Pyrethroid resistance in Anopheles gambiae s.s. and Anopheles arabiensis in western Kenya: phenotypic, metabolic and target site characterizations of three populations

Malaria transmission potential of Anopheles gambiae s.l. in indoor residual spraying areas with clothianidin 50 WG in northern Benin

The study objective was to assess the frequency of the kdr-L995F and ace-1 G280S genetic mutations in Anopheles gambiae s.l. mosquitoes and examine their ability to transmit Plasmodium falciparum in areas where indoor residual spraying (IRS) was implemented with Clothianidin 50 WG. The study was conducted in six communes in the Alibori and Donga departments of which four were IRS-treated and two were untreated and served as control. Post-IRS monthly samples of adult mosquitoes were collected in study communes using human landing catches (HLC). An. gambiae s.l. specimens were processed to detect kdr-L995F and ace-1 G280S mutations via PCR as well as Plasmodium falciparum infectivity through CSP ELISA. Our data revealed a high and similar allelic frequency for the kdr-L995F mutation in both treated and control communes (79% vs. 77%, p = 0.14) whilst allelic frequency of the ace-1 G280S mutation was lower across the study area (2-3%, p = 0.58). The sporozoite rate was 2.6% and 2.4% respectively in treated and untreated communes (p = 0.751). No association was found between Plasmodium falciparum infection in Anopheles gambiae s.l. vectors and carriage of kdr-L995F and ace-1 G280S mutations regardless of genotypes. The study findings underline the need for an integrated approach to malaria control, combining different control methods to effectively target transmission. Regular monitoring of insecticide resistance and genetic mutations is essential to guide control strategies.

Assessing insecticide susceptibility, diagnostic dose and time for the sand fly Phlebotomus argentipes, the vector of visceral leishmaniasis in India, using the CDC bottle bioassay

Visceral leishmaniasis (VL) is a vector-borne protozoan disease, which can be fatal if left untreated. Synthetic chemical insecticides are very effective tools for controlling of insect vectors, including the sand fly Phlebotomus argentipes, the vector of VL in the Indian subcontinent. However, repeated use of the same insecticide with increasing doses potentially can create high selection pressure and lead to tolerance and resistance development. The objective of this study was to determine the lethal concentrations and assess levels of susceptibility, diagnostic doses and times to death of laboratory-reared P. argentipes to five insecticides that are used worldwide to control vectors. Using the Center for Disease Control and Prevention (CDC) bottle bioassay, 20-30 sand flies were exposed in insecticide- coated 500-ml glass bottles. Flies were then observed for 24 hours and mortality was recorded. Dose-response survival curves were generated for each insecticide using QCal software and lethal concentrations causing 50%, 90% and 95% mortality were determined. A bioassay was also conducted to determine diagnostic doses and diagnostic times by exposing 20-30 flies in each bottle containing set concentrations of insecticide. Mortality was recorded at 10-minute intervals for 120 minutes to generate the survival curve. Phlebotomus argentipes are highly susceptible to alpha-cypermethrin, followed by deltamethrin, malathion, chlorpyrifos, and least susceptible to DDT. Also, the lowest diagnostic doses and diagnostic times were established for alpha-cypermethrin (3μg/ml for 40 minutes) to kill 100% of the flies. The susceptibility data, diagnostic doses and diagnostic times presented here will be useful as baseline reference points for future studies to assess insecticide susceptibility and resistance monitoring of field caught sand flies and to assist in surveillance as VL elimination is achieved in the region.

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

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

The resting behavior of malaria vectors in different ecological zones of Ghana and its implications for vector control

BACKGROUND

In sub-Saharan Africa there is widespread use of long-lasting insecticidal nets and indoor residual spraying to help control the densities of malaria vectors and decrease the incidence of malaria. This study was carried out to investigate the resting behavior, host preference and infection with Plasmodium falciparum of malaria vectors in Ghana in the context of the increasing insecticide resistance of malaria vectors in sub-Saharan Africa.

METHODS

Indoor and outdoor resting anopheline mosquitoes were sampled during the dry and rainy seasons in five sites in three ecological zones [Sahel savannah (Kpalsogo, Pagaza, Libga); coastal savannah (Anyakpor); and forest (Konongo)]. Polymerase chain reaction-based molecular diagnostics were used to determine speciation, genotypes for knockdown resistance mutations (L1014S and L1014F) and the G119S ace1 mutation, specific host blood meal origins and sporozoite infection in the field-collected mosquitoes.

RESULTS

Anopheles gambiae sensu lato (s.l.) predominated (89.95%, n = 1718), followed by Anopheles rufipes (8.48%, n = 162) and Anopheles funestus s.l. (1.57%, n = 30). Sibling species of the Anopheles gambiae s.l. revealed Anopheles coluzzii accounted for 63% (95% confidence interval = 57.10-68.91) and 27% (95% confidence interval = 21.66-32.55) was Anopheles gambiae s. s.. The mean resting density of An. gambiae s.l. was higher outdoors (79.63%; 1368/1718) than indoors (20.37%; 350/1718) (Wilcoxon rank sum test, Z = - 4.815, P < 0.0001). The kdr west L1014F and the ace1 mutation frequencies were higher in indoor resting An. coluzzii and An. gambiae in the Sahel savannah sites than in the forest and coastal savannah sites. Overall, the blood meal analyses revealed that a larger proportion of the malaria vectors preferred feeding on humans (70.2%) than on animals (29.8%) in all of the sites. Sporozoites were only detected in indoor resting An. coluzzii from the Sahel savannah (5.0%) and forest (2.5%) zones.

CONCLUSIONS

This study reports high outdoor resting densities of An. gambiae and An. coluzzii with high kdr west mutation frequencies, and the presence of malaria vectors indoors despite the use of long-lasting insecticidal nets and indoor residual spraying. Continuous monitoring of changes in the resting behavior of mosquitoes and the implementation of complementary malaria control interventions that target outdoor resting Anopheles mosquitoes are necessary in Ghana.

Relationship between temperature and Anopheles gambiae sensu lato mosquitoes' susceptibility to pyrethroids and expression of metabolic enzymes

Background

Malaria remains one of the most devastating diseases globally, and the control of mosquitoes as the vector is mainly dependent on chemical insecticides. Elevated temperatures associated with future warmer climates could affect mosquitoes' metabolic enzyme expression and increase insecticide resistance, making vector control difficult. Understanding how mosquito rearing temperatures influence their susceptibility to insecticide and expression of metabolic enzymes could aid in the development of novel tools and strategies to control mosquitoes in a future warmer climate. This study evaluated the effects of temperature on the susceptibility of Anopheles gambiae sensu lato (s.l.) mosquitoes to pyrethroids and their expression of metabolic enzymes.

Methods

Anopheles gambiae s.l. eggs obtained from laboratory-established colonies were reared under eight temperature regimes (25, 28, 30, 32, 34, 36, 38, and 40 °C). Upon adult emergence, 3- to 5-day-old female non-blood-fed mosquitoes were used for susceptibility tests following the World Health Organization (WHO) bioassay protocol. Batches of 20–25 mosquitoes from each temperature regime (25–34 °C) were exposed to two pyrethroid insecticides (0.75% permethrin and 0.05% deltamethrin). In addition, the levels of four metabolic enzymes (α-esterase, β-esterase, glutathione S-transferase [GST], and mixed-function oxidase [MFO]) were examined in mosquitoes that were not exposed and those that were exposed to pyrethroids.

Results

Mortality in An. gambiae s.l. mosquitoes exposed to deltamethrin and permethrin decreased at temperatures above 28 °C. In addition, mosquitoes reared at higher temperatures were more resistant and had more elevated enzyme levels than those raised at low temperatures. Overall, mosquitoes that survived after being exposed to pyrethroids had higher levels of metabolic enzymes than those that were not exposed to pyrethroids.

Conclusions

This study provides evidence that elevated temperatures decreased An. gambiae s.l. mosquitoes' susceptibility to pyrethroids and increased the expression of metabolic enzymes. This evidence suggests that elevated temperatures projected in a future warmer climate could increase mosquitoes' resistance to insecticides and complicate malaria vector control measures. This study therefore provides vital information, and suggests useful areas of future research, on the effects of temperature variability on mosquitoes that could guide vector control measures in a future warmer climate.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05273-z.

Behavioral responses of pyrethroid resistant and susceptible Anopheles gambiae mosquitoes to insecticide treated bed net

Background

Long-lasting insecticidal nets are an effective tool in reducing malaria transmission. However, with increasing insecticide resistance little is known about how physiologically resistant malaria vectors behave around a human-occupied bed net, despite their importance in malaria transmission. We used the Mbita bednet trap to assess the host-seeking behavior of insecticide-resistant Anopheles gambiae mosquitoes under semi-field conditions. The trap incorporates a mosquito netting panel which acts as a mechanical barrier that prevents host-seeking mosquitoes from reaching the human host baiting the trap.

Methods

Susceptible and pyrethroid-resistant colonies of female Anopheles gambiae mosquitoes aged 3–5 days old were used in this study. The laboratory-bred mosquitoes were color-marked with fluorescent powders and released inside a semi-field environment where a human subject slept inside a bednet trap erected in a traditional African hut. The netting panel inside the trap was either untreated (control) or deltamethrin-impregnated. The mosquitoes were released outside the hut. Only female mosquitoes were used. A window exit trap was installed on the hut to catch mosquitoes exiting the hut. A prokopack aspirator was used to collect indoor and outdoor resting mosquitoes. In addition, clay pots were placed outside the hut to collect outdoor resting mosquitoes. The F1 progeny of wild-caught mosquitoes were also used in these experiments.

Results

The mean number of resistant mosquitoes trapped in the deltamethrin-impregnated bed net trap was higher (mean = 50.21± 3.7) compared to susceptible counterparts (mean + 22.4 ± 1.31) (OR = 1.445; P<0.001). More susceptible mosquitoes were trapped in an untreated (mean = 51.9 ± 3.6) compared to a deltamethrin-treated bed net trap (mean = 22.4 ± 1.3) (OR = 2.65; P<0.001). Resistant mosquitoes were less likely to exit the house when a treated bed net was present compared to the susceptible mosquitoes. The number of susceptible mosquitoes caught resting outdoors (mean + 28.6 ± 2.22) when a treated bed net was hanged was higher than when untreated bednet was present inside the hut (mean = 4.6 ± 0.74). The susceptible females were 2.3 times more likely to stay outdoors away from the treated bed net (OR = 2.25; 95% CI = [1.7–2.9]; P<0.001).

Conclusion

The results show that deltamethrin-treatment of netting panels inside the bednet trap did not alter the host-seeking behavior of insecticide-resistant female An. gambiae mosquitoes. On the contrary, susceptible females exited the hut and remained outdoors when a treated net was used. However, further investigations of the behavior of resistant mosquitoes under natural conditions should be undertaken to confirm these observations and improve the current intervention which are threatened by insecticide resistance and altered vector behavior.

Insecticide Resistance in Aedes aegypti Varies Seasonally and Geographically in Texas/Mexico Border Cities

Insecticide use is the primary method of attempting to reduce or control the spread of mosquito-borne diseases. Insecticide resistance is a major concern as resistance will limit the efficacy of vector-control efforts. The lower Rio Grande Valley region of South Texas has had autochthonous transmission of multiple mosquito-borne diseases including those caused by dengue virus, chikungunya virus, and Zika virus. However, the current status of mosquito resistance to commonly used pesticides in this region is unknown. In this study, we collected field samples from multiple municipalities in South Texas and assessed resistance using the Centers for Disease Control and Prevention bottle bioassay. All populations exhibited characteristics of resistance, and permethrin was the most effective insecticide with an average mortality rate of 44.78%. Deltamethrin and sumethrin had significantly lower mortality rates of 20.31% and 32.16%, respectively, although neither of these insecticides are commonly used for vector-control activities in this region. Depending on which insecticide was used, there was little significance between each of the 7 cities. Seasonal variation in resistance was observed among the collection sites. Both deltamethrin and sumethrin exhibited an increase in susceptibility over the course of 10 months, while permethrin exhibited a decrease in susceptibility. These data highlight the need for further studies to determine if variations in resistance observed are repeated. The data and future findings may be useful in determining the most effective strategies for pesticide use and rotation.

Genetic diversity and population structure of the human malaria parasite Plasmodium falciparum surface protein Pfs47 in isolates from the lowlands in Western Kenya

Plasmodium falciparum parasites have evolved genetic adaptations to overcome immune responses mounted by diverse Anopheles vectors hindering malaria control efforts. Plasmodium falciparum surface protein Pfs47 is critical in the parasite’s survival by manipulating the vector’s immune system hence a promising target for blocking transmission in the mosquito. This study aimed to examine the genetic diversity, haplotype distribution, and population structure of Pfs47 and its implications on malaria infections in endemic lowlands in Western Kenya. Cross-sectional mass blood screening was conducted in malaria endemic regions in the lowlands of Western Kenya: Homa Bay, Kombewa, and Chulaimbo. Dried blood spots and slide smears were simultaneously collected in 2018 and 2019. DNA was extracted using Chelex method from microscopic Plasmodium falciparum positive samples and used to genotype Pfs47 using polymerase chain reaction (PCR) and DNA sequencing. Thirteen observed haplotypes of the Pfs47 gene were circulating in Western Kenya. Population-wise, haplotype diversity ranged from 0.69 to 0.77 and the nucleotide diversity 0.10 to 0.12 across all sites. All the study sites displayed negative Tajima’s D values although not significant. However, the negative and significant Fu’s Fs statistical values were observed across all the study sites, suggesting population expansion or positive selection. Overall genetic differentiation index was not significant (F_ST = -0.00891, P > 0.05) among parasite populations. All Nm values revealed a considerable gene flow in these populations. These results could have important implications for the persistence of high levels of malaria transmission and should be considered when designing potential targeted control interventions.

Larval ecology and bionomics of Anopheles funestus in highland and lowland sites in western Kenya

Background

An. funestus is a major Afrotropical vector of human malaria. This study sought to investigate the larval ecology, sporozoite infection rates and blood meal sources of An. funestus in western Kenya.

Methods

Larval surveys were carried out in Bungoma (Highland) and Kombewa (lowland) of western Kenya. Aquatic habitats were identified, characterized, georeferenced and carefully examined for mosquito larvae and predators. Indoor resting mosquitoes were sampled using pyrethrum spray catches. Adults and larvae were morphologically and molecularly identified to species. Sporozoite infections and blood meal sources were detected using real-time PCR and ELISA respectively.

Results

Of the 151 aquatic habitats assessed, 62/80 (78%) in Bungoma and 58/71(82%) in Kombewa were positive for mosquito larvae. Of the 3,193 larvae sampled, An. funestus larvae constitute 38% (1224/3193). Bungoma recorded a higher number of An. funestus larvae (85%, 95%, CI, 8.722–17.15) than Kombewa (15%, 95%, CI, 1.33–3.91). Molecular identification of larvae showed that 89% (n = 80) were An. funestus. Approximately 59%, 35% and 5% of An. funestus larvae co-existed with An. gambiae s.l, Culex spp and An. coustani in the same habitats respectively. Of 1,221 An. funestus s.l adults sampled, molecular identifications revealed that An. funestus constituted 87% (n = 201) and 88% (n = 179) in Bungoma and Kombewa, respectively. The Plasmodium falciparum sporozoite rate of An. funestus in Bungoma and Kombewa was 2% (3/174) and 1% (2/157), respectively, and the human blood index of An. funestus was 84% (48/57) and 89% (39/44) and for Bungoma and Kombewa, respectively.

Conclusion

Man-made ponds had the highest abundance of An. funestus larvae. Multiple regression and principal component analyses identified the distance to the nearest house as the key environmental factor associated with the abundance of An. funestus larvae in aquatic habitats. This study serves as a guide for the control of An. funestus and other mosquito species to complement existing vector control strategies.

Cypermethrin and ivermectin resistance in field populations of Rhipicephalus sanguineus sensu lato (Latrielle, 1806) collected from dogs in south India

The abuse of acaricides for control of Rhipicephalus sanguineus sensu lato ticks led to a strong selection pressure for acaricide resistance. Data on acaricide resistance in R. sanguineus s.l. populations from India is meager though veterinarians frequently report treatment failures. The present study was aimed to characterize the level of resistance in R. sanguineus s.l. against most commonly used drugs, cypermethrin and ivermectin in Andhra Pradesh, south India. Fourteen adult female tick populations were collected from naturally infested dogs at veterinary clinics, residence and stray dog in nine state municipal corporations/municipalities. Information on the history of dog treatment with acaricides was obtained by interviewing dog owners. The larval packet test (LPT) and larval immersion test (LIT) were used on the larvae of ticks to test for resistance to cypermethrin and ivermectin, respectively. Mortality data of each tick population was analyzed by probit analysis. Corresponding to the most susceptible population, thirteen (92.6%) and six (42.9%) tick populations were regarded as resistant to cypermethrin and ivermectin, respectively. The phenotypic level of cypermethrin (resistance ratios at LC50% varied from 1.55 to 13.29) and ivermectin (resistance ratios at LC50% ranged from 1.16 to 4.79) resistance varied distinctly between the populations. Additionally, all the populations were tested using PCR to demonstrate the frequency of the point mutation in sodium channel gene that corresponds with resistance of R. sanguineus s.l. to cypermethrin. A nucleotide substitution (T2134C) on domain III segment VI of the sodium channel gene was noticed in phenotypically resistant tick populations on DNA sequencing. Ivermectin resistance in the brown dog ticks is reported for the first time from India.

Characterization of resistance profile (intensity and mechanisms) of Anopheles gambiae in three communes of northern Benin, West Africa

Background

The selection and the spread of insecticide resistance in malaria vectors to the main classes of insecticides used in vector control tools are a major and ongoing challenge to malaria vector control programmes. This study aimed to determine the intensity of vector resistance to insecticides in three regions of Benin with different agro-ecological characteristics.

Methods

Larvae of Anopheles gambiae sensu lato (s.l.) were collected from September to November 2017 in different larval sites in three northern Benin communes: Parakou, Kandi and Malanville. Two to five-day-old, non-blood-fed, female mosquitoes were exposed to papers impregnated with deltamethrin, permethrin and bendiocarb at dosages of 1 × the diagnostic dose, 5 × and 10 × to determine the intensity of resistance in these vectors. Molecular frequencies of the kdr L1014F and ace-1R G119S insecticide resistance mutations and levels of detoxification enzymes were determined for mosquitoes sampled at each study site.

Results

Resistance to pyrethroids (permethrin and deltamethrin) was recorded in all three communes with mortality rates below 60% using the diagnostic dose (1x). The results obtained after exposure of An. gambiae to permethrin 10 × were 99% in Kandi, 98% in Malanville and 99% in Parakou. With deltamethrin 10x, mortality rates were 100% in Kandi, 96% in Malanville and 73% in Parakou. For the diagnostic dose of bendiocarb, suspected resistance was recorded in the communes of Malanville (97%) and Kandi (94%) while sensitivity was observed in Parakou (98%).Using the 10 × dose, mortality was 98% in Kandi, 100% in Malanville and 99% in Parakou. The frequencies of the kdr L1014F allele varied between 59 and 83% depending on the sites and species of the An. gambiae complex, while the frequency of the ace-1R G119S gene varied between 0 and 5%. Biochemical tests showed high levels of oxidase and esterase activity compared to the susceptible colony strain of An. gambiae sensu stricto (Kisumu strain).

Conclusion

Anopheles gambiae showed a generalized loss of susceptibility to permethrin and deltamethrin but also showed moderate to high intensity of resistance in different regions of Benin. This high intensity of resistance is a potential threat to the effectiveness of vector control.

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.

Insecticide resistance status in Anopheles gambiae (s.l.) in coastal Kenya

Background

The rapid and widespread evolution of insecticide resistance has emerged as one of the major challenges facing malaria control programs in sub-Saharan Africa. Understanding the insecticide resistance status of mosquito populations and the underlying mechanisms of insecticide resistance can inform the development of effective and site-specific strategies for resistance prevention and management. The aim of this study was to investigate the insecticide resistance status of Anopheles gambiae (s.l.) mosquitoes from coastal Kenya.

Methods

Anopheles gambiae (s.l.) larvae sampled from eight study sites were reared to adulthood in the insectary, and 3- to 5-day-old non-blood-fed females were tested for susceptibility to permethrin, deltamethrin, dichlorodiphenyltrichloroethane (DDT), fenitrothion and bendiocarb using the standard World Health Organization protocol. PCR amplification of rDNA intergenic spacers was used to identify sibling species of the An. gambiae complex. The An. gambiae (s.l.) females were further genotyped for the presence of the L1014S and L1014F knockdown resistance (kdr) mutations by real-time PCR.

Results

Anopheles arabiensis was the dominant species, accounting for 95.2% of the total collection, followed by An. gambiae (s.s.), accounting for 4.8%. Anopheles gambiae (s.l.) mosquitoes were resistant to deltamethrin, permethrin and fenitrothion but not to bendiocarb and DDT. The L1014S kdr point mutation was detected only in An. gambiae (s.s.), at a low allelic frequency of 3.33%, and the 1014F kdr mutation was not detected in either An. gambiae (s.s.) or An. arabiensis.

Conclusion

The findings of this study demonstrate phenotypic resistance to pyrethroids and organophosphates and a low level of the L1014S kdr point mutation that may partly be responsible for resistance to pyrethroids. This knowledge may inform the development of insecticide resistance management strategies along the Kenyan Coast.

Phenotypic Insecticide Resistance in Anopheles gambiae (Diptera: Culicidae): Specific Characterization of Underlying Resistance Mechanisms Still Matters

An effective control of malaria vectors requires an extensive knowledge of mechanisms underlying the resistance-phenotypes developed by these vectors against insecticides. We investigated Anopheles gambiae mosquitoes from Benin and Togo for their intensity of insecticide resistance and we discussed the involvement of genotyped mechanisms in the resistance-phenotypes observed. Three- to five-day-old adult mosquitoes emerged from field and laboratory An. gambiae larvae were assayed using WHO tube intensity tests against various doses of deltamethrin: 1× (0.05%); 2× (0.1%); 5× (0.25%); 7.5× (0.375%) and those of pirimiphos-methyl: 0.5× (0.125%); 1× (0.25%). Members of An. gambiae complex were screened in field populations using polymerase chain reaction (PCR) assays. The presence of kdrR(1014F/1014S) and ace-1R(119S) mutations was also investigated using TaqMan and PCR-RFLP techniques, respectively. Anopheles gambiae from field were very resistant to deltamethrin, whereas KisKdr and AcerKdrKis strains displayed 100% mortality rates at 2× the diagnostic dose. In contrast, the field mosquitoes displayed a low resistance-intensity against 1× the diagnostic dose of pirimiphos-methyl, whereas AcerKis and AcerKdrKis strains showed susceptibility at 0.5× the diagnostic dose. Anopheles gambiae s.s., Anopheles coluzzii, and Anopheles arabiensis were identified. Allelic frequencies of kdrR (1014F) and ace-1R (119S) mutations in the field populations varied from 0.65 to 1 and 0 to 0.84, respectively. The field An. gambiae displayed high-resistance levels against deltamethrin and pirimiphos-methyl when compared with those of the laboratory An. gambiae-resistant strains. These results exhibit the complexity of underlying insecticide resistance mechanisms in these field malaria vectors.

Insecticide resistance status of indoor and outdoor resting malaria vectors in a highland and lowland site in Western Kenya

BACKGROUND

Long Lasting Insecticidal Nets (LLINs) and indoor residual spraying (IRS) represent powerful tools for controlling malaria vectors in sub-Saharan Africa. The success of these interventions relies on their capability to inhibit indoor feeding and resting of malaria mosquitoes. This study sought to understand the interaction of insecticide resistance with indoor and outdoor resting behavioral responses of malaria vectors from Western Kenya.

METHODS

The status of insecticide resistance among indoor and outdoor resting anopheline mosquitoes was compared in Anopheles mosquitoes collected from Kisumu and Bungoma counties in Western Kenya. The level and intensity of resistance were measured using WHO-tube and CDC-bottle bioassays, respectively. The synergist piperonyl butoxide (PBO) was used to determine if metabolic activity (monooxygenase enzymes) explained the resistance observed. The mutations at the voltage-gated sodium channel (Vgsc) gene and Ace 1 gene were characterized using PCR methods. Microplate assays were used to measure levels of detoxification enzymes if present.

RESULTS

A total of 1094 samples were discriminated within Anopheles gambiae s.l. and 289 within An. funestus s.l. In Kisian (Kisumu county), the dominant species was Anopheles arabiensis 75.2% (391/520) while in Kimaeti (Bungoma county) collections the dominant sibling species was Anopheles gambiae s.s 96.5% (554/574). The An. funestus s.l samples analysed were all An. funestus s.s from both sites. Pyrethroid resistance of An.gambiae s.l F1 progeny was observed in all sites. Lower mortality was observed against deltamethrin for the progeny of indoor resting mosquitoes compared to outdoor resting mosquitoes (Mortality rate: 37% vs 51%, P = 0.044). The intensity assays showed moderate-intensity resistance to deltamethrin in the progeny of mosquitoes collected from indoors and outdoors in both study sites. In Kisian, the frequency of vgsc-L1014S and vgsc-L1014F mutation was 0.14 and 0.19 respectively in indoor resting malaria mosquitoes while those of the outdoor resting mosquitoes were 0.12 and 0.12 respectively. The ace 1 mutation was present in higher frequency in the F1 of mosquitoes resting indoors (0.23) compared to those of mosquitoes resting outdoors (0.12). In Kimaeti, the frequencies of vgsc-L1014S and vgsc-L1014F were 0.75 and 0.05 respectively for the F1 of mosquitoes collected indoors whereas those of outdoor resting ones were 0.67 and 0.03 respectively. The ace 1 G119S mutation was present in progeny of mosquitoes from Kimaeti resting indoors (0.05) whereas it was absent in those resting outdoors. Monooxygenase activity was elevated by 1.83 folds in Kisian and by 1.33 folds in Kimaeti for mosquitoes resting indoors than those resting outdoors respectively.

CONCLUSION

The study recorded high phenotypic, metabolic and genotypic insecticide resistance in indoor resting populations of malaria vectors compared to their outdoor resting counterparts. The indication of moderate resistance intensity for the indoor resting mosquitoes is alarming as it could have an operational impact on the efficacy of the existing pyrethroid based vector control tools. The use of synergist (PBO) in LLINs may be a better alternative for widespread use in these regions recording high insecticide resistance.

Western Kenyan Anopheles gambiae showing intense permethrin resistance harbour distinct microbiota

Background

Insecticide resistance poses a growing challenge to malaria vector control in Kenya and around the world. Following evidence of associations between the mosquito microbiota and insecticide resistance, the microbiota of Anopheles gambiae sensu stricto (s.s.) from Tulukuyi village, Bungoma, Kenya, with differing permethrin resistance profiles were comparatively characterized.

Methods

Using the CDC bottle bioassay, 133 2–3 day-old, virgin, non-blood fed female F₁ progeny of field-caught An. gambiae s.s. were exposed to five times (107.5 µg/ml) the discriminating dose of permethrin. Post bioassay, 50 resistant and 50 susceptible mosquitoes were subsequently screened for kdr East and West mutations, and individually processed for microbial analysis using high throughput sequencing targeting the universal bacterial and archaeal 16S rRNA gene.

Results

47 % of the samples tested (n = 133) were resistant, and of the 100 selected for further processing, 99 % were positive for kdr East and 1 % for kdr West. Overall, 84 bacterial taxa were detected across all mosquito samples, with 36 of these shared between resistant and susceptible mosquitoes. A total of 20 bacterial taxa were unique to the resistant mosquitoes and 28 were unique to the susceptible mosquitoes. There were significant differences in bacterial composition between resistant and susceptible individuals (PERMANOVA, pseudo-F = 2.33, P = 0.001), with presence of Sphingobacterium, Lysinibacillus and Streptococcus (all known pyrethroid-degrading taxa), and the radiotolerant Rubrobacter, being significantly associated with resistant mosquitoes. On the other hand, the presence of Myxococcus, was significantly associated with susceptible mosquitoes.

Conclusions

This is the first report of distinct microbiota in An. gambiae s.s. associated with intense pyrethroid resistance. The findings highlight differentially abundant bacterial taxa between resistant and susceptible mosquitoes, and further suggest a microbe-mediated mechanism of insecticide resistance in mosquitoes. These results also indicate fixation of the kdr East mutation in this mosquito population, precluding further analysis of its associations with the mosquito microbiota, but presenting the hypothesis that any microbe-mediated mechanism of insecticide resistance would be likely of a metabolic nature. Overall, this study lays initial groundwork for understanding microbe-mediated mechanisms of insecticide resistance in African mosquito vectors of malaria, and potentially identifying novel microbial markers of insecticide resistance that could supplement existing vector surveillance tools.

The first implementation of allele-specific primers for detecting the knockdown and acetylcholinesterase target site mutations in malaria vector, Anopheles sacharovi

Anopheles sacharovi, a primer malaria vector species of Turkey, have a significant public health importance. It is aimed to determine the insecticide resistance status in Anopheles sacharovi populations in the Aegean and Mediterranean regions of Turkey. A total of 1638 individuals were analysed from 15 populations. Bioassay results indicated all An. sacharovi populations were resistant to DDT, malathion, fenitrothion, bendiocarb, propoxur. Many populations have begun to have resistance against permethrin and deltamethrin. Biochemical analyses results revealed that glutathione-S-transferases and P450 monooxygenases might be responsible from the mechanisms of DDT resistance; esterases and acetylcholinesterase might be responsible for organophosphate and carbamate resistance; P450 monooxygenases and esterases might be responsible for pyrethroid resistance into populations sampled from the study area. Allele-specific primers detected L1014F and L1014S mutations that provide kdr resistance against pyrethroids and DDT. Increased acetylcholinesterase insensitivity was detected while Ace-1 G119S mutations were not detected in An. sacharovi populations by using allele-specific primers. Overall results indicate the presence of multiple resistance mechanisms in Turkish An. sacharovi field populations suggesting that populations might gain resistance against all possible insecticide in the future. Therefore, insecticide resistance management strategies are urgently needed for effective vector control implementation.

Phenotypic, genotypic and biochemical changes during pyrethroid resistance selection in Anopheles gambiae mosquitoes

The directional selection for insecticide resistance due to indiscriminate use of insecticides in public health and agricultural system favors an increase in the frequency of insecticide-resistant alleles in the natural populations. Similarly, removal of selection pressure generally leads to decay in resistance. Past investigations on the emergence of insecticide resistance in mosquitoes mostly relied on field survey of resistance in vector populations that typically had a complex history of exposure to various public health and agricultural pest control insecticides in nature, and thus the effect of specific insecticides on rate of resistance emergency or resistance decay rate is not known. This study examined the phenotypic, genotypic, and biochemical changes that had occurred during the process of selection for pyrethroid resistance in Anopheles gambiae, the most important malaria vector in Africa. In parallel, we also examined these changes in resistant populations when there is no selection pressure applied. Through repeated deltamethrin selection in adult mosquitoes from a field population collected in western Kenya for 12 generations, we obtained three independent and highly pyrethroid-resistant An. gambiae populations. Three susceptible populations from the same parental population were generated by removing selection pressure. These two lines of mosquito populations differed significantly in monooxygenase and beta-esterase activities, but not in Vgsc gene mutation frequency, suggesting metabolic detoxification mechanism plays a major role in generating moderate-intensity resistance or high-intensity resistance. Pre-exposure to the synergist piperonyl butoxide restored the susceptibility to insecticide among the highly resistant mosquitoes, confirming the role of monooxygenases in pyrethroid resistance. The rate of resistance decay to become fully susceptible from moderate-intensity resistance took 15 generations, supporting at least 2-years interval is needed when the rotational use of insecticides with different modes of action is considered for resistance management.

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.

Impact of indoor residual spraying with pirimiphos-methyl (Actellic 300CS) on entomological indicators of transmission and malaria case burden in Migori County, western Kenya

Indoor residual spraying (IRS) of insecticides is a major vector control strategy for malaria prevention. We evaluated the impact of a single round of IRS with the organophosphate, pirimiphos-methyl (Actellic 300CS), on entomological and parasitological parameters of malaria in Migori County, western Kenya in 2017, in an area where primary vectors are resistant to pyrethroids but susceptible to the IRS compound. Entomological monitoring was conducted by indoor CDC light trap, pyrethrum spray catches (PSC) and human landing collection (HLC) before and after IRS. The residual effect of the insecticide was assessed monthly by exposing susceptible An. gambiae s.s. Kisumu strain to sprayed surfaces in cone assays and measuring mortality at 24 hours. Malaria case burden data were extracted from laboratory records of four health facilities within the sprayed area and two adjacent unsprayed areas. IRS was associated with reductions in An. funestus numbers in the intervention areas compared to non-intervention areas by 88% with light traps (risk ratio [RR] 0.12, 95% CI 0.07-0.21, p < 0.001) and 93% with PSC collections (RR = 0.07, 0.03-0.17, p < 0.001). The corresponding reductions in the numbers of An. arabiensis collected by PSC were 69% in the intervention compared to the non-intervention areas (RR = 0.31, 0.14-0.68, p = 0.006), but there was no significant difference with light traps (RR = 0.45, 0.21-0.96, p = 0.05). Before IRS, An. funestus accounted for over 80% of Anopheles mosquitoes collected by light trap and PSC in all sites. After IRS, An. arabiensis accounted for 86% of Anopheles collected by PSC and 66% by CDC light trap in the sprayed sites while the proportion in non-intervention sites remained unchanged. No sporozoite infections were detected in intervention areas after IRS and biting rates by An. funestus were reduced to near zero. Anopheles funestus and An. arabiensis were fully susceptible to pirimiphos-methyl and resistant to pyrethroids. The residual effect of Actellic 300CS lasted ten months on mud and concrete walls. Malaria case counts among febrile patients within IRS areas was lower post- compared to pre-IRS by 44%, 65% and 47% in Rongo, Uriri and Nyatike health facilities respectively. A single application of IRS with Actellic 300CS in Migori County provided ten months protection and resulted in the near elimination of the primary malaria vector An. funestus and a corresponding reduction of malaria case count among out-patients. The impact was less on An. arabiensis, most likely due to their exophilic nature.

Resting behaviour of malaria vectors in highland and lowland sites of western Kenya: Implication on malaria vector control measures

BACKGROUND

Understanding the interactions between increased insecticide resistance and resting behaviour patterns of malaria mosquitoes is important for planning of adequate vector control. This study was designed to investigate the resting behavior, host preference and rates of Plasmodium falciparum infection in relation to insecticide resistance of malaria vectors in different ecologies of western Kenya.

METHODS

Anopheles mosquito collections were carried out during the dry and rainy seasons in Kisian (lowland site) and Bungoma (highland site), both in western Kenya using pyrethrum spray catches (PSC), mechanical aspiration (Prokopack) for indoor collections, clay pots, pit shelter and Prokopack for outdoor collections. WHO tube bioassay was used to determine levels of phenotypic resistance of indoor and outdoor collected mosquitoes to deltamethrin. PCR-based molecular diagnostics were used for mosquito speciation, genotype for knockdown resistance mutations (1014S and 1014F) and to determine specific host blood meal origins. Enzyme-linked Immunosorbent Assay (ELISA) was used to determine mosquito sporozoite infections.

RESULTS

Anopheles gambiae s.l. was the most predominant species (75%, n = 2706) followed by An. funestus s.l. (25%, n = 860). An. gambiae s.s hereafter (An. gambiae) accounted for 91% (95% CI: 89-93) and An. arabiensis 8% (95% CI: 6-9) in Bungoma, while in Kisian, An. arabiensis composition was 60% (95% CI: 55-66) and An. gambiae 39% (95% CI: 34-44). The resting densities of An. gambiae s.l and An. funestus were higher indoors than outdoor in both sites (An. gambiae s.l; F1, 655 = 41.928, p < 0.0001, An. funestus; F1, 655 = 36.555, p < 0.0001). The mortality rate for indoor and outdoor resting An. gambiae s.l F1 progeny was 37% (95% CI: 34-39) vs 67% (95% CI: 62-69) respectively in Bungoma. In Kisian, the mortality rate was 67% (95% CI: 61-73) vs 76% (95% CI: 71-80) respectively. The mortality rate for F1 progeny of An. funestus resting indoors in Bungoma was 32% (95% CI: 28-35). The 1014S mutation was only detected in indoor resitng An. arabiensis. Similarly, the 1014F mutation was present only in indoor resting An. gambiae. The sporozoite rates were highest in An. funestus followed by An. gambiae, and An. arabiensis resting indoors at 11% (34/311), 8% (47/618) and 4% (1/27) respectively in Bungoma. Overall, in Bungoma, the sporozoite rate for indoor resting mosquitoes was 9% (82/956) and 4% (8/190) for outdoors. In Kisian, the sporozoite rate was 1% (1/112) for indoor resting An. gambiae. None of the outdoor collected mosquitoes in Kisian tested positive for sporozoite infections (n = 73).

CONCLUSION

The study reports high indoor resting densities of An. gambiae and An. funestus, insecticide resistance, and persistence of malaria transmission indoors regardless of the use of long-lasting insecticidal nets (LLINs). These findings underline the difficulties of controlling malaria vectors resting and biting indoors using the current interventions. Supplemental vector control tools and implementation of sustainable insecticide resistance management strategies are needed in western Kenya.

Diagnostic dose determination and efficacy of chlorfenapyr and clothianidin insecticides against Anopheles malaria vector populations of western Kenya

BACKGROUND

Malaria vector control is dependent on chemical insecticides applied to walls by indoor residual spraying or on long-lasting insecticidal nets. The emergence and spread of insecticide resistance in major malaria vectors may compromise malaria control and elimination efforts. The aim of this study was to estimate a diagnostic dose for chlorfenapyr (class: pyrrole) and clothianidin (class: neonicotinoid) and assess the baseline susceptibility of three major Anopheles malaria vectors of western Kenya to these two insecticides.

METHODS

The Centers for Disease Control and Prevention (CDC) bottle assay was used to determine the diagnostic doses of chlorfenapyr and clothianidin insecticides against the susceptible Kisumu strain of Anopheles gambiae. Probit analysis was used to determine the lethal doses at which 50% (LD50) and 99% (LD99) of the susceptible mosquitoes would be killed 24, 48 and 72 h following exposure for 1 h. Insecticidal efficacy of chlorfenapyr, clothianidin and the pyrethroid deltamethrin was then evaluated against field collected female Anopheles mosquitoes sampled from Nyando, Bumula and Ndhiwa sub-Counties in western Kenya. Members of Anopheles funestus and An. gambiae complexes were identified using polymerase chain reaction (PCR).

RESULTS

The determined diagnostic doses of chlorfenapyr and clothianidin insecticides were 50 µg/bottle and 150 µg/bottle, respectively, for An. gambiae, Kisumu strain. When exposed to the diagnostic dose of each insecticide, Anopheles malaria vector populations in western Kenya were susceptible to both insecticides with 100% mortality observed after 72 h. Mortality of mosquitoes exposed to deltamethrin increased over time but did not reach 100%. Mortality of Anopheles arabiensis from Nyando exposed to deltamethrin was 83% at 24 h, 88% at 48 h and 94.5% at 72 h while An. funestus from Ndhiwa was 89% at 24 h, 91.5% at 48 h and 94.5% at 72 h.

CONCLUSION

Mosquitoes of western Kenya, despite being resistant to pyrethroids, are susceptible to chlorfenapyr and clothianidin. Field evaluations of the formulated product are needed.

Susceptibility of Anopheles gambiae complex mosquitoes to microbial larvicides in diverse ecological settings in western Kenya

The microbial larvicides Bacillus thuringiensis var. israelensis (Bti) and Bacillus sphaericus (Bs) (Bacillales: Bacillaceae) are well known for their efficacy and safety in mosquito control. In order to assess their potential value in future mosquito control strategies in western Kenya, the current study tested the susceptibility of five populations of Anopheles gambiae complex mosquitoes (Diptera: Culicidae), collected from five diverse ecological sites in this area, to Bti and Bs under laboratory conditions. In each population, bioassays were conducted with eight concentrations of larvicide (Bti/Bs) in four replicates and were repeated on three separate days. Larval mortality was recorded at 24 h or 48 h after the application of larvicide and subjected to probit analysis. A total of 2400 An. gambiae complex larvae from each population were tested for their susceptibility to Bti and Bs. The mean (± standard error of the mean, SEM) lethal concentration values of Bti required to achieve 50% and 95% larval mortality (LC₅₀ and LC₉₅ ) across the five populations were 0.062  (± 0.005) mg/L and 0.797 (± 0.087) mg/L, respectively. Corresponding mean  (± SEM) values for Bs were 0.058 (± 0.005) mg/L and 0.451 (± 0.053) mg/L, respectively. Statistical analysis indicated that the five populations of An. gambiae complex mosquitoes tested were fully susceptible to Bti and Bs, and there was no significant variation in susceptibility among the tested populations.

Influence of blood meal and age of mosquitoes on susceptibility to pyrethroids in Anopheles gambiae from Western Kenya

BACKGROUND

Physiological characteristics (age and blood feeding status) of malaria vectors can influence their susceptibility to the current vector control tools that target their feeding and resting behaviour. To ensure the sustainability of the current and future vector control tools an understanding of how physiological characteristics may contribute to insecticide tolerance in the field is fundamental for shaping resistance management strategies and vector control tools. The aim of this study was to determine whether blood meal and mosquito age affect pyrethroid tolerance in field-collected Anopheles gambiae from western Kenya.

METHODS

Wild mosquito larvae were reared to adulthood alongside the pyrethroid-susceptible Kisumu strain. Adult females from the two populations were monitored for deltamethrin resistance when they were young at 2-5 days old and older 14-16 days old and whether fed or unfed for each age group. Metabolic assays were also performed to determine the level of detoxification enzymes. Mosquito specimens were further identified to species level using the polymerase chain reaction (PCR) method.

RESULTS

Anopheles gambiae sensu stricto was the predominant species comprising 96% of specimens and 2.75% Anopheles arabiensis. Bioassay results showed reduced pyrethroid induced mortality with younger mosquitoes compared to older ones (mortality rates 83% vs. 98%), independently of their feeding status. Reduced mortality was recorded with younger females of which were fed compared to their unfed counterparts of the same age with a mortality rate of 35.5% vs. 83%. Older blood-fed females showed reduced susceptibility after exposure when compared to unfed females of the same age (mortality rates 86% vs. 98%). For the Kisumu susceptible population, mortality was straight 100% regardless of age and blood feeding status. Blood feeding status and mosquito age had an effect on enzyme levels in both populations, with blood fed individuals showing higher enzyme elevations compared to their unfed counterparts (P < 0.0001). The interaction between mosquito age and blood fed status had significant effect on mosquito mortality.

CONCLUSION

The results showed that mosquito age and blood feeding status confers increased tolerance to insecticides as blood feeding may be playing an important role in the toxicity of deltamethrin, allowing mosquitoes to rest on insecticide-treated materials despite treatment. These may have implications for the sustained efficacy of indoor residual spraying and insecticide-treated nets based control programmes that target indoor resting female mosquitoes of various gonotrophic status.

Evaluation of Pyrethroid Susceptibility in Culex pipiens of Northern Izmir Province, Turkey

BACKGROUND

Mosquitoes, being a nuisance species, are considered as one of the most important species in public health control programs due to their role as a vector in mosquito-borne diseases observed in humans and animals. We evaluated the susceptibility status of Culex pipiens collected from northern Izmir, Turkey in 2011-16.

METHODS

Mosquito larvae, collected from three different locations in northern İzmir, were reared in the laboratory. Adult susceptibility bioassays were performed using the WHO insecticide-impregnated papers including deltamethrin 0.05%, permethrin 0.75%, α-cypermethrin 0.05% and cyfluthrin 0.15%. In addition, adult bioassays were performed after the pre-exposure to piperonyl butoxide (PBO) to determine the contribution of P450 detoxification enzymes to the phenotypic resistance.

RESULTS

In all of the three populations, high levels of resistance were observed (mortalities<63%) to all of the four pyrethroids. Different pyrethroids but with the same mode of action can exhibit significantly different phenotypic resistance in a single population. PBO bioassays also showed that P450 detoxification enzymes can have diverse effects on different pyrethroids.

CONCLUSION

Using just one chemical in a class of insecticide can be misleading for resistance studies.

Insecticide susceptibility status of Anopheles gambiae (s.l.) in South-West Cameroon four years after long-lasting insecticidal net mass distribution

BACKGROUND

Members of the Anopheles gambiae (s.l.) complex are one of the major vectors of malaria in Africa. LLINs and IRS are the most effective tools used in vector control of malaria. However, their effectiveness may be hampered by the development and spread of insecticide resistance in the target vectors species. The objective of this study was to assess the susceptibility of Anopheles gambiae (s.l.) mosquitoes from South-West Cameroon to deltamethrin, permethrin and to malathion, four years after the mass deployment of LLINs.

METHODS

Anopheles larvae were collected from Limbe, Tiko and Buea, three cities of the Fako division and reared until adult emergence. Adult mosquitoes from field larvae were identified as belonging to the Anopheles gambiae (s.l.) complex using standard identification keys. Susceptibility of mosquito samples to deltamethrin, permethrin and malathion was assessed using WHO susceptibility tests protocol for adult mosquitoes. Molecular identification of tested samples was performed using the PCR SINE200 protocol and by PCR-RFLP. The kdr alleles were genotyped using the hot ligation oligonucleotide assay (HOLA).

RESULTS

Two species of the An. gambiae (s.l.) complex, An. coluzzii and An. gambiae (s.s.) were identified in all three study locations with high proportions of An. coluzzii in Limbe (84.06%) and Tiko (92.2%), while in Buea, An. coluzzii (55.6%) and An. gambiae (s.s.) (44.4%) occurred almost in the same proportions. Tested samples were found resistant to pyrethroids (deltamethrin and permethrin) in all locations (< 90% mortality), with > 3-fold increase of KDT₅₀ values compared with the Kisumu susceptible reference strain of An. gambiae (s.s.). However, the mosquito populations from Limbe and Buea were fully susceptible to malathion. The L1014F kdr was found in both An. coluzzii and An. gambiae (s.s.) with the highest frequencies found in An. gambiae (s.l.) populations from Tiko (94%) and Buea (90%) compared with the Limbe population (66%) (P = 0.00063, df = 2). No kdr L1014S was observed in analyzed samples.

CONCLUSIONS

These findings reemphasize the ongoing development of An. gambiae (s.l.) resistance to pyrethroids used in impregnating LLINs and suggest the use of malathion as an alternative insecticide for IRS in complementarity with LLINs.

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.

Emerging Pyrethroid Resistance among Anopheles arabiensis in Kenya

Vector control programs, particularly in the form of insecticide-treated bed nets (ITNs), are essential for achieving malaria elimination goals. Recent reports of increasing knockdown resistance (kdr) mutation frequencies for Anopheles arabiensis in Western Kenya heightens the concern on the future effectiveness of ITNs in Kenya. We examined resistance in An. arabiensis populations across Kenya through kdr mutations and World Health Organization–recommended bioassays. We detected two kdr alleles, L1014F and L1014S. Kdr mutations were found in five of the 11 study sites, with mutation frequencies ranging from 3% to 63%. In two Western Kenya populations, the kdr L1014F allele frequency was as high as 10%. The L1014S frequency was highest at Chulaimbo at 55%. Notably, the kdr L1014F mutation was found to be associated with pyrethroid resistance at Port Victoria, but kdr mutations were not significantly associated with resistance at Chulaimbo, which had the highest kdr mutation frequency among all sites. This study demonstrated the emerging pyrethroid resistance in An. arabiensis and that pyrethroid resistance may be related to kdr mutations. Resistance monitoring and management are urgently needed for this species in Kenya where resistance is emerging and its abundance is becoming predominant. Kdr mutations may serve as a biomarker for pyrethroid resistance in An. arabiensis.

Insecticide resistance in Anopheles arabiensis from Ethiopia (2012-2016): a nationwide study for insecticide resistance monitoring

Background

Indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) remain the cornerstones of malaria vector control. However, the development of insecticide resistance and its implications for operational failure of preventative strategies are of concern. The aim of this study was to characterize insecticide resistance among Anopheles arabiensis populations in Ethiopia and describe temporal and spatial patterns of resistance between 2012 and 2016.

Methods

Between 2012 and 2016, resistance status of An. arabiensis was assessed annually during the long rainy seasons in study sites from seven of the nine regions in Ethiopia. Insecticide resistance levels were measured with WHO susceptibility tests and CDC bottle bioassays using insecticides from four chemical classes (organochlorines, pyrethroids, organophosphates and carbamates), with minor variations in insecticides tested and assays conducted between years. In selected sites, CDC synergist assays were performed by pre-exposing mosquitoes to piperonyl butoxide (PBO). In 2015 and 2016, mosquitoes from DDT and deltamethrin bioassays were randomly selected, identified to species-level and screened for knockdown resistance (kdr) by PCR.

Results

Intense resistance to DDT and pyrethroids was pervasive across Ethiopia, consistent with historic use of DDT for IRS and concomitant increases in insecticide-treated net coverage over the last 15 years. Longitudinal resistance trends to malathion, bendiocarb, propoxur and pirimiphos-methyl corresponded to shifts in the national insecticide policy. By 2016, resistance to the latter two insecticides had emerged, with the potential to jeopardize future long-term effectiveness of vector control activities in these areas. Between 2015 and 2016, the West African (L1014F) kdr allele was detected in 74.1% (n = 686/926) of specimens, with frequencies ranging from 31 to 100% and 33 to 100% in survivors from DDT and deltamethrin bioassays, respectively. Restoration of mosquito susceptibility, following pre-exposure to PBO, along with a lack of association between kdr allele frequency and An. arabiensis mortality rate, both indicate metabolic and target-site mutation mechanisms are contributing to insecticide resistance.

Conclusions

Data generated by this study will strengthen the National Malaria Control Programme’s insecticide resistance management strategy to safeguard continued efficacy of IRS and other malaria control methods in Ethiopia.

Electronic supplementary material

The online version of this article (10.1186/s12936-017-2115-2) contains supplementary material, which is available to authorized users.

Characterization of a Sodium Channel Mutation in Permethrin-Resistant Rhipicephalus sanguineus (Acari: Ixodidae)

The brown dog tick, Rhipicephalus sanguineus (Latrielle) sensu lato, is an important ectoparasite of dogs and occasionally humans, capable of transmitting several pathogens, such as Rickettsia and Ehrlichia, which are of veterinary and medical importance. The brown dog tick is distributed worldwide and has an affinity for human habitations in much of its range. In some populations, lack of integrated pest management plans and overuse of pyrethroid pesticides and other sodium channel inhibitors has resulted in high levels of resistance to permethrin. Recently, a highly conserved region of the R. sanguineus sodium channel was sequenced, indicating that a single nucleotide polymorphism of thymine to cytosine on domain III segment VI of the sodium channel could confer resistance. A molecular assay targeting a point mutation in the sodium channel was developed and optimized to separate ticks expressing permethrin resistance from those from a susceptible colony. Thereafter, multiple field-collected phenotypically permethrin-resistant populations were evaluated using this molecular assay to determine genotype. As confirmed by DNA sequencing, a point mutation was present at a high rate in phenotypically resistant tick populations that was not present in the susceptible strain. These data suggest an additional permethrin resistance mechanism to metabolic resistance, which has been reported for this tick species, and confirm its association with phenotypic resistance. The results of this study further emphasize the need to preserve acaricide chemistry through rotation of active ingredients used to control ectoparasites.

Evaluation of active ingredients and larvicidal activity of clove and cinnamon essential oils against Anopheles gambiae (sensu lato)

BACKGROUND

Mosquitoes are well-known vectors of many diseases including malaria and lymphatic filariasis. Uses of synthetic insecticides are associated with high toxicity, resistance, environmental pollution and limited alternative, effective synthetic insecticides. This study was undertaken to evaluate the larvicidal efficacy of clove and cinnamon essential oils against laboratory Anopheles gambiae (sensu stricto) and wild An. arabiensis larvae.

METHODS

The standard WHO guideline for larvicides evaluation was used, and the GC-MS machine was used for active compounds percentage composition analysis and structures identification. Probit regression analysis was used for LC₅₀ and LC₉₅ calculations while a t-test was used to test for significant differences between laboratory-reared and wild larvae populations in each concentration of plant extract.

RESULTS

Mortality effect of clove and cinnamon essential oils against wild and laboratory-reared larvae had variations indicated by their LC₅₀ and LC₉₅ values. The mortality at different concentrations of cinnamon and clove post-exposure for wild and laboratory-reared larvae were dosage-dependent and were higher for cinnamon than for clove essential oils. The mortality effect following exposure to a blend of the two essential oils was higher for blends containing a greater proportion of cinnamon oil. In the chemical analysis of the active ingredients of cinnamon essential oil, the main chemical content was Eugenol, and the rarest was β-Linalool while for clove essential oil, the main chemical content was Eugenol and the rarest was Bicyclo.

CONCLUSION

The essential oils showed a larvicidal effect which was concentration-dependent for both laboratory and wild collected larvae. The active ingredient compositions triggered different responses in mortality. Further research in small-scale should be conducted with concentrated extracted compounds.

Insecticide resistance status of three malaria vectors, Anopheles gambiae (s.l.), An. funestus and An. mascarensis, from the south, central and east coasts of Madagascar

BACKGROUND

Insecticide-based vector control, which comprises use of insecticide-treated bed nets (ITNs) and indoor residual spraying (IRS), is the key method to malaria control in Madagascar. However, its effectiveness is threatened as vectors become resistant to insecticides. This study investigated the resistance status of malaria vectors in Madagascar to various insecticides recommended for use in ITNs and/or IRS.

METHODS

WHO tube and CDC bottle bioassays were performed on populations of Anopheles gambiae (s.l.), An. funestus and An. mascarensis. Adult female An. gambiae (s.l.) mosquitoes reared from field-collected larvae and pupae were tested for their resistance to DDT, permethrin, deltamethrin, alpha-cypermethrin, lambda-cyhalothrin, bendiocarb and pirimiphos-methyl. Resting An. funestus and An. mascarensis female mosquitoes collected from unsprayed surfaces were tested against permethrin, deltamethrin and pirimiphos-methyl. The effect on insecticide resistance of pre-exposure to the synergists piperonyl-butoxide (PBO) and S,S,S-tributyl phosphorotrithioate (DEF) also was assessed. Molecular analyses were done to identify species and determine the presence of knock-down resistance (kdr) and acetylcholinesterase resistance (ace-1 ^R ) gene mutations.

RESULTS

Anopheles funestus and An. mascarensis were fully susceptible to permethrin, deltamethrin and pirimiphos-methyl. Anopheles gambiae (s.l.) was fully susceptible to bendiocarb and pirimiphos-methyl. Among the 17 An. gambiae (s.l.) populations tested for deltamethrin, no confirmed resistance was recorded, but suspected resistance was observed in two sites. Anopheles gambiae (s.l.) was resistant to permethrin in four out of 18 sites (mortality 68-89%) and to alpha-cypermethrin (89% mortality) and lambda-cyhalothrin (80% and 85%) in one of 17 sites, using one or both assay methods. Pre-exposure to PBO restored full susceptibility to all pyrethroids tested except in one site where only partial restoration to permethrin was observed. DEF fully suppressed resistance to deltamethrin and alpha-cypermethrin, while it partially restored susceptibility to permethrin in two of the three sites. Molecular analysis data suggest absence of kdr and ace-1 ^R gene mutations.

CONCLUSION

This study suggests involvement of detoxifying enzymes in the phenotypic resistance of An. gambiae (s.l.) to pyrethroids. The absence of resistance in An. funestus and An. mascarensis to pirimiphos-methyl and pyrethroids and in An. gambiae (s.l.) to carbamates and organophosphates presents greater opportunity for managing resistance in Madagascar.

Diagnostic doses and times for Phlebotomus papatasi and Lutzomyia longipalpis sand flies (Diptera: Psychodidae: Phlebotominae) using the CDC bottle bioassay to assess insecticide resistance

BACKGROUND

Insecticide resistance to synthetic chemical insecticides is a worldwide concern in phlebotomine sand flies (Diptera: Psychodidae), the vectors of Leishmania spp. parasites. The CDC bottle bioassay assesses resistance by testing populations against verified diagnostic doses and diagnostic times for an insecticide, but the assay has been used limitedly with sand flies. The objective of this study was to determine diagnostic doses and diagnostic times for laboratory Lutzomyia longipalpis (Lutz & Nieva) and Phlebotomus papatasi (Scopoli) to ten insecticides, including pyrethroids, organophosphates, carbamates, and DDT, that are used worldwide to control vectors.

METHODS

Bioassays were conducted in 1,000-ml glass bottles each containing 10-25 sand flies from laboratory colonies of L. longipalpis or P. papatasi. Four pyrethroids, three organophosphates, two carbamates and one organochlorine, were evaluated. A series of concentrations were tested for each insecticide, and four replicates were conducted for each concentration. Diagnostic doses were determined only during the exposure bioassay for the organophosphates and carbamates. For the pyrethroids and DDT, diagnostic doses were determined for both the exposure bioassay and after a 24-hour recovery period.

RESULTS

Both species are highly susceptible to the carbamates as their diagnostic doses are under 7.0 μg/ml. Both species are also highly susceptible to DDT during the exposure assay as their diagnostic doses are 7.5 μg/ml, yet their diagnostic doses for the 24-h recovery period are 650.0 μg/ml for Lu. longipalpis and 470.0 μg/ml for P. papatasi.

CONCLUSIONS

Diagnostic doses and diagnostic times can now be incorporated into vector management programs that use the CDC bottle bioassay to assess insecticide resistance in field populations of Lu. longipalpis and P. papatasi. These findings provide initial starting points for determining diagnostic doses and diagnostic times for other sand fly vector species and wild populations using the CDC bottle bioassay.

Eave Screening and Push-Pull Tactics to Reduce House Entry by Vectors of Malaria

Long-lasting insecticidal nets and indoor residual spraying have contributed to a decline in malaria over the last decade, but progress is threatened by the development of physiological and behavioral resistance of mosquitoes against insecticides. Acknowledging the need for alternative vector control tools, we quantified the effects of eave screening in combination with a push-pull system based on the simultaneous use of a repellent (push) and attractant-baited traps (pull). Field experiments in western Kenya showed that eave screening, whether used in combination with an attractant-baited trap or not, was highly effective in reducing house entry by malaria mosquitoes. The magnitude of the effect varied for different mosquito species and between two experiments, but the reduction in house entry was always considerable (between 61% and 99%). The use of outdoor, attractant-baited traps alone did not have a significant impact on mosquito house entry but the high number of mosquitoes trapped outdoors indicates that attractant-baited traps could be used for removal trapping, which would enhance outdoor as well as indoor protection against mosquito bites. As eave screening was effective by itself, addition of a repellent was of limited value. Nevertheless, repellents may play a role in reducing outdoor malaria transmission in the peridomestic area.

Presence of the knockdown resistance mutation, Vgsc-1014F in Anopheles gambiae and An. arabiensis in western Kenya

Introduction

The voltage gated sodium channel mutation Vgsc-1014S (kdr-east) was first reported in Kenya in 2000 and has since been observed to occur at high frequencies in the local Anopheles gambiae s.s. population. The mutation Vgsc-1014F has never been reported from An. gambiae Complex complex mosquitoes in Kenya.

Findings

Molecularly confirmed An. gambiae s.s. (hereafter An. gambiae) and An. arabiensis collected from 4 different parts of western Kenya were genotyped for kdr from 2011 to 2013. Vgsc-1014F was observed to have emerged, apparently, simultaneously in both An. gambiae and An. arabiensis in 2012. A portion of the samples were submitted for sequencing in order to confirm the Vgsc-1014F genotyping results. The resulting sequence data were deposited in GenBank (Accession numbers: KR867642-KR867651, KT758295-KT758303). A single Vgsc-1014F haplotype was observed suggesting, a common origin in both species.

Conclusion

This is the first report of Vgsc-1014F in Kenya. Based on our samples, the mutation is present in low frequencies in both An. gambiae and An. arabiensis. It is important that we start monitoring relative frequencies of the two kdr genes so that we can determine their relative importance in an area of high insecticide treated net ownership.

Insecticidal decay effects of long-lasting insecticide nets and indoor residual spraying on Anopheles gambiae and Anopheles arabiensis in Western Kenya

Background

Indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) are the first-line tools for malaria prevention and control in Africa. Vector resistance to insecticides has been extensively studied, however the insecticidal effects of the nets and sprayed walls on pyrethroid resistant mosquitoes has not been studied thoroughly. We evaluated the bioefficacy of LLINs of different ages and lambda-cyhalothrin (ICON 10cs) on the sprayed mud walls for a period of time on malaria vector survivorship.

Methods

WHO tube bioassay was performed using diagnostic doses of lambda-cyhalothrin (0.05 %), permethrin (0.75 %) and deltamethrin (0.05 %). Cone bioassays were conducted on netting materials from 0 to 3 years old long-lasting insecticide-impregnated nets. Wall bioassays were performed monthly on mud slabs sprayed with lambdacyhalothrin over a period of seven months. All bioassays used An. gambiae mosquitoes collected from the field and the laboratory susceptible reference Kisumu strain. Concentration of the insecticides on the netting materials was examined using the gas chromatography method. Mosquitoes were identified to species level using PCR and genotyped for the kdr gene mutation frequencies.

Results

WHO bioassays results showed that populations from five sites were highly resistant to the pyrethroids (mortalities ranged from 52.5 to 75.3 %), and two sites were moderately resistant to these insecticides (80.4 – 87.2 %). Homozygote kdr mutations of L1014S ranged from 73 to 88 % in An. gambiae s.s. dominant populations whereas L1014S mutation frequencies were relatively low (7–31 %) in An. arabiensis dominant populations. There was a significant decrease (P < 0.05) in mosquito mortality with time after the spray with both lambda-cyhalothrin (75 % mortality after six months) and with the age of LLINs (60 % mortality after 24 month). Field collected mosquitoes were able to survive exposure to both IRS and LLINs even with newly sprayed walls (86.6–93.5 % mortality) and new LLINs (77.5–85.0 % mortality), Wild mosquitoes collected from the field had significantly lower mortality rates to LLINs (59.6–85.0 %) than laboratory reared susceptible strain (100 %). Insecticide concentration decreased significantly from 0.14 μg/ml in the new nets to 0.077 μg/ml in nets older than 18 months (P < 0.05).

Conclusion

This study confirms that insecticide decay and developing levels of resistance have a negative contribution to reduced efficacy of ITN and IRS in western Kenya. These factors contribute to decreased efficacy of pyrethroid insectides in ongoing malaria control programs. In order to mitigate against the impact of insecticide resistance and decay it is important to follow the WHO policy to provide the residents with new LLINs every three years of use while maintaining a high level of LLINs coverage and usage. There is also need for urgent development and deployment of non-pyrethroid based vector control tools.

Investigating a Non-Mesh Mosquito Net Among Outdoor Sleeping Nomadic Communities in Kenya

Rising reports of exophagic malaria vectors make even more pressing the need for alternatives to traditional, mesh, long-lasting insecticidal nets (LLINs) designed for indoor sleeping and often inadequate in the protection of outdoor-sleeping populations. This study tests and evaluates the retention, utilization, and durability of novel, non-mesh nets designed for outdoor use. Longitudinal, cross-sectional surveys were conducted, the physical condition of nets was assessed, and bio-efficacy and insecticide content were tested. At 22 months, retention was 98.0%; 97.1% of nets fell within the World Health Organization (WHO) category of being in “good” condition; none were in the “torn” category. At 18 months post-distribution, 100% of nets had at least WHO Pesticide Evaluation Scheme (WHOPES)-acceptable levels of insecticide, this proportion was 66.7% at 22 months. This novel mosquito net has the potential to provide a durable and context-specific tool to prevent malaria among traditionally hard-to-protect and highly vulnerable populations.

Development of Resistance to Pyrethroid in Culex pipiens pallens Population under Different Insecticide Selection Pressures

Current vector control programs are largely dependent on pyrethroids, which are the most commonly used and only insecticides recommended by the World Health Organization for insecticide-treated nets (ITNs). However, the rapid spread of pyrethroid resistance worldwide compromises the effectiveness of control programs and threatens public health. Since few new insecticide classes for vector control are anticipated, limiting the development of resistance is crucial for prolonging efficacy of pyrethroids. In this study, we exposed a field-collected population of Culex pipiens pallens to different insecticide selection intensities to dynamically monitor the development of resistance. Moreover, we detected kdr mutations and three detoxification enzyme activities in order to explore the evolutionary mechanism of pyrethroid resistance. Our results revealed that the level of pyrethroid resistance was proportional to the insecticide selection pressure. The kdr and metabolic resistance both contributed to pyrethroid resistance in the Cx. pipiens pallens populations, but they had different roles under different selection pressures. We have provided important evidence for better understanding of the development and mechanisms of pyrethroid resistance which may guide future insecticide use and vector management in order to avoid or delay resistance.

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.

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.

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

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

A look into the Medical and Veterinary Entomology crystal ball

Medical and Veterinary Entomology (MVE) represents a leading periodical in its field and covers many aspects of the biology and control of insects, ticks, mites and other arthropods of medical and veterinary importance. Since the first issue of the journal, researchers working in both developed and developing countries have published in MVE, with direct impact on current knowledge in the field. An increasing number of articles dealing with the epidemiology and transmission of vector-borne pathogens have been published in MVE, reflecting rapid changes in vector distribution, pathogen transmission and host-arthropod interactions. This article represents a gaze into the crystal ball in which we identify areas of increasing interest, discuss the main changes that have occurred in the epidemiology of parasitic arthropods since the first issue of MVE, and predict the principal scientific topics that might arise in the next 25 years for scientists working in medical and veterinary entomology.

Pyrethroid susceptibility of malaria vectors in four Districts of western Kenya

BACKGROUND

Increasing pyrethroid resistance in malaria vectors has been reported in western Kenya where long lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) are the mainstays of vector control. To ensure the sustainability of insecticide-based malaria vector control, monitoring programs need to be implemented. This study was designed to investigate the extent and distribution of pyrethroid resistance in 4 Districts of western Kenya (Nyando, Rachuonyo, Bondo and Teso). All four Districts have received LLINs while Nyando and Rachuonyo Districts have had IRS campaigns for 3-5 years using pyrethroids. This study is part of a programme aimed at determining the impact of insecticide resistance on malaria epidemiology.

METHODS

Three day old adult mosquitoes from larval samples collected in the field, were used for bioassays using the WHO tube bioassay, and mortality recorded 24 hours post exposure. Resistance level was assigned based on the 2013 WHO guidelines where populations with <90% mortality were considered resistant. Once exposed, samples were identified to species using PCR.

RESULTS

An. arabiensis comprised at least 94% of all An. gambiae s.l. in Bondo, Rachuonyo and Nyando. Teso was a marked contrast case with 77% of all samples being An. gambiae s.s. Mortality to insecticides varied widely between clusters even in one District with mortality to deltamethrin ranging from 45-100%, while to permethrin the range was 30-100%. Mortality to deltamethrin in Teso District was < 90% in 4 of 6 clusters tested in An arabiensis and <90% in An. gambiae s.s in 5 of 6 clusters tested. To permethrin, mortality ranged between 5.9-95%, with <90% mortality in 9 of 13 and 8 of 13 in An. arabiensis and An. gambiae s.s. respectively. Cluster specific mortality of An. arabiensis between permethin and deltamethrin were not correlated (Z = 2.9505, P = 0.2483).

CONCLUSION

High levels of pyrethroid resistance were observed in western Kenya. This resistance does not seem to be associated with either species or location. Insecticide resistance can vary within small geographical areas and such heterogeneity may make it possible to evaluate the impact of resistance on malaria and mosquito parameters within similar eco-epidemiological zones.

A push-pull system to reduce house entry of malaria mosquitoes

BACKGROUND

Mosquitoes are the dominant vectors of pathogens that cause infectious diseases such as malaria, dengue, yellow fever and filariasis. Current vector control strategies often rely on the use of pyrethroids against which mosquitoes are increasingly developing resistance. Here, a push-pull system is presented, that operates by the simultaneous use of repellent and attractive volatile odorants.

METHOD/RESULTS

Experiments were carried out in a semi-field set-up: a traditional house which was constructed inside a screenhouse. The release of different repellent compounds, para-menthane-3,8-diol (PMD), catnip oil e.o. and delta-undecalactone, from the four corners of the house resulted in significant reductions of 45% to 81.5% in house entry of host-seeking malaria mosquitoes. The highest reductions in house entry (up to 95.5%), were achieved by simultaneously repelling mosquitoes from the house (push) and removing them from the experimental set-up using attractant-baited traps (pull).

CONCLUSIONS

The outcome of this study suggests that a push-pull system based on attractive and repellent volatiles may successfully be employed to target mosquito vectors of human disease. Reductions in house entry of malaria vectors, of the magnitude that was achieved in these experiments, would likely affect malaria transmission. The repellents used are non-toxic and can be used safely in a human environment. Delta-undecalactone is a novel repellent that showed higher effectiveness than the established repellent PMD. These results encourage further development of the system for practical implementation in the field.

Reemergence of Anopheles funestus as a vector of Plasmodium falciparum in western Kenya after long-term implementation of insecticide-treated bed nets

Historically, the malaria vectors in western Kenya have been Anopheles funestus, Anopheles gambiae s.s., and Anopheles arabiensis. Of these species, An. funestus populations declined the most after the introduction of insecticide-treated bed nets (ITNs) in the 1990s in Asembo, and collections of An. funestus in the region remained low until at least 2008. Contrary to findings during the early years of ITN use in Asembo, the majority of the Anopheles collected here in 2010 and 2011 were An. funestus. Female An. funestus had characteristically high Plasmodium falciparum sporozoite rates and showed nearly 100% anthropophily. Female An. funestus were found more often indoors than outdoors and had relatively low mortality rates during insecticide bioassays. Together, these results are of serious concern for public health in the region, indicating that An. funestus may once again be contributing significantly to the transmission of malaria in this region despite the widespread use of ITNs/long-lasting insecticidal nets (LLINs).

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.

The efficacy of long-lasting nets with declining physical integrity may be compromised in areas with high levels of pyrethroid resistance

BACKGROUND

Long-lasting insecticide-treated mosquito nets (LLINs) are a primary malaria prevention strategy in sub-Saharan Africa. However, emergence of insecticide resistance threatens the effectiveness of LLINs.

METHODS

Cross-sectional surveys of LLINs were conducted in houses of seven and four villages in Gem and Bungoma Districts in western Kenya, respectively. Condition (number and area of holes in the nets), number and species of mosquitoes resting inside them, and insecticidal activity of nets were quantified. Mosquitoes collected inside nets were allowed to lay eggs and progeny tested for susceptibility to deltamethrin and permethrin, pyrethoids commonly deployed in LLINs in western Kenya.

RESULTS

In Gem, 83.3% of nets were less than three years old and 32.4% had at least one hole of any size; while in Bungoma, 92% were less than three years old and 48% had at least one hole. No anopheline and five Culex spp. mosquitoes were found resting inside nets in Gem regardless of the number and size of holes, while 552 Anopheles gambiae s.l., five Anopheles funestus s.l. and 137 Culex spp. were in nets in Bungoma. The number of mosquitoes resting inside nets increased with hole areas >50 cm in Bungoma. In WHO resistance assays, f1 offspring of samples collected in nets in Bungoma were 94 and 65% resistant to deltamethrin and permethrin, respectively. Nets from Bungoma retained strong activity against a susceptible laboratory strain, but not against f1 offspring of field-collected An. gambiae s.s. All An. gambiae s.s. samples collected in nets were homozygous for the kdr genotype L1014S.

CONCLUSIONS

In areas with pyrethroid resistant vectors, LLINs with modest hole areas permit mosquito entry and feeding, providing little protection against the vectors. LLIN formulations develop large holes within three years of use, diminishing their presupposed lifetime effectiveness.

High level of resistance in the mosquito Anopheles gambiae to pyrethroid insecticides and reduced susceptibility to bendiocarb in north-western Tanzania

Background

To control malaria in Tanzania, two primary vector control interventions are being scaled up: long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS). The main threat to effective malaria control is the selection of insecticide resistance. While resistance to pyrethroids, the primary insecticide used for LLINs and IRS, has been reported among mosquito vectors in only a few sites in Tanzania, neighbouring East African countries are recording increasing levels of resistance. To monitor the rapidly evolving situation, the resistance status of the malaria vector Anopheles gambiae s.l to different insecticides and the prevalence of the kdr resistance allele involved in pyrethroid resistance were investigated in north-western Tanzania, an area that has been subject to several rounds of pyrethroid IRS since 2006.

Methods

Household collections of anopheline mosquitoes were exposed to diagnostic dosages of pyrethroid, DDT, and bendiocarb using WHO resistance test kits. The relative proportions of An. gambiae s.s and Anopheles arabiensis were also investigated among mosquitoes sampled using indoor CDC light traps. Anophelines were identified to species and the kdr mutation was detected using real time PCR TaqMan assays.

Results

From the light trap collections 80% of An. gambiae s.l were identified as An. gambiae s.s and 20% as An. arabiensis. There was cross-resistance between pyrethroids and DDT with mortality no higher than 40% reported in any of the resistance tests. The kdr-eastern variant was present in homozygous form in 97% of An. gambiae s.s but was absent in An. arabiensis. Anopheles gambiae s.s showed reduced susceptibility to the carbamate insecticide, bendiocarb, the proportion surviving WHO tests ranging from 0% to 30% depending on season and location.

Conclusion

Anopheles gambiae s.s has developed phenotypic resistance to pyrethroids and DDT and kdr frequency has almost reached fixation. Unlike in coastal Tanzania, where the ratio of An. gambiae s.s to An. arabiensis has decreased in response to vector control, An. gambiae s.s persists at high frequency in north-western Tanzania, probably due to selection of pyrethroid resistance, and this trend is likely to arise in other areas as resistance spreads or is subject to local selection from IRS or LLINs.