Bendiocarb, a potential alternative against pyrethroid resistant Anopheles gambiae in Benin, West Africa

A high-throughput method for quantifying Drosophila fecundity

Measurements of Drosophila fecundity are used in a wide variety of studies, such as investigations of stem cell biology, nutrition, behavior, and toxicology. In addition, because fecundity assays are performed on live flies, they are suitable for longitudinal studies such as investigations of aging or prolonged chemical exposure. However, standard Drosophila fecundity assays have been difficult to perform in a high-throughput manner because experimental factors such as the physiological state of the flies and environmental cues must be carefully controlled to achieve consistent results. In addition, exposing flies to a large number of different experimental conditions (such as chemical additives in the diet) and manually counting the number of eggs laid to determine the impact on fecundity is time-consuming. We have overcome these challenges by combining a new multiwell fly culture strategy with a novel 3D-printed fly transfer device to rapidly and accurately transfer flies from one plate to another; the RoboCam, a low-cost, custom built robotic camera to capture images of the wells automatically; and an image segmentation pipeline to automatically identify and quantify eggs. We show that this method is compatible with robust and consistent egg laying throughout the assay period; and demonstrate that the automated pipeline for quantifying fecundity is very accurate (r2 = 0.98 for the correlation between the automated egg counts and the ground truth) In addition, we show that this method can be used to efficiently detect the effects on fecundity induced by dietary exposure to chemicals. Taken together, this strategy substantially increases the efficiency and reproducibility of high throughput egg laying assays that require exposing flies to multiple different media conditions.

Vectorial competence, insecticide resistance in Anopheles funestus and operational implications for malaria vector control strategies in Benin Republic

The primary reason for the failure of malaria vector control across endemic regions is the widespread insecticide resistance observed in Anopheles vectors. The most dominant African vectors of malaria parasites are Anopheles gambiae and Anopheles funestus mosquitoes. These species often exhibit divergent behaviours and adaptive changes underscoring the importance of deploying active and effective measures in their control. Unlike An. gambiae, An. funestus mosquitoes are poorly studied in Benin Republic. However, recent reports indicated that An. funestus can adapt and colonize various ecological niches owing to its resistance against insecticides and adaptation to changing breeding habitats. Unfortunately, scientific investigations on the contribution of An. funestus to malaria transmission, their susceptibility to insecticide and resistance mechanism developed are currently insufficient for the design of better control strategies. In an attempt to gather valuable information on An. funestus, the present review examines the progress made on this malaria vector species in Benin Republic and highlights future research perspectives on insecticide resistance profiles and related mechanisms, as well as new potential control strategies against An. funestus. Literature analysis revealed that An. funestus is distributed all over the country, although present in low density compared to other dominant malaria vectors. Interestingly, An. funestus is being found in abundance during the dry seasons, suggesting an adaptation to desiccation. Among the An. funestus group, only An. funestus sensu stricto (s.s.) and Anopheles leesoni were found in the country with An. funestus s.s. being the most abundant species. Furthermore, An. funestus s.s. is the only one species in the group contributing to malaria transmission and have adapted biting times that allow them to bite at dawn. In addition, across the country, An. funestus were found resistant to pyrethroid insecticides used for bed nets impregnation and also resistant to bendiocarb which is currently being introduced in indoor residual spraying formulation in malaria endemic regions. All these findings highlight the challenges faced in controlling this malaria vector. Therefore, advancing the knowledge of vectorial competence of An. funestus, understanding the dynamics of insecticide resistance in this malaria vector, and exploring alternative vector control measures, are critical for sustainable malaria control efforts in Benin Republic.

What can be learned from the residual efficacy of three formulations of insecticides (pirimiphos-methyl, clothianidin and deltamethrin mixture, and clothianidin alone) in large-scale in community trial in North Benin, West Africa?

BACKGROUND

In Alibori and Donga, two departments of high malaria incidence of Northern Benin, pirimiphos-methyl, mixture deltamethrin + clothianidin, as well as clothianidin were used at large scale for IRS. The present study aimed to assess the residual efficacy of these products.

METHODS

Immatures of Anopheles gambiae sensu lato (s.l.) collected in the communes of Kandi and Gogounou (Department of Alibori), Djougou and Copargo (Department of Donga) were reared until adulthood. Females aged 2-5 days were used for susceptibility tube tests following the WHO protocol. The tests were conducted with deltamethrin (0.05%), bendiocarb (0.1%), pirimiphos-methyl (0.25%) and clothianidin (2% weight per volume). For cone tests performed on cement and mud walls, the An. gambiae Kisumu susceptible strain was used. After the quality control of the IRS performed 1-week post-campaign, the evaluation of the residual activity of the different tested insecticides/mixture of insecticides was conducted on a monthly basis.

RESULTS

Over the three study years, deltamethrin resistance was observed in all the communes. With bendiocarb, resistance or possible resistance was observed. In 2019 and 2020, full susceptibility to pirimiphos-methyl was observed, while possible resistance to the same product was detected in 2021 in Djougou, Gogounou and Kandi. With clothianidin, full susceptibility was observed 4-6 days post-exposure. The residual activity lasted 4-5 months for pirimiphos-methyl, and 8-10 months for clothianidin and the mixture deltamethrin + clothianidin. A slightly better efficacy of the different tested products was observed on cement walls compared to the mud walls.

CONCLUSION

Overall, An. gambiae s.l. was fully susceptible to clothianidin, while resistance/possible resistance was observed the other tested insecticides. In addition, clothianidin-based insecticides showed a better residual activity compared to pirimiphos-methyl, showing thus their ability to provide an improved and prolonged control of pyrethroid resistant vectors.

Characterizing pyrethroid resistance and mechanisms in Anopheles gambiae (s.s.) and Anopheles arabiensis from 11 districts in Uganda

Insecticide resistance threatens recent progress on malaria control in Africa. To characterize pyrethroid resistance in Uganda, Anopheles gambiae (s.s.) and Anopheles arabiensis were analyzed from 11 sites with varied vector control strategies. Mosquito larvae were collected between May 2018 and December 2020. Sites were categorized as receiving no indoor-residual spraying ('no IRS', n ​= ​3); where IRS was delivered from 2009 to 2014 and in 2017 and then discontinued ('IRS stopped', n ​= ​4); and where IRS had been sustained since 2014 ('IRS active', n ​= ​4). IRS included bendiocarb, pirimiphos methyl and clothianidin. All sites received long-lasting insecticidal nets (LLINs) in 2017. Adult mosquitoes were exposed to pyrethroids; with or without piperonyl butoxide (PBO). Anopheles gambiae (s.s.) and An. arabiensis were identified using PCR. Anopheles gambiae (s.s.) were genotyped for Vgsc-995S/F, Cyp6aa1, Cyp6p4-I236M, ZZB-TE, Cyp4j5-L43F and Coeae1d, while An. arabiensis were examined for Vgsc-1014S/F. Overall, 2753 An. gambiae (s.l.), including 1105 An. gambiae (s.s.) and 1648 An. arabiensis were evaluated. Species composition varied by site; only nine An. gambiae (s.s.) were collected from 'IRS active' sites, precluding species-specific comparisons. Overall, mortality following exposure to permethrin and deltamethrin was 18.8% (148/788) in An. gambiae (s.s.) and 74.6% (912/1222) in An. arabiensis. Mortality was significantly lower in An. gambiae (s.s.) than in An. arabiensis in 'no IRS' sites (permethrin: 16.1 vs 67.7%, P ​< ​0.001; deltamethrin: 24.6 vs 83.7%, P ​< ​0.001) and in 'IRS stopped' sites (permethrin: 11.3 vs 63.6%, P ​< ​0.001; deltamethrin: 25.6 vs 88.9%, P ​< ​0.001). When PBO was added, mortality increased for An. gambiae (s.s.) and An. arabiensis. Most An. gambiae (s.s.) had the Vgsc-995S/F mutation (95% frequency) and the Cyp6p4-I236M resistance allele (87%), while the frequency of Cyp4j5 and Coeae1d were lower (52% and 55%, respectively). Resistance to pyrethroids was widespread and higher in An. gambiae (s.s.). Where IRS was active, An. arabiensis dominated. Addition of PBO to pyrethroids increased mortality, supporting deployment of PBO LLINs. Further surveillance of insecticide resistance and assessment of associations between genotypic markers and phenotypic outcomes are needed to better understand mechanisms of pyrethroid resistance and to guide vector control.

Evolution of the Ace-1 and Gste2 Mutations and Their Potential Impact on the Use of Carbamate and Organophosphates in IRS for Controlling Anopheles gambiae s.l., the Major Malaria Mosquito in Senegal

Widespread of insecticide resistance amongst the species of the Anopheles gambiae complex continues to threaten vector control in Senegal. In this study, we investigated the presence and evolution of the Ace-1 and Gste2 resistance genes in natural populations of Anopheles gambiae s.l., the main malaria vector in Senegal. Using historical samples collected from ten sentinel health districts, this study focused on three different years (2013, 2017, and 2018) marking the periods of shift between the main public health insecticides families (pyrethroids, carbamates, organophosphates) used in IRS to track back the evolutionary history of the resistance mutations on the Ace-1 and Gste2 loci. The results revealed the presence of four members of the Anopheles gambiae complex, with the predominance of An. arabiensis followed by An. gambiae, An. coluzzii, and An. gambiae–coluzzii hybrids. The Ace-1 mutation was only detected in An. gambiae and An. gambiae–coluzzii hybrids at low frequencies varying between 0.006 and 0.02, while the Gste2 mutation was found in all the species with a frequency ranging between 0.02 and 0.25. The Ace-1 and Gste2 genes were highly diversified with twenty-two and thirty-one different haplotypes, respectively. The neutrality tests on each gene indicated a negative Tajima’s D, suggesting the abundance of rare alleles. The presence and spread of the Ace-1 and Gste2 resistance mutations represent a serious threat to of the effectiveness and the sustainability of IRS-based interventions using carbamates or organophosphates to manage the widespread pyrethroids resistance in Senegal. These data are of the highest importance to support the NMCP for evidence-based vector control interventions selection and targeting.

Bioefficacy of bendiocarb WP80 in vector-borne and zoonotic diseases areas in borderline of Iran and Pakistan

Malaria and leishmaniasis are the public health problems in southern Iran. The main activity of vector control is indoor residual spraying using pyrethroids, using different insecticides as larviciding and impregnated bednets. The aim of study was to evaluate the biological assays of bendiocarb wettable powder (WP) at different surfaces of wall. The residual effect of bendiocarb WP80 at 400 milligram/meter square (mg/m²) was evaluated on various local surfaces of rooms such as mud and plaster as well as thatch roofs and wooden. World Health Organization standard cones using contact bioassays were carried out using laboratory reared sugar-fed, 48-72 h old females of Anopheles stephensi. Contact bioassays were carried out on sprayed surfaces for 150 days. Contact bioassay on surfaces treated with bendiocarb WP80 at different surfaces was estimated about 2 months. Fumigant tests of bendiocarb WP80 at 400 mg/m² revealed 50-93.83% mortality with 1 month persistency. The results showed that carbamate insecticide could be used as rotation with pyrethroids for malaria vector control. Monitoring and evaluation of environmental toxicology of pesticides is important for decision making for choosing appropriate pesticides for disease vector control.

Variable residual activity of K-Othrine® PolyZone and Actellic® 300 CS in semi-field and natural conditions in the Democratic Republic of the Congo

Background

Indoor Residual Spray (IRS) against vector mosquitoes is a primary means for combating malaria transmission. To combat increased patterns of resistance to chemicals against mosquito vectors, alternative candidate insecticide formulations should be screened. With mortality as the primary endpoint, the persistence of residual efficacy of a polymer-enhanced pyrethroid suspension concentrate containing deltamethrin (K-Othrine® PolyZone—KOPZ) applied at 25 mg active ingredient (ai)/m² was compared with a microencapsulated organophosphate suspension formulation of pirimiphos-methyl (Actellic® 300CS—ACS) applied at 1 g ai/m².

Methods

Following standard spray application, periodic contact bioassays were conducted for at least 38 weeks on four types of wall surfaces (unbaked clay, baked clay, cement, and painted cement) sprayed with either KOPZ or ACS in simulated semi-field conditions. Similarly, two types of existing walls in occupied houses (painted cement and baked clay) were sprayed and examined. A colonized strain of female Anopheles arabiensis mosquitoes were exposed to treated or untreated surfaces (controls) for 30 min. For each wall surface test period, 40 treatment mosquitoes (4 cones × 10) in semi-field and 90 (9 cones × 10) in ‘natural’ house conditions were used per wall. 30 mosquitoes (3 cones × 10) on a matching unsprayed surface served as the control. Insecticide, wall material, and sprayed location on wall (in houses) were compared by final mortality at 24 h.

Results

Insecticide, wall material, and sprayed location on wall surface produced significant difference for mean final mortality over time. In semi-field conditions, KOPZ produced a 72% mean mortality over a 38-week period, while ACS gave 65% (p < 0.001). Painted cement wall performed better than other wall surfaces throughout the study period (73% mean mortality). In the two occupied houses, KOPZ provided a mean mortality of 88%, significantly higher than ACS (p < 0.001). KOPZ provided an effective residual life (≥ 80% mortality) between 7.3 and 14 weeks on experimental walls and between 18.3 and 47.2 weeks in houses, while ACS persisted between 3 and 7.6 weeks under semi-field conditions and between 7.1 and 17.3 weeks in houses. Household painted cement walls provided a longer effective residual activity compared to baked clay for both formulations. Greater mortality was recorded at the top and middle sections of sprayed wall compared to the bottom portion near the floor.

Conclusion

KOPZ provided longer residual activity on all surfaces compared to ACS. Painted cement walls provided better residual longevity for both insecticides compared to other surfaces. Insecticides also performed better in an occupied house environment compared to semi-field constructed walls. This study illustrates the importance of collecting field-based observations to determine appropriate product active ingredient formulations and timing for recurring IRS cycles.

ImergardTMWP: A Non-Chemical Alternative for an Indoor Residual Spray, Effective against Pyrethroid-Resistant Anopheles gambiae (s.l.) in Africa

Malaria is the deadliest mosquito-borne disease and kills predominantly people in sub-Saharan Africa (SSA). The now widespread mosquito resistance to pyrethroids, with rapidly growing resistance to other insecticide classes recommended by the World Health Organization (WHO), may overturn the successes gained in mosquito control in recent years. It is of utmost importance to search for new, inexpensive, and safe alternatives, with new modes of action, that might improve the efficacy of current insecticides. The efficacy of a novel mechanical insecticidal mineral derived from volcanic rock, ImergardTMWP, was investigated to determine its efficacy as a stand-alone residual wall spray and as a mixture with deltamethrin (K-Othrine® Polyzone) in experimental huts in Cove, Benin. The evaluation was conducted with susceptible (Kisumu) and wild-type Anopheles gambiae (s.l.). Deltamethrin applied alone demonstrated 40-45% mortality (at 72 h post-exposure) during the first four months, which declined to 25% at six months for wild An. gambiae from Cove. ImergardTMWP alone and mixed with deltamethrin, under the same assay conditions, produced 79-82% and 73-81% mortality, respectively, during the same six-month period. ImergardTMWP met the 80% WHO bio-efficacy threshold for residual activity for the first five months with 78% residual activity at six months. ImergardTMWP can be used as a mixture with chemical insecticides or as a stand-alone pesticide for mosquito control in Africa.

Lessons learned, challenges and outlooks for decision-making after a decade of experience monitoring the impact of indoor residual spraying in Benin, West Africa

BACKGROUND

Since 2008, Indoor Residual Spraying (IRS) has been performed in Benin in 19 districts, including 4 in southern Benin, 9 in Atacora, and 8 in Atacora, Alibori and Donga in northern Benin. However, Benin still struggles with questions about IRS cost-benefit and epidemiological impact. Lessons learned and challenges from 10 years of IRS in Benin to be shared with the stakeholders involved in vector control implementation for decision-making.

METHODS

Entomological parameters have been assessed entomological parameters in IRS communes since 2008. In all IRS intervention communes, decreases in human biting rate (HBR) of Anopheles gambiae, blood feeding inhibition and entomological inoculation rate (EIR) as compared to control district have been measured.

RESULTS

EIR was reduced by 80-90%, which is encouraging, but should be observed with caution because: (i) the reduction may be insufficient to decrease epidemiological indicators given that the residual EIR in IRS districts is still higher than it is in some regions of stable malaria; (ii) the reduction in EIR is based on comparisons with control communes, but it is difficult to select control areas with the same environmental characteristics as intervention areas; (iii) despite the reduction, half of all mosquitoes that entered IRS-treated houses succeeded in taking human blood meals. Further, there are behaviours among Benin's population that limit IRS efficacy, including recent data showing that > 90% of people are not protected by IRS between 7 and 10 p.m. This is due to the fact that they remain outdoors and that most people are not protected from mosquito bites after 10 p.m. because they either sleep outdoors without IRS protection or indoors without an ITN. Moreover, people have large amounts of clothing hanging on walls where mosquitoes can rest instead of IRS-treated walls. Finally, other components are important to consider in implementing IRS among which: (i) Vector resistance management strategies are sometimes poorly understood; this is actually different from the need to replace one insecticide with another after the emergence of resistance; (ii) African countries should prepare to finance IRS themselves.

CONCLUSION

To curtail residual malaria transmission, additional interventions able to target vectors escaping IRS should be prioritized.

Upsurge of malaria transmission after indoor residual spraying withdrawal in Atacora region in Benin, West Africa

Background

In Benin, malaria vector control mostly relies on long-lasting, insecticidal-treated bed nets (LLINs) and indoor residual spraying (IRS) operations. From 2011 to 2016, an IRS programme has been implemented in Atacora region. However, in 2017 the programme was withdrawn from two other regions in the northern part of the country, with hopes that gains would be relatively sustained because of the seasonality of malaria transmission. What would be the vulnerability of populations to malaria after the withdrawal of IRS?

Methods

Monthly mosquito collections were performed through human landing captures (HLCs) for 24 months (from January to December 2016 during the last IRS campaign, and from January to December 2018, 2 years after the withdrawal of IRS). Vector mosquitoes biting density was sampled by HLC and was tested for presence of Plasmodium falciparum sporozoites. The carcass of these mosquitoes (abdomens, wing, legs) were subjected to molecular species identification using polymerase chain reaction (PCR) assays.

Results

It is noticed a drastic increase (~ 3 times higher) of vector abundance after the withdrawal of IRS. Mosquito biting rates in the 3 survey districts increased significantly after IRS was withdrawn. In 2018, after IRS cessation a significant increase of entomological inoculation rate was recorded, where each inhabitant received an average of 94.9 infected bites/year to 129.21 infected bites/year against an average of 17.15 infected bites/year to 24.82 infected bites/year in 2016.

Conclusion

It is obvious that the withdrawal of IRS confers a vulnerability of the population with regard to the malaria transmission. Robust monitoring is needed to better understand when and where IRS should be most adequate, or can be safely withdrawn. In case of withdrawal, adapted accompanying measures should be proposed according to the context not only to maintain the gains capitalized with IRS, but also to avoid any rebound of transmission.

Impact of vector control interventions on malaria transmission intensity, outdoor vector biting rates and Anopheles mosquito species composition in Tororo, Uganda

BACKGROUND

Long-lasting insecticidal nets (LLINs) and indoor residual spraying of insecticide (IRS) are widely recommended for the prevention of malaria in endemic regions. Data from human landing catches provide information on the impact of vector control on vector populations. Here, malaria transmission indoors and outdoors, before and after mass deployment of LLINs and IRS in Uganda was compared.

METHODS

The study took place in Tororo district, a historically high transmission area where universal LLIN distribution was conducted in November 2013 and May 2017 and 6 rounds of IRS implemented from December 2014 to July 2018. Human landing catches were performed in 8 houses monthly from October 2011 to September 2012 (pre-intervention period) and every 4 weeks from November 2017 to October 2018 (post-intervention period). Mosquitoes were collected outdoors from 18:00 to 22:00 h and indoors from 18:00 to 06:00 h. Female Anopheles were tested for the presence of Plasmodium falciparum sporozoites and species identification performed using gross dissection and polymerase chain reaction (PCR).

RESULTS

The interventions were associated with a decline in human biting rate from 19.6 to 2.3 female Anopheles mosquitoes per house per night (p < 0.001) and annual entomological inoculation rate from 129 to 0 infective bites per person per year (p < 0.001). The proportion of mosquitoes collected outdoors increased from 11.6 to 49.4% (p < 0.001). Prior to the interventions the predominant species was Anopheles gambiae sensu stricto (s.s.), which comprised an estimated 76.7% of mosquitoes. Following the interventions, the predominant species was Anopheles arabiensis, which comprised 99.5% of mosquitoes, with almost complete elimination of An. gambiae s.s. (0.5%).

CONCLUSIONS

Mass distribution of LLINs and 6 rounds of IRS dramatically decreased vector density and sporozoite rate resulting in a marked reduction in malaria transmission intensity in a historically high transmission site in Uganda. These changes were accompanied by a shift in vector species from An. gambiae s.s. to An. arabiensis and a relative increase in outdoor biting.

Pesticides and the evolution of the genetic structure of Anopheles coluzzii populations in some localities in Benin (West Africa)

Background

Changes in the natural habitats of insect groups are determined the genetic polymorphisms between individuals. The objective of this study was to establish the genetic structure of the Anopheles coluzzii populations in four localities of Benin.

Methods

Insecticide surveys and larval sampling were conducted on 4 study localities, including Cotonou, Ketou, Zagnanado, and Sô-Ava. Molecular characterizations were performed on the Anopheles mosquitoes collected with the allelic and genotypic frequencies of kdr gene determined. The multiple comparison Chi square test for proportions was performed with R version 3.3.3. Next, the observed heterozygosity, expected heterozygosity, and indices of fixation, and genetic differentiation were estimated. Finally, the Hardy–Weinberg equilibrium (EHW) was determined to assess whether panmixia exists in the different populations of mosquitoes of the agroecological zones under study.

Results

Carbamates, pyrethroids, organophosphorus and organochlorines use have been reported in all localities except Sô-Ava. Anopheles coluzzii was strongly represented across all study localities. The L1014F allele was observed in the localities of Kétou, Cotonou and Zagnanado. Likewise, insecticide selection pressure of homozygous resistant individuals (L1014F/L1014F) was significantly higher in Kétou, Cotonou and Zagnanado (p value < 0.05). Surprisingly in Sô-Ava, a relatively high frequency of the L1014F allele despite the reported absence of pesticide use was observed. All mosquito populations were found to be deficient in heterozygosity across the study sites (F_IS< 0). No genetic differentiation (F_ST< 0) was observed in the localities of Zagnanado and Kétou.

Conclusion

The survey on the use of insecticides showed that insecticide selection pressures differ across the investigated localities. It would be desirable to rotate or apply formulations of combined products with different modes of action. Doing so would enable a better management of resistant homozygous individuals, and mitigate the resistance effect of commonly used insecticides.

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

Transmission of Plasmodium falciparum malaria parasites occurs when nocturnal Anopheles mosquito vectors feed on human blood. In Africa, where malaria burden is highest, bednets treated with pyrethroid insecticide were highly effective in preventing mosquito bites and reducing transmission, and essential to achieving unprecedented reductions in malaria until 2015 (ref. ¹). Since then, progress has stalled², and with insecticidal bednets losing efficacy against pyrethroid-resistant Anopheles vectors^3,4, methods that restore performance are urgently needed to eliminate any risk of malaria returning to the levels seen before their widespread use throughout sub-Saharan Africa⁵. Here, we show that the primary malaria vector Anopheles gambiae is targeted and killed by small insecticidal net barriers positioned above a standard bednet in a spatial region of high mosquito activity but zero contact with sleepers, opening the way for deploying many more insecticides on bednets than is currently possible. Tested against wild pyrethroid-resistant A. gambiae in Burkina Faso, pyrethroid bednets with organophosphate barriers achieved significantly higher killing rates than bednets alone. Treated barriers on untreated bednets were equally effective, without significant loss of personal protection. Mathematical modelling of transmission dynamics predicted reductions in clinical malaria incidence with barrier bednets that matched those of 'next-generation' nets recommended by the World Health Organization against resistant vectors. Mathematical models of mosquito-barrier interactions identified alternative barrier designs to increase performance. Barrier bednets that overcome insecticide resistance are feasible using existing insecticides and production technology, and early implementation of affordable vector control tools is a realistic prospect.

The G119S Acetylcholinesterase (Ace-1) Target Site Mutation Confers Carbamate Resistance in the Major Malaria Vector Anopheles gambiae from Cameroon: A Challenge for the Coming IRS Implementation

Growing resistance is reported to carbamate insecticides in malaria vectors in Cameroon. However, the contribution of acetylcholinesterase (Ace-1) to this resistance remains uncharacterised. Here, we established that the G119S mutation is driving resistance to carbamates in Anopheles gambiae populations from Cameroon. Insecticide bioassay on field-collected mosquitoes from Bankeng, a locality in southern Cameroon, showed high resistance to the carbamates bendiocarb (64.8% ± 3.5% mortality) and propoxur (55.71% ± 2.9%) but a full susceptibility to the organophosphate fenitrothion. The TaqMan genotyping of the G119S mutation in field-collected adults revealed the presence of this resistance allele (39%). A significant correlation was observed between the Ace-1^R and carbamate resistance at allelic ((bendiocarb; odds ratio (OR) = 75.9; p < 0.0001) and (propoxur; OR = 1514; p < 0.0001)) and genotypic (homozygote resistant vs. homozygote susceptible (bendiocarb; OR = 120.8; p < 0.0001) and (propoxur; OR = 3277; p < 0.0001)) levels. Furthermore, the presence of the mutation was confirmed by sequencing an Ace-1 portion flanking codon 119. The cloning of this fragment revealed a likely duplication of Ace-1 in Cameroon as mosquitoes exhibited at least three distinct haplotypes. Phylogenetic analyses showed that the predominant Ace-1^R allele is identical to that from West Africa suggesting a recent introduction of this allele in Central Africa from the West. The spread of this Ace-1^R represents a serious challenge to future implementation of indoor residual spraying (IRS)-based interventions using carbamates or organophosphates in Cameroon.

Efficacy of insecticides used in indoor residual spraying for malaria control: an experimental trial on various surfaces in a "test house"

Background

Malaria is a public health problem in the Brazilian Amazon region. In integrated vector management for malaria (anopheline) control, indoor residual spraying (IRS) represents one of the main tools in the basic strategy applied in the Amazonian states. It is essential to understand the residual efficacy of insecticides on different surfaces to determine spray cycles, ensure their rational use, and prevent wastage. This study aimed to evaluate the residual efficacy of six insecticide formulations used in the National Malaria Control Programme on four different types of walls in a field simulation at a “test house”.

Methods

The tests were performed as a field-simulating evaluation at a “test house” built in the municipality of Macapá. Six insecticide formulations comprising four pyrethroids, a carbamate, and an organophosphate were used, and evaluated when applied on different wall surfaces: painted wood, unpainted wood, plastered cement, and unplastered cement. The insecticides were applied to the interior walls of the “test house” by a trained technician.

Results

In the bioassays performed with pyrethroids, deltamethrin water-dispersible granules (WG) performed particularly well, presenting residual bioefficacy of 8 months on both wood surfaces after the IRS, whereas alpha-cypermethrin suspension concentrate (SC) and etofenprox wettable powder (WP) demonstrated residual bioefficacy of 4 months on at least one of the wood surfaces; however, the pyrethroid lambda-cyhalothrin WP showed a low residual bioefficacy (< 3 months) on all tested surfaces, demonstrating its inefficiency for areas with a long transmission cycle of malaria. For the carbamate-bendiocarb WP, residual bioefficacy for 3 months was achieved only on wood surfaces. In general, the organophosphate pirimifos-methyl capsule suspension (CS) demonstrated the best result, with a mortality rate < 80% over a period of 6 months on all surfaces tested.

Conclusion

Insecticide efficiency varies among different types of surface; therefore, a “test house” is a valuable evaluation tool. This work highlights the usefulness of associating the residual efficacy of insecticides on the surfaces commonly found in houses in endemic areas, together with knowledge about the transmission cycle duration of the transmission cycle and the insecticide susceptibility of the vector. This association helps in the decision-making for the malaria control intervention regarding.

Systematic review of indoor residual spray efficacy and effectiveness against Plasmodium falciparum in Africa

Indoor residual spraying (IRS) is an important part of malaria control. There is a growing list of insecticide classes; pyrethroids remain the principal insecticide used in bednets but recently, novel non-pyrethroid IRS products, with contrasting impacts, have been introduced. There is an urgent need to better assess product efficacy to help decision makers choose effective and relevant tools for mosquito control. Here we use experimental hut trial data to characterise the entomological efficacy of widely-used, novel IRS insecticides. We quantify their impact against pyrethroid-resistant mosquitoes and use a Plasmodium falciparum transmission model to predict the public health impact of different IRS insecticides. We report that long-lasting IRS formulations substantially reduce malaria, though their benefit over cheaper, shorter-lived formulations depends on local factors including bednet use, seasonality, endemicity and pyrethroid resistance status of local mosquito populations. We provide a framework to help decision makers evaluate IRS product effectiveness.

Experimental huts trial of the efficacy of pyrethroids/piperonyl butoxide (PBO) net treatments for controlling multi-resistant populations of Anopheles funestus s.s. in Kpomè, Southern Benin

Background: Insecticides resistance in Anopheles mosquitoes limits Long-Lasting Insecticidal Nets (LLIN) used for malaria control in Africa, especially Benin. This study aimed to evaluate the bio-efficacy of current LLINs in an area where An. funestus s.l. and An. gambiae have developed multi-resistance to insecticides, and to assess in experimental huts the performance of a mixed combination of pyrethroids and piperonyl butoxide (PBO) treated nets on these resistant mosquitoes.

Methods: The study was conducted at Kpomè, Southern Benin. The bio-efficacy of LLINs against An. funestus and An. gambiae was assessed using the World Health Organization (WHO) cone and tunnel tests. A released/recapture experiment following WHO procedures was conducted to compare the efficacy of conventional LLINs treated with pyrethroids only and LLINs with combinations of pyrethroids and PBO. Prior to huts trials, we confirmed the level of insecticide and PBO residues in tested nets using high performance liquid chromatography (HPLC).

Results: Conventional LLINs (Type 2 and Type 4) have the lowest effect against local multi-resistant An. funestus s.s. and An. coluzzii populations from Kpomè. Conversely, when LLINs containing mixtures of pyrethroids and PBO (Type 1 and Type 3) were introduced in trial huts, we recorded a greater effect against the two mosquito populations (P < 0.0001). Tunnel test with An. funestus s.s. revealed mortalities of over 80% with this new generation of LLINs (Type 1 and Type 3),while conventional LLINs produced 65.53 ± 8.33% mortalities for Type 2 and 71.25 ±7.92% mortalities for Type 4. Similarly, mortalities ranging from 77 to 87% were recorded with the local populations of An. coluzzii.

Conclusion: This study suggests the reduced efficacy of conventional LLINs (Pyrethroids alone) currently distributed in Benin communities where Anopheles populations have developed multi-insecticide resistance. The new generation nets (pyrethroids+PBO) proved to be more effective on multi-resistant populations of mosquitoes.

Efficacy of a novel mode of action of an indoor residual spraying product, SumiShield® 50WG against susceptible and resistant populations of Anopheles gambiae (s.l.) in Benin, West Africa

Background

Scale-up of the distribution of long-lasting insecticide-treated bed nets and indoor residual spraying with insecticides over the last decade have contributed to the considerable decrease of malaria morbidity and mortality in sub-Saharan Africa. Due to the increasing pyrethroid resistance intensity and the spread of carbamate resistance in Anopheles gambiae (s.s.) mosquitoes and the limited number of insecticides recommended by the WHO for vector control, alternative insecticide formulations for IRS with long-lasting residual activity are required to sustain the gains obtained in most malaria-endemic countries.

Methods

SumiShield 50WG (clothianidin 300 mg ai/m²) developed by Sumitomo Chemical was evaluated alongside deltamethrin 25 mg ai/m² (K-Othrine 250 WG) against a pyrethroid resistant Anopheles gambiae (s.l.) population in experimental huts in Covè, Benin. Residual activity was also tested in cone bioassays with the susceptible An. gambiae “Kisumu” strain and the local wild resistant population.

Results

The results showed very low toxicity from deltamethrin (mortality rates ranged between 1–40%) against host-seeking resistant Anopheles populations. SumiShield in contrast gave an overall mean mortality of 91.7% at the 120 h observation across the eight- month observation period following spraying. The residual activity measured using cone tests was over the 80% WHO threshold for 24 weeks for resistant wild Anopheles population and 32 weeks for the susceptible strain “Kisumu” after the spraying.

Conclusions

SumiShield is a good candidate for IRS in areas of permanent malaria transmission and where Anopheles populations are resistant to other conventional insecticides such as pyrethroids. It would be interesting to complete experimental huts studies by assessing the efficacy and residual effect of SumiShield 50WG at community level (small-scale field testing) in an area where vectors are highly resistant to insecticides.

Relative performance of indoor vector control interventions in the Ifakara and the West African experimental huts

BACKGROUND

West African and Ifakara experimental huts are used to evaluate indoor mosquito control interventions, including spatial repellents and insecticides. The two hut types differ in size and design, so a side-by-side comparison was performed to investigate the performance of indoor interventions in the two hut designs using standard entomological outcomes: relative indoor mosquito density (deterrence), exophily (induced exit), blood-feeding and mortality of mosquitoes.

METHODS

Metofluthrin mosquito coils (0.00625% and 0.0097%) and Olyset® Net vs control nets (untreated, deliberately holed net) were evaluated against pyrethroid-resistant Culex quinquefasciatus in Benin. Four experimental huts were used: two West African hut designs and two Ifakara hut designs. Treatments were rotated among the huts every four nights until each treatment was tested in each hut 52 times. Volunteers rotated between huts nightly.

RESULTS

The Ifakara huts caught a median of 37 Culex quinquefasciatus/ night, while the West African huts captured a median of 8/ night (rate ratio 3.37, 95% CI: 2.30-4.94, P < 0.0001) and this difference in mosquito entry was similar for Olyset® Net and more pronounced for spatial repellents. Exophily was greater in the Ifakara huts with > 4-fold higher mosquito exit relative to the West African huts (odds ratio 4.18, 95% CI: 3.18-5.51, P < 0.0001), regardless of treatment. While blood-feeding rates were significantly higher in the West African huts, mortality appeared significantly lower for all treatments.

CONCLUSIONS

The Ifakara hut captured more Cx. quinquefasciatus that could more easily exit into windows and eave traps after failing to blood-feed, compared to the West African hut. The higher mortality rates recorded in the Ifakara huts could be attributable to the greater proportions of Culex mosquitoes exiting and probably dying from starvation, relative to the situation in the West African huts.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): West Nile fever

West Nile fever (WNF) has been assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7 on disease profile and impacts, Article 5 on the eligibility of WNF to be listed, Article 9 for the categorisation of WNF according to disease prevention and control rules as in Annex IV and Article 8 on the list of animal species related to WNF. The assessment has been performed following a methodology composed of information collection and compilation, expert judgement on each criterion at individual and, if no consensus was reached before, also at collective level. The output is composed of the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported. Details on the methodology used for this assessment are explained in a separate opinion. According to the assessment performed, WNF can be considered eligible to be listed for Union intervention as laid down in Article 5(3) of the AHL. The disease would comply with the criteria as in Sections 2 and 5 of Annex IV of the AHL, for the application of the disease prevention and control rules referred to in points (b) and (e) of Article 9(1). The animal species to be listed for WNF according to Article 8(3) criteria are several orders of birds and mammals as susceptible species and several families of birds as reservoir. Different mosquito species can serve as vectors.

Efficacy of Bendiocarb Used for Indoor Residual Spraying for Malaria Control in Madagascar: Results With Local Anopheles Species (Diptera: Culicidae) From Experimental Hut Trials

To control malaria in Madagascar, two primary vector control interventions are being scaled up: insecticide-treated nets and indoor residual spraying of bendiocarb, which was implemented in the Malagasy Central Highlands in 2009. The current efficacy of bendiocarb against Anopheles species was evaluated in a small-scale field trial. An experimental hut trial comparing the effectiveness of bendiocarb sprayed on five substrates (cement, wood, tin, mud, and vegetative materials) was carried out against Anopheles species in two study sites located in the eastern foothills of Madagascar. No significant difference was detected in either exophily or blood-feeding rates between treated and untreated huts. The mortality rate was significantly greater in treated huts compared to untreated huts. Efficacy up to 80% was found for 5 mo posttreatment. Although effective, bendiocarb has been used for 7 yr, and therefore an alternative insecticide may be needed to avoid the emergence of resistance.

Assessment of Ficam VC (Bendiocarb) Residual Activity on Different Wall Surfaces for Control of Anopheles gambiae s.s. (Diptera: Culicidae) in Northern Uganda

Insecticide decay rate on different wall surfaces is of importance to indoor residual spray (IRS) programs used as a malaria control intervention. Past IRS operations showed increasing populations of endophilic malaria vectors resting on indoor surfaces from various sites in Uganda following use of Ficam VC (bendiocarb) insecticide; variability of insecticide life was believed to be primarily due to wall surface type. Bendiocarb longevity was tested in the northern Uganda districts of Amuru, Apac, and Pader to assess its residual efficacy on three commonly encountered wall surfaces. Wall types included mud and wattle, plain brick, and painted plaster. A susceptible mosquito strain (Anopheles gambiae Kisumu) was used in all trials. Nine houses in each of the three districts were set with three test cones and one control cone per house, divided evenly among the three wall surfaces. Bioassays were run monthly through 6 mo. Painted plastered surfaces produced 100% mortality (at 24 h) through 6 mo. Plain brick surfaces killed 100% of test mosquitoes through 4 mo, while mud and wattle wall surfaces produced a 98% mortality rate at 3 mo post spray. The KD60 (knockdown at 60 min) for painted plastered surfaces was 100% for 6 mo, plain brick surface KD60 was 80% at 6 mo, and the mud and wattle surface KD60 was >80% at 3 mo. There was a significant effect on Ficam VC longevity by wall type and evidence of a relationship between test period and wall type on the KD60.

Investigation of mechanisms of bendiocarb resistance in Anopheles gambiae populations from the city of Yaoundé, Cameroon

Background

Resistance to the carbamate insecticide bendiocarb is emerging in Anopheles gambiae populations from the city of Yaoundé in Cameroon. However, the molecular basis of this resistance remains uncharacterized. The present study objective is to investigate mechanisms promoting resistance to bendiocarb in An. gambiae populations from Yaoundé.

Methods

The level of susceptibility of An. gambiae s.l. to bendiocarb 0.1 % was assessed from 2010 to 2013 using bioassays. Mosquitoes resistant to bendiocarb, unexposed and susceptible mosquitoes were screened for the presence of the Ace-1^R mutation using TaqMan assays. Microarray analyses were performed to assess the pattern of genes differentially expressed between resistant, unexposed and susceptible.

Results

Bendiocarb resistance was more prevalent in mosquitoes originating from cultivated sites compared to those from polluted and unpolluted sites. Both An. gambiae and Anopheles coluzzii were found to display resistance to bendiocarb. No G119S mutation was detected suggesting that resistance was mainly metabolic. Microarray analysis revealed the over-expression of several cytochrome P450 s genes including cyp6z3, cyp6z1, cyp12f2, cyp6m3 and cyp6p4. Gene ontology (GO) enrichment analysis supported the detoxification role of cytochrome P450 s with several GO terms associated with P450 activity significantly enriched in resistant samples. Other detoxification genes included UDP-glucosyl transferases, glutathione-S transferases and ABC transporters.

Conclusion

The study highlights the probable implication of metabolic mechanisms in bendiocarb resistance in An. gambiae populations from Yaoundé and stresses the need for further studies leading to functional validation of detoxification genes involved in this resistance.

Multiple insecticide resistance in an infected population of the malaria vector Anopheles funestus in Benin

Background

Knowledge on the spread and distribution of insecticide resistance in major malaria vectors such as Anopheles funestus is key to implement successful resistance management strategies across Africa. Here, by assessing the susceptibility status of an inland population of An. funestus Giles (Kpome) and investigating molecular basis of resistance, we show that multiple resistance and consistent plasmodium infection rate are present in Anopheles funestus populations from Kpome.

Methods

The insecticide susceptibility level of collected Anopheles funestus was assessed. Synergist (PBO) was used to screen resistance mechanisms. The TaqMan technique was used for genotyping of insecticide resistant alleles and detecting plasmodium infection levels. The nested PCR was used to further assess the plasmodium infection rate.

Results

The TaqMan analysis of plasmodial infections revealed an infection rate (18.2 %) of An. funestus in this locality. The WHO bioassays revealed a multiple phenotypic resistance profile for An. funestus in Kpome. This population is highly resistant to pyrethroids (permethrin and deltamethrin), organochlorines (DDT), and carbamates (bendiocarb). A reduced susceptibility was observed with dieldrin. Mortalities did not vary after pre-exposure to PBO for DDT indicating that cytochrome P450s play little role in DDT resistance in Kpome. In contrast, we noticed, a significant increase in mortalities when PBO was combined to permethrin suggesting the direct involvement of P450s in pyrethroid resistance. A high frequency of the L119F-GSTe2 DDT resistance marker was observed in the wild DDT resistant population (9 %RS and 91 %RR) whereas the A296S mutation was detected at a low frequency (1 %RS and 99 %SS).

Conclusion

The presence of multiple resistance in An. funestus populations in the inland locality of Kpome is established in this study as recently documented in the costal locality of Pahou. Data from both localities suggest that resistance could be widespread in Benin and this highlights the need for further studies to assess the geographical distribution of insecticide resistance across Benin and neighboring countries as well as a more comprehensive analysis of the resistance mechanisms involved.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1723-y) contains supplementary material, which is available to authorized users.

First evaluation of bendiocarb in experimental huts using different substrates in Madagascar

BACKGROUND

Indoor residual spraying with insecticide is recommended for malaria control in high-transmission settings. Determination of residual activity of insecticides is essential for the selection of appropriate indoor spraying policy. The present study was undertaken to evaluate the residual effect of bendiocarb, a carbamate insecticide used in Madagascar, on different indoor surfaces in order to elaborate future vector control interventions.

METHODS

The residual activity of bendiocarb was evaluated in both experimental huts and houses. Tests in experimental huts on different substrates represented a small scale-field trials. The houses IRS performed in parallel of experimental huts IRS, was done to compare semi-field results and field results. Bioassays according to the World Health Organization (WHO) standard protocol were carried out on different substrates impregnated with bendiocarb using susceptible strains of Anopheles arabiensis and Aedes albopictus.

RESULTS

Bendiocarb induced significantly high mortality in treated huts against exposed mosquito (p < 0.005) compared to untreated huts. The mortality is up to the WHO threshold of 80 % during 5 months post-treatment. Using a multivariate analysis, Ae. albopictus mortality decreased significantly from the 3rd month post-treatment. However, An. arabiensis mortality decreased significantly from the 4th month after treatment. Comparing mosquito mortality results from the mud experimental huts and the mud houses showed no significant difference regarding the persistence of bendiocarb on wall.

CONCLUSIONS

Current data suggest variable persistence of bendiocarb according to the type of wall surfaces, highlighting the importance of testing insecticide for IRS in local context before using them in large scale. Data from this study validate also the importance of using experimental huts as representative tool to evaluate the effectiveness of an insecticide.

Impact of Insecticide Resistance on the Effectiveness of Pyrethroid-Based Malaria Vectors Control Tools in Benin: Decreased Toxicity and Repellent Effect

Since the first evidence of pyrethroids resistance in 1999 in Benin, mutations have rapidly increased in mosquitoes and it is now difficult to design a study including a control area where malaria vectors are fully susceptible. Few studies have assessed the after effect of resistance on the success of pyrethroid based prevention methods in mosquito populations. We therefore assessed the impact of resistance on the effectiveness of pyrethroids based indoor residual spraying (IRS) in semi-field conditions and long lasting insecticidal nets (LLINs) in laboratory conditions. The results observed showed low repulsion and low toxicity of pyrethroids compounds in the test populations. The toxicity of pyrethroids used in IRS was significantly low with An. gambiae s.l (< 46%) but high for other predominant species such as Mansonia africana (93% to 97%). There were significant differences in terms of the repellent effect expressed as exophily and deterrence compared to the untreated huts (P<0.001). Furthermore, mortality was 23.71% for OlyseNet® and 39.06% for PermaNet®. However, with laboratory susceptible "Kisumu", mortality was 100% for both nets suggesting a resistance within the wild mosquito populations. Thus treatment with pyrethroids at World Health Organization recommended dose will not be effective at reducing malaria in the coming years. Therefore it is necessary to study how insecticide resistance decreases the efficacy of particular pyrethroids used in pyrethroid-based vector control so that a targeted approach can be adopted.

Six years of experience in entomological surveillance of indoor residual spraying against malaria transmission in Benin: lessons learned, challenges and outlooks

Background

From 2008 to 2013, a prevention intervention against malaria based on indoor residual spraying (IRS) was implemented in Benin. From 2008 to 2012, Ficam M®, a bendiocarb-containing product was used for house spraying, in association with pirimiphos methyl EC (Actellic EC) in 2013. This operation aimed to strengthen the effectiveness of treated nets so as to expedite the achievement of Millennium Development Goals (MDGs): the reduction of morbidity and mortality due to malaria by 75 % from 2000 to 2015.

Methods

Monitoring and evaluation (M&E) was implemented in order to evaluate the impact of IRS intervention on malaria transmission. Anopheles gambiae s.l. populations were sampled by human landing catch. In addition, window exit traps and pyrethrum spray catches were performed to assess exophagic behaviour of Anopheles vectors the main malaria vector in the treated areas. The residual activity of insecticide in the treated walls was also assessed using WHO bioassay test.

Results

The purpose of this project was to draw attention to new challenges and future prospects for the success of IRS in Benin. The main strength of the intervention was a large-scale operation in which more than 80 % of the houses were treated due to the strong adhesion of population. In addition, a significant reduction of the EIR in areas under IRS were observed. However, there were many challenges including the high cost of IRS implementation and the identification of suitable areas to implement IRS. This was because of the low and short residual effect of the insecticides recommended for IRS and the management strategy for vector resistance to insecticides. This indicated that challenges are accompanied by suggested solutions. For the cost of IRS to be accessible to states, then local organizations need to be created in partnership with the National Malaria Control Programme (NMCP) in order to ensure relevant planning and implementation of IRS.

Conclusion

As an anticipatory measure against vector resistance, this paper proposes various methods, such as periodic IRS based on a combination of two or three insecticides of different classes used in rotation every two or three years.

Contemporary evolution of resistance at the major insecticide target site gene Ace-1 by mutation and copy number variation in the malaria mosquito Anopheles gambiae

Functionally constrained genes are ideal insecticide targets because disruption is often fatal, and resistance mutations are typically costly. Synaptic acetylcholinesterase (AChE) is an essential neurotransmission enzyme targeted by insecticides used increasingly in malaria control. In Anopheles and Culex mosquitoes, a glycine–serine substitution at codon 119 of the Ace-1 gene confers both resistance and fitness costs, especially for 119S/S homozygotes. G119S in Anopheles gambiae from Accra (Ghana) is strongly associated with resistance, and, despite expectations of cost, resistant 119S alleles are increasing significantly in frequency. Sequencing of Accra females detected only a single Ace-1 119S haplotype, whereas 119G diversity was high overall but very low at non-synonymous sites, evidence of strong purifying selection driven by functional constraint. Flanking microsatellites showed reduced diversity, elevated linkage disequilibrium and high differentiation of 119S, relative to 119G homozygotes across up to two megabases of the genome. Yet these signals of selection were inconsistent and sometimes weak tens of kilobases from Ace-1. This unexpected finding is attributable to apparently ubiquitous amplification of 119S alleles as part of a large copy number variant (CNV) far exceeding the size of the Ace-1 gene, whereas 119G alleles were unduplicated. Ace-1 CNV was detectable in archived samples collected when the 119S allele was rare in Ghana. Multicopy amplification of resistant alleles has not been observed previously and is likely to underpin the recent increase in 119S frequency. The large CNV compromised localization of the strong selective sweep around Ace-1, emphasizing the need to integrate CNV analysis into genome scans for selection.

Use of insecticide quantification kits to investigate the quality of spraying and decay rate of bendiocarb on different wall surfaces in Kagera region, Tanzania

Background

Bendiocarb was introduced for the first time for Indoor Residual Spraying (IRS) in Tanzania in 2012 as part of the interim national insecticide resistance management plan. This move followed reports of increasingly alarming levels of pyrethroid resistance across the country. This study used the insecticide quantification kit (IQK) to investigate the intra-operational IRS coverage and quality of spraying, and decay rate of bendiocarb on different wall surfaces in Kagera region.

Methods

To assess intra-operational IRS coverage and quality of spraying, 104 houses were randomly selected out of 161,414 sprayed houses. A total of 509 samples (218 in Muleba and 291 in Karagwe) were obtained by scraping the insecticide samples from wall surfaces. To investigate decay rate, 66 houses (36 in Muleba and 30 in Karagwe) were selected and samples were collected monthly for a period of five months. Laboratory testing of insecticide concentration was done using IQK^TM [Innovative Vector Control Consortium].

Results

Of the 509 samples, 89.5% met the World Health Organization (WHO) recommended concentration (between 100–400 mg/m²) for IRS target dosage. The proportion of samples meeting WHO standards varied between Karagwe (84.3%) and Muleba (96.3%) (p < 0.001). Assessment of quality of spraying at house level revealed that Muleba (84.8%) had a significantly higher proportion of households that met the expected target dosage (100–400 mg/m²) compared to Karagwe (68.9%) (p < 0.001). The quality of spraying varied across different wall substrates in both districts. Evaluation of bendiocarb decay showed that the proportion of houses with recommended concentration declined from 96.9%, 93.5% and 76.2% at months one, two, and three post IRS, respectively (p-trend = 0.03). The rate of decay increased in the fourth and fifth month post spraying with only 55.9% and 26.3% houses meeting the WHO recommendations, respectively.

Conclusion

IQK is an important tool for assessing IRS coverage and quality of spraying. The study found adequate coverage of IRS; however, residual life of bendiocarb was observed to be three months. Results suggest that in order to maintain the recommended concentrations with bendiocarb, a second spray cycle should be carried out after three months.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-0859-5) contains supplementary material, which is available to authorized users.

Trends in the selection of insecticide resistance in Anopheles gambiae s.l. mosquitoes in northwest Tanzania during a community randomized trial of longlasting insecticidal nets and indoor residual spraying

Anopheles gambiae s.l. (Diptera: Culicidae) in Muleba, Tanzania has developed high levels of resistance to most insecticides currently advocated for malaria control. The kdr mutation has almost reached fixation in An. gambiae s.s. in Muleba. This change has the potential to jeopardize malaria control interventions carried out in the region. Trends in insecticide resistance were monitored in two intervention villages using World Health Organization (WHO) susceptibility test kits. Additional mechanisms contributing to observed phenotypic resistance were investigated using Centers for Disease Control (CDC) bottle bioassays with piperonylbutoxide (PBO) and S,S,S-tributyl phosphorotrithioate (DEF) synergists. Resistance genotyping for kdr and Ace-1 alleles was conducted using quantitative polymerase chain reaction (qPCR). In both study villages, high phenotypic resistance to several pyrethroids and DDT was observed, with mortality in the range of 12-23%. There was a sharp decrease in mortality in An. gambiae s.l. exposed to bendiocarb (carbamate) from 84% in November 2011 to 31% in December 2012 after two rounds of bendiocarb-based indoor residual spraying (IRS). Anopheles gambiae s.l. remained susceptible to pirimiphos-methyl (organophosphate). Bendiocarb-based IRS did not lead to the reversion of pyrethroid resistance. There was no evidence for selection for Ace-1 resistance alleles. The need to investigate the operational impact of the observed resistance selection on the effectiveness of longlasting insecticidal nets and IRS for malaria control is urgent.

Indoor residual spraying with microencapsulated DEET repellent (N, N-diethyl-m-toluamide) for control of Anopheles arabiensis and Culex quinquefasciatus

BACKGROUND

Evolution of insecticide resistance in Anopheles gambiae complex necessitates evaluation of alternative chemical classes to complement existing insecticides for long lasting insecticidal nets (LLIN) and indoor residual spraying (IRS). Microencapsulated (MC) DEET (N, N-diethyl-m-toluamide) is a formulation of the popular repellent, which gives long lasting activity when applied to nets. Its suitability for IRS use has not been evaluated before. This study assessed the efficacy of DEET MC, for IRS in experimental huts.

METHODS

DEET MC was tested alongside standard repellent and non-repellent residual insecticides: lambdacyhalothrin, permethrin, pirimiphos methyl and DDT. Residual formulations of these compounds were sprayed on plywood panels attached to walls of experimental huts to assess efficacy against pyrethroid resistant, wild free-flying Anopheles arabiensis and Culex quinquefasciatus. The panel treatments were rotated weekly between huts.

RESULTS

The overall mortalities of An. arabiensis induced by the various treatments (range: 76-86%) were significantly greater than mortality in the untreated control (8%, P < 0.001). Mortality of An. arabiensis in DEET sprayed huts (82%) was higher than in lambdacyhalothrin CS (76%, P = 0.043) but not significantly different to pirimiphos methyl CS (86%, P = 0.204) or DDT huts (81%, P = 0.703). Against Cx. quinquefasciatus DEET MC was less effective, inducing lower mortality (29%) than other treatments. An arabiensis blood feeding rates were higher in the unsprayed control (34%) than in sprayed huts (range between treatments: 19-22%, P < 0.002), and DEET provided equivalent or superior blood feeding inhibition (44%) to other insecticides. Against Cx. quinquefasciatus there was no significant reduction in blood-feeding for any treatment relative to the control. There was a significantly higher exiting of An. arabiensis from huts sprayed with DEET (98%), lambdacyhalothrin (98%) and permethrin (96%) relative to the control (80%, P < 0.01). Exiting rates of Cx. quinquefasciatus did not differ between treatment huts and the control.

CONCLUSION

Microencapsulated DEET acts like an insecticide at ambient temperature and induces mosquito mortality when applied to walls made from wooden panels. This trial demonstrated the potential of microencapsulated DEET to control An. arabiensis and warrants further studies of residual activity on interior substrates.

Good performances but short lasting efficacy of Actellic 50 EC Indoor Residual Spraying (IRS) on malaria transmission in Benin, West Africa

BACKGROUND

The National Malaria Control Program (NMCP) has been using pirimiphos methyl for the first time for indoor residual spraying (IRS) in Benin. The first round was a success with a significant decrease of entomological indicators of malaria transmission in the treated districts. We present the results of the entomological impact on malaria transmission. Entomologic parameters in the control area were compared with those in intervention sites.

METHODS

Mosquito collections were carried out in three districts in the Atacora-Dongo region of which two were treated with pirimiphos methyl (Actellic 50EC) (Tanguiéta and Kouandé) and the untreated (Copargo) served as control. Anopheles gambiae s.l. populations were sampled monthly by human landing catch. In addition, window exit traps and pyrethrum spray catches were performed to assess exophagic behavior of Anopheles vectors. In the three districts, mosquito collections were organized to follow the impact of pirimiphos methyl IRS on malaria transmission and possible changes in the behavior of mosquitoes. The residual activity of pirimiphos methyl in the treated walls was also assessed using WHO bioassay test.

RESULTS

A significant reduction (94.25%) in human biting rate was recorded in treated districts where an inhabitant received less than 1 bite of An. gambiae per night. During this same time, the entomological inoculation rate (EIR) dramatically declined in the treated area (99.24% reduction). We also noted a significant reduction in longevity of the vectors and an increase in exophily induced by pirimiphos methyl on An. gambiae. However, no significant impact was found on the blood feeding rate. Otherwise, the low residual activity of Actellic 50 EC, which is three months, is a disadvantage.

CONCLUSION

Pirimiphos methyl was found to be effective for IRS in Benin. However, because of the low persistence of Actellic 50EC used in this study on the treated walls, the recourse to another more residual formulation of pirimiphos methyl is required.

Laboratory and field evaluation of the impact of washings on the effectiveness of LifeNet®, Olyset® and PermaNet® 2.0 in two areas, where there is a high level of resistance of Anopheles gambiae to pyrethroids, Benin, West Africa

BACKGROUND

An investigation carried out in Benin has shown that, in some areas close to rivers where density of mosquitoes is high, long-lasting, insecticidal bed nets (LLINs) are permanently used. In such areas, LLINs are washed every month. Based on this situation, the 20-wash minimum efficacy advised by the manufacturers would be inadequate. The main goal of this study was to evaluate the effectiveness of LifeNet®, Olyset® and Permanet® 2.0 washed several times against Anopheles gambiae sensu stricto (s.s.) populations, which have developed high resistance to pyrethroids.

METHODS

Efficacy of LifeNet®, Olyset® and PermaNet® 2.0 washed 30 and 40 times was expressed in terms of blood-feeding inhibition rate, deterrence, induced exophily and mortality rates. This WHOPES phase II evaluation, conducted in experimental huts in Akron (southern Benin) and in Malanville (northern Benin), was accompanied by WHOPES Phase I evaluation.

RESULTS

Over 40 successive washes, LifeNet® induced a mortality rate over 80% in phase I. However, beyond 10 washes, Permanet® 2.0 and Olyset induced dramatically reduced mortality rates, respectively 12.5 and 2.5%. With regard to Phase II results, unwashed LifeNet®, LifeNet® and Olyset® washed 30 and 40 times induced a similar exophily rate per study site (at least 58% in Malanville and at least 71% in Akron). Regarding blood feeding inhibition, LifeNet® and Olyset® washed 30 and 40 times significantly reduced wild An. gambiae s.s. blood feeding showing a similar personal protection as unwashed LifeNet®. LifeNet® washed 30 and 40 times induced mortality rates significantly higher than those induced by Olyset® and Permanet® 2.0 (P < 0,05).

CONCLUSION

LifeNet®, followed by Olyset®, have shown good efficacy against host-seeking resistant An. gambiae s.s. population in experimental huts in Benin. Lifenet® have shown to be an effective and promising vector control tool to prevent malaria in areas where repeated washings is a common practice in the community.

CYP6 P450 enzymes and ACE-1 duplication produce extreme and multiple insecticide resistance in the malaria mosquito Anopheles gambiae

Malaria control relies heavily on pyrethroid insecticides, to which susceptibility is declining in Anopheles mosquitoes. To combat pyrethroid resistance, application of alternative insecticides is advocated for indoor residual spraying (IRS), and carbamates are increasingly important. Emergence of a very strong carbamate resistance phenotype in Anopheles gambiae from Tiassalé, Côte d'Ivoire, West Africa, is therefore a potentially major operational challenge, particularly because these malaria vectors now exhibit resistance to multiple insecticide classes. We investigated the genetic basis of resistance to the most commonly-applied carbamate, bendiocarb, in An. gambiae from Tiassalé. Geographically-replicated whole genome microarray experiments identified elevated P450 enzyme expression as associated with bendiocarb resistance, most notably genes from the CYP6 subfamily. P450s were further implicated in resistance phenotypes by induction of significantly elevated mortality to bendiocarb by the synergist piperonyl butoxide (PBO), which also enhanced the action of pyrethroids and an organophosphate. CYP6P3 and especially CYP6M2 produced bendiocarb resistance via transgenic expression in Drosophila in addition to pyrethroid resistance for both genes, and DDT resistance for CYP6M2 expression. CYP6M2 can thus cause resistance to three distinct classes of insecticide although the biochemical mechanism for carbamates is unclear because, in contrast to CYP6P3, recombinant CYP6M2 did not metabolise bendiocarb in vitro. Strongly bendiocarb resistant mosquitoes also displayed elevated expression of the acetylcholinesterase ACE-1 gene, arising at least in part from gene duplication, which confers a survival advantage to carriers of additional copies of resistant ACE-1 G119S alleles. Our results are alarming for vector-based malaria control. Extreme carbamate resistance in Tiassalé An. gambiae results from coupling of over-expressed target site allelic variants with heightened CYP6 P450 expression, which also provides resistance across contrasting insecticides. Mosquito populations displaying such a diverse basis of extreme and cross-resistance are likely to be unresponsive to standard insecticide resistance management practices.

Malaria control depends heavily on only four classes of insecticide to which Anopheles mosquitoes are increasingly resistant. It is important to manage insecticide application carefully to minimise increases in resistance, for example by using different compounds in combination or rotation. Recently, mosquitoes resistant to all available insecticides have been found in Tiassalé, West Africa, which could be problematic for resistance management, particularly if common genetic mechanisms are responsible (‘cross-resistance’). Tiassalé mosquitoes also exhibit extreme levels of resistance to the two most important classes, pyrethroids and carbamates. We investigated the genetic basis of extreme carbamate resistance and cross-resistance in Tiassalé, and the applicability of results in an additional population from Togo. We find that specific P450 enzymes are involved in both extreme and cross-resistance, including one, CYP6M2, which can cause resistance to three insecticide classes. However, amplification of a mutated version of the gene which codes for acetycholinesterase, the target site of both the carbamate and organophosphate insecticides, also plays an important role. Mechanisms involved in both extreme resistance and cross resistance are likely to be very resilient to insecticide management practices, and represent an alarming scenario for mosquito-targeted malaria control.

Insecticide resistance mechanisms associated with different environments in the malaria vector Anopheles gambiae: a case study in Tanzania

BACKGROUND

Resistance of mosquitoes to insecticides is a growing concern in Africa. Since only a few insecticides are used for public health and limited development of new molecules is expected in the next decade, maintaining the efficacy of control programmes mostly relies on resistance management strategies. Developing such strategies requires a deep understanding of factors influencing resistance together with characterizing the mechanisms involved. Among factors likely to influence insecticide resistance in mosquitoes, agriculture and urbanization have been implicated but rarely studied in detail. The present study aimed at comparing insecticide resistance levels and associated mechanisms across multiple Anopheles gambiae sensu lato populations from different environments.

METHODS

Nine populations were sampled in three areas of Tanzania showing contrasting agriculture activity, urbanization and usage of insecticides for vector control. Insecticide resistance levels were measured in larvae and adults through bioassays with deltamethrin, DDT and bendiocarb. The distribution of An. gambiae sub-species and pyrethroid target-site mutations (kdr) were investigated using molecular assays. A microarray approach was used for identifying transcription level variations associated to different environments and insecticide resistance.

RESULTS

Elevated resistance levels to deltamethrin and DDT were identified in agriculture and urban areas as compared to the susceptible strain Kisumu. A significant correlation was found between adult deltamethrin resistance and agriculture activity. The subspecies Anopheles arabiensis was predominant with only few An. gambiae sensu stricto identified in the urban area of Dar es Salaam. The L1014S kdr mutation was detected at elevated frequency in An gambiae s.s. in the urban area but remains sporadic in An. arabiensis specimens. Microarrays identified 416 transcripts differentially expressed in any area versus the susceptible reference strain and supported the impact of agriculture on resistance mechanisms with multiple genes encoding pesticide targets, detoxification enzymes and proteins linked to neurotransmitter activity affected. In contrast, resistance mechanisms found in the urban area appeared more specific and more related to the use of insecticides for vector control.

CONCLUSIONS

Overall, this study confirmed the role of the environment in shaping insecticide resistance in mosquitoes with a major impact of agriculture activities. Results are discussed in relation to resistance mechanisms and the optimization of resistance management strategies.

Impacts of agricultural practices on insecticide resistance in the malaria vector Anopheles arabiensis in Khartoum State, Sudan

Background

Agricultural pesticides may play a profound role in selection of resistance in field populations of mosquito vectors. The objective of this study is to investigate possible links between agricultural pesticide use and development of resistance to insecticides by the major malaria vector Anopheles arabiensis in northern Sudan.

Methodology/Principal Findings

Entomological surveys were conducted during two agricultural seasons in six urban and peri-urban sites in Khartoum state. Agro-sociological data were collected from 240 farmers subjected to semi-structured questionnaires based on knowledge attitude and practice (KAP) surveys. Susceptibility status of An. arabiensis (n=6000) was assessed in all sites and during each season using WHO bioassay tests to DDT, deltamethrin, permethrin, Malathion and bendiocarb. KAP analysis revealed that pesticide application was common practice among both urban and peri-urban farmers, with organophosphates and carbamates most commonly used. Selection for resistance is likely to be greater in peri-urban sites where farmers apply pesticide more frequently and are less likely to dispose of surpluses correctly. Though variable among insecticides and seasons, broad-spectrum mortality was slightly, but significantly higher in urban than peri-urban sites and most marked for bendiocarb, to which susceptibility was lowest. Anopheles arabiensis from all sites showed evidence of resistance or suspected resistance, especially pyrethroids. However, low-moderate frequencies of the L1014F kdr allele in all sites, which was very strongly associated with DDT, permethrin and deltamethrin survivorship (OR=6.14-14.67) suggests that resistance could increase rapidly.

Conclusions

Ubiquitous multiple-resistance coupled with presence of a clear mechanism for DDT and pyrethroids (kdr L1014F) in populations of An. arabiensis from Khartoum-Sudan suggests careful insecticide management is essential to prolong efficacy. Our findings are consistent with agricultural insecticide use as a source of selection for resistance and argue for coordination between the integrated vector control program and the Ministry of Agriculture to permit successful implementation of rational resistance management strategies.

The residual life of bendiocarb on different substrates under laboratory and field conditions in Benin, Western Africa

BACKGROUND

The efficacy of bendiocarb against pyrethroid resistant An. gambiae and the residual life of this insecticide on different substrates were evaluated under laboratory and field conditions.

METHODS

Bioassays according to the WHO (World Health Organization) standard protocol were carried out on different substrates impregnated with bendiocarb. Data were analyzed using a binomial regression model with R software.

RESULTS

A good efficacy of the bendiocarb against pyrethroid resistant An. gambiae and a high variation of its residual life according to the surfaces treated was observed. The probability that a female mosquito died after exposure to a treated substrate was below 80% after 13 weeks for the teak wood; 7 weeks for the wall made with a mixture of sand and cement and 6 weeks for walls made with red clay and those made with a mixture of the red clay and cement.

CONCLUSIONS

Considering the residual life of bendiocarb on walls made with red clay, the main substrates treated during IRS campaigns in rural areas in Benin, more than 2 treatments rounds per year would be necessary to achieve a long term efficacy of IRS using bendiocarb in these areas. Financial and logistical resources required to achieve such levels of coverage need more political will from leaders of African endemic countries. While waiting for innovative malaria control tool, alternative insecticides or combinations of insecticides have to be used for insecticide resistance management in Benin.

Acetylcholinesterase (Ace-1) target site mutation 119S is strongly diagnostic of carbamate and organophosphate resistance in Anopheles gambiae s.s. and Anopheles coluzzii across southern Ghana

BACKGROUND

With high DDT resistance present throughout much of West Africa, carbamates and organophosphates are increasingly important alternatives to pyrethroids for indoor residual spraying (IRS). Though less widespread, resistance to both of these alternative insecticide classes has also been documented within the Anopheles gambiae species pair (formerly the M and S molecular forms) in West Africa. To manage insecticide efficacy, it is important to predict how and where resistance is likely to occur and spread, which could be aided by using molecular diagnostics with high predictive value.

METHODS

Anopheles coluzzii and An. gambiae s.s. were collected from 18 sites throughout southern Ghana and bioassayed with bendiocarb, the most commonly applied carbamate, and an organophosphate, fenitrothion. The Ace-1 target site substitution G119S was genotyped by qPCR.

RESULTS

Fenitrothion induced higher mortality than bendiocarb, though phenotypes correlated strongly across populations. Ace-1 119S was found at much higher frequency in An. gambiae s.s than An. coluzzii, exceeding 90% in a population from Greater Accra, the highest frequency reported to date. Ace-1 G119S was very strongly associated with resistance to both insecticides, providing high predictive power for diagnosis, though with some evidence for a differential effect between molecular forms for bendiocarb. Sequencing of the gene revealed a lack of variation in resistant alleles precluding determination of origin, but Ace-1 copy number variation was detected for the first time in Ghana.

CONCLUSIONS

The results validate G119S as a useful diagnostic of organophosphate and carbamate resistance within and among populations, whilst highlighting the potential for an aggregate nature of Ace-1 genotypes, which may comprise both single-copy and duplicated genes. Further work is now required to determine the distribution and resistance-association of Ace-1 duplication.

A new long-lasting indoor residual formulation of the organophosphate insecticide pirimiphos methyl for prolonged control of pyrethroid-resistant mosquitoes: an experimental hut trial in Benin

Background

Indoor residual spraying (IRS) is widely used for malaria transmission control in sub-Saharan Africa. Resistance to pyrethroids in the mosquito Anopheles gambiae is a growing problem. There is an urgent need to develop long-lasting alternative insecticides to reduce selection pressure for pyrethroid resistance and to provide control with a single IRS application in countries with long transmission seasons.

Methods

Two capsule suspension formulations (CS) of the organophosphate pirimiphos methyl were evaluated as IRS treatments in experimental huts in an area of Benin where the mosquitoes Anopheles gambiae and Culex quinquefasciatus are resistant to pyrethroids but susceptible to organophosphates. The CS formulations were tested alongside an emulsifiable concentrate (EC) formulation of pirimiphos methyl and a CS formulation of the pyrethroid lambdacyhalothrin.

Results

The two CS formulations of pirimiphos methyl gave prolonged control of An. gambiae and Cx. quinquefasciatus. In cement huts application rates of 0.5 g/m² induced high mortality of An. gambiae for almost a year: overall mortality rates 87% (95% CI 82–91%) and 92% (95% CI 88–94%). In mud huts application rates of 1 g/m² induced high mortality of An. gambiae for 10 months: overall mortality rates 75% (95% CI 69–81%) and 76% (95% CI 68–83%). The EC formulation of pirimiphos methyl failed to control An. gambiae two months after spraying. The pyrethroid lambdacyhalothrin demonstrated prolonged residual activity in bioassay tests but failed to control pyrethroid resistant An. gambiae that entered the huts. Pirimiphos methyl CS was highly active against Culex quinquefasciatus and gave control for 10 months in cement huts and 6 months in mud huts.

Conclusion

Pirimiphos methyl CS (Actellic 300 CS) applied at 1 g/m² shows great promise for providing prolonged control of pyrethroid-resistant An gambiae and for delaying pyrethroid resistance. An alternative to DDT, giving year-round transmission control in sub-Saharan Africa is now a realistic prospect.

Bendiocarb resistance in Anopheles gambiae s.l. populations from Atacora department in Benin, West Africa: a threat for malaria vector control

BACKGROUND

Owing to pyrethroid resistance in An. gambiae, the carbamate and organophosphate insecticides are currently regarded as alternatives or supplements to pyrethroids for use on mosquito net treatments. Resistance monitoring is therefore essential to investigate the susceptibility of An. gambiae s.l to these alternative products.

METHODS

Two to three day old adult female Anopheles mosquitoes were reared from larvae collected in the five districts (Kouandé, Natitingou, Matéri, Péhunco, Tanguiéta) of the Atacora department. Mosquitoes were then exposed to WHO impregnated papers. The four treatments consisted of: carbamates (0.1% bendiocarb, 0.1% propoxur) and organophosphates (0.25% pirimiphosmethyl, 1% fenitrothion). PCR assays were run to determine the members of the An. gambiae complex, the molecular forms (M) and (S), as well as phenotypes for insensitive acetylcholinesterase (AChE1) due to ace-1(R) mutation.

RESULTS

Bioassays showed bendiocarb resistance in all populations of An. gambiae s.s. tested. Propoxur resistance was observed in Matéri, Péhunco and Tanguiéta, while it was suspected in Kouandé and Natitingou. As for the organophosphates, susceptibility to pirimiphos-methyl was assessed in all populations. Fenitrothion resistance was detected in Kouandé, Péhunco and Tanguiéta, while it was suspected in Matéri and Natitingou. The S-form was predominant in tested samples (94.44%). M and S molecular forms were sympatric but no M/S hybrids were detected. The ace-1(R) mutation was found in both S and M molecular forms with frequency from 3.6 to 12%. Although the homozygous resistant genotype was the most prevalent genotype among survivors, the genotypes could not entirely explain the bioassay results.

CONCLUSION

Evidence of bendiocarb resistance in An. gambiae populations is a clear indication that calls for the implementation of insecticide resistance management strategies. The ace-1(R) mutation could not entirely explain the resistance to bendiocarb observed and is highly suggestive of involvement of other resistance mechanisms such as metabolic detoxification.

A shift from indoor residual spraying (IRS) with bendiocarb to long-lasting insecticidal (mosquito) nets (LLINs) associated with changes in malaria transmission indicators in pyrethroid resistance areas in Benin

Background

Indoor residual spraying (IRS) was implemented in the department of Ouémé-Plateau, southern Benin, in 2008 and withdrawn in 2011, when long lasting insecticidal nets (LLINs) were distributed to the communities that were previously targeted by IRS. Did the LLIN strategy provide a better level of protection against malaria transmission than IRS?

Methods

Entomological surveillance was carried out to assess indicators of transmission risk during the last year of IRS and the first year after the LLIN intervention was put in place (2010–2011). Mosquito biting density was sampled by human landing collection (HLC). Females of Anopheles gambiae s.l. were dissected to estimate the parity rates and the blood meal index. A subsample of the An. gambiae s.l. collection was tested for presence of Plasmodium falciparum sporozoites. In addition, window exit traps and pyrethrum spray catches were performed to assess exophagic behavior of Anopheles vectors.

Results

There were significant increases in all the indicators following withdrawal of IRS. Vector biting density (p<0.001) and longevity (OR=3.81[3.01-4.82] 95% CI; p<0.001) of the An. gambiae s.l. increased significantly; so too did the blood meal index (OR=1.48 [1.1-1.99] 95% CI; p<0.001). Entomological inoculation rate, after IRS withdrawal at one surveillance site, Adjohoun, rose two fold (9.0 infected bites/person/9 months (Apr-Dec 2011) versus 3.66 infective bites/person during the 9 months preceding IRS (Apr-Dec 2010). A second site, Missérété, experienced a six-fold increase after IRS cessation (15.1 infective bites/person/9 months versus 2.41 during IRS). Exophily after IRS cessation decreased significantly in all areas (p<0.001) suggesting that mosquitoes were more likely to rest in houses with LLINs, than in houses subjected to IRS.

Conclusion

LLINs did not impact on indicators of transmission to the same levels as did IRS after IRS withdrawal.

Decreased proportions of indoor feeding and endophily in Anopheles gambiae s.l. populations following the indoor residual spraying and insecticide-treated net interventions in Benin (West Africa)

Background

In many parts of Africa as in Benin, the main strategies of vector control are based on the scaling-up of Long Lasting Insecticide Treated Nets (LLITNs) and indoor residual spraying (IRS). The need to understand the biological implications of IRS in large scale and full coverage of LLITNs is paramount. It is in this context that the present study was conducted. It aims to evaluate the effect of a large scale IRS using a non-pyrethroid insecticide and full coverage of deltamethrin treated nets on the behavior of An. gambiae s.l. in the intervention areas compared to untreated areas used as controls.

Methods

Mosquitoes were collected using human landing catches, pyrethrum spray catches and window exit traps to assess reduction of entry rate, endophily rate, endophagy rate and overall mortality rate in natural populations of An. gambiae s.l. before IRS and LLITNs intervention (2007) and after in 2008 and 2010.

Results

In the IRS arm, endophily rate was 67.13% before intervention and 4.5% after intervention, whereas in the control arm it was stable at 51.67% (P > 0 .05). In the LLITN arm endophily rates also decreased after intervention. After the IRS, no gravid mosquitoes were collected from all treated localities, but LLITN performance was not that spectacular. The proportion of mosquitoes biting indoors in the IRS arm decreased from 67.09% before intervention to 42.85% after intervention, compared to a low but significant decrease (71.31% to 57. 46%) in the LLITN arm.

The use of vector control tools and behavior of the host would be the main factors that modify the behavior of taking a human blood meal observed on An. gambiae s.l. inside human dwellings.

Conclusion

The impact on the behavior of An. gambiae s.l. observed with the bendiocarb used in IRS was highly effective compared with the free distribution of LLITNs in terms of mortality and the decrease of proportions of indoor feeding. Despite this efficacy, there is a need for complementary tools and research of alternative strategy oriented on effective health education, and the use of powerful tools such as IRS, LLITNs, larviciding and repellents.

Select small core structure carbamates exhibit high contact toxicity to "carbamate-resistant" strain malaria mosquitoes, Anopheles gambiae (Akron)

Acetylcholinesterase (AChE) is a proven target for control of the malaria mosquito (Anopheles gambiae). Unfortunately, a single amino acid mutation (G119S) in An. gambiae AChE-1 (AgAChE) confers resistance to the AChE inhibitors currently approved by the World Health Organization for indoor residual spraying. In this report, we describe several carbamate inhibitors that potently inhibit G119S AgAChE and that are contact-toxic to carbamate-resistant An. gambiae. PCR-RFLP analysis was used to confirm that carbamate-susceptible G3 and carbamate-resistant Akron strains of An. gambiae carry wild-type (WT) and G119S AChE, respectively. G119S AgAChE was expressed and purified for the first time, and was shown to have only 3% of the turnover number (k(cat)) of the WT enzyme. Twelve carbamates were then assayed for inhibition of these enzymes. High resistance ratios (>2,500-fold) were observed for carbamates bearing a benzene ring core, consistent with the carbamate-resistant phenotype of the G119S enzyme. Interestingly, resistance ratios for two oxime methylcarbamates, and for five pyrazol-4-yl methylcarbamates were found to be much lower (4- to 65-fold). The toxicities of these carbamates to live G3 and Akron strain An. gambiae were determined. As expected from the enzyme resistance ratios, carbamates bearing a benzene ring core showed low toxicity to Akron strain An. gambiae (LC(50)>5,000 μg/mL). However, one oxime methylcarbamate (aldicarb) and five pyrazol-4-yl methylcarbamates (4a-e) showed good to excellent toxicity to the Akron strain (LC(50) = 32-650 μg/mL). These results suggest that appropriately functionalized "small-core" carbamates could function as a resistance-breaking anticholinesterase insecticides against the malaria mosquito.

Evaluation of the efficacy of bendiocarb in indoor residual spraying against pyrethroid resistant malaria vectors in Benin: results of the third campaign

Background

Since 2008, the National Malaria Control Program (NMCP) has been engaged in the implementation of indoor residual spraying (IRS) in Benin. The first and second round was a success with a drastic decrease of malaria transmission in areas under IRS. We present here the results of the third round. The purpose of this study was to compare the results of the third round of IRS to those achieved during the first two rounds. A second success of IRS will enable the Government of Benin to extend the strategy to other areas.

Methods

Mosquito collections were carried out in the department of Ouémé where the homes of four districts were treated with bendiocarb. In these districts, more than 350 000 inhabitants were protected by IRS. A fifth untreated district served as control. In the five districts, mosquito collections were organized to follow the dynamics of malaria transmission and possible changes in the behavior of mosquitoes.

Results

A significant reduction in human biting rate was recorded after the third round of IRS, specifically in Adjohoun (89.78%), Dangbo (56.8%) and Missérété (93.22%) where an inhabitant received less than 2 bites of An. gambiae per night. During this same time, the entomological inoculation rate (EIR) declined dramatically in all areas under intervention (74.26% reduction). We also noted a significant reduction in longevity, the blood feeding rate of the vectors and an increase in exophily induced by bendiocarb on An. gambiae and Mansonia spp.

Conclusion

The present study showed, once again, the effectiveness of bendiocarb on anopheles populations resistant to pyrethroids. This product can be recommended in combination with other insecticides for the management of vector resistance to insecticides.

Impact of three years of large scale Indoor Residual Spraying (IRS) and Insecticide Treated Nets (ITNs) interventions on insecticide resistance in Anopheles gambiae s.l. in Benin

Background

In Benin, Indoor Residual Spraying (IRS) and long-lasting insecticidal nets (LLINs) are the cornerstones of malaria prevention. In the context of high resistance of Anopheles gambiae to pyrethroids, The National Malaria Control Program (NMCP) has undertaken a full coverage of IRS in a no-flood zone in the Oueme region, coupled with the distribution of LLINs in a flood zone. We assessed the impact of this campaign on phenotypic resistance, kdr (knock-down resistance) and ace-1^R (insensitive acetylcholinesterase) mutations.

Methods

Insecticides used for malaria vector control interventions were bendiocarb WP (0.4 g/m²) and deltamethrin (55 mg/m²), respectively for IRS and LLINs. Susceptibility status of An. gambiae was assessed using World Health Organization bioassay tests to DDT, permethrin, deltamethrin and bendiocarb in the Oueme region before intervention (2007) and after interventions in 2008 and 2010. An. gambiae specimens were screened for identification of species, molecular M and S forms and for the detection of the West African kdr (L1014F) as well as ace-1^R mutations using PCR techniques.

Results

The univariate logistic regression performed showed that kdr frequency has increased significantly during the three years in the intervention area and in the control area. Several factors (LLINs, IRS, mosquito coils, aerosols, use of pesticides for crop protection) could explain the selection of individual resistant An. gambiae. The Kdr resistance gene could not be the only mechanism of resistance observed in the Oueme region. The high susceptibility to bendiocarb is in agreement with a previous study conducted in Benin. However, the occurrence of ace-1^R heterozygous individuals even on sites far from IRS areas, suggests other factors may contribute to the selection of resistance other than those exerted by the vector control program.

Conclusion

The results of this study have confirmed that An.gambiae have maintained and developed the resistance to pyrethroids, but are still susceptible to bendiocarb. Our data clearly shows that selection of resistant individuals was caused by other insecticides than those used by the IRS and LLINs.

A modified experimental hut design for studying responses of disease-transmitting mosquitoes to indoor interventions: the Ifakara experimental huts

Differences between individual human houses can confound results of studies aimed at evaluating indoor vector control interventions such as insecticide treated nets (ITNs) and indoor residual insecticide spraying (IRS). Specially designed and standardised experimental huts have historically provided a solution to this challenge, with an added advantage that they can be fitted with special interception traps to sample entering or exiting mosquitoes. However, many of these experimental hut designs have a number of limitations, for example: 1) inability to sample mosquitoes on all sides of huts, 2) increased likelihood of live mosquitoes flying out of the huts, leaving mainly dead ones, 3) difficulties of cleaning the huts when a new insecticide is to be tested, and 4) the generally small size of the experimental huts, which can misrepresent actual local house sizes or airflow dynamics in the local houses. Here, we describe a modified experimental hut design - The Ifakara Experimental Huts- and explain how these huts can be used to more realistically monitor behavioural and physiological responses of wild, free-flying disease-transmitting mosquitoes, including the African malaria vectors of the species complexes Anopheles gambiae and Anopheles funestus, to indoor vector control-technologies including ITNs and IRS. Important characteristics of the Ifakara experimental huts include: 1) interception traps fitted onto eave spaces and windows, 2) use of eave baffles (panels that direct mosquito movement) to control exit of live mosquitoes through the eave spaces, 3) use of replaceable wall panels and ceilings, which allow safe insecticide disposal and reuse of the huts to test different insecticides in successive periods, 4) the kit format of the huts allowing portability and 5) an improved suite of entomological procedures to maximise data quality.

Aging partially restores the efficacy of malaria vector control in insecticide-resistant populations of Anopheles gambiae s.l. from Burkina Faso

Background

The operational impact of insecticide resistance on the effectiveness of long-lasting insecticide nets (LLINs) and indoor residual spraying (IRS) is poorly understood. One factor which may prolong the effectiveness of these tools in the field is the increase in insecticide susceptibility with mosquito age. In this study, LLINs and IRS were tested against young (three to five days) and old (17-19 days) pyrethroid resistant Anopheles gambiae s.l. from Burkina Faso.

Methods

Blood-fed adult Anopheles gambiae s.l. were collected from south-west Burkina Faso and identified to species/form level. Cohorts of the F1 progeny of An. gambiae s.s. S-forms were exposed to deltamethrin (0.05%) at three to five or 17-19 days post-emergence and tested for the frequency of the resistance allele 1014F. Isofemale lines of the M, S- form of An. gambiae s.s. and Anopheles arabiensis were exposed in WHO cone tests to either a) LLINs deployed in households for two years or (b) bendiocarb sprayed walls.

Results

Mortality rates in response to deltamethrin (0.05%) increased from levels indicative of strong resistance in three to five day old F1 mosquitoes, to near full susceptibility in the 17-19 day old cohort. On exposure to LLINs sampled from the field, the mortality rate in isofemale lines was higher in older mosquitoes than young (OR = 5.28, CI 95% = 2.81-9.92), although the mortality estimates were affected by the LLIN tested. In general, the LLINs sampled from the field performed poorly in WHO cone bioassays using either laboratory susceptible or field caught mosquito populations. Finally, there was a clear relationship between mortality and age on exposure to bendiocarb-sprayed walls, with older mosquitoes again proving more susceptible (OR = 3.39, CI 95% = 2.35-4.90).

Conclusions

Age is a key factor determining the susceptibility of mosquitoes to insecticides, not only in laboratory studies, but in response to field-based vector control interventions. This has important implications for understanding the epidemiological impact of resistance. If mosquitoes old enough to transmit malaria are still being suppressed with available insecticides, is resistance potentially having less of an impact than often assumed? However, the poor performance of LLINs used in this study in Burkina Faso, is a cause for concern and requires urgent investigation.

Dramatic decrease in malaria transmission after large-scale indoor residual spraying with bendiocarb in Benin, an area of high resistance of Anopheles gambiae to pyrethroids

In 2008, the National Malaria Control Program in Benin implemented a vector control intervention based on indoor residual spraying (IRS). Four districts of high resistance of Anopheles gambiae to pyrethroids were sprayed with bendiocarb. More than 350,000 inhabitants have been protected. Entomologic parameters in the control area were compared with those in intervention sites. The study has shown a drastic decrease in the An. gambiae biting rate in the sprayed areas. Results of an enzyme-linked immunosorbent assay were negative for Plasmodium falciparum antigen during the entire period of the intervention. No household members received infected bites (entomologic inoculation rate = 0 during January-July). Parous rates were low in areas covered by IRS because bendiocarb is not conducive to long-term mosquito survival. Bendiocarb was found to be a good alternative insecticide for IRS in Benin, in areas where An. gambiae has developed high resistance to pyrethroids.

Variations of insecticide residual bio-efficacy on different types of walls: results from a community-based trial in south Cameroon

BACKGROUND

Determination of residual activity of insecticides is essential information for the selection of appropriate indoor spraying operation. The present study was undertaken to evaluate the residual effect of three candidate insecticide formulations on different indoor surfaces in order to guide future interventions, in the context of Cameroon and other African countries.

METHODS

The study was conducted in the Ntougou neighbourhood in Yaoundé (capital city of Cameroon). Bendiocarb WP, lambda-cyhalothrin CS and deltamethrin WG were sprayed on the indoor wall surfaces of local cement, wood and mud houses. Their effects on the knockdown and mortality of the Kisumu susceptible strain of Anopheles gambiae s.s were assessed each month from March to September 2009, using the WHO plastic cones test. Knockdown and mortality rates were compared between different surfaces using Chi-square test. A Kaplan-Meir model was used to estimate the time of treatment failure.

RESULTS

With bendiocarb WP, the knockdown rates were frequently above 98% during 13 weeks after spraying, except on mud walls where it significantly decreased at the 13th week (P < 0.05). With lambda cyhalothrin CS, the knockdown rates remained 100% on wood surfaces during the 26 weeks trial. However, it significantly decreased on concrete and mud surfaces from the 11th (83%) and the 20th (88%) weeks respectively (P < 0.05). With deltamethrin WG, it remained high on concrete surfaces during 26 weeks (> 98%); while it varied between 60 and 100% on wood or mud surfaces. The survival estimates of bendiocarb WP treatments remaining effective in killing An. gambiae s.s. (mortality rate ≥ 80%) was > 13 weeks on cement and wood surfaces and 13 weeks on mud surfaces. Those of lambda-cyhalothrin CS were > 26 weeks on wood surfaces, and 20 weeks on concrete and mud surfaces. By contrast, those of deltamethrin WG were 26 weeks on concrete, 20 weeks on mud surfaces and 15 weeks on wood surfaces.

CONCLUSION

Current data suggest variable durations of spray cycles for each product, according to the type of wall surfaces, highlighting the importance of testing candidate products in local context before using them in large scale.