Insecticide Resistance Monitoring Research Articles

Early biting and insecticide resistance in the malaria vector Anopheles might compromise the effectiveness of vector control intervention in Southwestern Uganda.

BackgroundSouthwestern Uganda has high malaria heterogeneity despite moderate vector control and other interventions. Moreover, the early biting transmission and increased resistance to insecticides might compromise strategies relying on vector control. Consequently, monitoring of vector behaviour and insecticide efficacy is needed to assess the effectiveness of strategies aiming at malaria control. This eventually led to an entomological survey in two villages with high malaria prevalence in this region.MethodsDuring rainy, 2011 and dry season 2012, mosquitoes were collected in Engari and Kigorogoro, Kazo subcounty, using human landing collection, morning indoor resting collection, pyrethrum spray collection and larval collection. Circumsporozoite protein of Plasmodium falciparum sporozoites in female Anopheles mosquitoes was detected using ELISA assay. Bioassays to monitor Anopheles resistance to insecticides were performed.ResultsOf the 1,021 female Anopheles species captured, 62% (632) were Anopheles funestus and 36% (371) were Anopheles gambiae s.l. The most common species were Anopheles gambiae s.l. in Engari (75%) and A. funestus in Kigorogoro (83%). Overall, P. falciparum prevalence was 2.9% by ELISA. The daily entomological inoculation rates were estimated at 0.17 and 0.58 infected bites/person/night during rainy and dry season respectively in Engari, and 0.81 infected bites/person/night in Kigorogoro during dry season. In both areas and seasons, an unusually early evening biting peak was observed between 6 - 8 p.m. In Engari, insecticide bioassays showed 85%, 34% and 12% resistance to DDT during the rainy season, dry season and to deltamethrin during the dry season, respectively. In Kigorogoro, 13% resistance to DDT and to deltamethrin was recorded. There was no resistance observed to bendiocarb and pirimiphos methyl.ConclusionsThe heterogeneity of mosquito distribution, entomological indicators and resistance to insecticides in villages with high malaria prevalence highlight the need for a long-term vector control programme and monitoring of insecticide resistance in Uganda. The early evening biting habits of Anopheles combined with resistance to DDT and deltamethrin observed in this study suggest that use of impregnated bed nets alone is insufficient as a malaria control strategy, urging the need for additional interventions in this area of high transmission.

Monitoring resistance of Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) to new chemical insecticides in Punjab, Pakistan

Mealybug, Phenacoccus solenopsis Tinsley is a devastating pest of cotton and many other crops and ornamental plants. P. solenopsis has ability to develop resistance to most chemical classes of insecticides. The aim of present study was to monitor the resistance to new chemical insecticides in different field populations of P. solenopsis. Six populations of P. solenopsis, collected from Multan, Khanewal, Muzaffar Garh, Mailsi, Bahawalpur, and Sahiwal, were tested for resistance to selected new chemical insecticides by the leaf dip method. The resistance ratios were in the range of 4.0–30.9-fold for nitenpyram, 12.6–105.0-fold for acetamiprid, 3.4–79.0-fold for emamectin benzoate, 0.4–2.3 fold for indoxacarb, 12.1–28.9 for pyriproxyfen, 13.0–37.4 for cyromazine, 7.2–35.0 for methoxyfenozide, and 5.5–17.0-fold for lufenuron compared to the laboratory susceptible strain. Regular insecticide resistance monitoring and integrated management plans including the judicious use of insecticides with correct application rates and methods and rotation of insecticides with different modes of action are required to delay insecticide resistance development in P. solenopsis.

Resistance to Conventional and New Insecticides in House Flies (Diptera: Muscidae) From Poultry Facilities in Punjab, Pakistan.

House flies, Musca domestica L., are pests of poultry facilities and have the ability to develop resistance against different insecticides. This study was conducted to assess the resistance status of house flies to pyrethroid, organophosphate, and novel chemistry insecticides from poultry facilities in Punjab, Pakistan. Five adult house fly populations were studied for their resistance status to selected conventional and novel chemistry insecticides. For four pyrethroids, the range of resistance ratios was 14-55-fold for cypermethrin, 11-45-fold for bifenthrin, 0.84-4.06-fold for deltamethrin, and 4.42-24-fold for lambda-cyhalothrin when compared with a susceptible population. Very low levels of resistance were found to deltamethrin compared with the other pyrethroids. For the three organophosphate insecticides, the range of resistance ratios was 1.70-16-fold for profenofos, 7.50-60-fold for chlorpyrifos, and 4.37-53-fold for triazophos. Very low levels of resistance were found to profenofos compared with the other insecticides. For five novel chemistry insecticides, the range of resistance ratios was 1.20-16.00-fold for fipronil, 3.73-7.16-fold for spinosad, 3.06-23-fold for indoxacarb, 0.96-5.88-fold for abamectin, and 0.56-3.07-fold for emamectin benzoate. Rotation of insecticides with different modes of action showing no or very low resistance may prevent insecticide resistance in house flies. Regular insecticide resistance monitoring and integrated management plans on poultry farms are required to prevent resistance development, field control failures, and environmental pollution.

Mutations in the voltage-gated sodium channel gene of anophelines and their association with resistance to pyrethroids - a review.

Constant and extensive use of chemical insecticides has created a selection pressure and favored resistance development in many insect species worldwide. One of the most important pyrethroid resistance mechanisms is classified as target site insensitivity, due to conformational changes in the target site that impair a proper binding of the insecticide molecule. The voltage-gated sodium channel (NaV) is the target of pyrethroids and DDT insecticides, used to control insects of medical, agricultural and veterinary importance, such as anophelines. It has been reported that the presence of a few non-silent point mutations in the NaV gene are associated with pyrethroid resistance, termed as ‘kdr’ (knockdown resistance) for preventing the knockdown effect of these insecticides. The presence of these mutations, as well as their effects, has been thoroughly studied in Anopheles mosquitoes. So far, kdr mutations have already been detected in at least 13 species (Anopheles gambiae, Anopheles arabiensis, Anopheles sinensis, Anopheles stephensi, Anopheles subpictus, Anopheles sacharovi, Anopheles culicifacies, Anopheles sundaicus, Anopheles aconitus, Anopheles vagus, Anopheles paraliae, Anopheles peditaeniatus and Anopheles albimanus) from populations of African, Asian and, more recently, American continents. Seven mutational variants (L1014F, L1014S, L1014C, L1014W, N1013S, N1575Y and V1010L) were described, with the highest prevalence of L1014F, which occurs at the 1014 site in NaV IIS6 domain. The increase of frequency and distribution of kdr mutations clearly shows the importance of this mechanism in the process of pyrethroid resistance. In this sense, several species-specific and highly sensitive methods have been designed in order to genotype individual mosquitoes for kdr in large scale, which may serve as important tolls for monitoring the dynamics of pyrethroid resistance in natural populations. We also briefly discuss investigations concerning the course of Plasmodium infection in kdr individuals. Considering the limitation of insecticides available for employment in public health campaigns and the absence of a vaccine able to brake the life cycle of the malaria parasites, the use of pyrethroids is likely to remain as the main strategy against mosquitoes by either indoor residual spraying (IR) and insecticide treated nets (ITN). Therefore, monitoring insecticide resistance programs is a crucial need in malaria endemic countries.Electronic supplementary materialThe online version of this article (doi:10.1186/1756-3305-7-450) contains supplementary material, which is available to authorized users.

Resistance in the mealybug Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) in Pakistan to selected organophosphate and pyrethroid insecticides

The mealybug Phenacoccus solenopsis is a destructive pest of cotton with the potential to develop resistance to most chemical classes of insecticides. Six populations of P. solenopsis from cotton crops at six different locations in Pakistan were evaluated for resistance to selected organophosphate and pyrethroid insecticides. Resistance ratios (RRs) at LC50 were in the range of 2.7–13.3 fold for chlorpyrifos, 11.6–30.2 fold for profenofos and for the three pyrethroids tested were 10.6–46.4 for bifenthrin, 5.8–25.2 for deltamethrin and 4.1–25.0 for lambda-cyhalothrin. This is the first report of resistance to organophosphate and pyrethroid insecticides in Pakistani populations of P. solenopsis. Regular insecticide resistance monitoring programs are needed to prevent field control failures. Moreover, integrated approaches including the judicious use of insecticides and rotation of insecticides with different modes of action are needed to delay the development of insecticide resistance in P. solenopsis.

Erratum to: Standardization of a bottle assay—an indigenous method for laboratory and field monitoring of insecticide resistance and comparison with WHO adult susceptibility test

Erratum to: Standardization of a bottle assay—an indigenous method for laboratory and field monitoring of insecticide resistance and comparison with WHO adult susceptibility test

Insecticide resistance monitoring and correlation analysis to select appropriate insecticides against Nilaparvata lugens (Stål), a migratory pest in Korea

Nilaparvata lugens Stål is one of the important migratory pests of rice paddy fields in Korea. Resistance levels to nine insecticides were monitored in 12 local strains and correlation analysis was conducted to determine cross-resistance relationships among the tested insecticides. The local strains revealed 1.3- to 28.0-, 1.6- to 6.0-, 2.8- to 237.0-, 0.6- to 0.9-, and 0.7- to 1.3-fold resistance to carbamates, organophosphates, neonicotinoids, fipronil and etofenprox, respectively. Organophosphate insecticides revealed moderate correlations with benzofuranyl methylcarbamate (r=0.566–0.614, p>0.01). Three neonicotinoids were not correlated with each other, but imidacloprid and clothianidin were moderately correlated with several benzofuranyl methylcarbamate and organophosphate insecticides (r=0.590–0.705, p>0.05), indicating that unknown common factors (such as detoxification enzymes) might contribute to resistance to both insecticides. Fipronil and etofenprox exhibited low levels of resistance and cross-resistance with other insecticides, suggesting their potential as an effective insecticide for field application. Resistance level monitoring and correlation analysis would be valuable for the selection of appropriate insecticides to control insecticide-resistant N. lugens, a typical migratory pest in Korea.

Insecticide resistance in Anopheles arabiensis in Sudan: temporal trends and underlying mechanisms.

BackgroundMalaria vector control in Sudan relies mainly on indoor residual spraying (IRS) and the use of long lasting insecticide treated bed nets (LLINs). Monitoring insecticide resistance in the main Sudanese malaria vector, Anopheles arabiensis, is essential for planning and implementing an effective vector control program in this country.MethodsWHO susceptibility tests were used to monitor resistance to insecticides from all four WHO-approved classes of insecticide at four sentinel sites in Gezira state over a three year period. Insecticide resistance mechanisms were studied using PCR and microarray analyses.ResultsWHO susceptibility tests showed that Anopheles arabiensis from all sites were fully susceptible to bendiocarb and fenitrothion for the duration of the study (2008–2011). However, resistance to DDT and pyrethroids was detected at three sites, with strong seasonal variations evident at all sites. The 1014 F kdr allele was significantly associated with resistance to pyrethroids and DDT (P < 0.001) with extremely high effects sizes (OR > 7 in allelic tests). The 1014S allele was not detected in any of the populations tested. Microarray analysis of the permethrin-resistant population of An. arabiensis from Wad Medani identified a number of metabolic genes that were significantly over-transcribed in the field-collected resistant samples when compared to the susceptible Sudanese An. arabiensis Dongola strain. These included CYP6M2 and CYP6P3, two genes previously implicated in pyrethroid resistance in Anopheles gambiae s.s, and the epsilon-class glutathione-S-transferase, GSTe4.ConclusionsThese data suggest that both target-site mechanisms and metabolic mechanisms play an important role in conferring pyrethroid resistance in An. arabiensis from Sudan. Identification in An. arabiensis of candidate loci that have been implicated in the resistance phenotype in An. gambiae requires further investigation to confirm the role of these genes.

Changes in Insecticide Resistance of the Rice Striped Stem Borer (Lepidoptera: Crambidae)

Application of insecticides is the most important method to control Chilo suppressalis (Walker) (Lepidoptera: Crambidae), and continuous use of individual insecticides has driven the rapid development of insecticide resistance in C. suppressalis during the past 30 yr. Monitoring insecticide resistance provides information essential for integrated pest management. Insecticide resistance of field populations to monosultap, triazophos, chlorpyrifos, and abamectin in China was examined in 2010 and 2011. The results indicated that the resistance levels of 14 field populations to four insecticides were significantly different. Four populations showed moderate resistance, and other populations possessed low-level resistance or were susceptible to monosultap. Nine populations displayed an extremely high or a high level of resistance to triazophos, whereas four populations were sensitive to this agent. Five populations exhibited a low level of resistance to abamectin, while the others remained sensitive. When compared with historical data, resistance to monosultap and triazophos decreased significantly, and the percentage of populations with high-level or extremely high-level resistance was obviously reduced. By contrast, the resistance to abamectin increased slightly. The increasing and decreasing resistance levels reported in this study highlight the different evolutionary patterns of insecticide resistance in C. suppressalis. An overreliance on one or two insecticides may promote rapid development of resistance. Slow development of resistance to abamectin, which was used mainly in mixtures with other insecticides, implies that the use of insecticide mixtures may be an effective method to delay the evolution of resistance to insecticides.

Chemical composition and insecticidal activity of plant essential oils from Benin against Anopheles gambiae (Giles)

BackgroundInsecticide resistance in sub-Saharan Africa and especially in Benin is a major public health issue hindering the control of the malaria vectors. Each Anopheles species has developed a resistance to one or several classes of the insecticides currently in use in the field. Therefore, it is urgent to find alternative compounds to conquer the vector. In this study, the efficacies of essential oils of nine plant species, which are traditionally used to avoid mosquito bites in Benin, were investigated.MethodsEssential oils of nine plant species were extracted by hydrodistillation, and their chemical compositions were identified by GC-MS. These oils were tested on susceptible “kisumu” and resistant “ladji-Cotonou” strains of Anopheles gambiae, following WHO test procedures for insecticide resistance monitoring in malaria vector mosquitoes.ResultsDifferent chemical compositions were obtained from the essential oils of the plant species. The major constituents identified were as follows: neral and geranial for Cymbopogon citratus, Z-carveol, E-p-mentha-1(7),8-dien-2-ol and E-p-mentha-2,8-dienol for Cymbopogon giganteus, piperitone for Cymbopogon schoenanthus, citronellal and citronellol for Eucalyptus citriodora, p-cymene, caryophyllene oxide and spathulenol for Eucalyptus tereticornis, 3-tetradecanone for Cochlospermum tinctorium and Cochlospermum planchonii, methyl salicylate for Securidaca longepedunculata and ascaridole for Chenopodium ambrosioides. The diagnostic dose was 0.77% for C. citratus, 2.80% for E. tereticornis, 3.37% for E. citriodora, 4.26% for C. ambrosioides, 5.48% for C. schoenanthus and 7.36% for C. giganteus. The highest diagnostic doses were obtained with S. longepedunculata (9.84%), C. tinctorium (11.56%) and C. planchonii (15.22%), compared to permethrin 0.75%. A. gambiae cotonou, which is resistant to pyrethroids, showed significant tolerance to essential oils from C. tinctorium and S. longepedunculata as expected but was highly susceptible to all the other essential oils at the diagnostic dose.ConclusionsC. citratus, E. tereticornis, E. citriodora, C. ambrosioides and C. schoenanthus are potential promising plant sources for alternative compounds to pyrethroids, for the control of the Anopheles malaria vector in Benin. The efficacy of their essential oils is possibly based on their chemical compositions in which major and/or minor compounds have reported insecticidal activities on various pests and disease vectors such as Anopheles.

Geographic susceptibility of Chilo suppressalis Walker (Lepidoptera: Crambidae), to chlorantraniliprole in China

Chilo suppressalis Walker, the rice striped stem borer (RSSB) is one of the most serious lepidopteron insect pests of rice in Asia. The registration of chlorantraniliprole added an option for the chemical control of RSSBs. In this study, the geographic variability of susceptibility of field-collected RSSBs to chlorantraniliprole was assessed to establish a baseline for future comparisons of field population responses to the increased use of chlorantraniliprole products. Thirty-one RSSB colonies were established from 22 geographic areas in seven provinces of China, and were evaluated for chlorantraniliprole susceptibility. The range of mean lethal concentration (LC50 ) values in response to this chemical was between 0.821 and 17.720 mg (AI)/L across the populations. Susceptibility was significantly different (21.7-fold for LC50 and 55.6-fold for LC90 ) across the various geographic populations. No significant correlations were detected between the LC50 values of chlorantraniliprole and four conventional insecticides (monosultap, triazophos, chlorpyrifos and abamectin). These results revealed great geographical variations in chlorantraniliprole susceptibilities among different geographic populations of RSSBs, which might indicate the potential for RSSBs to develop resistance to chlorantraniliprole. The effective execution of insecticide resistance monitoring programs and management strategies is urgently needed in China to sustain RSSB susceptibility to chlorantraniliprole.

Mixed-function oxidases and esterases associated with permethrin, deltamethrin and bendiocarb resistance in Anopheles gambiae s.l. in the south-north transect Benin, West Africa

BackgroundInsecticide resistance monitoring is essential to help national programmers to implement more effective and sustainable malaria control strategies in endemic countries. The current study aimed at an exploring the involvement of detoxifying enzymes in the insecticide phenotype resistance in Anopheles gambiae s.l.from Benin, in order to guide future malaria vector control interventions.MethodsLarvae and pupae of Anopheles gambiae s.l. mosquitoes were collected from the breeding sites in Oueme, Atacora and Alibori provinces. CDC susceptibility tests were conducted on unfed female mosquitoes aged 2–5 days old. CDC bioassays were performed with stock solutions of permethrin (21.5 μg per bottle), deltamethrin (12.5 μg per bottle) and bendiocarb (12.5 μg per bottle). CDC biochemical assays using synergists were also conducted to assess the metabolic resistance.ResultsThe susceptibility of Anopheles gambiae Agbalilame and Kandi populations to permethrin and deltamethrin respectively, increased significantly when synergized by PBO, suggesting an implication of mono-oxygenases in resistance of Anopheles gambiae s.l. to pyrethroid. Esterases may play a role in bendiocarb resistance in Anopheles gambiae Tanguieta.ConclusionSynergists partially restored susceptibility to pyrethroid and carbamate insecticides and might help mitigate the impact of vector resistance in Anopheles gambiae Agbalilame, Kandi and Tanguieta populations. However, additional vector control tools are needed to further impact on malaria transmission in such settings.This will improve the implementation and management of future control programs against this important malaria vector in Benin and in Africa in general.

Insecticide resistance status in the major West Nile virus vector Culex pipiens from Greece

One of the largest West Nile virus outbreaks in Europe occurred in Greece in 2010. Use of insecticides against Culex pipiens was substantially scaled up, as an emergency tool. Although mosquito control has been based on insecticides for several decades in Greece, insecticide resistance data are not available. An examination was made of the resistance status of 13 Cx. pipiens populations from five regional units in Greece against four insecticides used for its control over a 3 year period. Bioassays demonstrated susceptibility of most populations to all insecticides, except for temephos in some regions, and deltamethrin and diflubenzuron on one occasion each. The authors also monitored the frequency of the pyrethroid target-site resistance mutations L1014F (kdr), as well as G119S and F290V in the Ace1 gene. Ace1 insensitivity mutations were found at low frequencies and always in heterozygocity. However, the frequency of kdr pyrethroid resistance mutations was higher (up to 63.0% in Thessaloniki). The high frequency of kdr mutations indicates a risk that needs to be addressed, should the use of pyrethroids be further extended. There was no strong evidence of significant resistance levels against Bacillus thuringiensis var. israelensis and diflubenzuron. Continued monitoring of insecticide resistance is recommended for the application of appropriate management tactics.

A potential threat to malaria elimination: extensive deltamethrin and DDT resistance to Anopheles sinensis from the malaria-endemic areas in China

BackgroundInsecticide resistance in malaria vectors is a growing concern in many countries and requires immediate attention because of the limited chemical arsenal available for vector control. There is lack of systematic and standard monitoring data of malaria vector resistance in the endemic areas, which is essential for the ambitious goal of malaria elimination programme of China.MethodsIn 2010, eight provinces from different malaria endemic region were selected for study areas. Bioassays were performed on F1 progeny of Anopheles sinensis reared from wild-caught females using the standard WHO susceptibility test with diagnostic concentrations of 0.25% deltamethrin and 4% DDT.ResultsFor An. sinensis, the results indicated that exposure to 0.25% deltamethrin of F1 families with mortalities ranging from 5.96% to 64.54% and less than 80% mortality to DDT at the diagnostic concentration of 4% across the study areas.ConclusionsAnopheles sinensis was completely resistant to both deltamethrin and DDT, and resistance to pyrethroid has risen strikingly compared to that recorded during 1990s. The results highlight the importance of longitudinal insecticide resistance monitoring and the urgent need for a better understanding of the status of insecticide resistance in this region.

Appropriate Larval Food Quality and Quantity forAedes albopictus(Diptera: Culicidae)

The Asian tiger mosquito Aedes albopictus (Skuse, 1894) is a globally invasive prominent vector of viral and parasitic pathogens. To soundly guide insecticide use in control programs it is crucial to use standardized test systems under rigorously controlled environmental conditions that allow for comparisons across laboratories. An acute standard test procedure (24 h) for insecticide resistance monitoring of mosquitoes has been published by the World Health Organization in 1998, but a standardized chronic test to monitor sublethal insecticide effects on the life cycle of mosquitoes does not yet exist. As a first step toward a standardized chronic bioassay (half-life-cycle-test), the exclusion of qualitative and quantitative food effects by means of standardized, optimal larval feeding would greatly facilitate inter-laboratory comparisons. Against this background we evaluated food qualities and quantities for the aquatic part of the A. albopictus life cycle under different thermal conditions. Five mg TetraMin (Tetra, Melle, Germany) larva(-1) at 25 degrees C rendered the lowest mortality and large pupae. Our fundamental data on A. albopictus feeding provide an opportunity to standardize experiments and thus support interlaboratory comparisons of studies on the ecotoxicology of this dangerous vector mosquito.

Resistance to conventional insecticides in Pakistani populations of Musca domestica L. (Diptera: Muscidae): a potential ectoparasite of dairy animals

The house fly, Musca domestica L., is an important hygienic pest of humans and dairy animals with the potential to develop resistance to most chemical classes of insecticides. Six adult house fly strains from dairy farms in Punjab, Pakistan were evaluated for resistance to selected insecticides from organochlorine, organophosphate, carbamate and pyrethroid classes. For a chlorocyclodiene and two organophosphates tested, the resistance ratios (RR) at LC50 were in the range of 5.60-22.02 fold for endosulfan, 7.66-23.24 fold for profenofos and 2.47-7.44 fold for chlorpyrifos. For two pyrethroids and one carbamate, the RR values at LC50 were 30.22-70.02 for cypermethrin, 5.73-18.31 for deltamethrin, and 4.39-15.50 for methomyl. This is the first report of resistance to different classes of insecticides in Pakistani dairy populations of house flies. Regular insecticide resistance monitoring programs on dairy farms are needed to prevent field control failures. Moreover, integrated approaches including the judicious use of insecticides are needed to delay the development of insecticide resistance in house flies.

Insecticide resistance monitoring of field‐collected Anopheles gambiae s.l. populations from Jinja, eastern Uganda, identifies high levels of pyrethroid resistance

Insecticide resistance in the malaria vector Anopheles gambiae s.l. (Diptera: Culicidae) threatens insecticide-based control efforts, necessitating regular monitoring. We assessed resistance in field-collected An. gambiae s.l. from Jinja, Uganda using World Health Organization (WHO) bioassays. Only An. gambiae s.s. and An. arabiensis (≈70%) were present. Female An. gambiae exhibited extremely high pyrethroid resistance (permethrin LT50 > 2 h; deltamethrin LT50 > 5 h). Female An. arabiensis were resistant to permethrin and exhibited reduced susceptibility to deltamethrin. However, while An. gambiae were DDT resistant, An. arabiensis were fully susceptible. Both species were fully susceptible to bendiocarb and fenitrothion. Kdr 1014S has increased rapidly in the Jinja population of An. gambiae s.s. and now approaches fixation (≈95%), consistent with insecticide-mediated selection, but is currently at a low frequency in An. arabiensis (0.07%). Kdr 1014F was also at a low frequency in An. gambiae. These frequencies preclude adequately-powered tests for an association with phenotypic resistance. PBO synergist bioassays resulted in near complete recovery of pyrethroid susceptibility suggesting involvement of CYP450s in resistance. A small number (0.22%) of An. gambiae s.s. ×An. arabiensis hybrids were found, suggesting the possibility of introgression of resistance alleles between species. The high levels of pyrethroid resistance encountered in Jinja threaten to reduce the efficacy of vector control programmes which rely on pyrethroid-impregnated bednets or indoor spraying of pyrethroids.

Pyrethroid Resistance in Anopheles gambiae, in Bomi County, Liberia, Compromises Malaria Vector Control

BackgroundLong Lasting Insecticidal Nets (LLIN) and Indoor Residual Spraying (IRS) have both proven to be effective malaria vector control strategies in Africa and the new technology of insecticide treated durable wall lining (DL) is being evaluated. Sustaining these interventions at high coverage levels is logistically challenging and, furthermore, the increase in insecticide resistance in African malaria vectors may reduce the efficacy of these chemical based interventions. Monitoring of vector populations and evaluation of the efficacy of insecticide based control approaches should be integral components of malaria control programmes. This study reports on entomological survey conducted in 2011 in Bomi County, Liberia.Methods Anopheles gambiae larvae were collected from four sites in Bomi, Liberia, and reared in a field insectary. Two to five days old female adult An gambiae s.l. were tested using WHO tube (n = 2027) and cone (n = 580) bioassays in houses treated with DL or IRS. A sample of mosquitoes (n = 169) were identified to species/molecular form and screened for the presence of knock down resistance (kdr) alleles associated with pyrethroid resistance.Results Anopheles gambiae s.l tested were resistant to deltamethrin but fully susceptible to bendiocarb and fenithrothion. The corrected mortality of local mosquitoes exposed to houses treated with deltamethrin either via IRS or DL was 12% and 59% respectively, suggesting that resistance may affect the efficacy of these interventions. The presence of pyrethroid resistance was associated with a high frequency of the 1014F kdr allele (90.5%) although this mutation alone cannot explain the resistance levels observed.ConclusionHigh prevalence of resistance to deltamethrin in Bomi County may reduce the efficacy of malaria strategies relying on this class of insecticide. The findings highlight the urgent need to expand and sustain monitoring of insecticide resistance in Liberian malaria vectors, evaluate the effectiveness of existing interventions and develop appropriate resistance management strategies.

The Efficacy of Vectron 20 WP, Etofenprox, for Indoor Residual Spraying in Areas of High Vector Resistance to Pyrethroids and Organochlorines in Zambia.

The selection of insecticide resistance in malaria vectors has the potential to compromise any insecticide-based vector control programme. To ensure that the insecticides used for indoor residual spraying and insecticide-treated nets in Zambia remain effective and their choice is evidence based, insecticide resistance surveillance and monitoring are essential. This study assessed and compared the residual efficacy of etofenprox (Vectron 20 WP), an ether pyrethroid, at 0.1 g/m2 with pyrethroids: bifenthrin (Bistar 10 WP) and lambda-cyhalothrin (Icon 10 CS) at 25 mg/m2 for indoor residual spraying. We also assessed the resistance status of etofenprox to local malaria vectors, An. funestus s.s and An. gambiae s.s, using World Health Organization standard protocols. The residual efficacy of Vectron 20 WP on cement, rendered walls of houses lasted for four months with 100% mortality. By the eighth month, the killing effect had reduced to 73.8% compared to 63.3% for bifenthrin and 77.0% for lambda-cyhalothrin. Susceptibility tests using standard World Health Organization assays on An. gambiae s.s showed susceptibility to etofenprox (0.1%) but some resistance was detected to Anopheles funestus s.s. The product is recommended as an ideal insecticide for indoor residual spraying for malaria control in Zambia as part of a resistance management programme in selected areas of the country.

Three years of insecticide resistance monitoring in Anopheles gambiae in Burkina Faso: resistance on the rise?

Background and methodsA longitudinal Anopheles gambiae s.l. insecticide-resistance monitoring programme was established in four sentinel sites in Burkina Faso. For three years, between 2008 and 2010, WHO diagnostic dose assays were used to measure the prevalence of resistance to all the major classes of insecticides at the beginning and end of the malaria transmission season. Species identification and genotyping for target site mutations was also performed and the sporozoite rate in adults determined.ResultsAt the onset of the study, resistance to DDT and pyrethroids was already prevalent in An. gambiae s.l. from the south-west of the country but mosquitoes from the two sites in central Burkina Faso were largely susceptible. Within three years, DDT and permethrin resistance was established in all four sites. Carbamate and organophosphate resistance remains relatively rare and largely confined to the south-western areas although a small number of bendiocarb survivors were found in all sites by the final round of monitoring. The ace-1R target site resistance allele was present in all localities and its frequency exceeded 20% in 2010 in two of the sites. The frequency of the 1014F kdr mutation increased throughout the three years and by 2010, the frequency of 1014F in all sites combined was 0.02 in Anopheles arabiensis, 0.56 in An. gambiae M form and 0.96 in An. gambiae S form. This frequency did not differ significantly between the sites. The 1014S kdr allele was only found in An. arabiensis but its frequency increased significantly throughout the study (P = 0.0003) and in 2010 the 1014S allele frequency was 0.08 in An. arabiensis. Maximum sporozoite rates (12%) were observed in Soumousso in 2009 and the difference between sites is significant for each year.ConclusionPyrethroid and DDT resistance is now established in An. gambiae s.l. throughout Burkina Faso. Results from diagnostic dose assays are highly variable within and between rounds of testing, and hence it is important that resistance monitoring is carried out on more than one occasion before decisions on insecticide procurement for vector control are made. The presence of 1014S in An. gambiae s.l., in addition to 1014F, is not unexpected given the recent report of 1014S in Benin but highlights the importance of monitoring for both mutations throughout the continent. Future research must now focus on the impact that this resistance is having on malaria control in Burkina Faso.