The sub-lethal effects of pyrethroid exposure on Anopheles gambiae s.l. life-history traits, behaviour, and the efficacy of insecticidal bednets

Malaria control progress in Africa has stalled. Though the reasons for this will be multifaceted, increasing and intense resistance to pyrethroids in Anopheles gambiae s.l. is almost certainly a contributing factor. Standard methods to monitor insecticide resistance and evaluate vector control tools primarily focus on the immediate and lethal effects on the mosquito. These methods disregard other important delayed and sub-lethal effects, despite their implications for malaria transmission. In response to growing concerns over the sustained effectiveness of current control tools, next-generation products are being developed and evaluated. These aim to target insecticide-resistant mosquitoes or mosquitoes that contribute to residual malaria transmission. Adaptations to current standard efficacy tests are needed to evaluate the novel modes of action of such products. The effect of insecticide exposure on the longevity, reproductive output and blood-feeding behaviour of a wild highly pyrethroid-resistant Anopheles gambiae s.l. population was evaluated. Mosquitoes were exposed to a range of insecticides and insecticidal bednets using laboratory tests and semi-field experimental hut trials. Benchtop video tests were evaluated for their feasibly in measuring the effectiveness of standard and next-generation nets. Subsequently, these tests were used to investigate the behaviour of field-populations of An. gambiae s.l. at the bednet interface in response to a human host. Following exposure to both pyrethroid-only and next-generation nets, evidence of sub-lethal impacts were limited or non-existent. The mosquitoes exposed to insecticidal nets did not suffer from reduced lifespan or altered reproductive output. Evidence of delayed mortality was only recorded when mosquitoes were exposed to extremely high levels of pyrethroids in WHO tube bioassays. Some mosquitoes were inhibited from blood-feeding in experimental hut trials, however, lab tests suggest this effect is absent by 8-hour post net-exposure. The efficacy of next-generation nets on the field population was dependant on the product. Brief contact with PermaNet 3.0 roof (pyrethroid + PBO) caused rapid knock-down and 100% mortality in all tests. Exposure to all other insecticidal nets, including Interceptor G2 (pyrethroid + chlorfenapyr), resulted in low 24-hour mortality in both lab and semi-field experiments. Following adaptations for the field, video tests were able to collect behavioural data on mosquito responses to insecticidal nets such as flying, resting, and probing behaviour. Responses were similar between untreated and pyrethroid- only netting. Extreme reductions in activity were observed following exposure to PermaNet 3.0, and Interceptor G2 showed signs of repellence. The results suggest community protection offered by first-generation LLINs is extremely low in this setting, however, pyrethroid-PBO nets appear to be effective at controlling the highly pyrethroid-resistant population. This work highlights the need for additional studies of sub-lethal effects in other field populations, with lower insecticide resistance levels or differing mechanisms, to establish if such measurements should be incorporated into the evaluation of novel vector control tools

Towards An Economics Policy Framework to Combat Malaria, in An Era of Insecticide Resistance

Malaria causes close to half a million deaths per year, the majority of which are in children under five years of age who live in sub-Saharan Africa. Despite significant progress in reducing malaria deaths in the past fifteen years, there is still a long way to go before universal coverage with key interventions like LLINs and IRS is reached, which is an essential step towards achieving malaria elimination. While severe resource constraints pose a fundamental challenge, growing resistance to insecticides used in LLIN and for IRS exacerbates this issue, and threatens to undermine the significant gains achieved to date. This IPPI Policy Brief draws from economic theory to analyse the case of insecticide resistance. It highlights some fundamental trade-offs brought about by the emergence of resistance to insecticides, as well as the lack of data that is necessary to analyse them. The paper also explores how the concept of market failure is applied in the field of malaria control, and where market inefficiencies have not yet been adequately addressed. Overall, while there is no doubt that significant additional funding is needed to combat malaria and hopefully to move closer to its elimination, there is an urgent need to use sound economic analysis to help develop and strengthen a global rationale for further public investment in malaria vector control and to better take account of insecticide resistance in the prioritisation and deployment of national, in-country programmes

Anopheles gambiae populations from Burkina Faso show minimal delayed mortality after exposure to insecticide-treated nets

Background: The efficacy of long-lasting insecticidal nets (LLINs) in preventing malaria in Africa is threatened by insecticide resistance. Bioassays assessing 24-hour mortality post-LLIN exposure have established that resistance to the concentration of pyrethroids used in LLINs is widespread. However, although mosquitoes may no longer be rapidly killed by LLIN exposure, a delayed mortality effect has been shown to reduce the transmission potential of mosquitoes exposed to nets. This has been postulated to partially explain the continued efficacy of LLINs against pyrethroid-resistant populations. Burkina Faso is one of a number of countries with very high malaria burdens and pyrethroid-resistant vectors, where progress in controlling this disease has stagnated. We measured the impact of LLIN exposure on mosquito longevity in an area of the country with intense pyrethroid resistance to establish whether pyrethroid exposure was still shortening mosquito lifespan in this setting. Methods: We quantified the immediate and delayed mortality effects of LLIN exposure using standard laboratory WHO cone tests, tube bioassays and experimental hut trials on Anopheles gambiae populations originating from the Cascades region of Burkina Faso using survival analysis and a Bayesian state-space model. Results: Following single and multiple exposures to a PermaNet 2.0 LLIN only one of the four mosquito populations tested showed evidence of delayed mortality. No delayed mortality was seen in experimental hut studies using LLINs. A delayed mortality effect was only observed in WHO tube bioassays when deltamethrin concentration was increased above the standard diagnostic dose. Conclusions: As mosquito pyrethroid-resistance increases in intensity, delayed effects from LLIN exposure are substantially reduced or absent. Given the rapid increase in resistance occurring in malaria vectors across Africa it is important to determine whether the failure of LLINs to shorten mosquito lifespan is now a widespread phenomenon as this will have important implications for the future of this pivotal malaria control tool

Piperonyl butoxide (PBO) combined with pyrethroids in long-lasting insecticidal nets (LLINs) to prevent malaria in Africa

This is a protocol for a Cochrane Review (Intervention). The objectives are as follows: 1. Evaluate whether adding PBO to pyrethroid LLINs increases the epidemiological and entomological effectiveness of the nets. 2. Compare the effects of PBO-LLINs currently in commercial development or on the market with their non-PBO equivalent in relation to: a. malaria infection (prevalence or incidence); b. entomological outcomes

Piperonyl butoxide (PBO) combined with pyrethroids in insecticide-treated nets to prevent malaria in Africa.

BackgroundPublic health strategies that target mosquito vectors, particularly pyrethroid long-lasting insecticidal nets (LLINs), have been largely responsible for the substantial reduction in the number of people in Africa developing malaria. The spread of insecticide resistance in Anopheles mosquitoes threatens these impacts. One way to control insecticide-resistant populations is by using insecticide synergists. Piperonyl butoxide (PBO) is a synergist that inhibits specific metabolic enzymes within mosquitoes and has been incorporated into pyrethroid-LLINs to form pyrethroid-PBO nets. Pyrethroid-PBO nets are currently produced by four LLIN manufacturers and, following a recommendation from the World Health Organization (WHO) in 2017, are being included in distribution campaigns in countries. This review examines epidemiological and entomological evidence on whether the addition of PBO to LLINs improves their efficacy.Objectives1. Evaluate whether adding PBO to pyrethroid LLINs increases the epidemiological and entomological effectiveness of the nets.2. Compare the effects of pyrethroid-PBO nets currently in commercial development or on the market with their non-PBO equivalent in relation to:a. malaria infection (prevalence or incidence);b. entomological outcomes.Search methodsWe searched the Cochrane Infectious Diseases Group (CIDG) Specialized Register; CENTRAL, MEDLINE, Embase, Web of Science, CAB Abstracts, and two clinical trial registers (ClinicalTrials.gov and WHO International Clinical Trials Registry Platform) up to 24 August 2018. We contacted organizations for unpublished data. We checked the reference lists of trials identified by the above methods.Selection criteriaWe included laboratory trials, experimental hut trials, village trials, and randomized clinical trials with mosquitoes from the Anopheles gambiae complex or Anopheles funestus group.Data collection and analysisTwo review authors assessed each trial for eligibility, extracted data, and determined the risk of bias for included trials. We resolved disagreements through discussion with a third review author. We analysed the data using Review Manager 5 and assessed the certainty of the evidence using the GRADE approach.Main resultsFifteen trials met the inclusion criteria: two laboratory trials, eight experimental hut trials, and five cluster-randomized controlled village trials.One village trial examined the effect of pyrethroid-PBO nets on malaria infection prevalence in an area with highly pyrethroid-resistant mosquitoes. The latest endpoint at 21 months post-intervention showed that malaria prevalence probably decreased in the intervention arm (OR 0.40, 95% CI 0.20 to 0.80; 1 trial, 1 comparison, moderate-certainty evidence).In highly pyrethroid-resistant areas ( 90% mosquito mortality), there may be little or no difference in the effect of unwashed pyrethroid-PBO nets compared to unwashed standard-LLINs on mosquito mortality (RR 1.20, 95% CI 0.64 to 2.26; 2791 mosquitoes, 2 trials, 2 comparisons, low-certainty evidence). This is similar for washed nets (RR 1.07, 95% CI 0.92 to 1.25; 2644 mosquitoes, 2 trials, 2 comparisons, low-certainty evidence). We do not know if unwashed pyrethroid-PBO nets have any effect on blood feeding success of susceptible mosquitoes (RR 0.50, 95% CI 0.11 to 2.32; 2791 mosquitoes, 2 trials, 2 comparisons, very low-certainty evidence). The same applies to washed nets (RR 1.28, 95% CI 0.81 to 2.04; 2644 mosquitoes, 2 trials, 2 comparisons, low-certainty evidence).In village trials comparing pyrethroid-PBO nets to LLINs, there was no difference in sporozoite rate (4 trials, 5 comparison) and mosquito parity (3 trials, 4 comparisons).Authors' conclusionsIn areas of high insecticide resistance, pyrethroid-PBO nets reduce mosquito mortality and blood feeding rates, and results from a single clinical trial demonstrate that this leads to lower malaria prevalence. Questions remain about the durability of PBO on nets, as the impact of pyrethroid-PBO LLINs on mosquito mortality was not sustained over 20 washes in experimental hut trials. There is little evidence to support higher entomological efficacy of pyrethroid-PBO nets in areas where the mosquitoes show lower levels of resistance to pyrethroids

Piperonyl butoxide (PBO) combined with pyrethroids in insecticide‐treated nets to prevent malaria in Africa

Background: Public health strategies that target mosquito vectors, particularly pyrethroid long-lasting insecticidal nets (LLINs), have been largely responsible for the substantial reduction in the number of people in Africa developing malaria. The spread of insecticide resistance in Anopheles mosquitoes threatens these impacts. One way to control insecticide-resistant populations is by using insecticide synergists. Piperonyl butoxide (PBO) is a synergist that inhibits specific metabolic enzymes within mosquitoes and has been incorporated into pyrethroid-LLINs to form pyrethroid-PBO nets. Pyrethroid-PBO nets are currently produced by four LLIN manufacturers and,following a recommendation from the World Health Organization (WHO) in 2017, are being included in distribution campaigns incountries. This review examines epidemiological and entomological evidence on whether the addition of PBO to LLINs improves theirefficacy. Objectives1. Evaluate whether adding PBO to pyrethroid LLINs increasesthe epidemiological and entomological effectiveness of the nets.2. Compare the effects of pyrethroid-PBO nets currently in commercial development or on the market with their non-PBO equivalentin relation to:a. malaria infection (prevalence or incidence);b. entomological outcomes. Search methods . We searched the Cochrane Infectious Diseases Group (CIDG) Specialized Register; CENTRAL, MEDLINE, Embase, Web of Science,CAB Abstracts, and two clinical trial registers (ClinicalTrials.gov and WHO International Clinical Trials Registry Platform) up to 24August 2018. We contacted organizations for unpublished data. We checked the reference lists of trials identified by the above methods

The Residual Efficacy of SumiShield™ 50WG and K-Othrine® WG250 IRS Formulations Applied to Different Building Materials against Anopheles and Aedes Mosquitoes

The Anopheles mosquitoes that transmit malaria are targeted by the use of indoor residual sprays (IRSs), insecticides applied to the walls of homes to kill mosquitoes that rest there when coming into houses in search of a blood meal. K-Othrine® is an IRS based on the pyrethroid deltamethrin and is widely used against mosquitoes that transmit malaria. SumiShield™ 50WG is an IRS based on the insecticide clothianidin, developed to kill mosquitoes that have become resistant to other forms of insecticide. These products were applied to cement, wood, and mud tiles, representative of typical building materials in areas where malaria is endemic. For 18 months, the ability of these treated surfaces to kill adult female mosquitoes exposed to them was measured. The clothianidin IRS was highly effective against insecticide susceptible and resistant strains of Anopheles gambiae and An. funestus, key malaria vector species, with an improved performance compared to deltamethrin IRS, though was not so effective against Aedes aegypti or Culex quinquefasciatus. Both IRS formulations were shown to be more effective and long-lasting on cement and mud than on wood tiles

Assessing the impact of the addition of pyriproxyfen on the durability of permethrin-treated bed nets in Burkina Faso: a compound-randomized controlled trial

Background Long-lasting insecticidal nets (LLINs) treated with pyrethroids are the foundation of malaria control in sub-Saharan Africa. Rising pyrethroid resistance in vectors, however, has driven the development of alternative net formulations. Here the durability of polyethylene nets with a novel combination of a pyrethroid, permethrin, and the insect juvenile hormone mimic, pyriproxyfen (PPF), compared to a standard permethrin LLIN, was assessed in rural Burkina Faso. Methods A compound-randomized controlled trial was completed in two villages. In one village 326 of the PPF-permethrin nets (Olyset Duo) and 327 standard LLINs (Olyset) were distributed to assess bioefficacy. In a second village, 170 PPF-permethrin nets and 376 LLINs were distributed to assess survivorship. Nets were followed at 6-monthly intervals for 3 years. Bioefficacy was assessed by exposing permethrin-susceptible and resistant Anopheles gambiae sensu lato mosquito strains to standard World Health Organization (WHO) cone and tunnel tests with impacts on fertility measured in the resistant strain. Insecticide content was measured using high-performance liquid chromatography. LLIN survivorship was recorded with a questionnaire and assessed by comparing the physical integrity using the proportionate hole index (pHI). Results The PPF-permethrin net met WHO bioefficacy criteria (≥ 80% mortality or ≥ 95% knockdown) for the first 18 months, compared to 6 months for the standard LLIN. Mean mosquito mortality for PPF-permethrin nets, across all time points, was 8.6% (CI 2.6–14.6%) higher than the standard LLIN. Fertility rates were reduced after PPF-permethrin net exposure at 1-month post distribution, but not later. Permethrin content of both types of nets remained within the target range of 20 g/kg ± 25% for 242/248 nets tested. The pyriproxyfen content of PPF-permethrin nets declined by 54%, from 10.4 g/kg (CI 10.2–10.6) to 4.7 g/kg (CI 3.5–6.0, p < 0.001) over 36 months. Net survivorship was poor, with only 13% of PPF-permethrin nets and 12% of LLINs still present in the original household after 36 months. There was no difference in the fabric integrity or survivorship between the two net types. Conclusion The PPF-permethrin net, Olyset Duo, met or exceeded the performance of the WHO-recommended standard LLIN (Olyset) in the current study but both net types failed the 3-year WHO bioefficacy criteria

Barrier bednets target malaria vectors and expand the range of usable insecticides

Transmission of Plasmodium falciparum malaria parasites occurs when nocturnal Anopheles mosquito vectors feed on human blood. In Africa, where malaria burden is highest, bednets treated with pyrethroid insecticide were highly effective in preventing mosquito bites and reducing transmission, and essential to achieving unprecedented reductions in malaria until 2015 (ref. ). Since then, progress has stalled , and with insecticidal bednets losing efficacy against pyrethroid-resistant Anopheles vectors , methods that restore performance are urgently needed to eliminate any risk of malaria returning to the levels seen before their widespread use throughout sub-Saharan Africa . Here, we show that the primary malaria vector Anopheles gambiae is targeted and killed by small insecticidal net barriers positioned above a standard bednet in a spatial region of high mosquito activity but zero contact with sleepers, opening the way for deploying many more insecticides on bednets than is currently possible. Tested against wild pyrethroid-resistant A. gambiae in Burkina Faso, pyrethroid bednets with organophosphate barriers achieved significantly higher killing rates than bednets alone. Treated barriers on untreated bednets were equally effective, without significant loss of personal protection. Mathematical modelling of transmission dynamics predicted reductions in clinical malaria incidence with barrier bednets that matched those of 'next-generation' nets recommended by the World Health Organization against resistant vectors. Mathematical models of mosquito-barrier interactions identified alternative barrier designs to increase performance. Barrier bednets that overcome insecticide resistance are feasible using existing insecticides and production technology, and early implementation of affordable vector control tools is a realistic prospect

Developing Consensus Standard Operating Procedures (SOPs) to Evaluate New Types of Insecticide-Treated Nets.

In response to growing concerns over the sustained effectiveness of pyrethroid-only based control tools, new products are being developed and evaluated. Some examples of these are dual-active ingredient (AI) insecticide-treated nets (ITNs) which contain secondary insecticides, or synergist ITNs which contain insecticide synergist, both in combination with a pyrethroid. These net types are often termed 'next-generation' insecticide-treated nets. Several of these new types of ITNs are being evaluated in large-scale randomized control trials (RCTs) and pilot deployment schemes at a country level. However, no methods for measuring the biological durability of the AIs or synergists on these products are currently recommended. In this publication, we describe a pipeline used to collate and interrogate several different methods to produce a singular 'consensus standard operating procedure (SOP)', for monitoring the biological durability of three new types of ITNs: pyrethroid + piperonyl butoxide (PBO), pyrethroid + pyriproxyfen (PPF), and pyrethroid + chlorfenapyr (CFP). This process, convened under the auspices of the Innovation to Impact programme, sought to align methodologies used for conducting durability monitoring activities of next-generation ITNs