for their susceptibility to CFP and PPF using the WHO bottle tests, and pyrethroids (alpha-cypermethrin, deltamethrin, and permethrin) and CTD using WHO tube tests. WHO cone tests were used to evaluate the efficacy of Interceptor® (which contains alpha-cypermethrin (ACM) only), Interceptor® G2, (CFP + ACM), and Royal Guard® nets (PPF + ACM). The ovaries of blood-fed An. gambiae from Ifangni exposed to a new PPF net were dissected, and egg development status was examined using Christopher's stages to determine the fertility status of the mosquitoes. Using a standardized protocol, the oviposition rate and oviposition inhibition rate were calculated from live blood-fed An. gambiae placed in oviposition chambers after exposure to PPF.

RESULTS

In all four mosquito populations, pyrethroid mortality ranged from 5 to 80%, while chlorfenapyr and clothianidin mortality ranged from 98 to 100%. At Ifangni, all mosquitoes exposed to Royal Guard® nets were infertile (100%) while the majority (74.9%) of mosquitoes exposed to Interceptor® nets had fully developed their eggs to Christopher's stage V. The oviposition inhibition rate after exposure of the mosquitoes to the PPF was 99% for the wild population of An. gambiae s.l. and the susceptible laboratory strain, An. gambiae sensu stricto (Kisumu).

CONCLUSIONS

The results of this study suggest that pyrethroid-resistant An. gambiae from the selected communes in southern Benin are susceptible to chlorfenapyr, clothianidin, and pyriproxyfen. In addition, based on bioassay results, new and unused Interceptor® G2 and Royal Guard® nets were effective on Ifangni's mosquito populations. Despite the availability of new effective insecticides, continued vigilance is needed in Benin. Therefore, monitoring of resistance to these insecticides will continue to periodically update the Benin national insecticide resistance database and management plan.

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

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

Field performance of three mosquito collection methods for assessing the entomological efficacy of dual-active ingredient long-lasting insecticidal nets

Selection of mosquito collection methods is of crucial importance to evaluate the impact of vector control tools on entomological outcomes. During a cluster randomised control trial evaluating the relative efficacy of two dual-active ingredient (a.i.) long-lasting insecticidal nets (LLINs) compared to pyrethroid-only LLINs, we assessed the performance of different mosquito collection methods: Human landing catches (HLC), Centers for Disease Control and Prevention (CDC) light traps, and pyrethrum spray catches (PSC). Anopheles mosquitoes were collected using three collection methods in 4 houses, in each of the 60 trial clusters at baseline and every quarter for 24 months using PSCs and HLCs, while CDC light traps were performed during two quarters only. Mean density of vectors collected per method per night was the highest with HLCs (15.9), followed by CDC light traps (6.8); with PSCs (1.1) collecting 10 times less mosquitoes than HLCs. All three collection methods collected fewer mosquitoes in the Interceptor G2® dual a.i. arm, compared to the other trial arms, although only HLCs and PSCs demonstrated strong evidence of this due to a greater number of collection rounds undertaken, than CDC light traps. The broadly similar results regarding the differential impact of the two dual a.i. LLINs showed by the three collection methods suggest that the more ethically acceptable, cheaper, and logistically simpler methods such as CDC light traps could be prioritised for use in large community trials for measuring the efficacy of vector control tools.

Efficacy of Pyrethroid-Pyriproxyfen and Pyrethroid-Chlorfenapyr Long-Lasting Insecticidal Nets (LLINs) for the Control of Non-Anopheles Mosquitoes: Secondary Analysis from a Cluster Randomised Controlled Trial (cRCT)

The efficacy of a vector control tool in reducing mosquito biting is crucial for its acceptability. The present study compared the vector density of Culex spp. And Mansonia spp. across clusters, which received two dual-active ingredient (a.i.) long-lasting insecticidal nets (LLINs) and a standard pyrethroid-only LLIN, and assessed the seasonality of these mosquito genera. A total of 85,723 Culex spp. and 144,025 Mansonia spp. were caught over the study period. The density of Culex and Mansonia was reduced in all three arms over the study period. There was no evidence of a significant reduction in the indoor or outdoor density of Culex spp. in either dual-a.i. LLIN arm as compared to the standard pyrethroid-only net arm. A similar trend was observed with Mansonia spp. A high density of Culex spp. was found both in rainy and dry seasons, while for Mansonia spp., this was mainly observed during the rainy season. These results suggest that the novel insecticides in the dual-a.i. LLINs did not have an additional impact on these species and that pyrethroids might still be effective on them. Further work is required to determine whether these species of mosquitoes have resistance to the insecticides tested in this trial.

Vector control for malaria prevention during humanitarian emergencies: a systematic review and meta-analysis

BACKGROUND

Humanitarian emergencies can lead to population displacement, food insecurity, severe health system disruptions, and malaria epidemics among individuals who are immunologically naive. We aimed to assess the impact of different vector control interventions on malaria disease burden during humanitarian emergencies.

METHODS

In this systematic review and meta-analysis, we searched ten electronic databases and two clinical trial registries from database inception to Oct 19, 2020, with no restrictions on language or study design. We also searched grey literature from 59 stakeholders. Studies were eligible if the population was affected by a humanitarian emergency in a malaria endemic region. We included studies assessing any vector control intervention and in which the primary outcome of interest was malaria infection risk. Reviewers (LAM, JF-A, KC, BP, and LP) independently extracted information from eligible studies, without masking of author or publication, into a database. We did random-effects meta-analyses to calculate pooled risk ratios (RRs) for randomised controlled trials, odds ratios (ORs) for dichotomous outcomes, and incidence rate ratios (IRR) for clinical malaria in non-randomised studies. Certainty of evidence was evaluated using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. This study is registered with PROSPERO, CRD42020214961.

FINDINGS

Of 12 475 studies screened, 22 studies were eligible for inclusion in our meta-analysis. All studies were conducted between Sept 1, 1989, and Dec 31, 2018, in chronic emergencies, with 616 611 participants from nine countries, evaluating seven different vector control interventions. Insecticide-treated nets significantly decreased Plasmodium falciparum incidence (RR 0·55 [95% CI 0·37-0·79]; high certainty) and Plasmodium vivax incidence (RR 0·69 [0·51-0·94]; high certainty). Evidence for an effect of indoor residual spraying on P falciparum (IRR 0·57 [95% CI 0·53-0·61]) and P vivax (IRR 0·51 [0·49-0·52]) incidence was of very low certainty. Topical repellents were associated with reductions in malaria infection (RR 0·58 [0·35-0·97]; moderate certainty). Moderate-to-high certainty evidence for an effect of insecticide-treated chaddars (equivalent to shawls or blankets) and insecticide-treated cattle on malaria outcomes was evident in some emergency settings. There was very low certainty evidence for the effect of insecticide-treated clothing.

INTERPRETATION

Study findings strengthen and support WHO policy recommendations to deploy insecticide-treated nets during chronic humanitarian emergencies. There is an urgent need to evaluate and adopt novel interventions for malaria control in the acute phase of humanitarian emergencies.

FUNDING

WHO Global Malaria Programme.

Efficacy of pyriproxyfen-pyrethroid long-lasting insecticidal nets (LLINs) and chlorfenapyr-pyrethroid LLINs compared with pyrethroid-only LLINs for malaria control in Benin: a cluster-randomised, superiority trial

BACKGROUND

New classes of long-lasting insecticidal nets (LLINs) combining mixtures of insecticides with different modes of action could put malaria control back on track after rebounds in transmission across sub-Saharan Africa. We evaluated the relative efficacy of pyriproxyfen-pyrethroid LLINs and chlorfenapyr-pyrethroid LLINs compared with standard LLINs against malaria transmission in an area of high pyrethroid resistance in Benin.

METHODS

We conducted a cluster-randomised, superiority trial in Zou Department, Benin. Clusters were villages or groups of villages with a minimum of 100 houses. We used restricted randomisation to randomly assign 60 clusters to one of three LLIN groups (1:1:1): to receive nets containing either pyriproxyfen and alpha-cypermethrin (pyrethroid), chlorfenapyr and alpha-cypermethrin, or alpha-cypermethrin only (reference). Households received one LLIN for every two people. The field team, laboratory staff, analyses team, and community members were masked to the group allocation. The primary outcome was malaria case incidence measured over 2 years after net distribution in a cohort of children aged 6 months-10 years, in the intention-to-treat population. This study is ongoing and is registered with ClinicalTrials.gov, NCT03931473.

FINDINGS

Between May 23 and June 24, 2019, 53 854 households and 216 289 inhabitants were accounted for in the initial census and included in the study. Between March 19 and 22, 2020, 115 323 LLINs were distributed to 54 030 households in an updated census. A cross-sectional survey showed that study LLIN usage was highest at 9 months after distribution (5532 [76·8%] of 7206 participants), but decreased by 24 months (4032 [60·6%] of 6654). Mean malaria incidence over 2 years after LLIN distribution was 1·03 cases per child-year (95% CI 0·96-1·09) in the pyrethroid-only LLIN reference group, 0·84 cases per child-year (0·78-0·90) in the pyriproxyfen-pyrethroid LLIN group (hazard ratio [HR] 0·86, 95% CI 0·65-1·14; p=0·28), and 0·56 cases per child-year (0·51-0·61) in the chlorfenapyr-pyrethroid LLIN group (HR 0·54, 95% CI 0·42-0·70; p<0·0001).

INTERPRETATION

Over 2 years, chlorfenapyr-pyrethroid LLINs provided greater protection from malaria than pyrethroid-only LLINs in an area with pyrethroid-resistant mosquitoes. Pyriproxyfen-pyrethroid LLINs conferred protection similar to pyrethroid-only LLINs. These findings provide crucial second-trial evidence to enable WHO to make policy recommendations on these new LLIN classes. This study confirms the importance of chlorfenapyr as an LLIN treatment to control malaria in areas with pyrethroid-resistant vectors. However, an arsenal of new active ingredients is required for successful long-term resistance management, and additional innovations, including pyriproxyfen, need to be further investigated for effective vector control strategies.

FUNDING

UNITAID, The Global Fund.

Small-scale field evaluation of PermaNet® Dual (a long-lasting net coated with a mixture of chlorfenapyr and deltamethrin) against pyrethroid-resistant Anopheles gambiae mosquitoes from Tiassalé, Côte d'Ivoire

BACKGROUND

Due to the rapid expansion of pyrethroid-resistance in malaria vectors in Africa, Global Plan for Insecticide Resistance Management (GPIRM) has recommended the development of long-lasting insecticidal nets (LLINs), containing insecticide mixtures of active ingredients with different modes of action to mitigate resistance and improve LLIN efficacy. This good laboratory practice (GLP) study evaluated the efficacy of the chlorfenapyr and deltamethrin-coated PermaNet^® Dual, in comparison with the deltamethrin and synergist piperonyl butoxide (PBO)-treated PermaNet^® 3.0 and the deltamethrin-coated PermaNet^® 2.0, against wild free-flying pyrethroid-resistant Anopheles gambiae sensu lato (s.l.), in experimental huts in Tiassalé, Côte d'Ivoire (West Africa).

METHODS

PermaNet^® Dual, PermaNet^® 3.0 and PermaNet^® 2.0, unwashed and washed (20 washes), were tested against free-flying pyrethroid-resistant An. gambiae s.l. in the experimental huts in Tiassalé, Côte d'Ivoire from March to August 2020. Complementary laboratory cone bioassays (daytime and 3-min exposure) and tunnel tests (nightly and 15-h exposure) were performed against pyrethroid-susceptible An. gambiae sensu stricto (s.s.) (Kisumu strain) and pyrethroid-resistant An. gambiae s.l. (Tiassalé strain).

RESULTS

PermaNet^® Dual demonstrated significantly improved efficacy, compared to PermaNet^® 3.0 and PermaNet^® 2.0, against the pyrethroid-resistant An. gambiae s.l. Indeed, the experimental hut trial data showed that the mortality and blood-feeding inhibition in the wild pyrethroid-resistant An. gambiae s.l. were overall significantly higher with PermaNet^® Dual compared with PermaNet^® 3.0 and PermaNet^® 2.0, for both unwashed and washed samples. The mortality with unwashed and washed samples were 93.6 ± 0.2% and 83.2 ± 0.9% for PermaNet^® Dual, 37.5 ± 2.9% and 14.4 ± 3.9% for PermaNet^® 3.0, and 7.4 ± 5.1% and 11.7 ± 3.4% for PermaNet^® 2.0, respectively. Moreover, unwashed and washed samples produced the respective percentage blood-feeding inhibition of 41.4 ± 6.9% and 43.7 ± 4.8% with PermaNet^® Dual, 51.0 ± 5.7% and 9.8 ± 3.6% with PermaNet^® 3.0, and 12.8 ± 4.3% and - 13.0 ± 3.6% with PermaNet^® 2.0. Overall, PermaNet^® Dual also induced higher or similar deterrence, exophily and personal protection when compared with the standard PermaNet^® 3.0 and PermaNet^® 2.0 reference nets, with both unwashed and washed net samples. In contrast to cone bioassays, tunnel tests predicted the efficacy of PermaNet^® Dual seen in the current experimental hut trial.

CONCLUSION

The deltamethrin-chlorfenapyr-coated PermaNet^® Dual induced a high efficacy and performed better than the deltamethrin-PBO PermaNet^® 3.0 and the deltamethrin-only PermaNet^® 2.0, testing both unwashed and 20 times washed samples against the pyrethroid-susceptible and resistant strains of An. gambiae s.l. The inclusion of chlorfenapyr with deltamethrin in PermaNet^® Dual net greatly improved protection and control of pyrethroid-resistant An. gambiae populations. PermaNet^® Dual thus represents a promising tool, with a high potential to reduce malaria transmission and provide community protection in areas compromised by mosquito vector resistance to pyrethroids.

Community-level impacts of the coronavirus pandemic on malaria prevention and health-seeking behaviours in rural Benin: A mixed methods study

Globally, negative impacts of the COVID-19 pandemic on malaria prevention and control efforts have been caused by delayed distributions of long-lasting insecticidal nets (LLIN), decreased outpatient attendance, and disruptions to malaria testing and treatment. Using a mixed methods approach, we aimed to evaluate the impact of COVID-19 on community-level malaria prevention and health-seeking practices in Benin more than one year after the start of the COVID-19 pandemic. We collected data through community-based cross-sectional surveys with 4200 households and ten focus group discussions (FGDs). Mixed effect logistic regression models accounting for a clustered sampling design were used to identify variables associated with main outcomes (good COVID-19 knowledge, LLIN usage and access, and avoidance of health centres). Consistent with the experiences of FGD participants, receiving information from radios or televisions was significantly associated with good COVID-19 knowledge and avoiding health centres because of the pandemic (p<0.001 for both). Qualitative findings also revealed varying and polarizing changes in health-seeking behaviours with participants noting that they either did not change their health-seeking behaviours or went to health centres less or more often because of the pandemic. LLIN usage and access did not decrease in the study area because of the pandemic (LLIN usage: 88% in 2019 to 99.9% in 2021; LLIN access: 62% in 2019 to 73% in 2021). An unexpected change and unintended challenge for sustained malaria prevention included families socially distancing in their homes, resulting in a shortage of LLINs. Our findings showed that there were minimal community-level impacts of the coronavirus pandemic on malaria prevention and health seeking behaviours in rural Benin, which highlights the importance of efforts to sustain malaria prevention and control interventions in the context of the COVID-19 pandemic.

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

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

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

Strain Characterisation for Measuring Bioefficacy of ITNs Treated with Two Active Ingredients (Dual-AI ITNs): Developing a Robust Protocol by Building Consensus

Durability monitoring of insecticide-treated nets (ITNs) containing a pyrethroid in combination with a second active ingredient (AI) must be adapted so that the insecticidal bioefficacy of each AI can be monitored independently. An effective way to do this is to measure rapid knock down of a pyrethroid-susceptible strain of mosquitoes to assess the bioefficacy of the pyrethroid component and to use a pyrethroid-resistant strain to measure the bioefficacy of the second ingredient. To allow robust comparison of results across tests within and between test facilities, and over time, protocols for bioefficacy testing must include either characterisation of the resistant strain, standardisation of the mosquitoes used for bioassays, or a combination of the two. Through a series of virtual meetings, key stakeholders and practitioners explored different approaches to achieving these goals. Via an iterative process we decided on the preferred approach and produced a protocol consisting of characterising mosquitoes used for bioefficacy testing before and after a round of bioassays, for example at each time point in a durability monitoring study. We present the final protocol and justify our approach to establishing a standard methodology for durability monitoring of ITNs containing pyrethroid and a second AI.

Durability of three types of dual active ingredient long-lasting insecticidal net compared to a pyrethroid-only LLIN in Tanzania: methodology for a prospective cohort study nested in a cluster randomized controlled trial

BACKGROUND

Progress achieved by long-lasting insecticidal nets (LLINs) against malaria is threatened by widespread selection of pyrethroid resistance among vector populations. LLINs with non-pyrethroid insecticides are urgently needed. This study aims to assess the insecticide and textile durability of three classes of dual-active ingredient (A.I.) LLINs using techniques derived from established WHO LLIN testing methods to set new standards of evaluation.

METHODS

A WHO Phase 3 active ingredients and textile durability study will be carried out within a cluster randomized controlled trial in 40 clusters in Misungwi district, Tanzania. The following treatments will be evaluated: (1) Interceptor®G2 combining chlorfenapyr and the pyrethroid alpha-cypermethrin, (2) Royal Guard® treated with pyriproxyfen and alpha-cypermethrin, (3) Olyset™ Plus which incorporates a synergist piperonyl butoxide and the pyrethroid permethrin, and (4) a reference standard alpha-cypermethrin only LLIN (Interceptor®). 750 nets will be followed in 5 clusters per intervention arm at 6, 12, 24 and 36 months post distribution for survivorship and hole index assessment. A second cohort of 1950 nets per net type will be identified in 10 clusters, of which 30 LLINs will be withdrawn for bio-efficacy and chemical analysis every 6 months up to 36 months and another 30 collected for experimental hut trials every year. Bio-efficacy will be assessed using cone bioassays and tunnel tests against susceptible and resistant laboratory strains of Anopheles gambiae sensu stricto. Efficacy of field-collected nets will be compared in six experimental huts. The main outcomes will be Anopheles mortality up to 72 h post exposure, blood feeding and egg maturation using ovary dissection to assess impact on fecundity.

CONCLUSIONS

Study findings will help develop bio-efficacy and physical durability criteria for partner A.I., in relation to the cRCT epidemiological and entomological outcomes, and refine preferred product characteristics of each class of LLIN. If suitable, the bioassay and hut outcomes will be fitted to transmission models to estimate correlation with cRCT outcomes.

TRIAL REGISTRATION NUMBER

NCT03554616.

Design and methods for a quasi-experimental pilot study to evaluate the impact of dual active ingredient insecticide-treated nets on malaria burden in five regions in sub-Saharan Africa

BACKGROUND

Vector control tools have contributed significantly to a reduction in malaria burden since 2000, primarily through insecticidal-treated bed nets (ITNs) and indoor residual spraying. In the face of increasing insecticide resistance in key malaria vector species, global progress in malaria control has stalled. Innovative tools, such as dual active ingredient (dual-AI) ITNs that are effective at killing insecticide-resistant mosquitoes have recently been introduced. However, large-scale uptake has been slow for several reasons, including higher costs and limited evidence on their incremental effectiveness and cost-effectiveness. The present report describes the design of several observational studies aimed to determine the effectiveness and cost-effectiveness of dual-AI ITNs, compared to standard pyrethroid-only ITNs, at reducing malaria transmission across a variety of transmission settings.

METHODS

Observational pilot studies are ongoing in Burkina Faso, Mozambique, Nigeria, and Rwanda, leveraging dual-AI ITN rollouts nested within the 2019 and 2020 mass distribution campaigns in each country. Enhanced surveillance occurring in select study districts include annual cross-sectional surveys during peak transmission seasons, monthly entomological surveillance, passive case detection using routine health facility surveillance systems, and studies on human behaviour and ITN use patterns. Data will compare changes in malaria transmission and disease burden in districts receiving dual-AI ITNs to similar districts receiving standard pyrethroid-only ITNs over three years. The costs of net distribution will be calculated using the provider perspective including financial and economic costs, and a cost-effectiveness analysis will assess incremental cost-effectiveness ratios for Interceptor® G2, Royal Guard®, and piperonyl butoxide ITNs in comparison to standard pyrethroid-only ITNs, based on incidence rate ratios calculated from routine data.

CONCLUSIONS

Evidence of the effectiveness and cost-effectiveness of the dual-AI ITNs from these pilot studies will complement evidence from two contemporary cluster randomized control trials, one in Benin and one in Tanzania, to provide key information to malaria control programmes, policymakers, and donors to help guide decision-making and planning for local malaria control and elimination strategies. Understanding the breadth of contexts where these dual-AI ITNs are most effective and collecting robust information on factors influencing comparative effectiveness could improve uptake and availability and help maximize their impact.

Pyriproxyfen-treated bed nets reduce reproductive fitness and longevity of pyrethroid-resistant Anopheles gambiae under laboratory and field conditions

Background

The efficacy of insecticide-treated nets (ITNs) containing the insect growth regulator pyriproxyfen (PPF) and pyrethroid insecticides (PPF-ITNs) is being assessed in clinical trials to determine whether they provide greater protection from malaria than standard pyrethroid-treated ITNs in areas where mosquitoes are resistant to pyrethroids. Understanding the entomological mode of action of this new ITN class will aide interpretation of the results from these trials.

Methods

Anopheles gambiae sensu lato (s.l.) mosquitoes from a susceptible laboratory strain were exposed to PPF-treated netting 24 h, 6 h, and immediately prior to, or 24 h post blood feeding, and the impact on fecundity, fertility and longevity recorded. Pyrethroid-resistant populations were exposed to nets containing permethrin and PPF (PPF-ITNs) in cone bioassays and daily mortality recorded. Mosquitoes were also collected from inside houses pre- and post-distribution of PPF-ITNs in a clinical trial conduced in Burkina Faso; female An. gambiae s.l. were then assessed for fecundity and fertility.

Results

PPF exposure reduced the median adult lifespan of insecticide-susceptible mosquitoes by 4 to 5 days in all exposure times (p < 0.05) other than 6 h pre-blood meal and resulted in almost complete lifelong sterilization. The longevity of pyrethroid-resistant mosquitoes was also reduced by at least 5 days after exposure to PPF-ITNs compared to untreated nets, but was unaffected by exposure to standard pyrethroid only ITNs. A total of 386 blood-fed or gravid An. gambiae s.l. females were collected from five villages between 1 and 12 months before distribution of PPF-ITNs. Of these mosquitoes, 75% laid eggs and the remaining 25% appeared to have normal ovaries upon dissection. In contrast, only 8.6% of the 631 blood-fed or gravid An. gambiae s.l. collected post PPF-ITN distribution successfully oviposited; 276 (43.7%) did not oviposit but had apparently normal ovaries upon dissection, and 301 (47.7%) did not oviposit and had abnormal eggs upon dissection. Egg numbers were also significantly lower (average of 138/female prior distribution vs 85 post distribution, p < 0.05).

Conclusion

Exposure to a mixture of PPF and pyrethroids on netting shortens the lifespan of mosquitoes and reduces reproductive output. Sterilization of vectors lasted at least one year under operational conditions. These findings suggest a longer effective lifespan of PPF-pyrethroid nets than reported previously.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03794-z.