A Chlorfenapyr Mixture Net Interceptor® G2 Shows High Efficacy and Wash Durability against Resistant Mosquitoes in West Africa

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Abstract

Background

Malaria control through use of long-lasting insecticidal nets (LN) is threatened by the selection of anopheline mosquitoes strongly resistant to pyrethroid insecticides. To sustain future effectiveness it is essential to identify and evaluate novel insecticides suitable for nets. Mixtures of two insecticides with contrasting mode of action have the potential to kill resistant vectors and restore transmission control provided the formulation can withstand regular washing over the net’s life span.

Method

The efficacy of a novel mixture LN, Interceptor ^® G2, that combines the pyrrole chlorfenapyr and pyrethroid alpha-cypermethrin was evaluated under controlled household conditions (experimental hut trial) and by laboratory bioassay against pyrethroid resistant An. gambiae in Benin before and after standardized washing. Comparison arms included standard alpha-cypermethrin LN, nets hand-treated with chlorfenapyr-only and untreated nets.

Results

The chlorfenapyr-alphacypermethrin LN demonstrated improved efficacy and wash resistance compared to a standard alpha-cypermethrin LN against pyrethroid resistant mosquitoes (resistance ratio 207). In experimental hut trial alpha-cypermethrin LN killed only 20% (95% CI 15–26%) of host-seeking An. gambiae whilst mixture LN killed 71% (95% CI 65–77%). Mixture LN washed 20 times killed 65% (95% CI 58–71%), and thus intensive washing reduced efficacy by only 6% (95% CI 1.3–11%). The chlorfenapyr net killed 76% (95% CI 70–81%). Personal protection and blood feeding inhibition did not differ between mixture and pyrethroid LN; however, the mixture LN was 2.5 (95% CI: 2.1–3.1) times more protective than untreated nets. Standard WHO cone bioassays conducted during day time hours failed to anticipate field efficacy but overnight tunnel tests successfully predicted mixture LN and chlorfenapyr net efficacy in field trials.

Conclusion

Interceptor ^® G2 LN demonstrates the potential to control transmission and provide community protection over the normal lifespan of long lasting nets where standard pyrethroid LN show signs of failing due to resistance.

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Most cited references 19

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Reduced Efficacy of Insecticide-treated Nets and Indoor Residual Spraying for Malaria Control in Pyrethroid Resistance Area, Benin

Raphael N’Guessan, Vincent Corbel, Martin Akogbeto … (2007)

During the last decade, pyrethroid-treated mosquito nets have become the main method of malaria prevention in many malaria-endemic African countries ( 1 , 2 ). In a few notable exceptions, usually those with a more developed health infrastructure, such as South Africa, a longstanding practice of applying indoor residual spraying (IRS) has been successful ( 3 ). The 2 approaches to malaria prevention, insecticide-treated nets (ITNs) and spraying (IRS), are not mutually exclusive, and in malaria-endemic areas where ITN coverage is still limited, the feasibility of introducing IRS to reduce transmission is being considered, for example, by the President’s Initiative Fund ( 4 ). Trials of IRS and ITNs have shown that in areas with pyrethroid-susceptible Anopheles gambiae the effectiveness of the 2 methods in controlling malaria does not differ ( 5 ). This comparability may not hold true for areas with pyrethroid-resistant populations. In southern Africa, for example, IRS with pyrethroid failed to control pyrethroid-resistant An. funestus and necessitated a switch to an alternative class of insecticide to which there was no resistance ( 6 ). During the last decade, pyrethroid resistance caused by the kdr mechanism has become widespread in An. gambiae in West Africa and is common in some areas ( 7 ). Whether kdr undermines the effectiveness of ITN in areas of high prevalence is unclear. An early experimental hut trial of ITNs in Côte d’Ivoire demonstrated a survival advantage of homozygotes for kdr resistance ( 8 ), whereas subsequent hut trials in adjacent resistant and susceptible populations showed no apparent difference in the effectiveness of ITNs between the 2 localities ( 9 ). Village randomized trials in Côte d’Ivoire showed that ITNs continued to prevent malaria despite a vector population that was kdr resistant ( 10 ). Whether kdr would undermine the effectiveness of IRS in the same way as resistance due to oxidases did against An. funestus in southern Africa ( 6 ) is unknown. To assess the practicability of applying IRS with pyrethroid in West Africa, we need to examine the effectiveness of this approach against a kdr-resistant population of An. gambiae is important. To get a clearer understanding of the influence of kdr resistance on the effectiveness of ITN, further experimental hut trials of ITNs against kdr-resistant populations need to be conducted. We describe 2 experimental hut trials in Benin. One compares the impact of IRS and ITN against a kd- resistant population in the southern part of the country; the other compares IRS and ITNs against a pyrethroid-resistant population several hundred kilometers to the north. Material and Methods Study Sites Ladji is a large village on the outskirts of Cotonou, the capital of Benin. The village floods during the rainy season. An. gambiae comprises the Mopti (M) cytotype and shows resistance to pyrethroids and DDT; kdr is present at high frequency ( 11 ). The nuisance mosquito Culex quinquefasciatus is also present and shows resistance to pyrethroids. Five experimental huts belonging to the Centre de Recherche Entomologique de Cotonou (CREC) are situated in the village. Malanville is in northern Benin, 800 km from Cotonou, in an irrigated rice-growing valley. The local An. gambiae comprises the M cytotype, but the kdr gene is almost absent and mosquitoes are susceptible to lambdacyalothrin and deltamethrin. Six experimental huts are present at Malanville. Experimental Huts The treated nets, residual spray treatments, and their respective untreated controls were evaluated in 4 experimental huts at each field site. Experimental huts are specially designed to test vector control product against freely entering mosquitoes under natural but controlled conditions. Huts were typical of the region. Each was made from concrete bricks, with a corrugated iron roof and a ceiling of thick polyethylene sheeting lined with hessian sackcloth on the interior surface, and each was built on a concrete base surrounded by a water-filled moat to exclude ants ( 12 ). Mosquito access was through 4 window slits, constructed from pieces of plywood fixed at an angle to create a funnel with a 1-cm gap, present on 3 sides of the huts. Mosquitoes had to fly upward to enter through the gaps and downwards to exit; this precluded or limited exodus through the aperture and enabled us to account for most entering mosquitoes. A veranda trap projected from the back wall of each hut. Movement of mosquitoes between a room and the veranda was unimpeded. Mosquito Net Treatments The nets were made of white, 100-denier polyester (SiamDutch Mosquito Netting Co., Bangkok, Thailand). Nets measured 2.0-m long, 1.6-m wide, and 1.8-m tall and had a surface area of 16.9 m2. To simulate badly torn nets, 80 holes, each measuring 2 × 2 cm, were cut in the sides and ends of each net. Insecticides used were formulations of lambdacyhalothrin (Icon, Syngenta, Switzerland): lambdacyhalothrin 2.5% CS, a microencapsulated suspension designed for ITNs, and lambdacyhalothrin 10% WP, a wettable powder designed for IRS. The lambdacyhalothin application rates of 18 mg/m2 for ITNs and 30 mg/m2 for IRS were within the ranges recommended by the manufacturer. Indoor residual treatments were applied with a hand-operated compression sprayer equipped with a flat fan nozzle. The cement walls and sackcloth ceilings were sprayed uniformly after masking the veranda and window slits with protective coverings. The control hut was sprayed with water only. The treated huts were left for 1 week before evaluations were started. Sleepers and Mosquito Collections Preliminary experiments showed the huts to be evenly attractive to mosquitoes. The treatments were randomly allocated to the 4 experimental huts at each site. The main trials were conducted from April to June 2005 at the Ladji site and from September to November 2005 at the Malanville site. Eight adult men employed by CREC slept overnight in the huts and collected mosquitoes from the huts in the mornings. Informed consent to participate in the study was given beforehand, and chemoprophylaxis was provided during the trial. Ethical approval was granted by the London School of Hygiene and Tropical Medicine (LSHTM) and Benin national ethics committees. The trial ran for 50 nights for 8 weeks at each site. The sleepers were rotated between huts to correct for possible variation in individual attractiveness. Each morning, mosquitoes were collected from the floors, walls, and ceilings of rooms, verandas, and nets with aspirators and torches. Mosquitoes were identified and scored as blood-fed or unfed and dead or live. Live mosquitoes were held in netted plastic cups and supplied with 10% honey solution for 24 h before delayed death was recorded. Male mosquitoes were not recorded. The entomologic impact of each treatment on mosquitoes was expressed relative to the control in terms of the following: deterrence, the proportional reduction in the number of mosquitoes entering a treated hut relative to that entering the control hut; induced exophily, the proportion of mosquitoes collected from the veranda trap of the treatment hut relative to the proportion in the veranda of the control hut; blood-feeding inhibition, the reduction in blood-feeding rate relative to the control hut; and mortality, the proportions of mosquitoes found dead in the hut at the time of collection and after a 24-h holding period. If a treatment deters a considerable number of mosquitoes from entering the hut, the values given by proportion blood-feeding or proportion killed in the treatment hut may underestimate the full personal protective effect and overestimate the full insecticidal efficacy of the treatment. The personal protective effect of a treatment is better described by the reduction in the number of blood-fed mosquitoes in the treatment hut relative to the number blood-fed in the control hut: % personal protection = 100 (Bu – Bt)/Bu where Bu = is the total number of blood-fed mosquitoes in the untreated control huts and Bt is the total number blood-fed mosquitoes in the huts with insecticide treatment. The overall insecticidal effect of a treatment needs to take into account that a considerable number of mosquitoes might be deterred from entering the hut and hence not be killed by the treatment. A mass killing effect is desirable to reduce transmission. The overall insecticidal effect of a treatment relative to the number of mosquitoes that would ordinarily enter an untreated hut can be estimated by using the following formula and expressed as a percentage: Overall insecticidal effect (%) = 100 (Kt – Ku)/(Tu – Ku) where Kt is the number killed in the treated hut, Ku is the number dying in the untreated control hut, and Tu is the total number collected from the control hut. Residual Activity of Insecticide Treatments To evaluate residual activity, World Health Organization (WHO) cone bioassays were undertaken monthly in the Ladji huts and bimonthly in the Malanville huts with a laboratory-susceptible strain of An. gambiae (Kisumu). An. gambiae females, 3–5 days old, were exposed within the cones to nets for 3 min or to sprayed walls and ceilings for 30 min. Approximately 50 mosquitoes in 5 replicates of 10 mosquitoes were tested on each substrate. Honey solution was provided during the 24-h holding period, and the temperature was kept at 25°C. Biochemical Assays Biochemical tests on individual mosquitoes were conducted to determine the activity of mixed function oxidases and nonspecific esterases present in pyrethroid-resistant and -susceptible samples of An. gambiae from the Ladji and Malanville sites. Tests were conducted on 3-day-old adult females (initially collected as larvae) in microtiter plates ( 13 ). Susceptible (Kisumu) and pyrethroid-resistant (Vkper) An. gambiae served as controls. Genotyping of An. gambiae was carried out to assess kdr frequency at both field stations ( 14 ). Adult Bioassay Data To determine whether a stronger pyrethroid resistance mechanism was present in the Ladji population than in the standard kdr strain Vkper, bioassays with 0.05% lambdacyalothrin-treated papers (18 mg/m2) were conducted in WHO resistance test kits by using a range of exposure times on batches of 25 unfed An. gambiae females 2–5 days of age. One hundred mosquitoes per exposure period were tested. Deaths were scored 24 h later. Log-time mortality curves were generated, and lethal time to kill 50% (LT50), estimated by using probit analysis. Data Analysis Proportional data from the hut trial (exophily, blood-feeding, deaths) were analyzed by using logistic regression (STATA 6 software, Stata Corporation, College Station, TX, USA). Deterrence rates were analyzed by comparing the number of mosquitoes entering each hut by using the Wilcoxon rank sum test. Biochemical activity was analyzed with Kruskal–Wallis and Wilcoxon rank sum tests. The level of resistance to lambdacyalothrin in insecticide bioassays was analyzed by using probit analysis. Results Insecticide Residual Activity Residual activity on ITN as measured by cone bioassay tests showed no decline during the 8 weeks of the trial. Activity of the IRS wettable powder formulation on sackcloth and cement showed a decline in performance by week 4. This trend continued until the end of the trial (Table 1). Table 1 Residual activity of lambdacyalothrin (insecticide)-treated nets (ITNs) and indoor residual spraying over 3 mo in experimental huts, Malanville and Ladji field stations* When and where substrate tested ITNs at 18 mg/m2 Indoor residual spraying at 30 mg/m2 Sides + top of net Ceiling Walls No. tested % Corrected mortality No. tested % Corrected mortality No. tested % Corrected mortality Wk 0 Malanville 77 100 33 100 60 100 Ladji 51 100 30 93.3 54 100 Wk 2 Ladji 52 100 22 100 41 100 Wk 4 Ladji 54 100 21 52.4 47 42.5 Wk 6 Ladji 57 100 25 80.0 45 31.1 Wk 8 Malanville 52 100 29 41.4 54 2.6 Ladji 44 97.7 8 25.0 39 18.5 *As determined by using World Health Organization cone bioassays and susceptible Anopheles gambiae (Kisumu). Efficacy of Treatments in Huts Over the 2-month trial, 1,395 An. gambiae, 3,070 Cx. quinquefasciatus, and small numbers of Mansonia uniformis, An. pharoensis, and Aedes aegypti were collected at Ladji. At Malanvile, 1,523 An. gambiae, 2,804 Mansonia sp., and smaller numbers of An. funestus and Ae. aegypti were collected. Only the malaria vector An. gambiae and the nuisance mosquito Cx. quinquefasciatus were analyzed further. Fewer An. gambiae entered the ITN- and IRS-treated huts than the respective control huts. The treatment induced reduction in hut entry was more evident in the resistance area than in the susceptible area (Table 2). The proportion deterred at each site did not differ between ITN or ITS treatments. Table 2 Experimental hut results of lambdacyhalothrin (insecticide)-treated nets (ITNs) and indoor residual spraying (IRS) against Anopheles gambiae, Ladji (pyrethroid resistance) and Malanville (pyrethroid susceptibility) field stations* Ladji (pyrethroid resistance) Malanville (pyrethroid susceptibility) ITN Untreated net Lambdacyhalothrin 18 mg/m2 Untreated net Lambdacyhalothrin 18 mg/m2 Total collected 689a 386b 363a 267b Deterred, % _ 44.0 ._ 26.4 Exiting, % (CI) 25.0† (21.7–28.2) 29.0† (24.5–33.5) 36.1 (31.1–41.0) 46.8‡ (40.8–52.8) Blood-fed, % (CI) 82.0† (79.1–84.9) 82.1† (78.3–85.9) 77.7† (73.4–81.9) 3.0‡ (0.9–5.0) Blood-feeding inhibition, % _ 0 _ 96.1 Personal protection, % (no. bloodfed) – (572) 44.6 (317) – (282) 97.2 (8) % Dead (CI) 13.6† (11.1–16.2) 29.8‡ 25.2–34.4) 3.6† (1.7–5.5) 98.5‡ (97.0–99.9) Insecticidal effect, % (no. dead) – (94) 3.0 (115) – (13) 68.9 (263) IRS Unsprayed hut Lambdacyhalothrin 30 mg/m2 Unsprayed hut Lambdacyhalothrin 30 mg/m2 Total collected 203† 117‡ 498† 395‡ Deterred, % _ 42.4 _ 20.7 Exiting, % (CI) 45.8† (38.9–52.7) 58.1† (49.2–67.1) 54.4† (50.0–58.8) 63.3† (58.5–68.0) Blood-fed, % (CI) 87.7† (83.2–92.2) 73.5‡ (65.5–81.5) 93.8† (91.6–95.9) 69.6‡ (65.1–74.2) Blood-feeding inhibition, % _ 16.2 _ 25.8 Personal protection, % (no. bloodfed) – (178) 51.7 (86) – (467) 41.1 (275) Dead, % (CI) 12.3† (7.8–16.8) 30.8† (22.4–39.1) 1.4† (0.4–2.4) 72.1‡ (67.7–76.6) Insecticidal effect, % (no. dead) – (25) 5.4 (36) – (7) 55.8 (285) *For each untreated–treated pair, values not sharing the same superscript are significantly different at the 5% level.. CI, 95% confidence interval. The untreated net was little or no barrier to blood-feeding of An. gambiae at either field site owing to the large number of holes cut in each net. Treating the holed net with pyrethroid led to a 96% reduction in the number of mosquitoes blood-feeding at the susceptible site (Malanville) but to no reduction in blood-feeding at the resistant site (Ladji). Inhibition of blood-feeding by IRS at either the resistant or susceptible site was limited (Table 2). Natural mortality of An. gambiae occurred in both types of control huts but was notably higher at Ladji than at Malanville. Both modes of treatment were highly insecticidal at Malanville: ITNs treated with 18 mg/m2 lambdacyhalothrin killed 99%, and IRS applied at 30 mg/m2 killed 72% of An. gambiae that entered the huts. At Ladji, the proportions of An. gambiae killed in either the ITN- or IRS-treated hut did not exceed 30% (Table 2). The proportion of An. gambiae collected from the veranda traps in the mornings was greater at Malanville than at Ladji and greater in the huts with untreated nets than in the unsprayed control huts. Relative to the controls, lambdacyalothrin-treated nets and IRS induced little or no exophily of the pyrethroid-resistant An. gambiae into the verandas of the Ladji huts, despite high survival rate of mosquitoes in huts. At Malanville, pyrethroid-induced exophily by ITN or IRS hut was not evident and may have been obscured by the high death rates among the mosquitoes. The personal protection derived from ITN was almost 100% in the susceptible area. Despite the low mortality rate and high rate of blood-feeding observed with ITN in the resistance area, the level of personal protection there was almost 50% because of the deterrent effect of lambdacyhalothin on mosquito entry into huts. The personal protective effect of IRS was low in both areas, and IRS was no barrier to blood-feeding. The overall insecticidal effect of pyrethroid-treated nets and IRS was negligible in the resistance area ( 55.8%). Table 3 breaks down the mortality data into 2-week blocks. Mortality associated with IRS treatments decreased week by week at both sites but started at a lower rate at the Ladji site because of the expression of resistance. Mortality associated with ITN treatments also showed a downward trend over time at Ladji but not at Malanville, where mosquitoes showed high susceptibility throughout the study. Table 3 Mortality rate of free-flying, naturally entering mosquitoes in huts, first 8 weeks of trial Ladji (pyrethroid-resistant Anopheles gambiae) Malanville (pyrethroid-susceptible An. gambiae) ITN IRS ITN IRS Wk No. % Corrected mortality No. % Corrected mortality No. % Corrected mortality No. % Corrected mortality 1–2 41 43.2 15 53.3 67 100 91 100 3–4 83 50.5 42 47.6 93 100 108 88.7 5–6 209 28.7 39 24.2 54 92.6 78 57.8 7–8 53 5.7 21 23.8 53 98.8 118 39.0 *ITN, insecticide-treated net; IRS, indoor residual spraying. Both ITN and IRS treatments at Ladji showed poor efficacy against Cx. quinquefasciatus (this species was not encountered in Malanville). Insecticide-induced deterrence was greater for ITN than for IRS (Table 4). Neither method killed many Culex nor stimulated repellency into verandas. The IRS treatment produced an unusually high level of blood-feeding inhibition. Table 4 Experimental hut results of lambdacyalothrin (insecticide)-treated nets (ITNs) and indoor residual spraying (IRS) against Culex quinquefasciatus, Ladji (pyrethroid resistance) field station* Results Treatments ITNs IRS Untreated net Lambdacyhalothrin 18 mg/m2 Unsprayed hut Lambdacyhalothrin 30 mg/m2 Total entered 845 598 858 769 Deterred, % _ 29.2 _ 10.4 Exiting, % (CI) 29.8 (26.7–32.9) 35.9 (32.1–39.8) 52.7 (49.3–56.0) 54.6 (51.1–58.1) Blood-fed, % (CI) 62.8 (59.6–56.1) 59.5 (55.6–63.5) 85.1 (82.7–87.5) 42.9 (39.4–46.4) Blood-feeding inhibition – NS – 49.6 Personal protection, % (no. blood-fed – (531) 33.1 (355) – (730) 54.8 (330) Dead, % (CI) 4.3 (2.9–5.6) 8.5 (6.3–10.8) 3.4 (2.2–4.6) 16.3 (13.7–18.9) Insecticidal effect, % (no. dead) – (36) 1.9 (51) – (29) 11.6 (125) *CI, 95% confidence interval. Biochemical Assays and kdr Genotyping An. gambiae from Ladji expressed a significantly higher level of oxidase activity than the standard susceptible (Kisumu) and the laboratory kdr (Vkper) strains, which had a similar level of oxidase activity. However, the pyrethroid-susceptible strain from Malanville showed a level of oxidase activity that was not significantly different from that of the Ladji strain. This finding would appear to rule out any contribution from oxidases to the pyrethroid resistance observed in An. gambiae from Ladji. The level of α-esterase activity in An. gambiae from Ladji was significantly higher than that expressed in Malanville or Kisumu strains, whereas the level of β-esterase activity in Ladji, Vkper, and Kisumu strains was similar and clearly played no part in resistance (Table 5). Overall, the mean level of esterase activity at Malanville was significantly lower than that of the susceptible reference strain (p 95%, a degree of personal protection associated with ITNs and IRS was still evident (45%–50%) relative to the untreated net or unsprayed hut owing to a partial deterrent effect of treatments on entry of mosquitoes rather than to any inhibition of blood-feeding once the insects were inside the huts. Indeed, on entering the huts, most mosquitoes did go on to blood-feed, and the deliberately holed ITN was no barrier to resistant mosquitoes. By contrast, in northern Benin, only 4% of the insecticide-susceptible mosquitoes that entered the hut fed through the holed ITN. The loss of personal protection and loss of mosquito mortality associated with resistance would presumably combine to make ITNs unattractive from the perspective of both the individual user and the malaria control manager. Incision of 80 holes per net is the standard for ITN trials in West Africa ( 8 , 9 , 12 ), and such nets have given a degree of personal protection in earlier trials. An ITN with no or few holes might be expected to give some protection against resistant mosquitoes from Ladji, but there were insufficient huts available to test this idea. These experimental hut results from southern Benin stand in contrast to results from an area of Côte d’Ivoire (Yaokoffikro) that had a comparable frequency of kdr (78%) to that of Ladji (83%) ( 15 ) and where lambdacyhalothrin-treated nets and other ITN showed continuing efficacy, with mortality rates of 45%–68% ( 8 , 16 – 19 ). We sought evidence that other resistance mechanisms than kdr might be contributing to the reduced efficacy of pyrethroids at Ladji. Metabolic resistance due to mixed function oxidases (MFO) has, for example, undermined attempts at malaria control with deltamethrin residual spraying in southern Africa caused by An. funestus ( 6 ), and elevated MFO activity in a strain of An. gambiae from Cameroon reduced the efficacy of permethrin-treated netting in laboratory tests ( 20 ). The combined elevated activity of MFOs, glutathione S–transferase, and esterases resulted in a failure of the S. Mexican IRS program against An. albimanus ( 21 ). Our examination of enzymatic activity in An. gambiae showed no evidence that MFO activity is any greater in mosquitoes from Ladji than in mosquitoes from Malanville, nor did esterase activity differ between Ladji and Vkper (kdr) strains. Thus, there was no evidence of metabolic resistance enhancing the resistance already caused by kdr in mosquitoes from Ladji. Lambdacyhalothin bioassay tests showed no evidence of resistance level differing between Ladji and Vkper strains, and we conclude that metabolic mechanisms made no contribution to the observations in Ladji. In East Africa a different type of kdr based on a leucine-to-serine mutation, which confers resistance to permethrin and DDT ( 22 ), has been detected in several countries. However, no mosquitoes of this genotype were detected in tests on samples of An. gambiae from Ladji ( 23 ). The complete absence of efficacy of lambdacyalothrin against Cx. quinquefasciatus in Ladji merely confirms earlier findings involving other types of pyrethroid in experimental huts in West Africa ( 6 , 9 , 16 , 18 ). The contribution of kdr to pyrethroid resistance in An. gambiae needs to be reappraised. While lambdacyhalothin-treated nets (reported here) and permethrin-treated nets reported earlier ( 24 ) were less effective in hut trials in the kdr area of Benin (Ladji) than in a corresponding area of Côte d’Ivoire (Yaokoffikro), pyrethroid-treated nets were more effective in the susceptible area of Benin (Malanville) than in the corresponding susceptible area of Côte d’Ivoire (M’Be) ( 9 ) for reasons that are presently unknown. Other differences between the biology of An. gambiae from Côte d’Ivoire and Benin exist. Ivorian An. gambiae with kdr is mainly of the S molecular form, whereas Beninoise An. gambiae is of the M form (V. Corbel, unpub. data). M and S forms differ in ecologic distribution and habitat. While mosquitoes of the M form with kdr might behave differently from those of the S form with kdr when exposed to pyrethroids, this is mere speculation. Moreover, the M form in Malanville showed higher vulnerability to ITN than did the corresponding S form in Côte d’Ivoire, a finding that seems at odds with a behavioral hypothesis. Our study provides persuasive evidence that pyrethroid resistance in Benin is capable of undermining control measures based on ITN. Nor is there reassurance to be taken from IRS, and any attempt to switch vector control strategy would seem doomed to fail. Whereas the earlier phase 3 malaria control trials of ITN in Côte d’Ivoire showed continuing effectiveness despite kdr at high levels ( 10 ), our phase 2 results from Benin give no grounds for optimism. However, only phase 3 can provide a definitive answer. Further phase 3 trials using pyrethroid-treated nets and IRS need to be undertaken in Benin in an area of pyrethroid resistance. The normal practice with phase 3 is to aim at complete community coverage. Coverage in real life is usually less than total, and the danger with the type of pyrethroid resistance found in Benin is that at lower levels of coverage the important mass protective effect of ITNs ( 25 , 26 ) may be lost and transmission may continue unabated among those who do not have ITNs. To establish whether this is true, phase 3 trials on resistant mosquito populations should ideally set the coverage level at <100%. If it is considered unacceptable to deny a section of the trial population access to ITNs, an alternative but much less rigorous approach would be to monitor malaria incidence among users and non-users of long-lasting insecticide nets (LLIN) during the proposed scaling up of LLIN coverage in Benin currently being considered. Pyrethroid resistance in Benin is far from homogeneous, and LLIN should give good protection wherever mosquito populations are susceptible. Use of LLIN should be encouraged but scale-up of treated nets may ultimately select for further resistance. The need to develop alternative insecticides to replace or supplement pyrethroids on nets is urgent and should be put on a par with the seeking of new antimalarial drugs or vaccines that have received far greater attention and resources in recent years.

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Role of cytochrome P450s in insecticide resistance: impact on the control of mosquito-borne diseases and use of insecticides on Earth

Jean-Philippe David, Hanafy Mahmoud Ismail, Alexia Chandor-Proust … (2013)

The fight against diseases spread by mosquitoes and other insects has enormous environmental, economic and social consequences. Chemical insecticides remain the first line of defence but the control of diseases, especially malaria and dengue fever, is being increasingly undermined by insecticide resistance. Mosquitoes have a large repertoire of P450s (over 100 genes). By pinpointing the key enzymes associated with insecticide resistance we can begin to develop new tools to aid the implementation of control interventions and reduce their environmental impact on Earth. Recent technological advances are helping us to build a functional profile of the P450 determinants of insecticide metabolic resistance in mosquitoes. Alongside, the cross-responses of mosquito P450s to insecticides and pollutants are also being investigated. Such research will provide the means to produce diagnostic tools for early detection of P450s linked to resistance. It will also enable the design of new insecticides with optimized efficacy in different environments.

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Loss of Household Protection from Use of Insecticide-Treated Nets against Pyrethroid-Resistant Mosquitoes, Benin

Alex Asidi, Raphael N’Guessan, Martin Akogbeto … (2012)

Insecticide-treated nets (ITNs) and long-lasting insecticidal nets (LLINs) are the primary interventions for preventing malaria in sub-Saharan Africa ( 1 , 2 ). Nets accumulate holes through wear and tear during the course of everyday use, but the pyrethroid treatment continues to provide personal protection and to reduce vector capacity through excito-repellency and the killing of mosquitoes that contact the net ( 3 , 4 ). During the last decade, pyrethroid resistance in Anopheles gambiae mosquitoes became widespread in western Africa and spread to or developed in eastern Africa ( 5 – 9 ). As coverage of LLINs expands across the continent under programs supported by the President’s Malaria Initiative and Global Fund ( 10 ), resistance will inevitably increase ( 11 – 13 ). Although resistance is perceived as a serious threat to the future of malaria control, the current distribution of resistance is patchy, and its severity seems to differ from 1 location to another. In the western African country of Benin, pyrethroid resistance has evolved in the M (Mopti) molecular form of An. gambiae mosquitoes that appears to combine the knockdown resistance (kdr) gene with oxidase mechanisms ( 14 , 15 ). Carriers of this resistance were not controlled by pyrethroid treatments in experimental hut trials of ITNs or the leading brands of LLINs, PermaNet 2.0 (Vestergaard Frandsen SA, Aarhus, Denmark) and Olyset (Sumitomo Chemicals, Osaka, Japan) ( 16 , 17 ). However, further west in Côte d’Ivoire, the kdr in An. gambiae S (Savannah) form mosquitoes conferred only limited resistance, and trials of ITNs continued to protect against mosquito blood feeding (biting) and malaria transmission by this species ( 18 – 20 ). Results from experimental hut trials in Benin raise an alarm. Of key concern is whether ITNs that are subject to wear and tear under everyday household conditions fail to protect ITN users now that An. gambiae mosquitoes are becoming resistant. Modern mosquito nets lack physical durability, and household nets can accrue an average of 12–20 holes during 1–2 years of use ( 21 ). Net replacement schemes struggle to meet demand at this level of deterioration and attrition. To assess protection conferred by in-use polyester nets, we compared nets in households of northern Benin, where An. gambiae mosquitoes are mostly susceptible to pyrethroids, with nets in households of southern Benin, where An. gambiae mosquitoes are mostly resistant ( 7 , 16 , 22 ). Residents of the selected households were all regular users of nets. Materials and Methods Study Sites Three suburbs (Ladji, Fifadji, and Abomey Calavi) of Cotonou in southern Benin support breeding of mosquitoes of An. gambiae M form that is mostly pyrethroid resistant with a high frequency of kdr (>90%) ( 14 , 22 ). Malanville, 800 km north of Cotonou, is situated in an area in which mosquitoes are mainly pyrethroid susceptible, where An. gambiae M form mosquitoes show a kdr frequency of 1 worn nets under regular use. The points of entry for mosquitoes were through open doors or eave gaps between walls and roofs. Nets were made of polyester, cotton, or nylon and contained holes of various sizes and number. Before inclusion, the nets were subjected to World Health Organization cone bioassays by using a laboratory-susceptible strain of An. gambiae to detect pyrethroid residue. Only untreated nets or nets that had lost their insecticide through washing were retained for the study. Household members gave informed consent to participate in the study and were provided with chemoprophylaxis throughout. The London School of Hygiene & Tropical Medicine and the Benin national ethics committees granted ethics approval. Mosquito Exit Window Traps Unidirectional window traps were fixed to window frames for collecting exiting mosquitoes. Each trap consisted of a 30-cm–sided metal frame covered with polyester netting, with 1 side drawn into a funnel to direct mosquitoes into the trap ( 23 ). The trap was fixed to a plywood sheet that could be fitted to window frames of differing sizes. The traps were placed before dusk and emptied of mosquitoes at 7 am. Treatment of Mosquito Nets Nets were treated with a microencapsulated formulation of lambdacyhalothrin (Icon 10 CS, Syngenta, Basel, Switzerland). The standard rate of 18 mg/m2 was used. Mosquito Collection We conducted the trials during May and June 2008 at the southern sites and during July and August 2008 at the northern site. Rooms of selected houses containing untreated nets were fitted with traps and monitored for 5 consecutive nights to assess baseline mosquito density and blood-feeding and death rates. Nets were then treated with lambdacyhalothrin and monitored for 5 additional nights. Houses that attracted too few mosquitoes during baseline monitoring were dropped. Each morning, mosquitoes were collected from the window traps by mouth aspirator and transferred to paper cups and provided with sugar solution. Indoor resting mosquitoes were then collected from white floor sheets after the windows were sealed off and the rooms were sprayed with a nonresidual pyrethroid. Mosquitoes were identified to species and recorded as blood fed or unfed by microscopy. Scoring of blood-feeding rates was pooled for window trap and room collections. Death rates of the exit trap collections were determined after a 24-hour holding period. An. gambiae mosquitoes were identified to species and molecular form by using the method of Favia et al. ( 24 ) and genotyped for kdr by using the method of Martinez-Torres et al. ( 25 ). Data Analysis We assessed the effect of pyrethroid-treated nets on the proportions of An. gambiae blood-feeding or killed mosquitoes using a random effects generalized linear mixed model, recording the proportions of female mosquitoes before treatment as the baseline (control) group and the proportions after treatment as the test group. The model comprised 4 independent variables: treatment, number of holes per net, total area of all holes in the net under test, and number of persons in the household. Random effects in the model also accounted for repeated sampling over several days and the number of persons sleeping in the room. Regional differences in the condition of nets and in household size between the sites with resistant and susceptible mosquitoes were analyzed by using the Wilcoxon rank sum test. All statistical analyses were conducted by using STATA 9 software (STATA Corp., College Station, TX, USA). Results Baseline Characteristics of Mosquito Nets and Sleepers Eleven households at the southern sites (where mosquitoes are resistant) and 5 households at the northern site (where mosquitoes are susceptible) participated in the study. Each household contributed 1 sleeping room and 1 net to the study. Numbers of holes per net recorded at the southern and northern sites did not differ (p = 0.41) (Table 1). The area of holes per net was significantly smaller for nets from the south (p = 0.0013) (Table 2). Household size in the south was twice that in the north (p = 0.025). Table 1 Baseline characteristics showing condition of selected mosquito nets in households in northern sites, where mosquitoes are pyrethroid susceptible, vs. southern sites, where mosquitoes are pyrethroid resistant, Benin, 2008* Variable Northern site Southern sites Difference (95% CI) p value Households, no. 5 11 NA NA Household members, average no. (range) 2.2 (1–3) 5.1 (2–7) 2.9 (1.4–4.4) 0.025 Holes in nets Average no. (range) 10.2 (5–13) 9.5 (5–25) 0.65 (–5.3 to 6.6) 0.41 Average size, cm2 (range) 28 (11–49) 11 (5–20) 15 (7–21) 0.0013 *NA, not applicable. Table 2 Protection against Anopheles gambiae s.l. mosquitoes for persons sleeping under in-use mosquito nets before and after treatment with 18 mg/m2 lambdacyalothrin in houses in northern vs. southern sites, Benin, 2008* Area, net condition Treatment of nets Blood fed, no. (%) OR (95% CI) p value aOR (95% CI) p value Northern (pyrethroid-susceptible mosquitoes) Before 810 (46) 1 1 After 1,041 (16) 0.22 (0.18–0.28) 10 Before 307 (48) 1 1 After 191 (24) 0.34 (0.23–0.51) 15 Before 751 (47) 1 1 After 824 (16) 0.21 (0.17–0.27) 10 Before 165 (27) 1 1 After 224 (25) 0.89 (0.56–1.42) 0.64 0.89 (0.56–1.41) 0.63 Size of holes, cm2 15 Before 153 (21) 1 1 After 235 (23) 1.10 (0.67–1.8) 0.70 1.09 (0.18–1.80) 0.713 *OR, odds ratio; aOR, OR adjusted for condition of nets and household size. Efficacy of Mosquito Nets Before and After Treatments During the 2-month trial, 692 An. gambiae mosquitoes; 2,271 Culex quinquefasciatus mosquitoes; and small numbers of Mansonia uniformis, An. pharoensis, and Aedes aegypti mosquitoes were collected at the southern sites. At the northern site, 1,856 An. gambiae mosquitoes, 1,051 Mansonia spp. mosquitoes, and small numbers of An. funestus and Ae. aegypti mosquitoes were collected. Only the malaria vector An. gambiae was analyzed further. The blood-feeding rate of An. gambiae mosquitoes under untreated nets was higher in the north (46%) than in the south (20%) (Table 2), probably because of the larger size of holes in nets in the north. At the northern site (susceptible mosquitoes), the odds of blood feeding were lower after treatment than before treatment with or without adjustment for other covariates (adjusted odds ratio 0.34; 95% CI 0.26–0.44; p 80% that are subject to high rates of malaria transmission, as in the southern provinces ( 22 , 31 ), the effects on the community of LLINs on malaria would be compromised among families who have poor-quality ITNs. Sustained protection by any LLIN depends on 2 factors: the rate of loss of insecticide residue from the fibers and the retention of textile integrity. Our research shows that the emphasis placed by the World Health Organization and net manufacturers on developing nets that retain insecticide after recurrent washing is overlooking the role of net durability on effectiveness. A net that retains insecticide after multiple washes or 3 years of use is of no benefit if, before this period, the physical condition of the net and the holes that accumulate mean that in locations with high levels of resistance the net has lost the capacity to protect. During household use, polyester- and polyethylene-based LLINs acquiring holes within the first year and are starting to be discarded after 2 years ( 21 , 28 , 32 ). LLIN manufacturers need to create new types of fiber or increase the tensile strength to give better resilience against tearing or acquiring holes. Any such product should have a strong commercial advantage. Resistance capable of undermining the effective use of LLINs is not confined to southern Benin. With the growing coverage of LLINs, the continuing selection of resistance in mosquitoes and its spread to mosquitoes in neighboring provinces and countries is inevitable. Restoring protection of LLINs requires innovation that combines pyrethroids and novel insecticides to which this form of An. gambiae mosquitoes shows no resistance.

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Author and article information

Contributors

John Vontas: Role: Editor

Journal

Journal ID (nlm-ta): PLoS One

Journal ID (iso-abbrev): PLoS ONE

Journal ID (publisher-id): plos

Journal ID (pmc): plosone

Title: PLoS ONE

Publisher: Public Library of Science (San Francisco, CA USA )

ISSN (Electronic): 1932-6203

Publication date (Electronic): 16 November 2016

Publication date Collection: 2016

Volume: 11

Issue: 11

Electronic Location Identifier: e0165925

Affiliations

[1 ]London School of Hygiene and Tropical Medicine, London, United Kingdom

[2 ]Centre de Recherche Entomologique de Cotonou, Ministry of Health, Cotonou, Benin

[3 ]Pan-African Malaria Vector Research Consortium (PAMVERC), London, United Kingdom

[4 ]Innovative Vector Control Consortium, Liverpool, United Kingdom

University of Crete, GREECE

Author notes

Competing Interests: The authors have declared that no competing interests exist.

Conceptualization: MR RN.
Data curation: MR RN.
Formal analysis: MR CN.
Funding acquisition: MR.
Investigation: AO RN.
Methodology: MR RN.
Project administration: RN.
Resources: DM.
Supervision: MR RN.
Validation: MR RN CN.
Visualization: MR.
Writing – original draft: MR RN.

* E-mail: mark.rowland@ 123456lshtm.ac.uk

Author information

Mark Rowland http://orcid.org/0000-0002-5567-9562

Article

Publisher ID: PONE-D-16-34045

DOI: 10.1371/journal.pone.0165925

PMC ID: 5112870

PubMed ID: 27851828

SO-VID: 1aafb339-2d25-43d8-9d14-cab0c3184b3a

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This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

History

Date received : 25 August 2016

Date accepted : 22 September 2016

Page count

Figures: 4, Tables: 3, Pages: 14

Funding

Funded by: funder-id http://dx.doi.org/10.13039/100000865, Bill and Melinda Gates Foundation;

Award Recipient :

ORCID: http://orcid.org/0000-0002-5567-9562

Mark Rowland

Funded by: Innovative Vector Control Consortium

Award Recipient :

ORCID: http://orcid.org/0000-0002-5567-9562

Mark Rowland

This work was supported by the Bill and Melinda Gates Foundation ( http://www.gatesfoundation.org/) and the Innovative Vector Control Consortium ( http://www.ivcc.com/). The institutional funders had no role in study design, data collection and analysis, preparation of the manuscript or decision to publish.

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Data Availability All relevant data are within the paper and its Supporting Information files.