Biochemical basis of permethrin resistance in Anopheles arabiensis from Lower Moshi, north-eastern Tanzania

Impact of MiraNet® long-lasting insecticidal net against Anopheles arabiensis wild population of Northern Tanzania

Despite high levels of pyrethroid resistance reported in malaria vectors, long-lasting insecticidal nets (LNs) still play a key role in controlling malaria transmission. This study tested the efficacy of MiraNet®, a pyrethroid-based LN against a wild population of Anopheles arabiensis in northern Tanzania. DuraNet® was used as a positive control in this evaluation. Standard WHO laboratory bioefficacy evaluations of MiraNet and DuraNet that were unwashed or had been washed 20 times indicated optimal knockdown and mortality for both net types against a susceptible strain of Anopheles gambiae s.s. Standard experimental hut evaluations were conducted to evaluate the efficacy of both nets against a wild population of An. arabiensis. The killing effect of MiraNet was 54.5% for unwashed and 50% for 20 times washed while DuraNet achieved 44.4% mortality for unwashed and 47.4% for 20 times washed against wild An. arabiensis. Both DuraNet and MiraNet exhibited significantly higher killing effects (> 44.4%). There was no significant difference in deterrence or induced exophily detected between the treatment arms for either net. Additionally, there were no adverse effects reported among hut sleepers. The results of this study indicate that the pyrethroid net MiraNet can be used effectively against wild populations of An. gambiae s.l. of low to moderate resistant levels from Northern Tanzania.

Trends of insecticide resistance monitoring in mainland Tanzania, 2004-2020

BACKGROUND

Insecticide resistance is a serious threat to the continued effectiveness of insecticide-based malaria vector control measures, such as long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS). This paper describes trends and dynamics of insecticide resistance and its underlying mechanisms from annual resistance monitoring surveys on Anopheles gambiae sensu lato (s.l.) populations conducted across mainland Tanzania from 2004 to 2020.

METHODS

The World Health Organization (WHO) standard protocols were used to assess susceptibility of the wild female An. gambiae s.l. mosquitoes to insecticides, with mosquitoes exposed to diagnostic concentrations of permethrin, deltamethrin, lambdacyhalothrin, bendiocarb, and pirimiphos-methyl. WHO test papers at 5× and 10× the diagnostic concentrations were used to assess the intensity of resistance to pyrethroids; synergist tests using piperonyl butoxide (PBO) were carried out in sites where mosquitoes were found to be resistant to pyrethroids. To estimate insecticide resistance trends from 2004 to 2020, percentage mortalities from each site and time point were aggregated and regression analysis of mortality versus the Julian dates of bioassays was performed.

RESULTS

Percentage of sites with pyrethroid resistance increased from 0% in 2004 to more than 80% in the 2020, suggesting resistance has been spreading geographically. Results indicate a strong negative association (p = 0.0001) between pyrethroids susceptibility status and survey year. The regression model shows that by 2020 over 40% of An. gambiae mosquitoes survived exposure to pyrethroids at their respective diagnostic doses. A decreasing trend of An. gambiae susceptibility to bendiocarb was observed over time, but this was not statistically significant (p = 0.8413). Anopheles gambiae exhibited high level of susceptibility to the pirimiphos-methyl in sampled sites.

CONCLUSIONS

Anopheles gambiae Tanzania's major malaria vector, is now resistant to pyrethroids across the country with resistance increasing in prevalence and intensity and has been spreading geographically. This calls for urgent action for efficient malaria vector control tools to sustain the gains obtained in malaria control. Strengthening insecticide resistance monitoring is important for its management through evidence generation for effective malaria vector control decision.

Eco-friendly biosynthesis of TiO2 nanoparticles using Desmostachya bipinnata extract: Larvicidal and pupicidal potential against Aedes aegypti and Spodoptera litura and acute toxicity in non-target organisms

The resistance to insecticides among insects, including mosquitoes and agricultural pests, and the impact of these compounds' environmental risks and health issues have motivated the proposition of eco-friendly alternatives. Thus, we aimed to explore the potential use of Desmostachya bipinnata for the biosynthesis of TiO₂NPs and evaluate their larvicidal and pupicidal activity of target (Aedes aegypti and Spodoptera litura) and acute toxicity in non-target organisms (Toxorhynchites splendens and Eisenia fetida), at distinct concentrations, after 24 h of exposure. The characterization of the biosynthesized TiO₂NPs was carried out by FT-IR, XRD, SEM, and EDX analysis. Under the UV-vis spectrum analysis, a sharp peak was recorded at 200 to 800 nm, which indicated the production of TiO₂NPs by the plant extract. The SEM analysis revealed that the synthesized TiO₂NPs were spherical with a diameter of 36.4 nm and were detected in the XRD spectrum analysis related to the TiO₂NPs. The highest percentage of mortality recorded at 900 μg/mL was 96 % and 94 % in the 2nd instar of A. aegypti and S. litura larvae, respectively, and exhibited the LC₅₀ and LC₉₀ values 5 of 458.79 and 531.01 μg/mL, respectively. The biosynthesized TiO₂NPs showed concentration-dependent increased pupal lethality for both A. aegypti and S. litura. We also observed increased detoxification enzyme activity (α esterase, β esterase, and glutathione-S-transferase) of A. aegypti and S. litura exposed to different concentrations of biosynthesized TiO₂NPs as histopathological changes in the midgut region of these animals. On the other hand, the mortality rate of non-target organisms (T. splendens and E. fetida) was lower when exposed to TiO₂NPs, compared to the high lethality induced by synthetic pesticides (cypermethrin and monocrotophos for E. fetida; and cypermethrin and temphos for T. splendens). Thus, our study provides pioneering evidence on the potential use of D. bipinnata-mediated TiO₂NPs for controlling mosquito vectors and agricultural pest management.

A non-inferiority and GLP-compliant study of broflanilide IRS (VECTRON™ T500), a novel meta-diamide insecticide against Anopheles arabiensis

Management of insecticide resistance in vector control requires development and evaluation of active ingredients (AIs) with new modes of action. VECTRON™ T500 is a wettable powder formulation used for Indoor Residual Spraying (IRS) containing 50% of broflanilide as an AI. This study evaluated the efficacy of VECTRON™ T500 sprayed on blocks of different substrates (concrete, mud and plywood) against pyrethroid susceptible and resistant Anopheles gambiae sensu stricto (s.s.) strains, and wild An. arabiensis. It also assessed the efficacy of VECTRON™ T500 in experimental huts plastered with mud and concrete against wild free-flying An. arabiensis; and non-inferiority to a World Health Organization listed indoor residual spraying product Actellic® 300CS in terms of mortality in Moshi, Tanzania.Monthly cone bioassays on blocks and in experimental huts (against pyrethroid susceptible and resistant An. gambiae s.s.) were conducted over a 12-month period after spraying of VECTRON™ T500 and Actellic® CS300. Collections of wild free-flying An. arabiensis were also done in the sprayed huts. The main outcome for cone bioassays was mortality while for the wild hut trial collections, it was mortality and blood feeding inhibition. Grouped logistic regressions with random effects were used to analyse all dichotomous outcome variables from wild collections.The results showed residual efficacy of VECTRON™ T500 of at least 80% mortality was longest on concrete, followed by plywood and then mud substrates for all mosquito strains. Furthermore, VECTRON™ T500 significantly increased the likelihood of mortality (OR:> 1.37, P<0.001) in wild collections of An. arabiensis compared to Actellic® 300CS. Blood feeding was not significantly different in the wild collection of An. arabiensis between VECTRON™ T500 and Actellic® 300CS arms.These results show that VECTRON™ T500 is efficacious against pyrethroid-resistant An. gambiae s.s. and non-inferior to Actellic® 300CS. Therefore, it should be an important addition to the current arsenal of insecticides used for insecticide resistance management and vector control.

Laboratory and semi-field efficacy evaluation of permethrin-piperonyl butoxide treated blankets against pyrethroid-resistant malaria vectors

To control pyrethroid-resistant malaria vectors, Indoor Residual Spraying (IRS) and Long-Lasting Insecticidal Nets (LLINs) that include additional ingredients to pyrethroid are being developed. Same progress needs to be made to the pyrethroid-treated blankets, which are more compatible with shelter structures found in emergency settings such as displaced populations. In the current study, efficacy of blankets treated with permethrin and piperonyl butoxide (PBO) was evaluated against pyrethroid-resistant Anopheles gambiae sensu stricto. Efficacy was compared with that of Olyset LLIN, Olyset Plus LLIN and untreated blanket in terms of mortality and blood-feeding inhibition against pyrethroid-resistant Anopheles gambiae mosquitoes. The current study indicates that, in emergency shelters such as migrant and refugee camps where LLINs cannot be used, PBO-permethrin blankets may provide protection against resistant mosquitoes if widely used. No side effects related to the use of the treated blankets were reported from the participants. These results need validation in a large-scale field trial to assess the epidemiological impact of the intervention, durability and acceptability of this new vector control strategy for malaria vector control.

Larvicidal, antioxidant and biotoxicity assessment of (2-(((2-ethyl-2 methylhexyl)oxy)carbonyl)benzoic acid isolated from Bacillus pumilus against Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus

Mosquitoes are a vector for many dreadful diseases known for their public health concern. The continued use of synthetic insecticides against vector control has led to serious environmental impacts, human health problems, and the development of insect resistance. Hence, alternative mosquito control methods are needed to protect the environment and human health. In the present study, the bioefficacy of (2-(((2-ethyl-2 methylhexyl)oxy)carbonyl) benzoic acid isolated from Bacillus pumilus were tested against Aedes aegypti, Culex quinquefasciatus and Anopheles stephensi. The isolated bioactive compound was characterized through thin layer chromatography (TLC), UV-visible spectroscopy (UV), Fourier-transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and gas chromatography-mass spectrometry analysis. The pure compound caused a high percent mortality rate in a dose-dependent manner, the obtained values were 96, 82, 69, 50 and 34%; 86, 72, 56, 43, and 44%; 100, 90, 83, 70 and 56% against Ae. aegypti, Cx. quinquefasciatus, and An. stephensi respectively. The effective lethal concentration values (LC₅₀) were 13.65, 14.90 and 9.64 ppm against Ae. aegypti, Cx. quinquefasciatus, An. Stephensi, respectively. The effect of (2-(((2-ethyl-2 methylhexyl)oxy)carbonyl) benzoic acid significantly increased the superoxide dismutase, catalase, α, β esterase and Glutathione-S-transferase level after 24 h of the treatment period. The comet assay confirmed that isolated compound causes DNA damage in all tested insects. Histopathological examinations of treated larvae showed shrunken body posture, damaged epithelial cells and microvillus as compared to control organisms. The biosafety of the isolated compound was assessed against G. affinis and did not produce mortality which confirmed that the activity of the isolated compound is species specific. The current study concludes that the critical success factors of new insecticidal agent development are based on the eco-compatibility and alternative tools for the pesticide producing industry.

Efficacy of bednets with dual insecticide-treated netting (Interceptor® G2) on side and roof panels against Anopheles arabiensis in north-eastern Tanzania

Background

Optimising insecticide use and managing insecticide resistance are important to sustain gains against malaria using long-lasting insecticidal nets (LLINs). Restricting insecticides to where mosquitoes are most likely to make multiple contacts could reduce the quantity of insecticide needed to treat the nets. Previous studies have shown that nets partially treated with a pyrethroid insecticide had equivalent mortality compared to a fully treated net. This study compared the efficacy of: (i) whole Interceptor® G2 nets (IG2; a dual-active LLIN containing alpha-cypermethrin and chlorfenapyr), (ii) nets with roof panels made of IG2 netting, (iii) nets with side panels made of IG2 netting and (iv) whole untreated nets as test nets.

Methods

The study was conducted in cow-baited experimental huts, Moshi Tanzania, using a four-arm Latin square design. Test nets had 30 holes cut in panels to simulate a typical net after 2–3 year use. The trial data were analysed using generalized linear models with mortality, blood-feeding, exophily and deterrence against wild free-flying Anopheles arabiensis as outcomes and test nets as predictors.

Results

Mortality was significantly higher in the nets with roof IG2 [27%, P = 0.001, odds ratio (OR) = 51.0, 95% CI = 4.8–546.2), side IG2 (44%, P < 0.001, OR = 137.6, 95% CI = 12.2–1553.2] and whole IG2 (53%, P < 0.001, OR = 223.0, 95% CI = 19.07–2606.0) nettings than the untreated (1%) nets. Mortality was also significantly higher in the whole IG2 net compared to the net with roof IG2 netting (P = 0.009, OR = 4.4, 95% CI = 1.4–13.3). Blood feeding was 22% in untreated, 10% in roof IG2, 14% in side IG2 and 19% in whole IG2 nets. Exiting was 92% in untreated, 89% in roof IG2, 97% in side IG2 and 94% whole IG2 nets.

Conclusion

The results show that although the roof-treated IG2 net induced greater mortality compared to untreated nets, its efficacy was reduced compared to whole IG2 nets. Therefore, there was no benefit to be gained from restricting dual-active ingredient IG2 netting to the roof of nets.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05454-w.

Effects of agricultural pesticides on the susceptibility and fitness of malaria vectors in rural south-eastern Tanzania

BACKGROUND

Agricultural pesticides may exert strong selection pressures on malaria vectors during the aquatic life stages and may contribute to resistance in adult mosquitoes. This could reduce the performance of key vector control interventions such as indoor-residual spraying and insecticide-treated nets. The aim of this study was to investigate effects of agrochemicals on susceptibility and fitness of the malaria vectors across farming areas in Tanzania.

METHODS

An exploratory mixed-methods study was conducted to assess pesticide use in four villages (V1-V4) in south-eastern Tanzania. Anopheles gambiae (s.l.) larvae were collected from agricultural fields in the same villages and their emergent adults examined for insecticide susceptibility, egg-laying and wing lengths (as proxy for body size). These tests were repeated using two groups of laboratory-reared An. arabiensis, one of which was pre-exposed for 48 h to sub-lethal aquatic doses of agricultural pesticides found in the villages.

RESULTS

Farmers lacked awareness about the linkages between the public health and agriculture sectors but were interested in being more informed. Agrochemical usage was reported as extensive in V1, V2 and V3 but minimal in V4. Similarly, mosquitoes from V1 to V3 but not V4 were resistant to pyrethroids and either pirimiphos-methyl or bendiocarb, or both. Adding the synergist piperonyl butoxide restored potency of the pyrethroids. Pre-exposure of laboratory-reared mosquitoes to pesticides during aquatic stages did not affect insecticide susceptibility in emergent adults of the same filial generation. There was also no effect on fecundity, except after pre-exposure to organophosphates, which were associated with fewer eggs and smaller mosquitoes. Wild mosquitoes were smaller than laboratory-reared ones, but fecundity was similar.

CONCLUSIONS

Safeguarding the potential of insecticide-based interventions requires improved understanding of how agricultural pesticides influence important life cycle processes and transmission potential of mosquito vectors. In this study, susceptibility of mosquitoes to public health insecticides was lower in villages reporting frequent use of pesticides compared to villages with little or no pesticide use. Variations in the fitness parameters, fecundity and wing length marginally reflected the differences in exposure to agrochemicals and should be investigated further. Pesticide use may exert additional life cycle constraints on mosquito vectors, but this likely occurs after multi-generational exposures.

Implementing OECD GLP principles for the evaluation of novel vector control tools: a case study with two novel LLINs, SafeNet® and SafeNet NF®

BACKGROUND

To sustain high universal Long-Lasting Insecticidal Nets (LLINs) coverage, affordable nets that provide equivalent or better protection than standard LLINs, are required. Test facilities evaluating new LLINs require compliance to Good Laboratory Practice (GLP) standards to ensure the quality and integrity of test data. Following GLP principles allows for the reconstruction of activities during the conduct of a study and minimizes duplication of efficacy testing. This case study evaluated the efficacy of two LLINs: SafeNet NF^® and SafeNet^® LLIN.

METHODS

The study was conducted according to GLP principles and followed World Health Organization guidelines for evaluating LLINs. The LLINs were assessed in experimental huts against wild, pyrethroid-resistant Anopheles arabiensis mosquitoes. Nets were either unwashed or washed 20 times and artificially holed to simulate a used torn net. Blood-feeding inhibition and mortality were compared with a positive control (Interceptor^® LLIN) and an untreated net.

RESULTS

Mosquito entry in the huts was reduced compared to negative control for the unwashed SafeNet NF, washed Safenet LLIN and the positive control arms. Similar exiting rates were found for all the treatment arms. Significant blood-feeding inhibition was only found for the positive control, both when washed and unwashed. All insecticide treatments induced significantly higher mortality compared to an untreated net. Compared to the positive control, the washed and unwashed SafeNet NF^® resulted in similar mortality. For the SafeNet^® LLINs the unwashed net had an equivalent performance, but the mortality for the washed net was significantly lower than the positive control. Internal audits of the study confirmed that all critical phases complied with Standard Operating Procedures (SOPs) and the study plan. The external audit confirmed that the study complied with GLP standards.

CONCLUSIONS

SafeNet NF^® and SafeNet^® LLIN offered equivalent protection to the positive control (Interceptor^® LLIN). However, further research is needed to investigate the durability, acceptability, and residual efficacy of these nets in the community. This study demonstrated that GLP-compliant evaluation of LLINs can be successfully conducted by African research institutions.

Composition of mosquito fauna and insecticide resistance status of Anopheles gambiae sensu lato in Itang special district, Gambella, Southwestern Ethiopia

Background

Anopheles arabiensis, member species of the Anopheles gambiae complex, is the primary vector of malaria and is widely distributed in Ethiopia. Anopheles funestus, Anopheles pharoensis and Anopheles nili are secondary vectors occurring with limited distribution in the country. Indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) are pillars for the interventions against malaria control and elimination efforts in Ethiopia. However, the emergence and widespread of insecticide resistance in An. gambiae sensu lato (s.l.), might compromise the control efforts of the country. The aim of this study was to investigate composition of mosquito fauna and insecticide resistance status of An. gambiae s.l. in Itang special district ( woreda), Gambella, southwestern Ethiopia.

Methods

Adult mosquitoes were sampled from September 2020 to February 2021 using the CDC light trap and pyrethrum spray catch (PSC). CDC light traps were placed in three selected houses for two consecutive days per month to collect mosquitoes indoor and outdoor from 6:00 P.M. to 06:00 A.M. and PSC was used to collect indoor resting mosquitoes from ten selected houses once in a month from October 2020 to February 2021. Moreover, mosquito larvae were also collected from different breeding sites and reared to adults to assess susceptibility status of populations of An. gambiae s.l. in the study area. Susceptibility tests were conducted on two to three days old non blood fed female An. gambiae s.l. using insecticide impregnated papers with deltamethrin (0.05%), alpha-cypermethrin (0.05%), propoxur (0.1%), pirimiphos-methyl (0.25%) and bendiocarb (0.1%) following World Health Organization (WHO) standard susceptibility test procedure. Molecular diagnostics were done for the identification of member species of An. gambiae s.l. and detection of knockdown resistance (kdr) allele using species specific polymerase chain reaction (PCR) and allele specific PCR.

Results

In total, 468 adult mosquitoes were collected from different houses. Culex mosquitoes were the most dominant (80.4%) followed by Anopheles mosquitoes. Three species of Anopheles (Anopheles coustani, An. pharoensis, and An. gambiae s.l.) were identified, of which An. coustani was the dominant (8.1%) species. Higher number of mosquitoes (231) were collected outdoor by CDC light traps. Out of 468 adult mosquitoes, 294 were blood fed, 46 were half-gravid and gravid whereas the remaining 128 were unfed. WHO bioassay tests revealed that the populations of An. gambiae s.l. in the study area are resistant against alpha-cypermethrin and deltamethrin, but susceptible to bendiocarb, pirimiphos-methyl and propoxur. Of the total 86 An. gambiae s.l. specimens assayed, 79 (92%) successfully amplified and identified as An. arabiensis. West African kdr (L1014F) mutation was detected with high kdr allele frequency ranging from 67 to 88%.

Conclusion

The detection of target site mutation, kdr L1014F allele, coupled with the phenotypic resistance against alpha-cypermethrin and deltamethrin call for continuous resistance monitoring.

Evidence of pyrethroid resistance in Anopheles amharicus and Anopheles arabiensis from Arjo-Didessa irrigation scheme, Ethiopia

Background

Indoor residual spraying and insecticide-treated nets are among the key malaria control intervention tools. However, their efficacy is declining due to the development and spread of insecticide resistant vectors. In Ethiopia, several studies reported resistance of An. arabiensis to multiple insecticide classes. However, such data is scarce in irrigated areas of the country where insecticides, pesticides and herbicides are intensively used. Susceptibility of An. gambiae s.l. to existing and new insecticides and resistance mechanisms were assessed in Arjo-Didessa sugarcane plantation area, southwestern Ethiopia.

Methods

Adult An. gambiae s.l. reared from larval/pupal collections of Arjo-Didessa sugarcane irrigation area and its surrounding were tested for their susceptibility to selected insecticides. Randomly selected An. gambiae s.l. (dead and survived) samples were identified to species using species-specific polymerase chain reaction (PCR) and were further analyzed for the presence of knockdown resistance (kdr) alleles using allele-specific PCR.

Results

Among the 214 An. gambiae s.l. samples analyzed by PCR, 89% (n = 190) were An. amharicus and 9% (n = 20) were An. arabiensis. Mortality rates of the An. gambiae s.l. exposed to deltamethrin and alphacypermethrin were 85% and 86.8%, respectively. On the other hand, mortalities against pirmiphos-methyl, bendiocarb, propoxur and clothianidin were 100%, 99%, 100% and 100%, respectively. Of those sub-samples (An. amharicus and An. arabiensis) examined for presence of kdr gene, none of them were found to carry the L1014F (West African) allelic mutation.

Conclusion

Anopheles amharicus and An. arabiensis from Arjo-Didessa sugarcane irrigation area were resistant to pyrethroids which might be synergized by extensive use of agricultural chemicals. Occurrence of pyrethroid resistant malaria vectors could challenge the ongoing malaria control and elimination program in the area unless resistance management strategies are implemented. Given the resistance of An. amharicus to pyrethroids, its behavior and vectorial capacity should be further investigated.

Forced Egg Laying Method to Establish F1 Progeny from Field Populations and Laboratory Strains of Anopheles Mosquitoes (Diptera: Culicidae) in Thailand

Successful monitoring of physiological resistance of malaria vectors requires about 150 female mosquitoes for a single set of tests. In some situations, the sampling effort is insufficient due to the low number of field-caught mosquitoes. To address this challenge, we demonstrate the feasibility of using the forced oviposition method for producing F1 from field-caught Anopheles mosquitoes. A total of 430 and 598 gravid Anopheles females from four laboratory strains and five field populations, respectively, were tested. After blood feeding, gravid mosquitoes were individually introduced into transparent plastic vials, containing moistened cotton balls topped with a 4 cm2 piece of filter paper. The number of eggs, hatching larvae, pupation, and adult emergence were recorded daily. The mean number of eggs per female mosquito ranged from 39.3 for Anopheles cracens to 93.6 for Anopheles dirus in the laboratory strains, and from 36.3 for Anopheles harrisoni to 147.6 for Anopheles barbirostris s.l. in the field populations. A relatively high egg hatching rate was found in An. dirus (95.85%), Anopheles minimus (78.22%), and An. cracens (75.59%). Similarly, a relatively high pupation rate was found for almost all test species ranging from 66% for An. minimus to 98.7% for Anopheles maculatus, and lowest for An. harrisoni (43.9%). Highly successful adult emergence rate was observed among 85-100% of pupae that emerged in all tested mosquito populations. The in-tube forced oviposition method is a promising method for the production of sufficient F1 progeny for molecular identification, vector competence, insecticide resistance, and bioassay studies.

Colonization and Authentication of the Pyrethroid-Resistant Anopheles gambiae s.s. Muleba-Kis Strain; an Important Test System for Laboratory Screening of New Insecticides

Simple Summary

Malaria control and prevention have traditionally relied on the use of insecticides in the form of treated bed nets or residual spraying in households. However, scaling up of these interventions—based on few available insecticide classes—resulted in the development and spread of insecticide resistance in malaria-transmitting mosquitoes. There is therefore an urgent need for introducing and applying new insecticides that are effective against these mosquitoes. Laboratories tasked with evaluating the efficacy of novel insecticides need to establish a large colony of resistant mosquitoes. In this study, we report the procedures used and challenges faced during the establishment and maintenance of a resistant mosquito strain in the laboratory which reflects the characteristics of the wild-resistant mosquito populations found in East Africa.

Abstract

Background: The emergence and spread of insecticide resistance in malaria vectors to major classes of insecticides call for urgent innovation and application of insecticides with novel modes of action. When evaluating new insecticides for public health, potential candidates need to be screened against both susceptible and resistant mosquitoes to determine efficacy and to identify potential cross-resistance to insecticides currently used for mosquito control. The challenges and lessons learned from establishing, maintaining, and authenticating the pyrethroid-resistant An. gambiae s.s. Muleba-Kis strain at the KCMUCo-PAMVERC Test Facility are described in this paper. Methods: Male mosquitoes from the F₁ generation of wild-pyrethroid resistant mosquitoes were cross-bred with susceptible female An. gambiae s.s. Kisumu laboratory strain followed by larval selection using a pyrethroid insecticide solution. Periodic screening for phenotypic and genotypic resistance was done. WHO susceptibility tests and bottle bioassays were used to assess the phenotypic resistance, while Taqman™ assays were used to screen for known target-site resistance alleles (kdr and ace-1). Additionally, the strains were periodically assessed for quality control by monitoring adult weight and wing length. Results: By out-crossing the wild mosquitoes with an established lab strain, a successful resistant insectary colony was established. Intermittent selection pressure using alphacypermethrin has maintained high kdr mutation (leucine-serine) frequencies in the selected colony. There was consistency in the wing length and weight measurements from the year 2016 to 2020, with the exception that one out of four years was significantly different. Mean annual wing length varied between 0.0142–0.0028 mm compared to values obtained in 2016, except in 2019 where it varied by 0.0901 mm. Weight only varied by approximately 0.001 g across four years, except in 2017 where it differed by 0.005 g. Routine phenotypic characterization on Muleba-Kis against pyrethroids using the WHO susceptibility test indicated high susceptibility when type I pyrethroids were used compared to type II pyrethroids. Dynamics on susceptibility status also depended on the lapse time when the selection was last done. Conclusions: This study described the procedure for introducing, colonizing, and maintaining a resistant An. gambiae s.s. strain in the laboratory with leucine to serine substitution kdr allele which reflects the features of the wild-resistant population in East Africa. Challenges in colonizing a wild-resistant mosquito strain were overcome by out-crossing between mosquito strains of desired traits followed by intermittent insecticide selection at the larval stage to select for the resistant phenotype.

A Systematic Review of the Effects of Temperature on Anopheles Mosquito Development and Survival: Implications for Malaria Control in a Future Warmer Climate

The rearing temperature of the immature stages can have a significant impact on the life-history traits and the ability of adult mosquitoes to transmit diseases. This review assessed published evidence of the effects of temperature on the immature stages, life-history traits, insecticide susceptibility, and expression of enzymes in the adult Anopheles mosquito. Original articles published through 31 March 2021 were systematically retrieved from Scopus, Google Scholar, Science Direct, PubMed, ProQuest, and Web of Science databases. After applying eligibility criteria, 29 studies were included. The review revealed that immature stages of An. arabiensis were more tolerant (in terms of survival) to a higher temperature than An. funestus and An. quadriannulatus. Higher temperatures resulted in smaller larval sizes and decreased hatching and pupation time. The development rate and survival of An. stephensi was significantly reduced at a higher temperature than a lower temperature. Increasing temperatures decreased the longevity, body size, length of the gonotrophic cycle, and fecundity of Anopheles mosquitoes. Higher rearing temperatures increased pyrethroid resistance in adults of the An. arabiensis SENN DDT strain, and increased pyrethroid tolerance in the An. arabiensis SENN strain. Increasing temperature also significantly increased Nitric Oxide Synthase (NOS) expression and decreased insecticide toxicity. Both extreme low and high temperatures affect Anopheles mosquito development and survival. Climate change could have diverse effects on Anopheles mosquitoes. The sensitivities of Anopeheles mosquitoes to temperature differ from species to species, even among the same complex. Notwithstanding, there seem to be limited studies on the effects of temperature on adult life-history traits of Anopheles mosquitoes, and more studies are needed to clarify this relationship.

Efficacy of indoor residual spraying with broflanilide (TENEBENAL), a novel meta-diamide insecticide, against pyrethroid-resistant anopheline vectors in northern Tanzania: An experimental hut trial

Novel chemistry for vector control is urgently needed to counter insecticide resistance in mosquitoes. Here a new meta-diamide insecticide, broflanilide (TENEBENALTM), was evaluated in East African experimental huts in Moshi, northern Tanzania. Two consecutive experimental hut trials with broflanilide 50WP were conducted; the first evaluating the efficacy of three concentrations, 50 mg/m2, 100 mg/m2, and 200 mg/m2 using a prototype formulation, and the second trial evaluating an improved formulation. The IRS treatments were applied on both mud and concrete surfaces and efficacy was monitored over time. The mortality, blood-feeding inhibition and exiting behaviour of free-flying wild mosquitoes was compared between treatment arms. Additionally, cone assays with pyrethroid-susceptible and resistant mosquito strains were conducted in the huts to determine residual efficacy. The first trial showed a dosage-mortality response of the prototype formulation and 3-8 months of residual activity, with longer activity on concrete than mud. The second trial with an improved formulation showed prolonged residual efficacy of the 100 mg/m2 concentration to 5-6 months on mud, and mosquito mortality on the concrete surface ranged between 94-100% for the full duration of the trial. In both trials, results with free-flying, wild Anopheles arabiensis echoed the mortality trend shown in cone assays, with the highest dose inducing the highest mortality and the improved formulation showing increased mortality rates. No blood-feeding inhibition or insecticide-induced exiting effects were observed with broflanilide. Broflanilide 50WP was effective against both susceptible and pyrethroid-resistant mosquito strains, demonstrating an absence of cross resistance between broflanilide and pyrethroids. The improved formulation, which has now been branded VECTRONTM T500, resulted in a prolonged residual efficacy. These results indicate the potential of this insecticide as an addition to the arsenal of IRS products needed to maintain both control of malaria and resistance management of malaria-transmitting mosquitoes.

Insecticide Resistance Development in the Filth Fly Pupal Parasitoid, Spalangia cameroni (Hymenoptera: Pteromalidae), Using Laboratory Selections

Filth flies remain one of the most prevalent pest groups affecting the animal production industry. Spalangia spp. and Muscidifurax spp. are beneficial parasitic wasps that often are utilized to manage filth fly populations such as house flies, Musca domestica L. (Diptera: Muscidae), and stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae). These wasps search for filth fly pupae as hosts in areas potentially treated with insecticides, which may result in nontarget insecticide selection effects. However, research regarding resistance development in parasitic wasps such as S. cameroni Perkins (Hymenoptera: Pteromalidae) is limited. Therefore, a study was conducted to determine the potential of S. cameroni to develop resistance to the commonly used insecticide permethrin, as well as compare permethrin susceptibility among several S. cameroni strains. After 10 selected generations, susceptibility was significantly lower for the selected strain when compared with that of its unselected parent strain. A comparison of several parasitoid strains collected from different U.S. states indicated that permethrin susceptibility was not significantly different between a baseline strain and more recently established field strains. The potential implications of this previously unrecognized nontarget insecticide exposure effect on filth fly parasitoids are discussed.

Species diversity and insecticide resistance within the Anopheles hyrcanus group in Ubon Ratchathani Province, Thailand

Background

Members of the Anopheles hyrcanus group have been incriminated as important malaria vectors. This study aims to identify the species and explore the insecticide susceptibility profile within the Anopheles hyrcanus group in Ubon Ratchathani Province, northeastern Thailand where increasing numbers of malaria cases were reported in 2014.

Methods

Between 2013 and 2015, five rounds of mosquito collections were conducted using human landing and cattle bait techniques during both the rainy and dry seasons. Anopheles mosquitoes were morphologically identified and their insecticide susceptibility status was investigated. Synergist bioassays were carried out with An. hyrcanus (s.l.) due to their resistance to all insecticides. An ITS2-PCR assay was conducted to identify to species the Hyrcanus group specimens.

Results

Out of 10,361 Anopheles females collected, representing 18 taxa in 2 subgenera, 71.8% were morphologically identified as belonging to the Hyrcanus Group (subgenus Anopheles), followed by An. barbirostris group (7.9%), An. nivipes (6.5%), An. philippinensis (5.9%) and the other 14 Anopheles species. Specimens of the Hyrcanus Group were more prevalent during the rainy season and were found to be highly zoophilic. Anopheles hyrcanus (s.l.) was active throughout the night, with an early peak of activity between 18:00 h and 21:00 h. ITS2-PCR assay conducted on 603 DNA samples from specimens within the Hyrcanus Group showed the presence of five sisters species. Anopheles peditaeniatus was the most abundant species (90.5%, n = 546), followed by An. nitidus (4.5%, n = 27), An. nigerrimus (4.3%, n = 26), An. argyropus (0.5%, n = 3), and An. sinensis (0.2%, n = 1). All An. hyrcanus (s.l.) specimens that were found resistant to insecticides (deltamethrin 0.05%, permethrin 0.75% and DDT 4% and synergist tests) belonged to An. peditaeniatus. The degree of resistance in An. peditaeniatus to each of these three insecticides was approximately 50%. Addition of PBO (Piperonyl butoxide), but not DEF (S.S.S-tributyl phosphotritioate), seemed to restore susceptibility, indicating a potential role of oxidases as a detoxifying enzyme resistance mechanism.

Conclusions

A better understanding of mosquito diversity related to host preference, biting activity and insecticide resistance status will facilitate the implementation of locally adapted vector control strategies.

The Impact of Insecticide Pre-Exposure on Longevity, Feeding Succession, and Egg Batch Size of Wild Anopheles gambiae s.l

Background

Insecticide resistance among the vector population is the main threat to existing control tools available. The current vector control management options rely on applications of recommended public health insecticides, mainly pyrethroids through long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS). Regular monitoring of insecticide resistance does not provide information on important factors that affect parasite transmission. Such factors include vector longevity, vector competence, feeding success, and fecundity. This study investigated the impacts of insecticide resistance on longevity, feeding behaviour, and egg batch size of Anopheles gambiae s.l.

Method

The larval sampling was conducted in rice fields using a standard dipper (350 ml) and reared to adults in field insectary. A WHO susceptibility test was conducted using standard treated permethrin (0.75%) and deltamethrin (0.05%) papers. The susceptible Kisumu strain was used for reference. Feeding succession and egg batch size were monitored for all survivors and control.

Results

The results revealed that mortality rates declined by 52.5 and 59.5% for permethrin and deltamethrin, respectively. The mortality rate for the Kisumu susceptible strain was 100%. The survival rates of wild An. gambiae s.l. was between 24 and 27 days. However, the Kisumu susceptible strain blood meal feeding was significantly higher than resistant colony (t = 2.789, df = 21, P=0.011). Additionally, the susceptible An. gambiae s.s. laid more eggs than the resistant An.gambiae s.l. colony (Χ² = 1366, df = 1, P ≤ 0.05).

Conclusion

It can, therefore, be concluded that the wild An. gambiae s.l. had increased longevity, blood feeding, and small egg batch size compared to Kisumu susceptible colonies.

Insecticide resistance in indoor and outdoor-resting Anopheles gambiae in Northern Ghana

BACKGROUND

Selection pressure from continued exposure to insecticides drives development of insecticide resistance and changes in resting behaviour of malaria vectors. There is need to understand how resistance drives changes in resting behaviour within vector species. The association between insecticide resistance and resting behaviour of Anopheles gambiae sensu lato (s.l.) in Northern Ghana was examined.

METHODS

F1 progenies from adult mosquitoes collected indoors and outdoors were exposed to DDT, deltamethrin, malathion and bendiocarb using WHO insecticide susceptibility tests. Insecticide resistance markers including voltage-gated sodium channel (Vgsc)-1014F, Vgsc-1014S, Vgsc-1575Y, glutathione-S-transferase epsilon 2 (GSTe2)-114T and acetylcholinesterase (Ace1)-119S, as well as blood meal sources were investigated using PCR methods. Activities of metabolic enzymes, acetylcholine esterase (AChE), non-specific β-esterases, glutathione-S-transferase (GST) and monooxygenases were measured from unexposed F1 progenies using microplate assays.

RESULTS

Susceptibility of Anopheles coluzzii to deltamethrin 24 h post-exposure was significantly higher in indoor (mortality = 5%) than outdoor (mortality = 2.5%) populations (P = 0.02). Mosquitoes were fully susceptible to malathion (mortality: indoor = 98%, outdoor = 100%). Susceptibility to DDT was significantly higher in outdoor (mortality = 9%) than indoor (mortality = 0%) mosquitoes (P = 0.006). Mosquitoes were also found with suspected resistance to bendiocarb but mortality was not statistically different (mortality: indoor = 90%, outdoor = 95%. P = 0.30). Frequencies of all resistance alleles were higher in F1 outdoor (0.11-0.85) than indoor (0.04-0.65) mosquito populations, while Vgsc-1014F in F0 An. gambiae sensu stricto (s.s) was significantly associated with outdoor-resting behaviour (P = 0.01). Activities of non-specific β-esterase enzymes were significantly higher in outdoor than indoor mosquitoes (Mean enzyme activity: Outdoor = : 1.70/mg protein; Indoor = 1.35/mg protein. P < 0.0001). AChE activity was also more elevated in outdoor (0.62/mg protein) than indoor (0.57/mg protein) mosquitoes but this was not significant (P = 0.08). Human blood index (HBI) was predominantly detected in indoor (18%) than outdoor mosquito populations (3%).

CONCLUSIONS

The overall results did not establish that there was a significant preference of resistant malaria vectors to solely rest indoors or outdoors, but varied depending on the resistant alleles present. Phenotypic resistance was higher in indoor than outdoor-resting mosquitoes, but genotypic and metabolic resistance levels were higher in outdoor than the indoor populations. Continued monitoring of changes in resting behaviour within An. gambiae s.l. populations is recommended.

Patterns of pesticide usage in agriculture in rural Tanzania call for integrating agricultural and public health practices in managing insecticide-resistance in malaria vectors

BACKGROUND

Unrestricted use of pesticides in agriculture is likely to increase insecticide resistance in mosquito vectors. Unfortunately, strategies for managing insecticide resistance in agriculture and public health sectors lack integration. This study explored the types and usage of agricultural pesticides, and awareness and management practices among retailers and farmers in Ulanga and Kilombero districts in south-eastern Tanzania, where Anopheles mosquitoes are resistant to pyrethroids.

METHODS

An exploratory sequential mixed-methods approach was employed. First, a survey to characterize pesticide stocks was conducted in agricultural and veterinary (agrovet) retail stores. Interviews to assess general knowledge and practices regarding agricultural pesticides were performed with 17 retailers and 30 farmers, followed by a survey involving 427 farmers. Concurrently, field observations were done to validate the results.

RESULTS

Lambda-cyhalothrin, cypermethrin (both pyrethroids) and imidacloprids (neonicotinoids) were the most common agricultural insecticides sold to farmers. The herbicide glyphosate (amino-phosphonates) (59.0%), and the fungicides dithiocarbamate and acylalanine (54.5%), and organochlorine (27.3%) were also readily available in the agrovet shops and widely used by farmers. Although both retailers and farmers had at least primary-level education and recognized pesticides by their trade names, they lacked knowledge on pest control or proper usage of these pesticides. Most of the farmers (54.4%, n = 316) relied on instructions from pesticides dealers. Overall, 93.7% (400) farmers practised pesticides mixing in their farms, often in close proximity to water sources. One-third of the farmers disposed of their pesticide leftovers (30.0%, n = 128) and most farmers discarded empty pesticide containers into rivers or nearby bushes (55.7%, n = 238).

CONCLUSION

Similarities of active ingredients used in agriculture and malaria vector control, poor pesticide management practices and low-levels of awareness among farmers and pesticides retailers might enhance the selection of insecticide resistance in malaria vectors. This study emphasizes the need for improving awareness among retailers and farmers on proper usage and management of pesticides. The study also highlights the need for an integrated approach, including coordinated education on pesticide use, to improve the overall management of insecticide resistance in both agricultural and public health sectors.

Potential Roles of Environmental and Socio-Economic Factors in the Distribution of Insecticide Resistance in Anopheles gambiae sensu lato (Culicidae: Diptera) Across Togo, West Africa

Vector control strategies recommended by the World Health Organization are threatened by resistance of Anopheles mosquitoes to insecticides. Information on the distribution of resistant genotypes of malaria vectors is increasingly needed to address the problem. Ten years of published and unpublished data on malaria vector susceptibility/resistance and resistance genes have been collected across Togo. Relationships between the spatial distribution of resistance status and environmental, socio-economic, and landscape features were tested using randomization tests, and calculating Spearman rank and Pearson correlation coefficients between mosquito mortality and different gridded values. Anopheles gambiae sensu lato was resistant to DDT, pyrethroids, and the majority of carbamates and organophosphates. Three sibling species were found (i.e., An. gambiae, Anopheles coluzzii, and Anopheles arabiensis) with four resistance genes, including kdr (L1014F, L1014S, and N1575Y) and ace1 (G119S). The most frequent resistance gene was L1014F. Overall, no association was found between the susceptibility/resistance status and environmental features, suggesting that evolution of resistance may be most closely related to extreme selection from local insecticide use. Nevertheless, further research is necessary for firm conclusions about this lack of association, and the potential role of landscape characteristics such as presence of crops and percentage of tree cover.

First report of AChE1 (G119S) mutation and multiple resistance mechanisms in Anopheles gambiae s.s. in Nigeria

Susceptibility and PBO synergist bioassays were done using 3–5 days old female Anopheles mosquito collected from Lagos State, Nigeria with WHO test papers DDT (4%), permethrin (0.75%), Bendiocarb (1%) and PBO (4%) according to standard procedures. The activities of cytochrome P450s, glutathione S-transferase and carboxylesterases were determined using biochemical assays. The presence of kdr-w, kdr-e and Ace-1^R mutations were examined using molecular assays. Resistance to DDT and permethrin in An gambiae s.s from the four Local Government Areas (LGAs) was recorded while suspected resistance to bendiocarb was recorded in mosquitoes from Alimosho and Kosofe LGAs. PBO synergist reduced the knockdown time and also recorded significantly (P < 0.05) higher 24 hrs percentage mortality compared to non-synergized bioassays. Increased activities of detoxifying enzymes was recorded in wild mosquito compared to the insecticides susceptible laboratory strain and this was significant (P < 0.05) in P450s, esterase α and β. Kdr-w was detected in An. gambiae s.s from all the LGAs, kdr-e (L1014S) was detected in Alimosho, Kosofe and Ibeju-Lekki, while the Ace-1^R gene was detected in Alimosho and Kosofe. Results from this study provide evidence for resistance of An. gambiae from Lagos State to multiple classes of neurotoxic insecticides with multiple resistance mechanisms to these insecticides.

Absence of knockdown mutations in pyrethroid and DDT resistant populations of the main malaria vectors in Colombia

BACKGROUND

Knockdown resistance (kdr) is a well-characterized target-site insecticide resistance mechanism that is associated with DDT and pyrethroid resistance. Even though insecticide resistance to pyrethroids and DDT have been reported in Anopheles albimanus, Anopheles benarrochi sensu lato (s.l.), Anopheles darlingi, Anopheles nuneztovari s.l., and Anopheles pseudopunctipennis s.l. malaria vectors in Latin America, there is a knowledge gap on the role that kdr resistance mechanisms play in this resistance. The aim of this study was to establish the role that kdr mechanisms play in pyrethroid and DDT resistance in the main malaria vectors in Colombia, in addition to previously reported metabolic resistance mechanisms, such as mixed function oxidases (MFO) and nonspecific esterases (NSE) enzyme families.

METHODS

Surviving (n = 62) and dead (n = 67) An. nuneztovari s.l., An. darlingi and An. albimanus mosquitoes exposed to diagnostic concentrations of DDT and pyrethroid insecticides were used to amplify and sequence a ~ 225 bp fragment of the voltage-gated sodium channels (VGSC) gene. This fragment spanning codons 1010, 1013 and 1014 at the S6 segment of domain II to identify point mutations, which have been associated with insecticide resistance in different species of Anopheles malaria vectors.

RESULTS

No kdr mutations were detected in the coding sequence of this fragment in 129 samples, 62 surviving mosquitoes and 67 dead mosquitoes, of An. darlingi, An. nuneztovari s.l. and An. albimanus.

CONCLUSION

Mutations in the VGSC gene, most frequently reported in other species of the genus Anopheles resistant to pyrethroid and DDT, are not associated with the low-intensity resistance detected to these insecticides in some populations of the main malaria vectors in Colombia. These results suggest that metabolic resistance mechanisms previously reported in these populations might be responsible for the resistance observed.

Dynamics and monitoring of insecticide resistance in malaria vectors across mainland Tanzania from 1997 to 2017: a systematic review

Background

Malaria still claims substantial lives of individuals in Tanzania. Insecticide-treated nets (ITNs) and indoor residual spray (IRS) are used as major malaria vector control tools. These tools are facing great challenges from the rapid escalating insecticide resistance in malaria vector populations. This review presents the information on the dynamics and monitoring of insecticide resistance in malaria vectors in mainland Tanzania since 1997. The information is important to policy-makers and other vector control stakeholders to reflect and formulate new resistance management plans in the country.

Methods

Reviewed articles on susceptibility and mechanisms of resistance in malaria vectors to insecticides across mainland Tanzania were systematically searched from the following databases: PubMed, Google scholar, HINARI and AGORA. The inclusion criteria were articles published between 2000 and 2017, reporting susceptibility of malaria vectors to insecticides, mechanisms of resistance in the mainland Tanzania, involving field collected adult mosquitoes, and mosquitoes raised from the field collected larvae. Exclusion criteria were articles reporting insecticide resistance in larval bio-assays, laboratory strains, and unpublished data. Reviewed information include year of study, malaria vectors, insecticides, and study sites. This information was entered in the excel sheet and analysed.

Results

A total of 30 articles met the selection criteria. The rapid increase of insecticide resistance in the malaria vectors across the country was reported since year 2006 onwards. Insecticide resistance in Anopheles gambiae sensu lato (s.l.) was detected in at least one compound in each class of all recommended insecticide classes. However, the Anopheles funestus s.l. is highly resistant to pyrethroids and DDT. Knockdown resistance (kdr) mechanism in An. gambiae s.l. is widely studied in the country. Biochemical resistance by detoxification enzymes (P450s, NSE and GSTs) in An. gambiae s.l. was also recorded. Numerous P450s genes associated with metabolic resistance were over transcribed in An. gambiae s.l. collected from agricultural areas. However, no study has reported mechanisms of insecticide resistance in the An. funestus s.l. in the country.

Conclusion

This review has shown the dynamics and monitoring of insecticide resistance in malaria vector populations across mainland Tanzanian. This highlights the need for devising improved control approaches of the malaria vectors in the country.

Enhanced mortality in deltamethrin-resistant Aedes aegypti in Thailand using a piperonyl butoxide synergist

Aedes aegypti is the primary vector of dengue viruses in Thailand. Control of this mosquito continues to rely heavily on use of insecticides in various forms and applications. The synergistic effect of piperonyl butoxide (PBO), combined with deltamethrin against eight populations of Ae. aegypti collected from different regions in Thailand is presented. The standard WHO adult contact bioassays found all populations with low to moderate levels of resistance to deltamethrin alone (using a 0.05% discriminating concentration), with final mortalities ranging from 15.6 to 70%, while a laboratory strain was fully susceptible (100% mortality). Pre-exposure of female mosquitoes to 4% PBO for 1 h, followed immediately by exposure to deltamethrin for 1 h, significantly increased mortality in seven populations (64.8-98.1%) with the exception of mosquitoes derived from Lampang Province. The knockdown time (KDT) synergist ratios between deltamethrin only and PBO + deltamethrin ranged from 1.7 to 2.8 for KDT₅₀ and 1.9 to 4.0 for KDT₉₅. Between deltamethrin alone and mosquitoes exposed to PBO + deltamethrin, all resistant populations produced significant differences (P < 0.05) in final 24-h mortality, except marginally for Lampang (P = 0.053). The synergistic effects of PBO with deltamethrin-resistant Ae. aegypti suggest a combination of this synergist with deltamethrin or other pyrethroid compounds can significantly enhance the effectiveness of these insecticides against pyrethroid-resistant Ae. aegypti found commonly in Thailand.

Malaria Vectors Insecticides Resistance in Different Agroecosystems in Western Kenya

Background

Malaria vector control efforts have taken malaria related cases down to appreciable number per annum after large scale of intervention tools. Insecticides-based tools remain the major control option for malaria vectors in Kenya and, therefore, the potential of such programs to be compromised by the reported insecticide resistance is of major concern. The objective of this study was to evaluate the status of insecticide resistance in malaria vectors in different agro ecosystems from western Kenya.

Methods

The study was carried out in the lowlands and highlands of western Kenya namely; Ahero, Kisian, Chulaimbo, Emutete, Emakakha, Iguhu, and Kabula. World Health Organization tube bioassays was conducted using standard diagnostic dosages of Lambdacyhalothrin, Deltamethrin, Permethrin, DDT, Bendiocarb, and Malathion tested on Anopheles mosquitoes collected from seven sites; Ahero, Kisian, Chulaimbo, Emutete, Emakakha, Iguhu, and Kabula. Biochemical assays, where the enzymatic activity of three enzymes (monooxygenases, esterases, and glutathione S-transferases) were performed on susceptible and resistant mosquito populations. Wild mosquito populations were identified to species level using polymerase chain reaction (PCR). The species of the wild mosquito populations were identified to species level using PCR. Real-time PCR was performed on the susceptible and resistant mosquitoes after the WHO tube bioassays to determine the presence of knockdown resistance (kdr) allele.

Results

WHO susceptibility tests indicated that Anopheles gambiae showed resistance to Pyrethroids and DDT in all the study sites, to Bendiocarb in Iguhu and Kabula and susceptible to Malathion (100% mortality) in all the study sites. There was an elevation of monooxygenases and esterases enzymatic activities in resistant An. gambiae mosquito populations exposed to Lambdacyhalothrin, Permethrin, Deltamethrin and DDT but no elevation in glutathione S-transferases. A high frequency of L1014S allele was detected in An. gambiae s.s. population, but there was no kdr allele found in Anopheles arabiensis mosquitoes.

Conclusion

An. gambiae mosquitoes from western Kenya have developed phenotypic resistance to pyrethroids and DDT. Therefore, there is a need for further research covering different climatic zones with different agroeconomic activities for detailed report on current status of insecticide resistance in malaria vectors.

Multiple insecticide resistance in Anopheles gambiae from Tanzania: a major concern for malaria vector control

BACKGROUND

Malaria vector control in Tanzania is based on use of long-lasting insecticide treated nets (LLINs) and indoor residual spraying (IRS), which both rely on the use of chemical insecticides. The effectiveness of these control tools is endangered by the development of insecticide resistance in the major malaria vectors. This study was carried out to monitor the susceptibility status of major malaria vectors to insecticides used for IRS and LLINs in mainland Tanzania.

METHODS

Mosquito larvae were collected in 20 sites of Tanzania mainland in 2015. Phenotypic resistance was determined using standard WHO susceptibility tests. Molecular assay were used to determine distribution of Anopheles gambiae sub-species. A microplate assay approach was used for identifying enzyme levels on single mosquitoes from each sites compared with a susceptible reference strain, An. gambiae sensu stricto (s.s.) Kisumu strain.

RESULTS

Anopheles arabiensis was the dominant malaria specie in the country, accounting for 52% of the sibling species identified, while An. gambiae s.s. represented 48%. In Arumeru site, the dominant species was An. arabiensis, which was resistant to both pyrethroids (permethrin and deltamethrin), and pirimiphos-methyl, and had significant elevated levels of GSTs, non-specific esterases, and oxidase enzymes. An. arabiensis was also a dominant species in Kilombero and Kondoa sites, both were resistant to permethrin and deltamethrin with significant activity levels of oxidase enzymes. Resistance to bendiocarb was recorded in Ngara site where specie composition is evenly distributed between An. gambiae s.s. and An.arabiensis. Also bendiocarb resistance was recorded in Mbozi site, where An. gambiae s.s. is the dominant species.

CONCLUSIONS

Overall, this study confirmed resistance to all four insecticide classes in An. gambiae sensu lato in selected locations in Tanzania. Results are discussed in relation to resistance mechanisms and the optimization of resistance management strategies.

Mapping insecticide resistance and characterization of resistance mechanisms in Anopheles arabiensis (Diptera: Culicidae) in Ethiopia

Background

The emergence and spread of insecticide resistance in the major African malaria vectors Anopheles gambiae (s.s.) and An. arabiensis may compromise the current vector control interventions and threatens the global malaria control and elimination efforts.

Methods

Insecticide resistance was monitored in several study sites in Ethiopia from 2013 to 2015 using papers impregnated with discriminating concentrations of DDT, deltamethrin, bendiocarb, propoxur, malathion, fenitrothion and pirimiphos-methyl, following the WHO insecticide susceptibility test procedure. Mosquitoes sampled from different localities for WHO bioassay were morphologically identified as An. gambiae (s.l.) using standard taxonomic keys. Samples were identified to species using species-specific polymerase chain reaction (PCR) and screened for the presence of target site mutations L1014F, L1014S and N1575Y in the voltage gated sodium channel (VGSC) gene and G119S in the acethylcholinesterase (AChE) gene using allele-specific PCR. Biochemical assays were performed to assess elevated levels of acetylcholinesterases, carboxylcholinesterases, glutathione-S-transferases (GSTs) and cytochrome P450s monooxygenases in wild populations of An. arabiensis, compared to the fully susceptible Sekoru An. arabiensis laboratory strain.

Results

Populations of An. arabiensis were resistant to DDT and deltamethrin but were susceptible to fenitrothion in all the study sites. Reduced susceptibility to malathion, pirimiphos-methyl, propoxur and bendiocarb was observed in some of the study sites. Knockdown resistance (kdr L1014F) was detected in all mosquito populations with allele frequency ranging from 42 to 91%. Elevated levels of glutathione-S-transferases (GSTs) were detected in some of the mosquito populations. However, no elevated levels of monooxygenases and esterases were detected in any of the populations assessed.

Conclusions

Anopheles arabiensis populations from all surveyed sites in Ethiopia exhibited resistance against DDT and pyrethroids. Moreover, some mosquito populations exhibited resistance to propoxur and possible resistance to bendiocarb. Target site mutation kdr L1014F was detected in all mosquito populations while elevated levels of glutathione-S-transferases (GSTs) was detected in some mosquito populations. The reduced susceptibility of An. arabiensis to propoxur and bendiocarb, which are currently used for indoor residual spraying (IRS) in Ethiopia, calls for continuous resistance monitoring, in order to plan and implement evidence based insecticide resistance management.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2342-y) contains supplementary material, which is available to authorized users.

Efficacy of PermaNet® 3.0 and PermaNet® 2.0 nets against laboratory-reared and wild Anopheles gambiae sensu lato populations in northern Tanzania

BACKGROUND

Mosquitoes have developed resistance against pyrethroids, the only class of insecticides approved for use on long-lasting insecticidal nets (LLINs). The present study sought to evaluate the efficacy of the pyrethroid synergist PermaNet® 3.0 LLIN versus the pyrethroid-only PermaNet® 2.0 LLIN, in an East African hut design in Lower Moshi, northern Tanzania. In this setting, resistance to pyrethroid insecticides has been identified in Anopheles gambiae mosquitoes.

METHODS

Standard World Health Organization bioefficacy evaluations were conducted in both laboratory and experimental huts. Experimental hut evaluations were conducted in an area where there was presence of a population of highly pyrethroid-resistant An. arabiensis mosquitoes. All nets used were subjected to cone bioassays and then to experimental hut trials. Mosquito mortality, blood-feeding inhibition and personal protection rate were compared between untreated nets, unwashed LLINs and LLINs that were washed 20 times.

RESULTS

Both washed and unwashed PermaNet® 2.0 and PermaNet® 3.0 LLINs had knockdown and mortality rates of 100% against a susceptible strain of An. gambiae sensu stricto. The adjusted mortality rate of the wild mosquito population after use of the unwashed PermaNet® 3.0 and PermaNet® 2.0 nets was found to be higher than after use of the washed PermaNet® 2.0 and PermaNet® 3.0 nets.

CONCLUSIONS

Given the increasing incidence of pyrethroid resistance in An. gambiae mosquitoes in Tanzania, we recommend that consideration is given to its distribution in areas with pyrethroid-resistant malaria vectors within the framework of a national insecticide-resistance management plan.

Direct interaction of avermectin with epidermal growth factor receptor mediates the penetration resistance in Drosophila larvae

With the widespread use of avermectins (AVMs) for managing parasitic and agricultural pests, the resistance of worms and insects to AVMs has emerged as a serious threat to human health and agriculture worldwide. The reduced penetration of AVMs is one of the main reasons for the development of the resistance to the chemicals. However, the detailed molecular mechanisms remain elusive. Here, we use the larvae of Drosophila melanogaster as the model organism to explore the molecular mechanisms underlying the development of penetration resistance to AVMs. We clearly show that the chitin layer is thickened and the efflux transporter P-glycoprotein (P-gp) is overexpressed in the AVM-resistant larvae epidermis. We reveal that the activation of the transcription factor Relish by the over-activated epidermal growth factor receptor (EGFR)/AKT/ERK pathway induces the overexpression of the chitin synthases DmeCHS1/2 and P-gp in the resistant larvae. Interestingly, we discover for the first time, to the best of our knowledge, that AVM directly interacts with EGFR and leads to the activation of the EGFR/AKT/ERK pathway, which activates the transcription factor Relish and induces the overexpression of DmeCHS1/2 and P-gp. These findings provide new insights into the molecular mechanisms underlying the development of penetration resistance to drugs.

Comparability between insecticide resistance bioassays for mosquito vectors: time to review current methodology?

BACKGROUND

Insecticides play an integral role in the control of mosquito-borne diseases. With resistance to insecticides on the rise, surveillance of the target population for optimal choice of insecticides is a necessity. The Centers for Disease Control and Prevention (CDC) bottle assay and the World Health Organization (WHO) susceptibility test are the most frequently used methods in insecticide resistance monitoring. However, the two bioassays differ in terms of insecticide delivery and how insecticide susceptibility is measured. To evaluate how equivalent data from the two assays are, we compared the two methods side-by-side.

METHODS

We did a literature search from 1998 to December 2014 to identify publications that performed both assays on the same mosquito population and compared the results. We then tested the WHO and CDC bioassays on laboratory strains of Aedes aegypti, Anopheles stephensi, An. gambiae and An. arabiensis with different insecticide resistance levels against permethrin, λ-cyhalothrin, DDT, bendiocarb and malathion. In addition, we also measured the relationship between time-to-knockdown and 24 h mortality.

RESULTS

Both published data and results from the present laboratory experiments showed heterogeneity in the comparability of the two bioassays. Following their standard procedures, the two assays showed poor agreement in detecting resistance at the WHO cut-off mark of 90% (Cohen's κ = 0.06). There was better agreement when 24 h mortality was recorded in the CDC bottle assay and compared with that of the WHO susceptibility test (Cohen's κ = 0.5148). Time-to-knockdown was shown to be an unreliable predictor of 24 h mortality.

CONCLUSION

Even though the two assays can detect insecticide resistance, they may not be used interchangeably. While the diagnostic dose in the WHO susceptibility test does not allow for detecting shifts at low or extreme resistance levels, time-to-knockdown measured in the CDC bottle assay is a poor predictor of 24 h mortality. Therefore, dose-response assays could provide the most flexibility. New standardized bioassays are needed that produce consistent dose-response measurements with a minimal number of mosquitoes.

Synergy between rhinacanthins from Rhinacanthus nasutus in inhibition against mosquito cytochrome P450 enzymes

The cytochrome P450 monooxygenases play a major role in insecticide detoxification and become a target for development of insecticide synergists. In this study, a collection of rhinacanthins (rhinacanthin-D, -E, -G, -N, -Q, and -H/I) purified from Rhinacanthus nasutus, in addition to previously purified rhinacanthin-B and -C, were isolated. These compounds displayed various degrees of inhibition against benzyloxyresorufin-O-debenzylation mediated by CYP6AA3 and CYP6P7 which were implicated in pyrethroid resistance in Anopheles minimus malaria vector. Inhibition modes and kinetics were determined for each of rhinacanthins. Cell-based inhibition assays by rhinacanthins employing 3-(4, 5-dimethylthiazol-2-y-l)-2, 5-diphenyltetrazolium bromide (MTT) cytotoxicity test were explored their synergistic effects with cypermethrin toxicity on CYP6AA3- and CYP6P7-expressing Spodoptera frugiperda (Sf9) cells. Rhinacanthin-B, -D, -E, -G, and -N exhibited mechanism-based inhibition against CYP6AA3, an indication of irreversible inhibition, while rhinacanthin-B, -D, -G, and -N were mechanism-based inhibitors of CYP6P7. There was structure-function relationship of these rhinacanthins in inhibition effects against both enzymes. In vitro enzymatic inhibition assays revealed that there were synergistic interactions among rhinacanthins, except rhinacanthin-B and -Q, in inhibition against both enzymes. These rhinacanthins exerted synergism with cypermethrin toxicity on Sf9 cells expressing each of the two P450 enzymes via P450 inhibition and in addition could interact in synergy to further increase cypermethrin toxicity. The inhibition potentials, synergy among rhinacanthins in inhibition against the P450 detoxification enzymes, and synergism with cypermethrin toxicity of the R. nasutus constituents of reported herein could be beneficial to implement effective resistance management of mosquito vector control.

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.

Impact of agriculture on the selection of insecticide resistance in the malaria vector Anopheles gambiae: a multigenerational study in controlled conditions

Background

Resistance of mosquitoes to insecticides is mainly attributed to their adaptation to vector control interventions. Although pesticides used in agriculture have been frequently mentioned as an additional force driving the selection of resistance, only a few studies were dedicated to validate this hypothesis and characterise the underlying mechanisms. While insecticide resistance is rising dramatically in Africa, deciphering how agriculture affects resistance is crucial for improving resistance management strategies. In this context, the multigenerational effect of agricultural pollutants on the selection of insecticide resistance was examined in Anopheles gambiae.

Methods

An urban Tanzanian An. gambiae population displaying a low resistance level was used as a parental strain for a selection experiment across 20 generations. At each generation larvae were selected with a mixture containing pesticides and herbicides classically used in agriculture in Africa. The resistance levels of adults to deltamethrin, DDT and bendiocarb were compared between the selected and non-selected strains across the selection process together with the frequency of kdr mutations. A microarray approach was used for pinpointing transcription level variations selected by the agricultural pesticide mixture at the adult stage.

Results

A gradual increase of adult resistance to all insecticides was observed across the selection process. The frequency of the L1014S kdr mutation rose from 1.6% to 12.5% after 20 generations of selection. Microarray analysis identified 90 transcripts over-transcribed in the selected strain as compared to the parental and the non-selected strains. Genes encoding cuticle proteins, detoxification enzymes, proteins linked to neurotransmitter activity and transcription regulators were mainly affected. RT-qPCR transcription profiling of candidate genes across multiple generations supported their link with insecticide resistance.

Conclusions

This study confirms the potency of agriculture in selecting for insecticide resistance in malaria vectors. We demonstrated that the recurrent exposure of larvae to agricultural pollutants can select for resistance mechanisms to vector control insecticides at the adult stage. Our data suggest that in addition to selected target-site resistance mutations, agricultural pollutants may also favor cuticle, metabolic and synaptic transmission-based resistance mechanisms. These results emphasize the need for integrated resistance management strategies taking into account agriculture activities.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0480-z) contains supplementary material, which is available to authorized users.

Insecticide resistance and its underlying mechanisms in field populations of Aedes aegypti adults (Diptera: Culicidae) in Singapore

Background

In Singapore, dose–response bioassays of Aedes aegypti (L.) adults have been conducted, but the mechanisms underlying resistance to insecticides remain unclear. In this study, we evaluated insecticide resistance and its underlying mechanism in field populations of Ae. aegypti adults.

Methods

Seven populations of Ae. aegypti were collected from public residential areas and assays were conducted according to WHO guidelines to determine their susceptibility to several commonly used insecticides.

Results

Various levels of pyrethroid resistance (RR₅₀ = 3.76 to 142.06-fold) and low levels of pirimiphos-methyl resistance (RR₅₀ = 1.01 to 1.51-fold) were detected. The insecticide susceptibility profile of Ae. aegypti adults was homogenous among the different study sites. Addition of the synergists piperonyl butoxide, S,S,S,-tributyl phosphorotrithioate, and triphenyl phosphate generally failed to enhance the toxicity of the insecticides investigated, suggesting an insignificant role of metabolic-based insecticide resistance and possible involvement of target site resistance. Further biochemical investigation of specific metabolic enzyme activities provided further evidence that detoxifying enzymes such as mono-oxygenases, esterases, glutathione S-transferases and altered acethylcholinesterases generally did not contribute to the resistance observed.

Conclusions

This study confirmed the presence of pyrethroid resistance among Ae. aegypti adults in Singapore and documented the early onset of organophosphate resistance.

Distribution and spread of pyrethroid and DDT resistance among the Anopheles gambiae complex in Tanzania

The development of insecticide resistance is a threat to the control of malaria in Africa. We report the findings of a national survey carried out in Tanzania in 2011 to monitor the susceptibility of malaria vectors to pyrethroid, organophosphate, carbamate and DDT insecticides, and compare these findings with those identified in 2004 and 2010. Standard World Health Organization (WHO) methods were used to detect knock-down and mortality rates in wild female Anopheles gambiae s.l. (Diptera: Culicidae) collected from 14 sentinel districts. Diagnostic doses of the pyrethroids deltamethrin, lambdacyhalothrin and permethrin, the carbamate propoxur, the organophosphate fenitrothion and the organochlorine DDT were used. Anopheles gambiae s.l. was resistant to permethrin in Muleba, where a mortality rate of 11% [95% confidence interval (CI) 6-19%] was recorded, Muheza (mortality rate of 75%, 95% CI 66-83%), Moshi and Arumeru (mortality rates of 74% in both). Similarly, resistance was reported to lambdacyhalothrin in Muleba, Muheza, Moshi and Arumeru (mortality rates of 31-82%), and to deltamethrin in Muleba, Moshi and Muheza (mortality rates of 28-75%). Resistance to DDT was reported in Muleba. No resistance to the carbamate propoxur or the organophosphate fenitrothion was observed. Anopheles gambiae s.l. is becoming resistant to pyrethoids and DDT in several parts of Tanzania. This has coincided with the scaling up of vector control measures. Resistance may impair the effectiveness of these interventions and therefore demands close monitoring and the adoption of a resistance management strategy.

Genetic basis of pyrethroid resistance in a population of Anopheles arabiensis, the primary malaria vector in Lower Moshi, north-eastern Tanzania

BACKGROUND

Pyrethroid resistance has been slower to emerge in Anopheles arabiensis than in An. gambiae s.s and An. funestus and, consequently, studies are only just beginning to unravel the genes involved. Permethrin resistance in An. arabiensis in Lower Moshi, Tanzania has been linked to elevated levels of both P450 monooxygenases and β-esterases. We have conducted a gene expression study to identify specific genes linked with metabolic resistance in the Lower Moshi An. arabiensis population.

METHODS

Microarray experiments employing an An. gambiae whole genome expression chip were performed on An. arabiensis, using interwoven loop designs. Permethrin-exposed survivors were compared to three separate unexposed mosquitoes from the same or a nearby population. A subsection of detoxification genes were chosen for subsequent quantitative real-time PCR (qRT-PCR).

RESULTS

Microarray analysis revealed significant over expression of 87 probes and under expression of 85 probes (in pairwise comparisons between permethrin survivors and unexposed sympatric and allopatric samples from Dar es Salaam (controls). For qRT-PCR we targeted over expressed ABC transporter genes (ABC '2060'), a glutathione-S-transferase, P450s and esterases. Design of efficient, specific primers was successful for ABC '2060'and two P450s (CYP6P3, CYP6M2). For the CYP4G16 gene, we used the primers that were previously used in a microarray study of An. arabiensis from Zanzibar islands. Over expression of CYP4G16 and ABC '2060' was detected though with contrasting patterns in pairwise comparisons between survivors and controls. CYP4G16 was only up regulated in survivors, whereas ABC '2060' was similar in survivors and controls but over expressed in Lower Moshi samples compared to the Dar es Salaam samples. Increased transcription of CYP4G16 and ABC '2060' are linked directly and indirectly respectively, with permethrin resistance in Lower Moshi An. arabiensis.

CONCLUSIONS

Increased transcription of a P450 (CYP4G16) and an ABC transporter (ABC 2060) are linked directly and indirectly respectively, with permethrin resistance in Lower Moshi An. arabiensis. Our study provides replication of CYP4G16 as a candidate gene for pyrethroid resistance in An. arabiensis, although its role may not be in detoxification, and requires further investigation.

Laboratory and experimental hut evaluation of a long-lasting insecticide treated blanket for protection against mosquitoes

Background

Long-lasting insecticide treated blankets (LLIBs) may provide additional protection against malaria where use of long lasting insecticidal nets (LLIN) is low or impractical such as in disaster or emergency situations.

Methods

Initial efficacy testing of a new candidate LLIB was carried out at LSHTM and KCMUCo, before and after washing, in cone and ball bioassays and arm-in-cage tests against pyrethroid susceptible Anopheles gambiae. A small scale field trial was conducted using veranda-trap experimental huts in northern Tanzania against wild An. arabiensis and Culex quinquefasciatus mosquitoes. Treatments included unwashed and 5 times washed permethrin treated LLIB and blankets hand-treated with permethrin (ITB), untreated blankets, and a holed unwashed Olyset net.

Results

Cone test mortality was 75% for LLIB when unwashed, but decreased to 32% after 5 washes and <10% after 10 washes. In arm-in-cage tests protection against biting was 100% for LLIBs regardless of the number of washes while reduction in landings was 79% when unwashed, 75% after 5 washes, but declined to 41% after 10 and 33% after 20 washes. In ball bioassays using pyrethroid resistant An. arabiensis, mortality was low in all treatments (<35%) and there was no significant difference in mortality between Olyset net, LLIB or ITB (p > 0.05). Percentage mortality of An. arabiensis in huts with LLIB unwashed (26%) was not statistically different to Olyset net (31%, p = 0.5). The 5 times washed LLIB reduced blood-feeding by 49% which was equivalent to Olyset net (p > 0.086). There was no significant difference in percentage blood-feeding between LLIB and ITB unwashed or 5 times washed (p = 0.147 and p = 0.346 respectively). The 5 times washed LLIB reduced blood-feeding of Culex quinquefasciatus by 40%, although the Olyset provided the greatest protection with 85% inhibition. ELISA analysis of a sub-sample of blood fed mosquitoes showed that not all had fed on humans in the huts, therefore blood-feeding inhibition may have been underestimated.

Conclusions

This trial demonstrated the potential of LLIBs to provide substantial personal protection even against pyrethroid resistant mosquitoes. LLIBs may prove particularly useful where LLINs are unsuitable or net usage is low.

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.

Co-occurrence and distribution of East (L1014S) and West (L1014F) African knock-down resistance in Anopheles gambiae sensu lato population of Tanzania

Objective

Insecticide resistance molecular markers can provide sensitive indicators of resistance development in Anopheles vector populations. Assaying these makers is of paramount importance in the resistance monitoring programme. We investigated the presence and distribution of knock-down resistance (kdr) mutations in Anopheles gambiae s.l. in Tanzania.

Methods

Indoor-resting Anopheles mosquitoes were collected from 10 sites and tested for insecticide resistance using the standard WHO protocol. Polymerase chain reaction-based molecular diagnostics were used to genotype mosquitoes and detect kdr mutations.

Results

The An. gambiae tested were resistance to lambdacyhalothrin in Muheza, Arumeru and Muleba. Out of 350 An. gambiae s.l. genotyped, 35% were An. gambiae s.s. and 65% An. arabiensis. L1014S and L1014F mutations were detected in both An. gambiae s.s. and An. arabiensis. L1014S point mutation was found at the allelic frequency of 4–33%, while L1014F was at the allelic frequency 6–41%. The L1014S mutation was much associated with An. gambiae s.s. (χ² = 23.41; P < 0.0001) and L1014F associated with An. arabiensis (χ² = 11.21; P = 0.0008). The occurrence of the L1014S allele was significantly associated with lambdacyhalothrin resistance mosquitoes (Fisher exact P < 0.001).

Conclusion

The observed co-occurrence of L1014S and L1014F mutations coupled with reports of insecticide resistance in the country suggest that pyrethroid resistance is becoming a widespread phenomenon among our malaria vector populations. The presence of L1014F mutation in this East African mosquito population indicates the spreading of this gene across Africa. The potential operational implications of these findings on malaria control need further exploration.

Objectif

Les marqueurs moléculaires de la résistance aux insecticides peuvent fournir des indicateurs sensibles du développement de la résistance dans les populations de vecteurs Anopheles. Le test de ces indicateurs est d'une importance énorme dans le programme de surveillance de la résistance. Nous avons étudié la présence et la répartition des mutations de résistance knockdown (kdr) chez Anopheles gambiae s.l. en Tanzanie.

Méthodes

Des anophèles d'intérieur, au repos ont été collectées dans 10 sites et testées pour la résistance aux insecticides en utilisant le protocole standard de l'OMS. Les diagnostics moléculaires basés sur la PCR ont été utilisés pour le génotypage des moustiques et la détection des génotypes kdr.

Résultats

Les An. gambiae testées étaient résistantes à la lambdacyhalothrine à Muheza, Arumeru et Muleba. Sur 350 An. gambiae s.l. génotypées, 35% étaient An. gambiae s.s. et 65% étaient An. arabiensis. Les mutations L1014S et L1014F ont été détectées à la fois chez An. gambiae s.s. et An. arabiensis. La mutation ponctuelle L1014S a été trouvée à la fréquence allélique de 4 à 33%, tandis que L1014F était à la fréquence allélique de 6 à 14%. La mutation L1014S a été fortement associée à An. gambiae s.s. (Chi carré = 23,41; P<0,0001) et L1014F était associée à An. arabiensis (chi carré = 11,21; P = 0,0008). L'allèle L1014S était significativement associé aux moustiques résistants à la lambdacyhalothrine (Fisher P exact <0,001).

Conclusion

La cooccurrence des mutations L1014S et L1014F couplées à des rapports sur la résistance aux insecticides suggèrent que la résistance aux pyréthrinoïdes est en train de devenir un phénomène répandu dans les populations de vecteurs du paludisme en Tanzanie. La présence de la mutation L1014F dans cette population de moustiques en Afrique de l'Est indique la propagation de ce gène à travers l'Afrique. L'investigation des implications opérationnelles potentielles de ces résultats sur le contrôle du paludisme devraient être approfondie.

Objetivo

Los marcadores moleculares de resistencia a insecticidas pueden ser indicadores sensibles del desarrollo de resistencias en las poblaciones de los vectores Anopheles. Evaluar dichos marcadores es crucial para los programas de monitorización de resistencias. Hemos investigado la presencia y la distribución de las mutaciones de resistencia knockdown (kdr) en Anopheles gambiae s.l. en Tanzania.

Métodos

Se recolectaron mosquitos Anopheles intradomiciliarios de 10 lugares diferentes y se evaluaron en busca de resistencia a insecticidas utilizando el protocolo estándar de la OMS. Mediante un diagnóstico molecular basado en la PCR se genotiparon los mosquitos y se detectaron los genotipos kdr.

Resultados

Los An. gambiae evaluados eran resistentes a lambdacialotrina en Muheza, Arumeru y Muleba. De 350 An. gambiae s.l. genotipados, 35% eran An. gambiae s.s. y 65% eran An. arabiensis. Se detectaron mutaciones L1014S y L1014F tanto en An. gambiae s.s. como en An. arabiensis. La mutación puntual L1014S se encontró con una frecuencia alélica de 4-33%, mientras que L1014F tenía una frecuencia alélica de 6-14%. La mutación L1014S estaba ampliamente asociada a An. gambiae s.s. (Chi-Cuadrado = 23.41; P < 0.0001) y la L1014F estaba asociada con An. arabiensis (Chi-Square = 11.21; P = 0.0008). El alelo L1014S estaba significativamente asociado con mosquitos resistentes a la lambdacialotrina (P < 0.001).

Conclusión

La simultaneidad de mutaciones de L1014S y L1014F junto con informes de resistencia a los insecticidas sugiere que la resistencia a piretroides se está convirtiendo en un fenómeno común entre las poblaciones del vector de la malaria en Tanzania. La presencia de la mutación L1014F en estas poblaciones del Este de África indican la diseminación del gen a lo largo del continente africano. Determinar las implicaciones potenciales a nivel operativo de estos hallazgos sobre el control de la malaria requiere de más estudios.

Indoor application of attractive toxic sugar bait (ATSB) in combination with mosquito nets for control of pyrethroid-resistant mosquitoes

Background

Attractive toxic sugar bait (ATSB) sprayed onto vegetation has been successful in controlling Anopheles mosquitoes outdoors. Indoor application of ATSB has yet to be explored. The purpose of this study was to determine whether ATSB stations positioned indoors have the potential to kill host-seeking mosquitoes and constitute a new approach to control of mosquito-borne diseases.

Methods

Insecticides were mixed with dyed sugar solution and tested as toxic baits against Anopheles arabiensis, An. Gambiae s.s. and Culex quinquefasciatus in feeding bioassay tests to identify suitable attractant-insecticide combinations. The most promising ATSB candidates were then trialed in experimental huts in Moshi, Tanzania. ATSB stations were hung in huts next to untreated mosquito nets occupied by human volunteers. The proportions of mosquitoes killed in huts with ATSB treatments relative to huts with non-insecticide control treatments huts were recorded, noting evidence of dye in mosquito abdomens.

Results

In feeding bioassays, chlorfenapyr 0.5% v/v, boric acid 2% w/v, and tolfenpyrad 1% v/v, mixed in a guava juice-based bait, each killed more than 90% of pyrethroid-susceptible An. Gambiae s.s. and pyrethroid-resistant An. arabiensis and Cx. quinquefasciatus. In the hut trial, mortality rates of the three ATSB treatments ranged from 41-48% against An. arabiensis and 36-43% against Cx. quinquefasciatus and all were significantly greater than the control mortalities: 18% for An. arabiensis, 7% for Cx. quinquefasciatus (p<0.05). Mortality rates with ATSB were comparable to those with long lasting insecticidal nets previously tested against the same species in this area.

Conclusions

Indoor ATSB shows promise as a supplement to mosquito nets for controlling mosquitoes. Indoor ATSB constitute a novel application method for insecticide classes that act as stomach poisons and have not hitherto been exploited for mosquito control. Combined with LLIN, indoor use of ATSB has the potential to serve as a strategy for managing insecticide resistance.

Towards malaria elimination and its implication for vector control, disease management and livelihoods in Tanzania

Over the years, malaria has remained the number one cause of morbidity and mortality in Tanzania. Population based studies have indicated a decline in overall malaria prevalence among under-fives from 18.1% in 2008 to 9.7% in 2012. The decline of malaria infection has occurred in all geographical zones of the country. Malaria mortality and cumulative probability of deaths have also shown a marked decline from 2000 to 2010. During the same period, area specific studies in Muheza, Korogwe, Muleba and Mvomero have also reported a similar declining trend in malaria prevalence and incidence. The decline in malaria prevalence has been observed to coincide with a decline in transmission indices including anopheline mosquito densities. The decline in malaria prevalence has been attributed to a combination of factors including improved access to effective malaria treatment with artemisinin combination therapy and protection from mosquito bites by increased availability of insecticide treated bednets and indoor residual spraying. The objective of this paper was to review the changing landscape of malaria and its implication for disease management, vector control, and livelihoods in Tanzania. It seeks to examine the links within a broad framework that considers the different pathways given the multiplicity of interactions that can produce unexpected outcomes and trade-offs. Despite the remarkable decline in malaria burden, Tanzania is faced with a number of challenges. These include the development of resistance of malaria vectors to pyrethroids, changing mosquito behaviour and livelihood activities that increase mosquito productivity and exposure to mosquito bites. In addition, there are challenges related to health systems, community perceptions, community involvement and sustainability of funding to the national malaria control programme. This review indicates that malaria remains an important and challenging disease that illustrates the interactions among ecosystems, livelihoods, and health systems. Livelihoods and several sectoral development activities including construction, water resource development and agricultural practices contribute significantly to malaria mosquito productivity and transmission. Consequently, these situations require innovative and integrative re-thinking of the strategies to prevent and control malaria. In conclusion, to accelerate and sustain malaria control in Tanzania, the prevention strategies must go hand in hand with an intersectoral participation approach that takes into account ecosystems and livelihoods that have the potential to increase or decrease malaria transmission.

The dynamics of pyrethroid resistance in Anopheles arabiensis from Zanzibar and an assessment of the underlying genetic basis

Background

The emergence of pyrethroid resistance in the malaria vector, Anopheles arabiensis, threatens to undermine the considerable gains made towards eliminating malaria on Zanzibar. Previously, resistance was restricted to the island of Pemba while mosquitoes from Unguja, the larger of the two islands of Zanzibar, were susceptible. Here, we characterised the mechanism(s) responsible for resistance on Zanzibar using a combination of gene expression and target-site mutation assays.

Methods

WHO resistance bioassays were conducted using 1-5d old adult Anopheles gambiae s.l. collected between 2011 and 2013 across the archipelago. Synergist assays with the P450 inhibitor piperonyl-butoxide were performed in 2013. Members of the An. gambiae complex were PCR-identified and screened for target-site mutations (kdr and Ace-1). Gene expression in pyrethroid resistant An. arabiensis from Pemba was analysed using whole-genome microarrays.

Results

Pyrethroid resistance is now present across the entire Zanzibar archipelago. Survival to the pyrethroid lambda-cyhalothrin in bioassays conducted in 2013 was 23.5-54.3% on Unguja and 32.9-81.7% on Pemba. We present evidence that resistance is mediated, in part at least, by elevated P450 monoxygenases. Whole-genome microarray scans showed that the most enriched gene terms in resistant An. arabiensis from Pemba were associated with P450 activity and synergist assays with PBO completely restored susceptibility to pyrethroids in both islands. CYP4G16 was the most consistently over-expressed gene in resistant mosquitoes compared with two susceptible strains from Unguja and Dar es Salaam. Expression of this P450 is enriched in the abdomen and it is thought to play a role in hydrocarbon synthesis. Microarray and qPCR detected several additional genes putatively involved in this pathway enriched in the Pemba pyrethroid resistant population and we hypothesise that resistance may be, in part, related to alterations in the structure of the mosquito cuticle. None of the kdr target-site mutations, associated with pyrethroid/DDT resistance in An. gambiae elsewhere in Africa, were found on the islands.

Conclusion

The consequences of this resistance phenotype are discussed in relation to future vector control strategies on Zanzibar to support the ongoing malaria elimination efforts on the islands.

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.

Comparison of the standard WHO susceptibility tests and the CDC bottle bioassay for the determination of insecticide susceptibility in malaria vectors and their correlation with biochemical and molecular biology assays in Benin, West Africa

Background

The detection of insecticide resistance in natural populations of Anopheles vectors is absolutely necessary for malaria control. In the African region, the WHO insecticide susceptibility test is the most common method for assessing resistance status. In order to search for a simple, rapid and more reliable technique in the assessment of insecticide resistance in malaria vectors, we compared the WHO tests with the CDC bottle bioassay in the Ouemé province of southern Benin where insecticide resistance has been widely reported.

Methods

Larvae and pupae of Anopheles gambiae s.l. mosquitoes were collected from the breeding sites in Ouemé. WHO and CDC susceptibility tests were conducted simultaneously on unfed female mosquitoes aged 2–5 days old. WHO bioassays were performed with impregnated papers of deltamethrin (0.05%) and bendiocarb (0.1%), whereas CDC bioassays were performed with stock solutions of deltamethrin (12.5 μg per bottle) and bendiocarb (12.5 μg per bottle). PCR techniques were used to detect species, Kdr and Ace-1 mutations. CDC biochemical assays using synergists were also conducted to assess the metabolic resistance.

Results

A slight decrease in mortality rates was observed with 97.95% and 98.33% obtained from CDC and WHO bioassays respectively in populations of mosquitoes from Adjara and Dangbo. PCR revealed that all specimens tested were Anopheles gambiae s.s. The Kdr mutation was found at high frequency in all populations and both the Kdr mutation and mono-oxygenase enzymes were implicated as mechanisms of pyrethroid resistance in An. gambiae from Misserete.

Conclusion

This study emphasizes that both WHO and CDC bioassays give similar results with regards to the susceptibility of mosquitoes to insecticides in southern Benin. There were complementarities between both methods, however, some specificity was noted for each of the two methods used. Both Kdr and metabolic mechanisms were implicated in the resistance.