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Efficacy of Fludora® Fusion (a mixture of deltamethrin and clothianidin) for indoor residual spraying against pyrethroid-resistant malaria vectors: laboratory and experimental hut evaluation

PMCID: PMC7488472

PMID: 32917255

Abstract

Background A new generation of IRS insecticides which can provide improved and prolonged control of pyrethroid-resistant malaria vector populations are being developed. Fludora® Fusion is a new IRS insecticide containing a mixture of deltamethrin and clothianidin, a neonicotinoid. Methods The efficacy of Fludora® Fusion IRS was evaluated over 11–12 months on concrete and mud substrates in laboratory bioassays and experimental huts against wild free-flying pyrethroid-resistant Anopheles gambiae ( sensu lato ) in Cové, Benin. A comparison was made with the two active ingredients of the mixture; clothianidin and deltamethrin, applied alone. CDC bottle bioassays were also performed to investigate resistance to clothianidin in the wild vector population. Results Fludora® Fusion induced > 80% laboratory cone bioassay mortality with both susceptible and pyrethroid-resistant An. gambiae ( s.l. ) for 7–9 months on concrete block substrates and 12 months on mud block substrates. The vector population at the experimental hut site was fully susceptible to clothianidin in CDC bottle bioassays. Overall mortality rates of wild free-flying pyrethroid-resistant An. gambiae ( s.l. ) entering the experimental huts during the 11-month trial were < 15% with deltamethrin and significantly higher with Fludora® Fusion (69–71%) and clothianidin alone (72–78%). Initial high experimental hut mortality rates with Fludora® Fusion (> 80%) only declined by 50% after 8 months. Monthly in situ wall cone bioassay mortality of susceptible mosquitoes was > 80% for 9–12 months with Fludora® Fusion and clothianidin alone. Fludora® Fusion induced significantly higher levels of early exiting of mosquitoes compared to clothianidin alone (55–60% vs 37–38%, P < 0.05). Conclusions Indoor residual spraying with Fludora® Fusion induced high and prolonged mortality of wild pyrethroid-resistant malaria vectors for 7–10 months mostly due to the clothianidin component and substantial early exiting of mosquitoes from treated huts due to the pyrethroid component. Fludora® Fusion is an important addition to the current portfolio of IRS insecticides with the potential to significantly reduce transmission of malaria by pyrethroid-resistant mosquito vectors.

Full Text

Indoor residual spraying remains a core strategy in the fight against malaria due to its ability to rapidly reduce transmission [1]. It constitutes the application of a residual insecticide to potential resting surfaces of malaria vectors; usually the inner walls, ceiling and eaves of human habitats and domestic animal shelters. There has been a substantial increase in the use of IRS over the last two decades [2, 3] and this has contributed significantly to reductions in malaria morbidity and mortality observed in many endemic countries [4]. The effectiveness of IRS for malaria vector control however depends on several factors; mainly the continued susceptibility of local vectors to the insecticides deployed and the duration of its action on treated home wall substrates. For some decades, IRS relied heavily on a rather limited number of classes of insecticides [2, 3] most of which were short-lived on home wall substrates (2–5 months) thus requiring multiple resource-demanding IRS campaign rounds when used in areas with stable malaria transmission [5]. Malaria vectors have also developed resistance to these conventional insecticides which is now widespread and increasing in intensity across Africa [6] and this, together with their short residual effect is driving the development of a new generation of long-lasting IRS insecticides to which local vectors are largely susceptible.
The neonicotinoid, clothianidin is a new repurposed insecticide which was recently added to the WHO’s list of pre-qualified insecticides for use in indoor residual spraying [7]. Clothianidin presents a new mode action which differs from that of conventional public health insecticides acting as an agonist on nicotinic acetylcholine receptors (nAChR) [8]. Owing to its novel mode of action, it shows potential to provide improved control of vector populations that have developed resistance to older public health insecticides. The addition of clothianidin to the portfolio of IRS insecticides also provides an opportunity to mitigate the development and spread of insecticide resistance in malaria vectors through the rotational use of IRS insecticides and the development of mixture IRS co-formulations [9]. Insecticide mixtures for IRS need to be explored with new public health insecticides when they become available because mixtures have the dual potential to improve malaria vector control through the combined effect of both active ingredients and contribute to insecticide resistance management, especially in areas where resistance to both active ingredients is not yet established [10].
In a previous experimental hut study in Benin, a tank mix of clothianidin and deltamethrin induced high and prolonged mortality (8–9 months) in wild pyrethroid-resistant An. gambiae (s.l.) owing to the clothianidin component and early exiting of mosquitoes from experimental huts due to the pyrethroid component [11]. The encouraging results from this early proof of concept study led to the development of Fludora® Fusion (Bayer CropScience, Monheim, Germany), a new IRS formulation of a wettable powder product containing 500 g/kg of clothianidin and 62.5 g/kg of deltamethrin in water-soluble sachets. In the present study, the efficacy of Fludora® Fusion was evaluated against pyrethroid-resistant An. gambiae (s.l.) in southern Benin under both laboratory and experimental hut conditions. Its residual effect was assessed on mud and concrete wall substrates. A comparison was made with the two active ingredients of the mixture; clothianidin and deltamethrin, applied alone.
Residual efficacy of the insecticides on the treated mud and concrete block substrates were assessed in monthly cone bioassays following WHO guidelines [12]. Forty (40) 2–5-day-old laboratory maintained susceptible Anopheles gambiae Kisumu and pyrethroid-resistant Anopheles gambiae (s.l.) Cové mosquitoes were exposed for 30 min to each treatment and substrate in replicates of 10 mosquitoes per block. The laboratory cone bioassays were conducted at monthly intervals for up to 12 months. Based on the delayed effect of clothianidin on mosquito mortality as demonstrated in previous studies [11], mortality was recorded every 24 hours up to 120 hours post-exposure across all experiments.
The hut trial was performed at the CREC/LSHTM experimental hut station in Cové, Southern Benin. The field site is located in an irrigated valley producing rice almost year-round and providing suitable breeding habitats for mosquitoes. The rainy season extends from March to October and the dry season from November to February. The vector population consists of both An. coluzzii and An. gambiae (sensu stricto) with the latter occurring at lower frequencies (~23%) and mostly in the dry season [13]. The vector population is highly resistant to pyrethroids. Molecular genotyping and microarray studies have demonstrated a high frequency of the L1014F allele (> 90%) and overexpression of the cytochrome P450s CYP6P3, associated with pyrethroid detoxification [13]. The trial ran for 11 months between December 2015 and November 2016 in 7 experimental huts of West African design. The experimental huts are made from concrete bricks with a corrugated iron roof. Inner walls were plastered with either concrete or mud and the ceilings fitted with palm thatch matting. Each hut was built on a concrete plinth surrounded by a water-filled moat to prevent the entry of scavenging ants and had a wooden framed veranda trap to capture exiting mosquitoes. Mosquito entry occurred via four window slits each measuring 1 cm and situated on three sides of the hut.
The trial followed the WHO guidelines for evaluation of IRS products [12]. Treatments were randomly allocated to experimental huts. Seven consenting adult human volunteers slept in the huts from 21:00 to 5:00 h each trial night to attract mosquitoes and were rotated between huts on successive nights to adjust for variation in individual attractiveness to mosquitoes. In the morning, mosquitoes were collected from the room and veranda using aspirators and brought to the laboratory where they were identified and scored as fed or unfed and dead or alive. Live mosquitoes were provided with 10% glucose solution and mortality scored every 24 h for up to 120 h.
To assess the residual activity of the treatments on the treated experimental hut walls, WHO cone bioassays were conducted using 2–5-day-old, female mosquitoes of the insecticide susceptible An. gambiae Kisumu strain. Bioassays were performed 3 days after application of treatments and at monthly intervals thereafter over 12 months. A total of 50 mosquitoes were tested per hut in cohorts of 10 per cone on each treated wall/ceiling surface. Mosquitoes were exposed to treated surfaces for 30 min following WHO guidelines [12]. Mortality was recorded every 24 h up to 120 h post-exposure.
Experimental hut data were entered in Excel and transferred to Stata 15.1 for analysis. Proportional data (exiting rate, blood-feeding and mortality) were analysed using logistic regression while adjusting for the effects of sleeper attractiveness to mosquitoes. Cone bioassay mortality was pooled for each treatment and substrate at each time point and compared against an 80% cut-off criteria following WHO guidelines [12].
Mortality of susceptible An. gambiae Kisumu and pyrethroid-resistant An. gambiae (s.l.) Cové strains in laboratory cone bioassays with control untreated mud and concrete blocks did not exceed 20% at any time point (Figs. 1 and 2). For both types of substrates, mortality rates for each treatment were generally higher/more persistent with the Kisumu strain compared to the pyrethroid-resistant Cové strain. Mortality with deltamethrin-treated blocks was > 80% for the first 2–4 months with the susceptible Kisumu strain after which it declined sharply (Figs. 1a and 2a), but with pyrethroid-resistant Cové strain, mortality on both substrate-types did not exceed 50% at any time point (Figs. 1b and 2b). On concrete blocks, mortality with clothianidin was > 80% for 7 months with the susceptible Kisumu strain (Fig. 1a) and only 2 months with the pyrethroid-resistant Cové strain (Fig. 1b). Meanwhile, Fludora® Fusion induced more residual mortality on concrete blocks which remained > 80% for 9 months with the susceptible Kisumu strain (Fig. 1a) and 7 months with the pyrethroid-resistant Cové strain (Fig. 1b). With mud blocks, mortality rates with clothianidin and Fludora® Fusion were very stable remaining above 90% for both mosquito strains for up to 12 months (Fig. 2).
Results from the CDC bottle bioassays are presented in Fig. 3. Mortality rates with the control were less than 10% even up to 120 h post-exposure. Knockdown and mortality of wild F1 An. gambiae (s.l.) mosquitoes emerging from larvae collected from the Cové experimental hut site were respectively 52% and 39% with deltamethrin-treated bottles thus confirming the levels of pyrethroid resistance in the Cové vector population. With clothianidin-treated bottles, knockdown and mortality after 24 h were respectively 98% and 100%, demonstrating full susceptibility to the insecticide. Meanwhile, with the susceptible An. gambiae Kisumu strain, mortality was > 95% at 24 h with both insecticides. More detailed results on CDC bottle bioassays are available in the supplementary information (Additional file 1: Table S1).
The experimental hut results are summarised in Table 1. A total of 57,518 wild free-flying pyrethroid-resistant female An. gambiae (s.l.) mosquitoes were collected in the experimental huts over the 11-month trial indicating an average of 8217 mosquitoes per hut (Table 1). For both concrete and mud-walled huts, exiting rates with deltamethrin (62–64%) and Fludora® Fusion (55–60%) was significantly higher than the control hut (40%) and clothianidin-only huts (37–38%) (P < 0.05). The exiting rates observed with Fludora® Fusion-treated huts are therefore attributable to the deltamethrin component in the mixture. Blood-feeding rates were high across all insecticide treatments tested (87–92%); hence, there was no evidence of blood-feeding inhibition with any of these treatments regardless of the wall substrate (Table 1).
Mortality rates of wild free-flying pyrethroid-resistant An. gambiae (s.l.) which entered the experimental huts during the 11-month trial are presented in Table 1 and Figs. 4, 5.
Delayed mortality. Figure 5 presents the mortality rates of wild pyrethroid-resistant An. gambiae (s.l.) in the experimental huts recorded every 24 h up to 120 h. Mortality with the control was < 2% after 24 h and increased to only 4% after 120 h. For both types of wall substrates, Fludora® Fusion and clothianidin alone demonstrated a substantial increase in delayed mortality with the numbers of days after collection from the treated huts. Mortality in huts treated with clothianidin and Fludora® Fusion increased steadily from approximately 20–25% after 24 h to 69–78% after 120 h. This trend was not strongly expressed in huts treated with deltamethrin alone where mortality did not exceed 20% after 120 h for both substrate types, thus demonstrating that the delayed mortality effect observed with Fludora® Fusion is largely due to the clothianidin component of the mixture.
Monthly mortality of wild mosquitoes. Figure 6 shows monthly mortality rates of wild free-flying, pyrethroid-resistant An. gambiae (s.l.) entering IRS-treated experimental huts over 11 months for both concrete and mud-walled huts. For both substrates, Fludora® Fusion and clothianidin killed at least 80% of wild, pyrethroid-resistant An. gambiae (s.l.) entering experimental huts for the first 4 months, after which mortality declined progressively to < 40% after 9 months. Conversely, the proportion of wild pyrethroid-resistant An. gambiae (s.l.) killed in the deltamethrin-treated huts did not exceed 25% throughout the trial.
Cone bioassay mortality on treated hut walls Quarterly mortality rates (pooled for every 3 months) of the susceptible, laboratory-maintained An. gambiae Kisumu strain following exposure to mud and concrete IRS-treated experimental hut walls in in situ cone bioassays are presented in Fig. 7. Cone bioassay mortality on control untreated hut walls did not exceed 5% for all four quarters. With deltamethrin-treated huts, cone bioassay mortality was > 80% only for the first 3 months after which it declined sharply. Cone bioassay mortality remained > 80% for 9 months with Fludora® Fusion and 12 months with clothianidin on concrete-walled huts and 12 months with both insecticides on mud-walled huts.
The results from chemical analysis of filter papers performed at BioGenius GmBH, Germany, are presented in Table 2. The average AI content in filter papers for each treatment and wall substrate type were within an acceptable deviation of 22% from the target dose showing that the treatments were correctly applied.
This study evaluated the efficacy of Fludora® Fusion, a clothianidin and deltamethrin mixture, for indoor residual spraying in laboratory studies and in an experimental hut trial against a vector population in Benin which is highly resistant to pyrethroids. The low mortality response with deltamethrin IRS in the experimental huts is very typical of studies conducted in this area of Benin [11, 14] thus demonstrating the redundancy of solo pyrethroid products for IRS and further highlighting the need for novel non-pyrethroid IRS insecticides. Studies performed with Fludora® Fusion in West Africa have so far reported its efficacy on treated surfaces only in cone bioassays [15, 16] which generally do not take into consideration the behaviour of vector mosquitoes. Our study demonstrates for the first time the efficacy of Fludora® Fusion against wild free-flying pyrethroid-resistant malaria vectors in household settings in Benin. At all levels of evaluation, Fludora® Fusion clearly showed greatly improved and prolonged overall mortality of pyrethroid susceptible and resistant strains of An. gambiae (s.l.) compared to deltamethrin on both mud and concrete substrates. The results confirm previous findings across Africa [11, 15, 16], thus demonstrating the suitability of Fludora® Fusion for indoor residual spraying in Benin and other malaria-endemic areas which are characterised by high intensities of pyrethroid resistance in local mosquito vectors.
Previous experimental hut studies also showed significantly reduced mortality with an IRS mixture of chlorfenapyr and alpha-cypermethrin compared to chlorfenapyr alone (43% vs 63% [14] and 18–22% vs 38–46% [17]) which was also attributed to the irritant effect of the pyrethroid in the mixture. Alternatively, the small difference in performance between Fludora® Fusion and clothianidin-solo IRS in huts could also be due to differences in the types of IRS formulations (WG for clothianidin and WP for Fludora® Fusion) as has been previously reported with some pyrethroid IRS insecticides [18, 19]. However, contrary to the chlorfenapyr and alpha-cypermethrin mixture, the mortality achieved with Fludora® Fusion was only a few points lower than that of clothianidin alone (70–71% vs 72–78%) and this is less expected to result in operationally significant differences in the impact on clinical malaria when used in IRS campaigns. Also, the high insecticide induced exiting rate observed with Fludora® Fusion compared to clothianidin alone in this study and previously [11] is important for reducing indoor resting and biting which may contribute to lowering transmission intensities.
While a universal diagnostic dose for clothianidin is yet to be established for malaria vectors, our results showed full susceptibility to clothianidin in CDC bottle bioassays at 90 µg/bottle in a vector population that is highly resistant pyrethroids [13]. Recent studies in neighbouring countries in West Africa also reported full susceptibility to clothianidin [20, 21] at even lower doses [21] in pyrethroid-resistant malaria vectors despite high levels of resistance to other neonicotinoids widely used in agriculture in the region [21]. As new active ingredients are introduced into the public health portfolio of insecticides, strategies to delay the development of resistance to these insecticides in malaria vectors must be taken into consideration before large-scale use in order to extend their useful life [9]. Modelling studies have suggested that the use of an insecticide in a mixture especially when it is effective at killing mosquitoes, has potential to prompt slower evolution of resistance to the insecticide compared to when it is used alone [22]. Considering its effectiveness, the co-formulation of clothianidin into the Fludora® Fusion mixture might be a better option for delaying the development and spread of resistance to clothianidin in malaria vectors compared to formulations with clothianidin alone. Further studies to investigate this hypothesis under large scale use would be necessary.
Contrary to a previous study in Benin [15], we observed a longer residual effect of Fludora® Fusion on mud walls compared to concrete surfaces. This could be attributed to the addition of a small amount of cement in the mud paste used for moulding the blocks and wall plaster used in our study. Previous studies examining the content of mud from Cové, Benin, demonstrated a high silt content which makes it less suitable for construction (unpublished data). The addition of a small amount of cement is in line with a common local practice in Benin and other parts of Africa which is aimed at improving the durability and aesthetic of inner wall plastering in earthen houses [23]. This could have changed the characteristics of the mud substrates making them more stable, much less porous and more suitable for Fludora® Fusion compared to the traditional mud-plastered wall. This finding shows that Fludora® Fusion could be highly effective in semi-urban and usually more populated areas in many African settings where the addition of cement to mud for construction and plastering of earthen houses is becoming common [23].
Based on results from our studies and others, Fludora® Fusion was added to the WHO’s list of pre-qualified IRS formulations for vector control becoming the first IRS insecticide to contain a mixture of active ingredients [7]. Fludora® Fusion was recently deployed for IRS in Benin by the National Malaria Control Programme in the 2020 IRS campaign; studies to assess its impact under operational conditions are on-going.

Sections

"[{\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR1\", \"CR2\", \"CR3\", \"CR4\", \"CR2\", \"CR3\", \"CR5\", \"CR6\"], \"section\": \"Background\", \"text\": \"Indoor residual spraying remains a core strategy in the fight against malaria due to its ability to rapidly reduce transmission [1]. It constitutes the application of a residual insecticide to potential resting surfaces of malaria vectors; usually the inner walls, ceiling and eaves of human habitats and domestic animal shelters. There has been a substantial increase in the use of IRS over the last two decades [2, 3] and this has contributed significantly to reductions in malaria morbidity and mortality observed in many endemic countries [4]. The effectiveness of IRS for malaria vector control however depends on several factors; mainly the continued susceptibility of local vectors to the insecticides deployed and the duration of its action on treated home wall substrates. For some decades, IRS relied heavily on a rather limited number of classes of insecticides [2, 3] most of which were short-lived on home wall substrates (2\\u20135\\u00a0months) thus requiring multiple resource-demanding IRS campaign rounds when used in areas with stable malaria transmission [5]. Malaria vectors have also developed resistance to these conventional insecticides which is now widespread and increasing in intensity across Africa [6] and this, together with their short residual effect is driving the development of a new generation of long-lasting IRS insecticides to which local vectors are largely susceptible.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR7\", \"CR8\", \"CR9\", \"CR10\"], \"section\": \"Background\", \"text\": \"The neonicotinoid, clothianidin is a new repurposed insecticide which was recently added to the WHO\\u2019s list of pre-qualified insecticides for use in indoor residual spraying [7]. Clothianidin presents a new mode action which differs from that of conventional public health insecticides acting as an agonist on nicotinic acetylcholine receptors (nAChR) [8]. Owing to its novel mode of action, it shows potential to provide improved control of vector populations that have developed resistance to older public health insecticides. The addition of clothianidin to the portfolio of IRS insecticides also provides an opportunity to mitigate the development and spread of insecticide resistance in malaria vectors through the rotational use of IRS insecticides and the development of mixture IRS co-formulations [9]. Insecticide mixtures for IRS need to be explored with new public health insecticides when they become available because mixtures have the dual potential to improve malaria vector control through the combined effect of both active ingredients and contribute to insecticide resistance management, especially in areas where resistance to both active ingredients is not yet established [10].\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR11\"], \"section\": \"Background\", \"text\": \"In a previous experimental hut study in Benin, a tank mix of clothianidin and deltamethrin induced high and prolonged mortality (8\\u20139\\u00a0months) in wild pyrethroid-resistant An. gambiae (s.l.) owing to the clothianidin component and early exiting of mosquitoes from experimental huts due to the pyrethroid component [11]. The encouraging results from this early proof of concept study led to the development of Fludora\\u00ae Fusion (Bayer CropScience, Monheim, Germany), a new IRS formulation of a wettable powder product containing 500 g/kg of clothianidin and 62.5 g/kg of deltamethrin in water-soluble sachets. In the present study, the efficacy of Fludora\\u00ae Fusion was evaluated against pyrethroid-resistant An. gambiae (s.l.) in southern Benin under both laboratory and experimental hut conditions. Its residual effect was assessed on mud and concrete wall substrates. A comparison was made with the two active ingredients of the mixture; clothianidin and deltamethrin, applied alone.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR12\", \"CR11\"], \"section\": \"Residual efficacy on treated block substrates\", \"text\": \"Residual efficacy of the insecticides on the treated mud and concrete block substrates were assessed in monthly cone bioassays following WHO guidelines [12]. Forty (40) 2\\u20135-day-old laboratory maintained susceptible Anopheles gambiae Kisumu and pyrethroid-resistant Anopheles gambiae (s.l.) Cov\\u00e9 mosquitoes were exposed for 30 min to each treatment and substrate in replicates of 10 mosquitoes per block. The laboratory cone bioassays were conducted at monthly intervals for up to 12 months. Based on the delayed effect of clothianidin on mosquito mortality as demonstrated in previous studies [11], mortality was recorded every 24 hours up to 120 hours post-exposure across all experiments.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR13\", \"CR13\"], \"section\": \"Study site and experimental huts\", \"text\": \"The hut trial was performed at the CREC/LSHTM experimental hut station in Cov\\u00e9, Southern Benin. The field site is located in an irrigated valley producing rice almost year-round and providing suitable breeding habitats for mosquitoes. The rainy season extends from March to October and the dry season from November to February. The vector population consists of both An. coluzzii and An. gambiae (sensu stricto) with the latter occurring at lower frequencies (~23%) and mostly in the dry season [13]. The vector population is highly resistant to pyrethroids. Molecular genotyping and microarray studies have demonstrated a high frequency of the L1014F allele (>\\u200990%) and overexpression of the cytochrome P450s CYP6P3, associated with pyrethroid detoxification [13]. The trial ran for 11\\u00a0months between December 2015 and November 2016 in 7 experimental huts of West African design. The experimental huts are made from concrete bricks with a corrugated iron roof. Inner walls were plastered with either concrete or mud and the ceilings fitted with palm thatch matting. Each hut was built on a concrete plinth surrounded by a water-filled moat to prevent the entry of scavenging ants and had a wooden framed veranda trap to capture exiting mosquitoes. Mosquito entry occurred via four window slits each measuring 1 cm and situated on three sides of the hut.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR12\"], \"section\": \"Hut trial procedure\", \"text\": \"The trial followed the WHO guidelines for evaluation of IRS products [12]. Treatments were randomly allocated to experimental huts. Seven consenting adult human volunteers slept in the huts from 21:00 to 5:00 h each trial night to attract mosquitoes and were rotated between huts on successive nights to adjust for variation in individual attractiveness to mosquitoes. In the morning, mosquitoes were collected from the room and veranda using aspirators and brought to the laboratory where they were identified and scored as fed or unfed and dead or alive. Live mosquitoes were provided with 10% glucose solution and mortality scored every 24 h for up to 120 h.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR12\"], \"section\": \"Residual activity of insecticide treatments\", \"text\": \"To assess the residual activity of the treatments on the treated experimental hut walls, WHO cone bioassays were conducted using 2\\u20135-day-old, female mosquitoes of the insecticide susceptible An. gambiae Kisumu strain. Bioassays were performed 3 days after application of treatments and at monthly intervals thereafter over 12\\u00a0months. A total of 50 mosquitoes were tested per hut in cohorts of 10 per cone on each treated wall/ceiling surface. Mosquitoes were exposed to treated surfaces for 30 min following WHO guidelines [12]. Mortality was recorded every 24 h up to 120 h post-exposure.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR12\"], \"section\": \"Data analysis\", \"text\": \"Experimental hut data were entered in Excel and transferred to Stata 15.1 for analysis. Proportional data (exiting rate, blood-feeding and mortality) were analysed using logistic regression while adjusting for the effects of sleeper attractiveness to mosquitoes. Cone bioassay mortality was pooled for each treatment and substrate at each time point and compared against an 80% cut-off criteria following WHO guidelines [12].\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"Fig1\", \"Fig2\", \"Fig1\", \"Fig2\", \"Fig1\", \"Fig2\", \"Fig1\", \"Fig1\", \"Fig1\", \"Fig1\", \"Fig2\"], \"section\": \"Laboratory cone bioassay results\", \"text\": \"Mortality of susceptible An. gambiae Kisumu and pyrethroid-resistant An. gambiae (s.l.) Cov\\u00e9 strains in laboratory cone bioassays with control untreated mud and concrete blocks did not exceed 20% at any time point (Figs.\\u00a01 and 2). For both types of substrates, mortality rates for each treatment were generally higher/more persistent with the Kisumu strain compared to the pyrethroid-resistant Cov\\u00e9 strain. Mortality with deltamethrin-treated blocks was\\u2009>\\u200980% for the first 2\\u20134\\u00a0months with the susceptible Kisumu strain after which it declined sharply (Figs.\\u00a01a and 2a), but with pyrethroid-resistant Cov\\u00e9 strain, mortality on both substrate-types did not exceed 50% at any time point (Figs.\\u00a01b and 2b). On concrete blocks, mortality with clothianidin was >\\u200980% for 7\\u00a0months with the susceptible Kisumu strain (Fig.\\u00a01a) and only 2 months with the pyrethroid-resistant Cov\\u00e9 strain (Fig.\\u00a01b). Meanwhile, Fludora\\u00ae Fusion induced more residual mortality on concrete blocks which remained\\u2009>\\u200980% for 9 months with the susceptible Kisumu strain (Fig.\\u00a01a) and 7 months with the pyrethroid-resistant Cov\\u00e9 strain (Fig.\\u00a01b). With mud blocks, mortality rates with clothianidin and Fludora\\u00ae Fusion were very stable remaining above 90% for both mosquito strains for up to 12 months (Fig.\\u00a02).\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"Fig3\", \"MOESM1\"], \"section\": \"Susceptibility of vector population to clothianidin\", \"text\": \"Results from the CDC bottle bioassays are presented in Fig.\\u00a03. Mortality rates with the control were less than 10% even up to 120 h post-exposure. Knockdown and mortality of wild F1 An. gambiae (s.l.) mosquitoes emerging from larvae collected from the Cov\\u00e9 experimental hut site were respectively 52% and 39% with deltamethrin-treated bottles thus confirming the levels of pyrethroid resistance in the Cov\\u00e9 vector population. With clothianidin-treated bottles, knockdown and mortality after 24 h were respectively 98% and 100%, demonstrating full susceptibility to the insecticide. Meanwhile, with the susceptible An. gambiae Kisumu strain, mortality was\\u2009>\\u200995% at 24 h\\u00a0with both insecticides. More detailed results on CDC bottle bioassays are available in the supplementary information (Additional file 1: Table S1).\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"Tab1\", \"Tab1\", \"Tab1\"], \"section\": \"Mosquito entry and exiting\", \"text\": \"The experimental hut results are summarised in Table\\u00a01. A total of 57,518 wild free-flying pyrethroid-resistant female An. gambiae (s.l.) mosquitoes were collected in the experimental huts over the 11-month trial indicating an average of 8217 mosquitoes per hut (Table\\u00a01). For both concrete and mud-walled huts, exiting rates with deltamethrin (62\\u201364%) and Fludora\\u00ae Fusion (55\\u201360%) was significantly higher than the control hut (40%) and clothianidin-only huts (37\\u201338%) (P\\u2009<\\u20090.05). The exiting rates observed with Fludora\\u00ae Fusion-treated huts are therefore attributable to the deltamethrin component in the mixture. Blood-feeding rates were high across all insecticide treatments tested (87\\u201392%); hence, there was no evidence of blood-feeding inhibition with any of these treatments regardless of the wall substrate (Table\\u00a01).\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"Tab1\", \"Fig4\", \"Fig5\"], \"section\": \"Mortality rates\", \"text\": \"Mortality rates of wild free-flying pyrethroid-resistant An. gambiae (s.l.) which entered the experimental huts during the 11-month trial are presented in Table\\u00a01 and Figs.\\u00a04, 5.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"Fig5\"], \"section\": \"Mortality rates\", \"text\": \"Delayed mortality. Figure\\u00a05 presents the mortality rates of wild pyrethroid-resistant An. gambiae (s.l.) in the experimental huts recorded every 24 h up to 120 h. Mortality with the control was <\\u20092% after 24 h and increased to only 4% after 120 h. For both types of wall substrates, Fludora\\u00ae Fusion and clothianidin alone demonstrated a substantial increase in delayed mortality with the numbers of days after collection from the treated huts. Mortality in huts treated with clothianidin and Fludora\\u00ae Fusion increased steadily from approximately 20\\u201325% after 24 h to 69\\u201378% after 120\\u00a0h. This trend was not strongly expressed in huts treated with deltamethrin alone where mortality did not exceed 20% after 120\\u00a0h for both substrate types, thus demonstrating that the delayed mortality effect observed with Fludora\\u00ae Fusion is largely due to the clothianidin component of the mixture.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"Fig6\"], \"section\": \"Residual efficacy of IRS treatments\", \"text\": \"Monthly mortality of wild mosquitoes. Figure\\u00a06 shows monthly mortality rates of wild free-flying, pyrethroid-resistant An. gambiae (s.l.) entering IRS-treated experimental huts over 11 months for both concrete and mud-walled huts. For both substrates, Fludora\\u00ae Fusion and clothianidin killed at least 80% of wild, pyrethroid-resistant An. gambiae (s.l.) entering experimental huts for the first 4 months, after which mortality declined progressively to <\\u200940% after 9 months. Conversely, the proportion of wild pyrethroid-resistant An. gambiae (s.l.) killed in the deltamethrin-treated huts did not exceed 25% throughout the trial.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"Fig7\"], \"section\": \"Residual efficacy of IRS treatments\", \"text\": \"Cone bioassay mortality on treated hut walls Quarterly mortality rates (pooled for every 3 months) of the susceptible, laboratory-maintained An. gambiae Kisumu strain following exposure to mud and concrete IRS-treated experimental hut walls in in situ cone bioassays are presented in Fig.\\u00a07. Cone bioassay mortality on control untreated hut walls did not exceed 5% for all four quarters. With deltamethrin-treated huts, cone bioassay mortality was >\\u200980% only for the first 3 months after which it declined sharply. Cone bioassay mortality remained >\\u200980% for 9 months with Fludora\\u00ae Fusion and 12 months with clothianidin on concrete-walled huts and 12 months with both insecticides on mud-walled huts.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"Tab2\"], \"section\": \"Quality of IRS applications\", \"text\": \"The results from chemical analysis of filter papers performed at BioGenius GmBH, Germany, are presented in Table\\u00a02. The average AI content in filter papers for each treatment and wall substrate type were within an acceptable deviation of 22% from the target dose showing that the treatments were correctly applied.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR11\", \"CR14\", \"CR15\", \"CR16\", \"CR11\", \"CR15\", \"CR16\"], \"section\": \"Discussion\", \"text\": \"This study evaluated the efficacy of Fludora\\u00ae Fusion, a clothianidin and deltamethrin mixture, for indoor residual spraying in laboratory studies and in an experimental hut trial against a vector population in Benin which is highly resistant to pyrethroids. The low mortality response with deltamethrin IRS in the experimental huts is very typical of studies conducted in this area of Benin [11, 14] thus demonstrating the redundancy of solo pyrethroid products for IRS and further highlighting the need for novel non-pyrethroid IRS insecticides. Studies performed with Fludora\\u00ae Fusion in West Africa have so far reported its efficacy on treated surfaces only in cone bioassays [15, 16] which generally do not take into consideration the behaviour of vector mosquitoes. Our study demonstrates for the first time the efficacy of Fludora\\u00ae Fusion against wild free-flying pyrethroid-resistant malaria vectors in household settings in Benin. At all levels of evaluation, Fludora\\u00ae Fusion clearly showed greatly improved and prolonged overall mortality of pyrethroid susceptible and resistant strains of An. gambiae (s.l.) compared to deltamethrin on both mud and concrete substrates. The results confirm previous findings across Africa [11, 15, 16], thus demonstrating the suitability of Fludora\\u00ae Fusion for indoor residual spraying in Benin and other malaria-endemic areas which are characterised by high intensities of pyrethroid resistance in local mosquito vectors.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR14\", \"CR17\", \"CR18\", \"CR19\", \"CR11\"], \"section\": \"Discussion\", \"text\": \"Previous experimental hut studies also showed significantly reduced mortality with an IRS mixture of chlorfenapyr and alpha-cypermethrin compared to chlorfenapyr alone (43% vs 63% [14] and 18\\u201322% vs 38\\u201346% [17]) which was also attributed to the irritant effect of the pyrethroid in the mixture. Alternatively, the small difference in performance between Fludora\\u00ae Fusion and clothianidin-solo IRS in huts could also be due to differences in the types of IRS formulations (WG for clothianidin and WP for Fludora\\u00ae Fusion) as has been previously reported with some pyrethroid IRS insecticides [18, 19]. However, contrary to the chlorfenapyr and alpha-cypermethrin mixture, the mortality achieved with Fludora\\u00ae Fusion was only a few points lower than that of clothianidin alone (70\\u201371% vs 72\\u201378%) and this is less expected to result in operationally significant differences in the impact on clinical malaria when used in IRS campaigns. Also, the high insecticide induced exiting rate observed with Fludora\\u00ae Fusion compared to clothianidin alone in this study and previously [11] is important for reducing indoor resting and biting which may contribute to lowering transmission intensities.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR13\", \"CR20\", \"CR21\", \"CR21\", \"CR21\", \"CR9\", \"CR22\"], \"section\": \"Discussion\", \"text\": \"While a universal diagnostic dose for clothianidin is yet to be established for malaria vectors, our results showed full susceptibility to clothianidin in CDC bottle bioassays at 90 \\u00b5g/bottle in a vector population that is highly resistant pyrethroids [13]. Recent studies in neighbouring countries in West Africa also reported full susceptibility to clothianidin [20, 21] at even lower doses [21] in pyrethroid-resistant malaria vectors despite high levels of resistance to other neonicotinoids widely used in agriculture in the region [21]. As new active ingredients are introduced into the public health portfolio of insecticides, strategies to delay the development of resistance to these insecticides in malaria vectors must be taken into consideration before large-scale use in order to extend their useful life [9]. Modelling studies have suggested that the use of an insecticide in a mixture especially when it is effective at killing mosquitoes, has potential to prompt slower evolution of resistance to the insecticide compared to when it is used alone [22]. Considering its effectiveness, the co-formulation of clothianidin into the Fludora\\u00ae Fusion mixture might be a better option for delaying the development and spread of resistance to clothianidin in malaria vectors compared to formulations with clothianidin alone. Further studies to investigate this hypothesis under large scale use would be necessary.\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR15\", \"CR23\", \"CR23\"], \"section\": \"Discussion\", \"text\": \"Contrary to a previous study in Benin [15], we observed a longer residual effect of Fludora\\u00ae Fusion on mud walls compared to\\u00a0concrete surfaces. This could be attributed to the addition of a small amount of cement in the mud paste used for moulding the blocks and wall plaster used in our study. Previous studies examining the content of mud from Cov\\u00e9, Benin, demonstrated a high silt content which makes it less suitable for construction (unpublished data). The addition of a small amount of cement is in line with a common local practice in Benin and other parts of Africa which is aimed at improving the durability and aesthetic of inner wall plastering in earthen houses [23]. This could have changed the characteristics of the mud substrates making them more stable, much less porous and more suitable for Fludora\\u00ae Fusion compared to the traditional mud-plastered wall. This finding shows that Fludora\\u00ae Fusion could be highly effective in semi-urban and usually more populated areas in many African settings where the addition of cement to mud for construction and plastering of earthen houses is becoming common [23].\"}, {\"pmc\": \"PMC7488472\", \"pmid\": \"32917255\", \"reference_ids\": [\"CR7\"], \"section\": \"Discussion\", \"text\": \"Based on results from our studies and others, Fludora\\u00ae Fusion was added to the WHO\\u2019s list of pre-qualified IRS formulations for vector control becoming the first IRS insecticide to contain a mixture of active ingredients [7]. Fludora\\u00ae Fusion was recently deployed for IRS in Benin by the National Malaria Control Programme in the 2020 IRS campaign; studies to assess its impact under operational conditions are on-going.\"}]"

Metadata

"{\"issue-copyright-statement\": \"\\u00a9 The Author(s) 2020\"}"