A Small-Scale Field Trial of Pyriproxyfen-Impregnated Bed Nets against Pyrethroid-Resistant Anopheles gambiae s.s. in Western Kenya

Pyrethroid resistance is becoming a major problem for vector control programs, because at present, there are few suitable chemical substitutes for pyrethroids, as when used on bed nets the insecticide must have low mammalian toxicity as well as high activity to mosquitoes. Pyriproxyfen (PPF) is one of the most active chemicals among the juvenile hormone mimic (JHM) group. Sterilizing mosquitoes by using PPF could be a potential control measure for pyrethroid-resistant malaria vectors. We investigated the sterilizing effects of two types of PPF-impregnated bed nets – a 1% PPF-impregnated net and a 1% PPF +2% permethrin-impregnated net (Olyset Duo) – to pyrethroid-resistant wild population of Anopheles gambiae s.s. in western Kenya. High mortality of blood-fed mosquitos was observed 3 days post-collection, in the houses where PPF-impregnated nets were used, indicating the effect of PPF on the longevity of mosquitos that came in contact with the net. Reduction in the number of ovipositing females, number of eggs, and number of progeny per female were also observed in the houses in which both Olyset Duo and PPF-impregnated nets were used. This is the first field study showing the high sterilizing efficacy of PPF against wild pyrethroid-resistant An. gambiae s.s. population. In addition, we recognized the necessity of combined use of permethrin with PPF, in order to reduce the risk of mosquito bites and provide a level of personal protection. Further studies on wild pyrethroid-resistant mosquito populations such as An. arabiensis and An. funestus s.s. would provide more information on the practical use of the PPF-impregnated bed nets.     

Olyset Duo? (a Pyriproxyfen and Permethrin Mixture Net): An Experimental Hut Trial against Pyrethroid Resistant Anopheles gambiae and Culex quinquefasciatus in Southern Benin

Background

Alternative compounds which can complement pyrethroids on long-lasting insecticidal nets (LN) in the control of pyrethroid resistant malaria vectors are urgently needed. Pyriproxyfen (PPF), an insect growth regulator, reduces the fecundity and fertility of adult female mosquitoes. LNs containing a mixture of pyriproxyfen and pyrethroid could provide personal protection through the pyrethroid component and reduce vector abundance in the next generation through the sterilizing effect of pyriproxyfen.

Method

The efficacy of Olyset Duo, a newly developed mixture LN containing pyriproxyfen and permethrin, was evaluated in experimental huts in southern Benin against pyrethroid resistant Anopheles gambiae and Culex quinquefasciatus. Comparison was made with Olyset Net® (permethrin alone) and a LN with pyriproxyfen alone (PPF LN). Laboratory tunnel tests were performed to substantiate the findings in the experimental huts.

Results

Overall mortality of wild pyrethroid resistant An. gambiae s.s. was significantly higher with Olyset Duo than with Olyset Net (50% vs. 27%, P = 0.01). Olyset DUO was more protective than Olyset Net (71% vs. 3%, P<0.001). The oviposition rate of surviving blood-fed An. gambiae from the control hut was 37% whereas none of those from Olyset Duo and PPF LN huts laid eggs. The tunnel test results were consistent with the experimental hut results. Olyset Duo was more protective than Olyset Net in the huts against wild pyrethroid resistant Cx. quinquefasciatus although mortality rates of this species did not differ significantly between Olyset Net and Olyset Duo. There was no sterilizing effect on surviving blood-fed Cx. quinquefasciatus with the PPF-treated nets.

Conclusion

Olyset Duo was superior to Olyset Net in terms of personal protection and killing of pyrethroid resistant An. gambiae, and sterilized surviving blood-fed mosquitoes. Mixing pyrethroid and pyriproxyfen on a LN shows potential for malaria control and management of pyrethroid resistant vectors by preventing further selection of pyrethroid resistant phenotypes.     

Towards a species-selective acetylcholinesterase inhibitor to control the mosquito vector of malaria, Anopheles gambiae

Anopheles gambiae is the major mosquito vector of malaria in sub-Saharan Africa. At present, insecticide-treated nets (ITNs) impregnated with pyrethroid insecticides are widely used in malaria-endemic regions to reduce infection; however the emergence of pyrethroid-resistant mosquitoes has significantly reduced the effectiveness of the pyrethroid ITNs. An acetylcholinesterase (AChE) inhibitor that is potent for An. gambiae but weakly potent for the human enzyme could potentially be safely deployed on a new class of ITNs. In this paper we provide a preliminary pharmacological characterization of An. gambiae AChE, discuss structural features of An. gambiae and human AChE that could lead to selective inhibition, and describe compounds with 130-fold selectivity for inhibition of An. gambiae AChE relative to human AChE.     

Experimental and molecular genetic analysis of the impact of pyrethroid and non-pyrethroid insecticide impregnated bednets for mosquito control in an area of pyrethroid resistance

Experimental huts in C?te d'Ivoire were used to evaluate the pyrethroid alpha-cypermethrin, the non-ester pyrethroid etofenprox, the organophosphate pirimiphos-methyl and the carbamate carbosulfan on bednets against pyrethroid-resistant Anopheles gambiae Giles. To test for selection for the resistance gene by the treated nets, A. gambiae collected live or dead from the huts were kept and analysed for the presence of the kdr gene using a new polymerase chain reaction followed by sequence-specific oligonucleotide probing (PCR-SSOP) for kdr-genotyping. Deliberately holed bednets freshly treated with pirimiphos-methyl or carbosulfan caused over 90% kill of A. gambiae s.s. and Culex spp. However, the mortality with alpha-cypermethrin or etofenprox treated nets was similar to that with untreated nets. Bloodfeeding of A. gambiae s.s. on the sleepers under the nets was only significantly reduced by alpha-cypermethrin and carbosulfan. Tests of the residual activity of the bednets after seven months showed that pirimiphos-methyl had lost its efficacy while carbosulfan still performed well. Once again the pyrethroid treated nets gave similar results to the untreated nets. Selection for the kdr-allele by alpha-cypermethrin and etofenprox, but not by carbosulfan, was indicated by PCR-SSOP genotyping of mosquitoes. Thus carbamates such as carbosulfan, or organophosphates of longer persistence than pirimiphos-methyl and of low mammalian toxicity, would seem to be a promising alternative to be used on bednets, particularly in areas of pyrethroid resistance.     

Can anything be done to maintain the effectiveness of pyrethroid-impregnated bednets against malaria vectors?

Pyrethroid-treated bednets are the most promising available method of controlling malaria in the tropical world. Every effort should be made to find methods of responding to, or preventing, the emergence of pyrethroid resistance in the Anopheles vectors. Some cases of such resistance are known, notably in An. gambiae in West Africa where the kdr type of resistance has been selected, probably because of the use of pyrethroids on cotton. Because pyrethroids are irritant to mosquitoes, laboratory studies on the impact of, and selection for, resistance need to be conducted with free-flying mosquitoes in conditions that are as realistic as possible. Such studies are beginning to suggest that, although there is cross-resistance to all pyrethroids, some treatments are less likely to select for resistance than others are. Organophosphate, carbamate and phenyl pyrazole insecticides have been tested as alternative treatments for nets or curtains. Attempts have been made to mix an insect growth regulator and a pyrethroid on netting to sterilize pyrethroid-resistant mosquitoes that are not killed after contact with the netting. There seems to be no easy solution to the problem of pyrethroid resistance management, but further research is urgently needed.     

Towards a species-selective acetylcholinesterase inhibitor to control the mosquito vector of malaria, Anopheles gambiae

Anopheles gambiae is the major mosquito vector of malaria in sub-Saharan Africa. At present, insecticide-treated nets (ITNs) impregnated with pyrethroid insecticides are widely used in malaria-endemic regions to reduce infection; however the emergence of pyrethroid-resistant mosquitoes has significantly reduced the effectiveness of the pyrethroid ITNs. An acetylcholinesterase (AChE) inhibitor that is potent for An. gambiae but weakly potent for the human enzyme could potentially be safely deployed on a new class of ITNs. In this paper we provide a preliminary pharmacological characterization of An. gambiae AChE, discuss structural features of An. gambiae and human AChE that could lead to selective inhibition, and describe compounds with 130-fold selectivity for inhibition of An. gambiae AChE relative to human AChE.     

Assessing the impact of low-technology emanators alongside long-lasting insecticidal nets to control malaria

Malaria control in sub-Saharan Africa relies on the widespread use of long-lasting insecticidal nets (LLINs) or the indoor residual spraying of insecticide. Disease transmission may be maintained even when these indoor interventions are universally used as some mosquitoes will bite in the early morning and evening when people are outside. As countries seek to eliminate malaria, they can target outdoor biting using new vector control tools such as spatial repellent emanators, which emit airborne insecticide to form a protective area around the user. Field data are used to incorporate a low-technology emanator into a mathematical model of malaria transmission to predict its public health impact across a range of scenarios. Targeting outdoor biting by repeatedly distributing emanators alongside LLINs increases the chance of elimination, but the additional benefit depends on the level of anthropophagy in the local mosquito population, emanator effectiveness and the pre-intervention proportion of mosquitoes biting outdoors. High proportions of pyrethroid-resistant mosquitoes diminish LLIN impact because of reduced mosquito mortality. When mosquitoes are highly anthropophagic, this reduced mortality leads to more outdoor biting and a reduced additional benefit of emanators, even if emanators are assumed to retain their effectiveness in the presence of pyrethroid resistance. Different target product profiles are examined, which show the extra epidemiological benefits of spatial repellents that induce mosquito mortality.This article is part of the theme issue ‘Novel control strategies for mosquito-borne diseases’.     

Permethrin resistance in Aedes aegypti: Genomic variants that confer knockdown resistance,recovery, and death

Pyrethroids are one of the few classes of insecticides available to control Aedes aegypti, the major vector of dengue, chikungunya, and Zika viruses. Unfortunately, evolving mechanisms of pyrethroid resistance in mosquito populations threaten our ability to control disease outbreaks. Two common pyrethroid resistance mechanisms occur in Ae. aegypti: 1) knockdown resistance, which involves amino acid substitutions at the pyrethroid target site—the voltage-gated sodium channel (VGSC)—and 2) enhanced metabolism by detoxification enzymes. When a heterogeneous population of mosquitoes is exposed to pyrethroids, different responses occur. During exposure, a proportion of mosquitoes exhibit immediate knockdown, whereas others are not knocked-down and are designated knockdown resistant (kdr). When these individuals are removed from the source of insecticide, the knocked-down mosquitoes can either remain in this status and lead to dead or recover within a few hours. The proportion of these phenotypic responses is dependent on the pyrethroid concentration and the genetic background of the population tested. In this study, we sequenced and performed pairwise genome comparisons between kdr, recovered, and dead phenotypes in a pyrethroid-resistant colony from Tapachula, Mexico. We identified single-nucleotide polymorphisms (SNPs) associated with each phenotype and identified genes that are likely associated with the mechanisms of pyrethroid resistance, including detoxification, the cuticle, and insecticide target sites. We identified high association between kdr and mutations at VGSC and moderate association with additional insecticide target site, detoxification, and cuticle protein coding genes. Recovery was associated with cuticle proteins, the voltage-dependent calcium channel, and a different group of detoxification genes. We provide a list of detoxification genes under directional selection in this field-resistant population. Their functional roles in pyrethroid metabolism and their potential uses as genomic markers of resistance require validation.     

Combined pyrethroid and carbamate 'two-in-one' treated mosquito nets: field efficacy against pyrethroid-resistant Anopheles gambiae and Culex quinquefasciatus

A new approach is proposed in the treatment of mosquito nets, using a 'two-in-one' combination of pyrethroid and non-pyrethroid insecticides applied to different parts of bednets. The objectives are mainly to overcome certain limitations of pyrethroid-impregnated bednets currently recommended for malaria control purposes. Apart from developing alternatives to pyrethroid dependency, we sought to counteract pyrethroid irritant effects on mosquitoes (excito-repellency) and resistance to pyrethroids. The idea takes advantage of the presumed host-seeking behaviour of mosquitoes confronted by a net draped over a bed, whereby the mosquito may explore the net from the top downwards. Thus, nets could be more effective if treated on the upper part with residual non-irritant insecticide (carbamate or organophosphate) and with a pyrethroid on the lower part. Sequential exposure to different insecticides with distinct modes of action is equivalent to the use of a mixture as a potential method of managing insecticide resistance. We also intended to improve the control of nuisance mosquitoes, especially Culex quinquefasciatus Say (Diptera: Culicidae) that often survive pyrethroids, in order to encourage public compliance with use of insecticide-treated nets (ITNs). Polyester bednets were pretreated with residual pyrethroid (bifenthrin 50 mg/m2 or deltamethrin 25 mg/m2) on the lower half and with carbamate (carbosulfan 300 mg/m2) on the upper half to minimize contact with net users. Unreplicated examples of these 'two-in-one' treated nets were field-tested against wild mosquitoes, in comparison with an untreated net and bednets treated with each insecticide alone, including PermaNet wash-resistant formulation of deltamethrin 50 mg/m2. Overnight tests involved volunteers sleeping under the experimental bednets in verandah-trap huts at Yaokofikro, near Bouaké in C te d'Ivoire, where the main malaria vector Anopheles gambiae Giles, as well as Culex quinquefasciatus Say, are highly resistant to pyrethroids. Efficacy of these ITNs was assessed in the huts by four entomological criteria: deterrency and induced exophily (effects on hut entry and exit), blood-feeding and mortality rates (immediate and delayed). Overall, the best impact was achieved by the bednet treated with carbosulfan alone, followed by 'two-in-one' treatments with carbosulfan plus pyrethroid. Blood-feeding rates were 13% An. gambiae and 17% Cx. quinquefasciatus in huts with untreated nets, but only 3% with carbosulfan ITNs, 7-11% with combined ITN treatment, 6-8% An. gambiae and 12-14% Cx. quinquefasciatus with pyrethroid alone. Mosquitoes that entered the huts were killed sooner by nets with combined treatment than by pyrethroid alone. Mortality-rates in response to ITNs with carbosulfan (alone or combined with pyrethroid) were significantly greater for Cx. quinquefasciatus, but not for An. gambiae, compared to ITNs with only pyrethroid. About 20% of sleepers reported potential side-effects (headache and/or sneezing) from use of ITN treated with carbosulfan alone. Further development of this new 'two-in-one' ITN concept requires a range of investigations (choice of effective products, cost-benefit analysis, safety, etc.) leading to factory production of wash-resistant insecticidal nets treated with complementary insecticides.     

Efficacy of etofenprox against insecticide susceptible and resistant mosquito strains containing characterized resistance mechanisms

tofenprox is a non-ester pyrethroid insecticide with comparable toxicity and a similar mode of action to other pyrethroids. Cross-resistance studies on mosquitoes showed no effect of carboxylesterase, elevated esterase, altered acetylcholinesterase or glutathione S -transferase-based resistance mechanisms on etofenprox toxicity, when compared to standard susceptible strains of Anopheles and Culex . Cross-resistance to etofenprox occurred in a pyrethroid-resistant strain of Culex quinquefasciatus with both oxidase and 'kdr'-like resistance mechanisms.
Dose–response data for susceptible mosquito strains suggest that, in standard W.H.O. susceptibility tests of adult mosquitoes, appropriate discriminating concentrations of etofenprox for detection of resistance would be 0.1% for Culex and 0.25% for Anopheles .     

Cuticle Thickening in a Pyrethroid-Resistant Strain of the Common Bed Bug,Cimex lectularius L. (Hemiptera: Cimicidae)

Thickening of the integument as a mechanism of resistance to insecticides is a well recognised phenomenon in the insect world and, in recent times, has been found in insects exhibiting pyrethroid-resistance. Resistance to pyrethroid insecticides in the common bed bug, Cimex lectularius L., is widespread and has been frequently inferred as a reason for the pest’s resurgence. Overexpression of cuticle depositing proteins has been demonstrated in pyrethroid-resistant bed bugs although, to date, no morphological analysis of the cuticle has been undertaken in order to confirm a phenotypic link. This paper describes examination of the cuticle thickness of a highly pyrethroid-resistant field strain collected in Sydney, Australia, in response to time-to-knockdown upon forced exposure to a pyrethroid insecticide. Mean cuticle thickness was positively correlated to time-to-knockdown, with significant differences observed between bugs knocked-down at 2 hours, 4 hours, and those still unaffected at 24 hours. Further analysis also demonstrated that the 24 hours survivors possessed a statistically significantly thicker cuticle when compared to a pyrethroid-susceptible strain of C. lectularius. This study demonstrates that cuticle thickening is present within a pyrethroid-resistant strain of C. lectularius and that, even within a stable resistant strain, cuticle thickness will vary according to time-to-knockdown upon exposure to an insecticide. This response should thus be considered in future studies on the cuticle of insecticide-resistant bed bugs and, potentially, other insects.     

Comparison of bednets treated with alphacypermethrin, permethrin or lambdacyhalothrin against Anopheles gambiae in the Gambia

In the Gambian village of Saruja, where malaria is transmitted mainly by mosquitoes of the Anopheles gambiae complex, a trial was undertaken of the acceptability and efficacy of bednets treated with one of three pyrethroid insecticides – alphacypermethrin 40 mg/m², permethrin 500 mg/m² and lambdacyhalothrin 10 mg/m^2. Fewer mosquitoes were found alive under nets treated with insecticide than under control nets. Significantly more dead mosquitoes were found under nets treated with alphacypermethrin than under nets treated with permethrin or lambdacyhalothrin. Side-effects were reported by a proportion of the users of nets treated with each of the insecticides, but none were severe and their prevalence was similar between treatment groups. Unwashed nets treated with alphacypermethrin were more effective at killing anopheline mosquitoes in bioassays than nets treated with permethrin or lambdacyhalothrin. Killing activity was reduced when nets were washed, irrespective of which insecticide was used. Bednets treated with alphacypermethrin are well accepted, effectively killed anopheline mosquitoes and should therefore be evaluated for personal protection against malaria transmission.     

The insecticidal activity of essential oil constituents against pyrethroid-resistant Anopheles funestus (Diptera: Culicidae)

Malaria vector control relies on the use of insecticides for indoor residual spraying and long-lasting bed nets. However, insecticide resistance to pyrethroids among others, has escalated. Anopheles funestus, one of the major African malaria vectors, has attained significant levels of resistance to pyrethroids. Overexpressed P450 monooxygenases have been previously identified in pyrethroid resistant An. funestus. The escalating resistance against conventional insecticides signals an urgent need for identification of novel insecticides. Essential oils have gained recognition as promising sources of alternative natural insecticides. This study investigated six essential oil constituents, farnesol, (−)-α-bisabolol, cis-nerolidol, trans-nerolidol, methyleugenol, santalol (α and β isomers) and essential oil of sandalwood, for the adulticidal effects against pyrethroid-resistant An. funestus strain. The susceptibility against these terpenoids were evaluated on both pyrethroid-susceptible and resistant An. funestus. Furthermore, the presence of overexpressed monooxygenases in resistant An. funestus was confirmed. Results showed that both the pyrethroid-susceptible and resistant An. funestus were susceptible to three EOCs; cis-nerolidol, trans-nerolidol and methyleugenol. On the other hand, the pyrethroid-resistant An. funestus survived exposure to both farnesol and (−)-α-bisabolol. This study however does not show any direct association of the overexpressed Anopheles monooxygenases and the efficacy of farnesol and (−)-α-bisabolol. The enhanced activity of these terpenoids against resistant An. funestus that has been pre-exposed to a synergist, piperonyl butoxide, suggests their potential effectiveness in combination with monooxygenase inhibitors. This study proposes that cis-nerolidol, trans-nerolidol and methyleugenol are potential agents for further investigation as novel bioinsecticides against pyrethroid-resistant An. funestus strain.     

The Repellent DEET Potentiates Carbamate Effects via Insect Muscarinic Receptor Interactions: An Alternative Strategy to Control Insect Vector-Borne Diseases

Insect vector-borne diseases remain one of the principal causes of human mortality. In addition to conventional measures of insect control, repellents continue to be the mainstay for personal protection. Because of the increasing pyrethroid-resistant mosquito populations, alternative strategies to reconstitute pyrethroid repellency and knock-down effects have been proposed by mixing the repellent DEET (N,N-Diethyl-3-methylbenzamide) with non-pyrethroid insecticide to better control resistant insect vector-borne diseases. By using electrophysiological, biochemichal, in vivo toxicological techniques together with calcium imaging, binding studies and in silico docking, we have shown that DEET, at low concentrations, interacts with high affinity with insect M1/M3 mAChR allosteric site potentiating agonist effects on mAChRs coupled to phospholipase C second messenger pathway. This increases the anticholinesterase activity of the carbamate propoxur through calcium-dependent regulation of acetylcholinesterase. At high concentrations, DEET interacts with low affinity on distinct M1/M3 mAChR site, counteracting the potentiation. Similar dose-dependent dual effects of DEET have also been observed at synaptic mAChR level. Additionally, binding and in silico docking studies performed on human M1 and M3 mAChR subtypes indicate that DEET only displays a low affinity antagonist profile on these M1/M3 mAChRs. These results reveal a selective high affinity positive allosteric site for DEET in insect mAChRs. Finally, bioassays conducted on Aedes aegypti confirm the synergistic interaction between DEET and propoxur observed in vitro, resulting in a higher mortality of mosquitoes. Our findings reveal an unusual allosterically potentiating action of the repellent DEET, which involves a selective site in insect. These results open exciting research areas in public health particularly in the control of the pyrethroid-resistant insect-vector borne diseases. Mixing low doses of DEET and a non-pyrethroid insecticide will lead to improvement in the efficiency treatments thus reducing both the concentration of active ingredients and side effects for non-target organisms. The discovery of this insect specific site may pave the way for the development of new strategies essential in the management of chemical use against resistant mosquitoes.     

Behavioral Cost & Overdominance in Anopheles gambiae

In response to the widespread use of control strategies such as Insecticide Treated Nets (ITN), Anopheles mosquitoes have evolved various resistance mechanisms. Kdr is a mutation that provides physiological resistance to the pyrethroid insecticides family (PYR). In the present study, we investigated the effect of the Kdr mutation on the ability of female An. gambiae to locate and penetrate a 1cm-diameter hole in a piece of netting, either treated with insecticide or untreated, to reach a bait in a wind tunnel. Kdr homozygous, PYR-resistant mosquitoes were the least efficient at penetrating an untreated damaged net, with about 51% [39-63] success rate compared to 80% [70-90] and 78% [65-91] for homozygous susceptible and heterozygous respectively. This reduced efficiency, likely due to reduced host-seeking activity, as revealed by mosquito video-tracking, is evidence of a recessive behavioral cost of the mutation. Kdr heterozygous mosquitoes were the most efficient at penetrating nets treated with PYR insecticide, thus providing evidence for overdominance, the rarely-described case of heterozygote advantage conveyed by a single locus. The study also highlights the remarkable capacity of female mosquitoes, whether PYR-resistant or not, to locate holes in bed-nets.     

Molecular surveillance of resistance to pyrethroids insecticides in Colombian Aedes aegypti populations

IntroductionIn Colombia, organochloride, organophosphate, carbamate, and pyrethroid insecticides are broadly used to control Aedes aegypti populations. However, Colombian mosquito populations have shown variability in their susceptibility profiles to these insecticides, with some expressing high resistance levels.Materials and methodsIn this study, we analyzed the susceptibility status of ten Colombian field populations of Ae. aegypti to two pyrethroids; permethrin (type-I pyrethroid) and lambda-cyhalothrin (type-II pyrethroid). In addition, we evaluated if mosquitoes pressured with increasing lambda-cyhalothrin concentrations during some filial generations exhibited altered allelic frequency of these kdr mutations and the activity levels of some metabolic enzymes.ResultsMosquitoes from all field populations showed resistance to lambda-cyhalothrin and permethrin. We found that resistance profiles could only be partially explained by kdr mutations and altered enzymatic activities such as esterases and mixed-function oxidases, indicating that other yet unknown mechanisms could be involved. The molecular and biochemical analyses of the most pyrethroid-resistant mosquito population (Acacías) indicated that kdr mutations and altered metabolic enzyme activity are involved in the resistance phenotype expression.ConclusionsIn this context, we propose genetic surveillance of the mosquito populations to monitor the emergence of resistance as an excellent initiative to improve mosquito-borne disease control measures.     

Independent selection of multiple mechanisms for pyrethroid resistance in Guatemalan Anopheles albimanus (Diptera: Culicidae).

Isofemale lines were established containing either, both, or neither of the elevated esterase and oxidase resistance mechanisms conferring pyrethroid resistance in a Guatemalan strain of Anopheles albimanus (Wiedemann). Plots of esterase and oxidase levels for individual mosquitoes from these single families correlated with data obtained using oxidase and esterase synergists in bioassays run in the bottle format. Mixed populations of pyrethroid-resistant A. albimanus adult females were selected using DDT, permethrin, or malathion; and the esterase and oxidase levels of the individual progeny were plotted. These data showed that the 3 classes of insecticide selected the 2 mechanisms differently. These results are discussed in terms of the problem of multiresistance surveillance in the field, especially concerning pyrethroid insecticides and the interaction of agricultural and public health insecticide application.     

Behaviour of Anopheles albimanus in relation to pyrethroid-treated bednets

Abstract. Responses of the malaria vector Anopheles albimanus to pyrethroid impregnated bednets made of cotton or nylon, compared with untreated nets, were investigated in houses occupied by two people inside and/or outside two bednets, in coastal Chiapas, México. The pyrethroid used was lambdacyhalothrin 30mga.i./m². Bioassay mortality rates of An.albimanus exposed to treated nets for 3 or 15min, rose from 40–55% to 90–100% for nylon nets 3–19 weeks post-treatment, but were consistently lower for treated cotton nets. An.albimanus females (collected unfed on human bait) were released in houses surrounded by curtains for trapping mosquitoes that exited from the house. Floor sheets were used in and around each experimental house for retrieving any mosquitoes knocked-down and/or killed. During post-treatment assessment for 17 weeks, An.albimanus blood-feeding success rates were 23–24% with untreated nets, 14–18% with treated cotton nets and 8–15% with treated nylon nets, significantly reduced when both human baits were inside the treated bednets, but not when one or more baits were outside the treated bednet(s) within the house. Proportions of mosquitoes leaving houses <3 h post-release were 53–59% from houses with untreated bednets versus 65–78% with treated bednets. Except in one case (when both humans were outside treated cotton nets), these increased early exit rates were significant, whether or not the human baits were inside the treated bednets indoors. Mortality rates of An.albimanus females exiting overnight (22.00–06.00 hours) averaged 15–39% from houses with treated cotton and 16–46% with treated nylon nets, very significantly greater than the control mean rates of 6–8% mortality with untreated nets. Observations on wild-caught An. albimanus females marked with fluorescent powder and released indoors revealed that few mosquitoes (3–11 %) actually contacted the bednets unless both human baits remained under them - when contact rates were 22% on treated nylon, 23% on treated cotton and 42% on untreated nets (P= 0.05). The mean resting time was significantly longer on untreated (14.4min) than on treated nylon (5.8min) or cotton (9.5 min) bednets, whereas mean resting times on other surfaces indoors were 16.5–19.8min. Proportions exiting within 2h of release were significantly more from houses with treated houses (33–35%) than with untreated nets (8%). However, mortality rates of mosquitoes that landed on treated nets were very significantly greater (90–100%) than after landing on untreated nets (10%). Thus, despite some excito-repellency, lambdacyhalothrin-impregnated bednets (especially made of nylon) proved to be effective as an alternative to house-spraying against An.albimanus.     

Experimental hut evaluation of bednets treated with an organophosphate (chlorpyrifos-methyl) or a pyrethroid (lambdacyhalothrin) alone and in combination against insecticide-resistant Anopheles gambiae and Culex quinquefasciatus mosquitoes

Background

Pyrethroid resistant mosquitoes are becoming increasingly common in parts of Africa. It is important to identify alternative insecticides which, if necessary, could be used to replace or supplement the pyrethroids for use on treated nets. Certain compounds of an earlier generation of insecticides, the organophosphates may have potential as net treatments.

Methods

Comparative studies of chlorpyrifos-methyl (CM), an organophosphate with low mammalian toxicity, and lambdacyhalothrin (L), a pyrethroid, were conducted in experimental huts in Côte d'Ivoire, West Africa. Anopheles gambiae and Culex quinquefasciatus mosquitoes from the area are resistant to pyrethroids and organophosphates (kdr and insensitive acetylcholinesterase Ace.1 ^R ). Several treatments and application rates on intact or holed nets were evaluated, including single treatments, mixtures, and differential wall/ceiling treatments.

Results and Conclusion

All of the treatments were effective in reducing blood feeding from sleepers under the nets and in killing both species of mosquito, despite the presence of the kdr and Ace.1 ^R genes at high frequency. In most cases, the effects of the various treatments did not differ significantly. Five washes of the nets in soap solution did not reduce the impact of the insecticides on A. gambiae mortality, but did lead to an increase in blood feeding. The three combinations performed no differently from the single insecticide treatments, but the low dose mixture performed encouragingly well indicating that such combinations might be used for controlling insecticide resistant mosquitoes. Mortality of mosquitoes that carried both Ace.1 ^R and Ace.1 ^S genes did not differ significantly from mosquitoes that carried only Ace.1 ^S genes on any of the treated nets, indicating that the Ace.1 ^R allele does not confer effective resistance to chlorpyrifos-methyl under the realistic conditions of an experimental hut.     

Pyrethroid resistance reduces the efficacy of space sprays for dengue control on the island of Martinique (Caribbean)

Background

Dengue fever is reemerging on the island of Martinique and is a serious threat for the human population. During dengue epidemics, adult Aedes aegypti control with pyrethroid space sprays is implemented in order to rapidly reduce transmission. Unfortunately, vector control programs are facing operational challenges with the emergence of pyrethroid resistant Ae. aegypti populations.

Methodology/Principal Findings

To assess the impact of pyrethroid resistance on the efficacy of treatments, applications of deltamethrin and natural pyrethrins were performed with vehicle-mounted thermal foggers in 9 localities of Martinique, where Ae. aegypti populations are strongly resistant to pyrethroids. Efficacy was assessed by monitoring mortality rates of naturally resistant and laboratory susceptible mosquitoes placed in sentinel cages. Before, during and after spraying, larval and adult densities were estimated. Results showed high mortality rates of susceptible sentinel mosquitoes treated with deltamethrin while resistant mosquitoes exhibited very low mortality. There was no reduction of either larval or adult Ae. aegypti population densities after treatments.

Conclusions/Significance

This is the first documented evidence that pyrethroid resistance impedes dengue vector control using pyrethroid-based treatments. These results emphasize the need for alternative tools and strategies for dengue control programs.