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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Quantifying the impact of interventions against Plasmodium vivax: A model for country-specific use

In order to evaluate the impact of various intervention strategies on Plasmodium vivax dynamics in low endemicity settings without significant seasonal pattern, we introduce a simple mathematical model that can be easily adapted to reported case numbers similar to that collected by surveillance systems in various countries. The model includes case management, vector control, mass drug administration and reactive case detection interventions and is implemented in both deterministic and stochastic frameworks. It is available as an R package to enable users to calibrate and simulate it with their own data. Although we only illustrate its use on fictitious data, by simulating and comparing the impact of various intervention combinations on malaria risk and burden, this model could be a useful tool for strategic planning, implementation and resource mobilization.

Modeling resource allocation strategies for insecticide-treated bed nets to achieve malaria eradication

Large reductions in the global malaria burden have been achieved, but plateauing funding poses a challenge for progressing towards the ultimate goal of malaria eradication. Using previously published mathematical models of Plasmodium falciparum and Plasmodium vivax transmission incorporating insecticide-treated nets (ITNs) as an illustrative intervention, we sought to identify the global funding allocation that maximized impact under defined objectives and across a range of global funding budgets. The optimal strategy for case reduction mirrored an allocation framework that prioritizes funding for high-transmission settings, resulting in total case reductions of 76% and 66% at intermediate budget levels, respectively. Allocation strategies that had the greatest impact on case reductions were associated with lesser near-term impacts on the global population at risk. The optimal funding distribution prioritized high ITN coverage in high-transmission settings endemic for P. falciparum only, while maintaining lower levels in low-transmission settings. However, at high budgets, 62% of funding was targeted to low-transmission settings co-endemic for P. falciparum and P. vivax. These results support current global strategies to prioritize funding to high-burden P. falciparum-endemic settings in sub-Saharan Africa to minimize clinical malaria burden and progress towards elimination, but highlight a trade-off with 'shrinking the map' through a focus on near-elimination settings and addressing the burden of P. vivax.

In-vitro susceptibility and ex-vivo evaluation of macrocyclic lactone endectocides sub-lethal concentrations against Plasmodium vivax oocyst development in Anopheles arabiensis

BACKGROUND

Asymptomatic malaria transmission has become a public health concern across malaria-endemic Africa including Ethiopia. Specifically, Plasmodium vivax is more efficient at transmitting earlier in the infection and at lower densities than Plasmodium falciparum. Consequently, a greater proportion of individuals infected with P. vivax can transmit without detectable gametocytaemia. Mass treatment of livestock with macrocyclic lactones (MLs), e.g., ivermectin and doramectin, was suggested as a complementary malaria vector tool because of their insecticidal effects. However, the effects of MLs on P. vivax in Anopheles arabiensis has not yet been fully explored. Hence, comparative in-vitro susceptibility and ex-vivo studies were conducted to evaluate the effects of ivermectin, doramectin and moxidectin sub-lethal concentrations on P. vivax oocyst development in An. arabiensis.

METHODS

The 7-day sub-lethal concentrations of 25% (LC25) and 5% (LC5) were determined from in-vitro susceptibility tests on female An. arabiensis in Hemotek® membrane feeding assay. Next, an ex-vivo study was conducted using P. vivax gametocytes infected patient's blood spiked with the LC25 and LC5 of the MLs. At 7-days post-feeding, each mosquito was dissected under a dissection stereo microscope, stained with 0.5% (w/v) mercurochrome solution, and examined for the presence of P. vivax oocysts. Statistical analysis was based on a generalized mixed model with binomially distributed error terms.

RESULTS

A 7-day lethal concentration of 25% (LC25, in ng/mL) of 7.1 (95% CI: [6.3;8.0]), 20.0 (95%CI:[17.8;22.5]) and 794.3 (95%CI:[716.4;1516.3]) were obtained for ivermectin, doramectin and moxidectin, respectively. Similarly, a lethal concentration of 5% (LC5, in ng/mL) of 0.6 (95% CI: [0.5;0.7]), 1.8 (95% CI:[1.6;2.0]) and 53.7 (95% CI:[ 48.4;102.5]) were obtained respectively for ivermectin, doramectin and moxidectin. The oocyst prevalence in treatment and control groups did not differ significantly (p > 0.05) from each other. Therefore, no direct effect of ML endectocides on P. vivax infection in An. arabiensis mosquitoes was observed at the sub-lethal concentration (LC25 and LC5).

CONCLUSIONS

The effects of ivermectin and doramectin on malaria parasite is more likely via indirect effects, particularly by reducing the vectors lifespan and causing mortality before completing the parasite's sporogony cycle or reducing their vector capacity as it affects the locomotor activity of the mosquito.

Effectiveness of long-lasting insecticidal nets with pyriproxyfen-pyrethroid, chlorfenapyr-pyrethroid, or piperonyl butoxide-pyrethroid versus pyrethroid only against malaria in Tanzania: final-year results of a four-arm, single-blind, cluster-randomised trial

BACKGROUND

New classes of long-lasting insecticidal nets (LLINs) containing two active ingredients have been recently recommended by WHO in areas where malaria vectors are resistant to pyrethroids. This policy was based on evidence generated by the first 2 years of our recently published trial in Tanzania. In this Article, we report the final third-year trial findings, which are necessary for assessing the long-term effectiveness of new classes of LLIN in the community and the replacement intervals required.

METHODS

A third year of follow-up of a four-arm, single-blind, cluster-randomised controlled trial of dual active ingredient LLINs was conducted between July 14, 2021, and Feb 10, 2022, in Misungwi, Tanzania. Restricted randomisation was used to assign 84 clusters to the four LLIN groups (1:1:1:1) to receive either standard pyrethroid (PY) LLINs (reference), chlorfenapyr-PY LLINs, pyriproxyfen-PY LLINs, or piperonyl butoxide (PBO)-PY LLINs. All households received one LLIN for every two people. Data collection was done in consenting households in the cluster core area with at least one child between 6 months and 15 years of age who permanently resided in the selected household. Exclusion criteria were householders absent during the visit, living in the cluster buffer area, no adult caregiver capable of giving informed consent, or eligible children who were severely ill. Field staff and study participants were masked to allocation, and those analysing data were not. The primary 24-month endpoint was reported previously; here, we present the secondary outcome, malaria infection prevalence in children at 36 months post LLIN distribution, reported in the intention-to-treat analysis. The trial was registered with ClinicalTrials.gov (NCT03554616) and is now complete.

FINDINGS

Overall usage of study nets was 1023 (22·3%) of 4587 people at 36 months post distribution. In the standard PY LLIN group, malaria infection was prevalent in 407 (37·4%) of 1088 participants, compared with 261 (22·8%) of 1145 in the chlorfenapyr-PY LLIN group (odds ratio 0·57, 95% CI 0·38-0·86; p=0·0069), 338 (32·2%) of 1048 in the PBO-PY LLIN group (0·95, 0·64-1·42; p=0·80), and 302 (28·8%) of 1050 in the pyriproxyfen-PY LLIN group (0·82, 0·55-1·23; p=0·34). None of the participants or caregivers reported side-effects.

INTERPRETATION

Despite low coverage, the protective efficacy against malaria offered by chlorfenapyr-PY LLINs was superior to that provided by standard PY LLINs over a 3-year LLIN lifespan. Appropriate LLIN replacement strategies to maintain adequate usage of nets will be necessary to maximise the full potential of these nets.

FUNDING

Department for International Development, UK Medical Research Council, Wellcome Trust, Department of Health and Social Care, and Bill & Melinda Gates Foundation via the Innovative Vector Control Consortium.

Inference for entomological semi-field experiments: Fitting a mathematical model assessing personal and community protection of vector-control interventions

The effectiveness of vector-control tools is often assessed by experiments as a reduction in mosquito landings using human landing catches (HLCs). However, HLCs alone only quantify a single characteristic and therefore do not provide information on the overall impacts of the intervention product. Using data from a recent semi-field study which used time-stratified HLCs, aspiration of non-landing mosquitoes, and blood feeding, we suggest a Bayesian inference approach for fitting such data to a stochastic model. This model considers both personal protection, through a reduction in biting, and community protection, from mosquito mortality and disarming (prolonged inhibition of blood feeding). Parameter estimates are then used to predict the reduction of vectorial capacity induced by etofenpox-treated clothing, picaridin topical repellents, transfluthrin spatial repellents and metofluthrin spatial repellents, as well as combined interventions for Plasmodium falciparum malaria in Anopleles minimus. Overall, all interventions had both personal and community effects, preventing biting and killing or disarming mosquitoes. This led to large estimated reductions in the vectorial capacity, with substantial impact even at low coverage. As the interventions aged, fewer mosquitoes were killed; however the impact of some interventions changed from killing to disarming mosquitoes. Overall, this inference method allows for additional modes of action, rather than just reduction in biting, to be parameterised and highlights the tools assessed as promising malaria interventions.

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).

Can the performance of pyrethroid-chlorfenapyr nets be reduced when combined with pyrethroid-piperonyl butoxide (PBO) nets?

BACKGROUND

Pyrethroid-chlorfenapyr (CFP) and pyrethroid-piperonyl butoxide (PBO) nets are being scaled across endemic countries to improve control of malaria transmitted by pyrethroid-resistant mosquitoes. CFP is a pro-insecticide requiring activation by mosquito cytochrome P450 monooxygenase enzymes (P450s) while PBO improves pyrethroid potency by inhibiting the action of these enzymes in pyrethroid-resistant mosquitoes. The inhibitory action of PBO against P450s may thus reduce the efficacy of pyrethroid-CFP nets when applied inside the same household as pyrethroid-PBO nets.

METHODS

Two experimental hut trials were performed to evaluate the entomological impact of two different types of pyrethroid-CFP ITN (Interceptor® G2, PermaNet® Dual) when applied alone and in combination with pyrethroid-PBO ITNs (DuraNet® Plus, PermaNet® 3.0) against a pyrethroid-resistant vector population in southern Benin. In both trials, all net types were tested as single and double net treatments. Bioassays were also performed to assess the resistance profile of the vector population at the hut site and investigate interactions between CFP and PBO.

RESULTS

The vector population was susceptible to CFP but exhibited a high intensity of pyrethroid resistance that was overcame by PBO pre-exposure. Vector mortality was significantly lower in huts with combinations of pyrethroid-CFP nets plus pyrethroid-PBO nets compared to huts with two pyrethroid-CFP nets (74% vs. 85% for Interceptor® G2 and 57% vs. 83% for PermaNet® Dual, p < 0.001). PBO pre-exposure reduced the toxicity of CFP in bottle bioassays suggesting this effect may be partly attributable to antagonism between CFP and PBO. Higher levels of vector mortality were observed in huts with net combinations that included pyrethroid-CFP nets compared to those that did not and highest mortality was achieved when pyrethroid-CFP nets were applied alone as two nets together (83-85%).

CONCLUSIONS

This study shows evidence of a reduced performance of pyrethroid-CFP nets when combined with pyrethroid-PBO ITNs compared to when applied alone and higher efficacy with net combinations that included pyrethroid-CFP nets. These findings suggest that in similar contexts, prioritizing distribution of pyrethroid-CFP nets over other net types would maximize vector control impact.

How many mosquito nets are needed to maintain universal coverage: an update

BACKGROUND

Insecticide-treated nets (ITNs) have served as the cornerstone of malaria vector control in sub-Saharan Africa for the past two decades. Over 2.5 billion ITNs have been delivered since 2004 primarily through periodic mass distribution campaigns scheduled at approximately three-year intervals, aligning with the expected lifespan of nets. Recent work indicates that ITN retention times are less than two years in most countries, raising key questions for quantification approaches and delivery frequency for ITN distribution. This paper models several quantification approaches for five typical ITN distribution strategies, estimates the proportion of the population with access to an ITN, and presents recommended quantification approaches to meet global targets for ITN access and use.

METHODS

A stock and flow model with annual timesteps was used to model ITN distribution and resulting ITN access for 2020-2035 under five scenarios in 40 countries: (1) three-year mass campaigns, (2) full-scale annual continuous distribution, (3) three-year mass campaigns plus continuous distribution in the years between campaigns, (4) three-year mass campaigns at different quantification approaches, (5) two-year mass campaigns at different quantification approaches. All scenarios included ITN distribution to pregnant women at antenatal clinics and infants at immunization visits.

RESULTS

The current status quo of conducting mass campaigns every three years using a population/1.8 quantifier is insufficient to achieve or maintain targets of 80% population access to ITNs in most malaria-endemic countries, given most estimated retention times are less than three years. Tailored three- or two-year mass campaigns were less efficient than annual continuous distribution strategies in nearly all settings. For countries with at least 2.5 year median ITN retention times, full scale continuous distribution provided better ITN access while needing 20-23% fewer ITNs compared to current mass campaigns.

CONCLUSION

Given variation in ITN retention times across countries, tailored quantification approaches for mass campaigns and continuous distribution strategies are warranted. Continuous distribution strategies are likely to offer more efficient ways to maintain ITN coverage, with fewer nets, where ITN retention times are at least two and a half years. National malaria programmes and their funding partners should work to increase the number of ITNs available to those vulnerable to malaria, while at the same time working to extend the useful life of these critical commodities.

Characterising the intensity of insecticide resistance: A novel framework for analysis of intensity bioassay data

Insecticide resistance is a growing problem that risks harming the progress made by vector control tools in reducing the malaria burden globally. New methods for quantifying the extent of resistance in wild populations are urgently needed to guide deployment of interventions to improve disease control. Intensity bioassays measure mosquito mortality at a range of insecticide doses and characterise phenotypic resistance in regions where resistance is already detected. These data are increasingly being collected but tend to exhibit high measurement error and there is a lack of formal guidelines on how they should be analysed or compared. This paper introduces a novel Bayesian framework for analysing intensity bioassay data, which uses a flexible statistical model able to capture a wide variety of relationships between mortality and insecticide dose. By accounting for background mortality of mosquitoes, our approach minimises the impact of this source of measurement noise resulting in more precise quantification of resistance. It outputs a range of metrics for describing the intensity and variability in resistance within the sample and quantifies the level of measurement error in the assay. The functionality is illustrated with data from laboratory-reared mosquitoes to show how the lethal dose varies within and between different strains. The framework can also be used to formally test hypotheses by explicitly considering the high heterogeneity seen in these types of data in field samples. Here we show that the intensity of resistance (as measured by the median lethal dose (LC50) of insecticide) increases over 7 years in mosquitoes from one village in Burkina Faso but remains constant in another. This work showcases the benefits of statistically rigorous analysis of insecticide bioassay data and highlights the additional information available from this and other dose-response data.

Influence of amino acid size at the P3 position of N-Cbz-tripeptide Michael acceptors targeting falcipain-2 and rhodesain for the treatment of malaria and human african trypanosomiasis

In recent decades, several structure-activity relationship (SAR) studies provided potent inhibitors of the cysteine proteases falcipain-2 (FP-2) and rhodesain (RD) from Plasmodium falciparum and Trypanosoma brucei rhodesiense, respectively. Whilst the roles of the warhead and residues targeting the P1 and P2 pockets of the proteases were extensively investigated, the roles of the amino acids occupying the S3 pocket were not widely assessed. Herein we report the synthesis and biological evaluation of a set of novel Michael acceptors bearing amino acids of increasing size at the P3 site (1a-g/2a-g, SPR20-SPR33) against FP-2, RD, P. falciparum, and T. brucei. Overall, the Michael acceptors bearing small amino acids at the P3 site exhibited the most potent inhibitory properties towards FP-2. In contrast, analogues with bulky residues at the P3 position were very potent rhodesain inhibitors. In cell based assays, single-digit micromolar EC₅₀ values against the two protozoa were observed. These findings can be a starting point for the development of peptide-based FP-2 and RD inhibitors.

Perennial malaria chemoprevention with and without malaria vaccination to reduce malaria burden in young children: a modelling analysis

BACKGROUND

A recent WHO recommendation for perennial malaria chemoprevention (PMC) encourages countries to adapt dose timing and number to local conditions. However, knowledge gaps on the epidemiological impact of PMC and possible combination with the malaria vaccine RTS,S hinder informed policy decisions in countries where malaria burden in young children remains high.

METHODS

The EMOD malaria model was used to predict the impact of PMC with and without RTS,S on clinical and severe malaria cases in children under the age of two years (U2). PMC and RTS,S effect sizes were fit to trial data. PMC was simulated with three to seven doses (PMC-3-7) before the age of eighteen months and RTS,S with three doses, shown to be effective at nine months. Simulations were run for transmission intensities of one to 128 infectious bites per person per year, corresponding to incidences of < 1 to 5500 cases per 1000 population U2. Intervention coverage was either set to 80% or based on 2018 household survey data for Southern Nigeria as a sample use case. The protective efficacy (PE) for clinical and severe cases in children U2 was calculated in comparison to no PMC and no RTS,S.

RESULTS

The projected impact of PMC or RTS,S was greater at moderate to high transmission than at low or very high transmission. Across the simulated transmission levels, PE estimates of PMC-3 at 80% coverage ranged from 5.7 to 8.8% for clinical, and from 6.1 to 13.6% for severe malaria (PE of RTS,S 10-32% and 24.6-27.5% for clinical and severe malaria, respectively. In children U2, PMC with seven doses nearly averted as many cases as RTS,S, while the combination of both was more impactful than either intervention alone. When operational coverage, as seen in Southern Nigeria, increased to a hypothetical target of 80%, cases were reduced beyond the relative increase in coverage.

CONCLUSIONS

PMC can substantially reduce clinical and severe cases in the first two years of life in areas with high malaria burden and perennial transmission. A better understanding of the malaria risk profile by age in early childhood and on feasible coverage by age, is needed for selecting an appropriate PMC schedule in a given setting.

Modelling the relative cost-effectiveness of the RTS,S/AS01 malaria vaccine compared to investment in vector control or chemoprophylaxis

BACKGROUND

The World Health Organization has recommended a 4-dose schedule of the RTS,S/AS01 (RTS,S) vaccine for children in regions of moderate to high P. falciparum transmission. Faced with limited supply and finite resources, global funders and domestic malaria control programs will need to examine the relative cost-effectiveness of RTS,S and identify target areas for vaccine implementation relative to scale-up of existing interventions.

METHODS

Using an individual-based mathematical model of P. falciparum, we modelled the cost-effectiveness of RTS,S across a range of settings in sub-Saharan Africa, incorporating various rainfall patterns, insecticide-treated net (ITN) use, treatment coverage, and parasite prevalence bands. We compare age-based and seasonal RTS,S administration to increasing ITN usage, switching to next generation ITNs in settings experiencing insecticide-resistance, and introduction of seasonal malaria chemoprevention (SMC) in areas of seasonal transmission.

RESULTS

For RTS,S to be the most cost-effective intervention option considered, the maximum cost per dose was less than $9.30 USD in 90.9% of scenarios. Nearly all (89.8%) values at or above $9.30 USD per dose were in settings with 60% established bed net use and / or with established SMC, and 76.3% were in the highest PfPR_2-10 band modelled (40%). Addition of RTS,S to strategies involving 60% ITN use, increased ITN usage or a switch to PBO nets, and SMC, if eligible, still led to significant marginal case reductions, with a median of 2,653 (IQR: 1,741 to 3,966) cases averted per 100,000 people annually, and 82,270 (IQR: 54,034 to 123,105) cases averted per 100,000 fully vaccinated children (receiving at least three doses).

CONCLUSIONS

Use of RTS,S results in reductions in malaria cases and deaths even when layered upon existing interventions. When comparing relative cost-effectiveness, scale up of ITNs, introduction of SMC, and switching to new technology nets should be prioritized in eligible settings.

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

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

Entomological monitoring data driving decision-making for appropriate and sustainable malaria vector control in Côte d'Ivoire

BACKGROUND

Entomological surveillance provides critical information on vectors for appropriate malaria vector control and strategic decision-making. The widely documented insecticide resistance of malaria vectors in Côte d'Ivoire requires that any vector control intervention deployment be driven by entomological data to optimize its effectiveness and appropriate resource allocations. To achieve this goal, this study documents the results of monthly vector surveillance and insecticide susceptibility tests conducted in 2019 and a review of all previous entomological monitoring data used to guide vector control decision making. Furthermore, susceptibility to pirimiphos-methyl and clothianidin was assessed in addition to chlorfenapyr and pyrethroids (intensity and piperonyl butoxide (PBO) synergism) tests previously reported. Vector bionomic data were conducted monthly in four sites (Sakassou, Béoumi, Dabakala and Nassian) that were selected based on their reported high malaria incidence. Adult mosquitoes were collected using human landing catches (HLCs), pyrethrum spray catches (PSCs), and human-baited CDC light traps to assess vector density, behaviour, species composition and sporozoite infectivity.

RESULTS

Pirimiphos-methyl and clothianidin susceptibility was observed in 8 and 10 sites, respectively, while previous data reported chlorfenapyr (200 µg/bottle) susceptibility in 13 of the sites, high pyrethroid resistance intensity and increased mortality with PBO pre-exposure at all 17 tested sites. Anopheles gambiae sensu lato was the predominant malaria vector collected in all four bionomic sites. Vector density was relatively higher in Sakassou throughout the year with mean biting rates of 278.2 bites per person per night (b/p/n) compared to Béoumi, Dabakala and Nassian (mean of 48.5, 81.4 and 26.6 b/p/n, respectively). The mean entomological inoculation rate (EIR) was 4.44 infective bites per person per night (ib/p/n) in Sakassou, 0.34 ib/p/n in Beoumi, 1.17 ib/p/n in Dabakala and 1.02 ib/p/n in Nassian. The highest EIRs were recorded in October in Béoumi (1.71 ib/p/n) and Nassian (3.22 ib/p/n), in July in Dabakala (4.46 ib/p/n) and in May in Sakassou (15.6 ib/p/n).

CONCLUSION

Based on all results and data review, the National Malaria Control Programme developed and implemented a stratified insecticide-treated net (ITN) mass distribution in 2021 considering new generation ITNs. These results also supported the selection of clothianidin-based products and an optimal spraying time for the first indoor residual spraying (IRS) campaign in Sakassou and Nassian in 2020.

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

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

Collateral Effects of Insecticide-Treated Nets on Human and Environmental Safety in an Epidemiological Model for Malaria with Human Risk Perception

Malaria remains a major health problem in many parts of the world, including Sub-Saharan Africa. Insecticide-treated nets, in combination with other control measures, have been effective in reducing malaria incidence over the past two decades. Nevertheless, there are concerns about improper handling and misuse of nets, producing possible health effects from intoxication and collateral environmental damage. The latter is caused, for instance, from artisanal fishing. We formulate a model of impulsive differential equations to describe the interplay between malaria dynamics, human intoxication, and ecosystem damage; affected by human awareness to these risks and levels of net usage. Our results show that an increase in mosquito net coverage reduces malaria prevalence and increases human intoxications. In addition, a high net coverage significantly reduces the risk perception to disease, naturally increases the awareness for intoxications from net handling, and scarcely increases the risk perception to collateral damage from net fishing. According to our model, campaigns aiming at reducing disease prevalence or intoxications are much more successful than those creating awareness to ecosystem damage. Furthermore, we can observe from our results that introducing closed fishing periods reduces environmental damage more significantly than strategies directed towards increasing the risk perception for net fishing.

The mosquito vectors that sustained malaria transmission during the Magude project despite the combined deployment of indoor residual spraying, insecticide-treated nets and mass-drug administration

The "Magude project" aimed but failed to interrupt local malaria transmission in Magude district, southern Mozambique, by using a comprehensive package of interventions, including indoor residual spraying (IRS), pyrethroid-only long-lasting insecticide treated nets (LLINs) and mass-drug administration (MDA). Here we present detailed information on the vector species that sustained malaria transmission, their association with malaria incidence and behaviors, and their amenability to the implemented control interventions. Mosquitoes were collected monthly between May 2015 and October 2017 in six sentinel sites in Magude district, using CDC light traps both indoors and outdoors. Anopheles arabiensis was the main vector during the project, while An. funestus s.s., An. merus, An. parensis and An. squamosus likely played a secondary role. The latter two species have never previously been found positive for Plasmodium falciparum in southern Mozambique. The intervention package successfully reduced vector sporozoite rates in all species throughout the project. IRS was effective in controlling An. funestus s.s. and An. parensis, which virtually disappeared after its first implementation, but less effective at controlling An. arabiensis. Despite suboptimal use, LLINs likely provided significant protection against An. arabiensis and An. merus that sought their host largely indoors when people where in bed. Adding IRS on top of LLINs and MDA likely added value to the control of malaria vectors during the Magude project. Future malaria elimination attempts in the area could benefit from i) increasing the use of LLINs, ii) using longer-lasting IRS products to counteract the increase in vector densities observed towards the end of the high transmission season, and iii) a higher coverage with MDA to reduce the likelihood of human infection. However, additional interventions targeting vectors that survive IRS and LLINs by biting outdoors or indoors before people go to bed, will be likely needed to achieve local malaria elimination.

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

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

Estimating the effectiveness of malaria vector control interventions has typically relied on resource-intensive cluster randomised trials. Here, the authors estimate changes in malaria prevalence using entomological data from experimental hut trials, which may provide an alternative route to approval of interventions in some situations.

The potential impact of Anopheles stephensi establishment on the transmission of Plasmodium falciparum in Ethiopia and prospective control measures

BACKGROUND

Sub-Saharan Africa has seen substantial reductions in cases and deaths due to malaria over the past two decades. While this reduction is primarily due to an increasing expansion of interventions, urbanisation has played its part as urban areas typically experience substantially less malaria transmission than rural areas. However, this may be partially lost with the invasion and establishment of Anopheles stephensi. A. stephensi, the primary urban malaria vector in Asia, was first detected in Africa in 2012 in Djibouti and was subsequently identified in Ethiopia in 2016, and later in Sudan and Somalia. In Djibouti, malaria cases have increased 30-fold from 2012 to 2019 though the impact in the wider region remains unclear.

METHODS

Here, we have adapted an existing model of mechanistic malaria transmission to estimate the increase in vector density required to explain the trends in malaria cases seen in Djibouti. To account for the observed plasticity in An. stephensi behaviour, and the unknowns of how it will establish in a novel environment, we sample behavioural parameters in order to account for a wide range of uncertainty. This quantification is then applied to Ethiopia, considering temperature-dependent extrinsic incubation periods, pre-existing vector-control interventions and Plasmodium falciparum prevalence in order to assess the potential impact of An. stephensi establishment on P. falciparum transmission. Following this, we estimate the potential impact of scaling up ITN (insecticide-treated nets)/IRS (indoor residual spraying) and implementing piperonyl butoxide (PBO) ITNs and larval source management, as well as their economic costs.

RESULTS

We estimate that annual P. falciparum malaria cases could increase by 50% (95% CI 14-90) if no additional interventions are implemented. The implementation of sufficient control measures to reduce malaria transmission to pre-stephensi levels will cost hundreds of millions of USD.

CONCLUSIONS

Substantial heterogeneity across the country is predicted and large increases in vector control interventions could be needed to prevent a major public health emergency.

Protective efficacy of holed and aging PBO-pyrethroid synergist-treated nets on malaria infection prevalence in north-western Tanzania

Two billion pyrethroid long-lasting insecticidal nets (LLINs) have been distributed since 2004 for malaria prevention in Sub-Saharan Africa. Current malaria control strategies rely on an assumed effective 3-year lifespan for LLINs. PBO synergist LLINs are a newly recommended class of net but there is limited information on their life span and long-term protective efficacy in communities. To assess their operational survival, a cohort of 390 PBO LLINs (Olyset Plus) and 367 standard pyrethroid LLIN (Olyset net) from 396 households were followed for 36 months in Western Tanzania. To assess the association between the condition of the LLIN and malaria infection, nets from at least 480 randomly selected households were assessed during malaria prevalence cross-sectional surveys at 4, 9, 16, 21, 28, and 33 months post-distribution. Information on the presence and condition of nets, and demographic information from the household, were collected to evaluate factors influencing net durability. After 3 years less than 17% of nets distributed still remained in the households. The fabric condition was not associated with malaria infection in either type of net. The difference between the net types was highest when nets were between 1-2 years old, when PBO nets appeared to be similarly protective as nets less than a year old, whereas standard nets were considerably less protective as they aged, regardless of fabric condition. There was no statistical difference in the estimated median functional survival time between net types with 1.6 years (95% CI 1.38-1.87) for PBO LLIN and 1.9 years (95% CI 1.67-2.06) for standard LLINs. After 3 years, there was a loss of 55% of permethrin (pyrethroid) content for both nets, and 97% of PBO content was lost in PBO LLIN. These results highlight that functional survival is less than the recommended 3 years for both net types. However, even as the nets age, the PBO nets remained more protective than standard nets, regardless of their condition.