Evaluation of piperonyl butoxide in enhancing the efficacy of pyrethroid insecticides against resistant Anopheles gambiae s.l. in Ghana

Modeling the synergistic interplay between malaria dynamics and economic growth

The mosquito-borne disease (malaria) imposes significant challenges on human health, healthcare systems, and economic growth/productivity in many countries. This study develops and analyzes a model to understand the interplay between malaria dynamics, economic growth, and transient events. It uncovers varied effects of malaria and economic parameters on model outcomes, highlighting the interdependence of the reproduction number (R0) on both malaria and economic factors, and a reciprocal relationship where malaria diminishes economic productivity, while higher economic output is associated with reduced malaria prevalence. This emphasizes the intricate interplay between malaria dynamics and socio-economic factors. The study offers insights into malaria control and underscores the significance of optimizing external aid allocation, especially favoring an even distribution strategy, with the most significant reduction observed in an equal monthly distribution strategy compared to longer distribution intervals. Furthermore, the study shows that controlling malaria in high mosquito biting areas with limited aid, low technology, inadequate treatment, or low economic investment is challenging. The model exhibits a backward bifurcation implying that sustainability of control and mitigation measures is essential even when R0 is slightly less than one. Additionally, there is a parameter regime for which long transients are feasible. Long transients are critical for predicting the behavior of dynamic systems and identifying factors influencing transitions; they reveal reservoirs of infection, vital for disease control. Policy recommendations for effective malaria control from the study include prioritizing sustained control measures, optimizing external aid allocation, and reducing mosquito biting.

Temperature, mosquito feeding status and mosquito density influence the measured bio-efficacy of insecticide-treated nets in cone assays

BACKGROUND

The WHO cone bioassay is routinely used to evaluate the bioefficacy of insecticide-treated nets (ITNs) for product pre-qualification and confirmation of continued ITN performance during operational monitoring. Despite its standardized nature, variability is often observed between tests. We investigated the influence of temperature in the testing environment, mosquito feeding status and mosquito density on cone bioassay results.

METHODS

Cone bioassays were conducted on MAGNet (alphacypermethrin) and Veeralin (alphacypermethrin and piperonyl butoxide (PBO)) ITNs, using laboratory-reared pyrethroid-resistant Anopheles funestus sensu stricto (FUMOZ strain) mosquitoes. Three experiments were conducted using standard cone bioassays following WHO-recommended test parameters, with one variable changed in each bioassay: (i) environmental temperature during exposure: 22-23 °C, 26-27 °C, 29-30 °C and 32-33 °C; (ii) feeding regimen before exposure: sugar starved for 6 h, blood-fed or sugar-fed; and (iii) mosquito density per cone: 5, 10, 15 and 20 mosquitoes. For each test, 15 net samples per treatment arm were tested with four cones per sample (N = 60). Mortality after 24, 48 and 72 h post-exposure to ITNs was recorded.

RESULTS

There was a notable influence of temperature, feeding status and mosquito density on An. funestus mortality for both types of ITNs. Mortality at 24 h post-exposure was significantly higher at 32-33 °C than at 26-27 °C for both the MAGNet [19.33% vs 7%; odds ratio (OR): 3.96, 95% confidence interval (CI): 1.99-7.87, P < 0.001] and Veeralin (91% vs 47.33%; OR: 22.20, 95% CI: 11.45-43.05, P < 0.001) ITNs. Mosquito feeding status influenced the observed mortality. Relative to sugar-fed mosquitoes, The MAGNet ITNs induced higher mortality among blood-fed mosquitoes (7% vs 3%; OR: 2.23, 95% CI: 0.94-5.27, P = 0.068) and significantly higher mortality among starved mosquitoes (8% vs 3%, OR: 2.88, 95% CI: 1.25-6.63, P = 0.013); in comparison, the Veeralin ITNs showed significantly lower mortality among blood-fed mosquitoes (43% vs 57%; OR: 0.56, 95% CI: 0.38-0.81, P = 0.002) and no difference for starved mosquitoes (58% vs 57%; OR: 1.05, 95% CI: 0.72-1.51, P = 0.816). Mortality significantly increased with increasing mosquito density for both the MAGNet (e.g. 5 vs 10 mosquitoes: 7% vs 12%; OR: 1.81, 95% CI: 1.03-3.20, P = 0.040) and Veeralin (e.g. 5 vs 10 mosquitoes: 58% vs 71%; OR 2.06, 95% CI: 1.24-3.42, P = 0.005) ITNs.

CONCLUSIONS

The results of this study highlight that the testing parameters temperature, feeding status and mosquito density significantly influence the mortality measured in cone bioassays. Careful adherence to testing parameters outlined in WHO ITN testing guidelines will likely improve the repeatability of studies within and between product testing facilities.

Insecticide susceptibility status of Anopheles gambiae mosquitoes and the effect of pre-exposure to a piperonyl butoxide (PBO) synergist on resistance to deltamethrin in northern Namibia

BACKGROUND

Pyrethroid-based indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) have been employed as key vector control measures against malaria in Namibia. However, pyrethroid resistance in Anopheles mosquitoes may compromise the efficacy of these interventions. To address this challenge, the World Health Organization (WHO) recommends the use of piperonyl butoxide (PBO) LLINs in areas where pyrethroid resistance is confirmed to be mediated by mixed function oxidase (MFO).

METHODS

This study assessed the susceptibility of Anopheles gambiae sensu lato (s.l.) mosquitoes to WHO tube bioassays with 4% DDT and 0.05% deltamethrin insecticides. Additionally, the study explored the effect of piperonyl butoxide (PBO) synergist by sequentially exposing mosquitoes to deltamethrin (0.05%) alone, PBO (4%) + deltamethrin (0.05%), and PBO alone. The Anopheles mosquitoes were further identified morphologically and molecularly.

RESULTS

The findings revealed that An. gambiae sensu stricto (s.s.) (62%) was more prevalent than Anopheles arabiensis (38%). The WHO tube bioassays confirmed resistance to deltamethrin 0.05% in the Oshikoto, Kunene, and Kavango West regions, with mortality rates of 79, 86, and 67%, respectively. In contrast, An. arabiensis displayed resistance to deltamethrin 0.05% in Oshikoto (82% mortality) and reduced susceptibility in Kavango West (96% mortality). Notably, there was reduced susceptibility to DDT 4% in both An. gambiae s.s. and An. arabiensis from the Kavango West region. Subsequently, a subsample from PBO synergist assays in 2020 demonstrated a high proportion of An. arabiensis in Oshana (84.4%) and Oshikoto (73.6%), and 0.42% of Anopheles quadriannulatus in Oshana. Non-amplifiers were also present (15.2% in Oshana; 26.4% in Oshikoto). Deltamethrin resistance with less than 95% mortality, was consistently observed in An. gambiae s.l. populations across all sites in both 2020 and 2021. Following pre-exposure to the PBO synergist, susceptibility to deltamethrin was fully restored with 100.0% mortality at all sites in 2020 and 2021.

CONCLUSIONS

Pyrethroid resistance has been identified in An. gambiae s.s. and An. arabiensis in the Kavango West, Kunene, and Oshikoto regions, indicating potential challenges for pyrethroid-based IRS and LLINs. Consequently, the data highlights the promise of pyrethroid-PBO LLINs in addressing resistance issues in the region.

Mechanisms of piperonyl butoxide cytotoxicity and its enhancement with imidacloprid and metals in Chinese hamster ovary cells

The widespread use of chemicals and the presence of chemical and metal residues in various foods, beverages, and other consumables have raised concerns about the potential for enhanced toxicity. This study assessed the cytotoxic effects of Piperonyl butoxide (PBO) and its enhancement by combination with major contamination chemicals including Imidacloprid and metals, using different cytotoxic and genotoxic assays in Chinese hamster ovary (CHO) cells. PBO exhibited elevated cytotoxic effects in poly (ADP-ribose) polymerase (PARP) deficient CHO mutants but not in Glutathione S-transferase deficient CHO mutants. PBO cytotoxicity was enhanced by PARP inhibitor, Olaparib. PBO cytotoxicity was also enhanced with co-exposure to Imidacloprid, Lead Chloride, or Sodium Selenite. PBO induces γH2AX foci formation and apoptosis. The induction of DNA damage markers was elevated with PARP deficiency and co-exposure to Imidacloprid, Lead Chloride, or Sodium Selenite. Moreover, PBO triggers to form etch pits on plastic surfaces. These results revealed novel mechanisms of PBO cytotoxicity associated with PARP and synergistic effects with other environmental pollutants. The toxicological mechanisms underlying exposure to various combinations at different concentrations, including concentrations below the permitted limit of intake or the level of concern, require further study.

Estimating malaria transmission risk through surveillance of human-vector interactions in northern Ghana

BACKGROUND

Vector bionomics are important aspects of vector-borne disease control programs. Mosquito-biting risks are affected by environmental, mosquito behavior and human factors, which are important for assessing exposure risk and intervention impacts. This study estimated malaria transmission risk based on vector-human interactions in northern Ghana, where indoor residual spraying (IRS) and insecticide-treated nets (ITNs) have been deployed.

METHODS

Indoor and outdoor human biting rates (HBRs) were measured using monthly human landing catches (HLCs) from June 2017 to April 2019. Mosquitoes collected were identified to species level, and Anopheles gambiae sensu lato (An. gambiae s.l.) samples were examined for parity and infectivity. The HBRs were adjusted using mosquito parity and human behavioral observations.

RESULTS

Anopheles gambiae was the main vector species in the IRS (81%) and control (83%) communities. Indoor and outdoor HBRs were similar in both the IRS intervention (10.6 vs. 11.3 bites per person per night [b/p/n]; z = -0.33, P = 0.745) and control communities (18.8 vs. 16.4 b/p/n; z = 1.57, P = 0.115). The mean proportion of parous An. gambiae s.l. was lower in IRS communities (44.6%) than in control communities (71.7%). After adjusting for human behavior observations and parity, the combined effect of IRS and ITN utilization (IRS: 37.8%; control: 57.3%) on reducing malaria transmission risk was 58% in IRS + ITN communities and 27% in control communities with ITNs alone (z = -4.07, P < 0.001). However, this also revealed that about 41% and 31% of outdoor adjusted bites in IRS and control communities respectively, occurred before bed time (10:00 pm). The mean directly measured annual entomologic inoculation rates (EIRs) during the study were 6.1 infective bites per person per year (ib/p/yr) for IRS communities and 16.3 ib/p/yr for control communities. After considering vector survival and observed human behavior, the estimated EIR for IRS communities was 1.8 ib/p/yr, which represents about a 70% overestimation of risk compared to the directly measured EIR; for control communities, it was 13.6 ib/p/yr (16% overestimation).

CONCLUSION

Indoor residual spraying significantly impacted entomological indicators of malaria transmission. The results of this study indicate that vector bionomics alone do not provide an accurate assessment of malaria transmission exposure risk. By accounting for human behavior parameters, we found that high coverage of ITNs alone had less impact on malaria transmission indices than combining ITNs with IRS, likely due to observed low net use. Reinforcing effective communication for behavioral change in net use and IRS could further reduce malaria transmission.

Multi-country evaluation of the durability of pyrethroid plus piperonyl-butoxide insecticide-treated nets: study protocol

BACKGROUND

Mass distributions of long-lasting insecticidal nets (LLINs) have contributed to large reductions in the malaria burden. However, this success is in jeopardy due in part to the increasing pyrethroid-resistant mosquito population as well as low LLINs coverage in various areas because the lifespan of LLINs is often shorter than the interval between replenishment campaigns. New insecticide-treated nets (ITNs) containing pyrethroid and piperonyl-butoxide (PBO) have shown a greater reduction in the incidence of malaria than pyrethroid LLINs in areas with pyrethroid-resistant mosquitoes. However, the durability (attrition, bio-efficacy, physical integrity and chemical retainment) of pyrethroid-PBO ITNs under operational settings has not been fully characterized. This study will measure the durability of pyrethroid-PBO ITNs to assess whether they meet the World Health Organization (WHO) three years of operational performance criteria required to be categorized as "long-lasting".

METHODS

A prospective household randomized controlled trial will be conducted simultaneously in Tanzania, India and Côte d'Ivoire to estimate the field durability of three pyrethroid-PBO ITNs (Veeralin®, Tsara® Boost, and Olyset® Plus) compared to a pyrethroid LLIN: MAGNet®. Durability monitoring will be conducted up to 36 months post-distribution and median survival in months will be calculated. The proportion of ITNs: (1) lost (attrition), (2) physical integrity, (3) resistance to damage score, (4) meeting WHO bio-efficacy (≥ 95% knockdown after 1 h or ≥ 80% mortality after 24 h for WHO cone bioassay, or ≥ 90% blood-feeding inhibition or ≥ 80% mortality after 24 h for WHO Tunnel tests) criteria against laboratory-reared resistant and susceptible mosquitoes, and insecticidal persistence over time will be estimated. The non-inferiority of Veeralin® and Tsara® Boost to the first-in-class, Olyset® Plus will additionally be assessed for mortality, and the equivalence of 20 times washed ITNs compared to field aged ITNs will be assessed for mortality and blood-feeding inhibition endpoints in the Ifakara Ambient Chamber Test, Tanzania.

CONCLUSION

This will be the first large-scale prospective household randomized controlled trial of pyrethroid-PBO ITNs in three different countries in East Africa, West Africa and South Asia, simultaneously. The study will generate information on the replenishment intervals for PBO nets.

Assessing the impact of insecticide-treated nets in the face of insecticide resistance on malaria control

The mosquito-borne disease, malaria, continues to impose a devastating health and economic burden worldwide. In malaria-endemic areas, insecticide-treated nets (ITNs) have been useful in curtailing the burden of the disease. However, mosquito resistance to insecticides, decay in ITN efficacy, net attrition, etc., undermine the effectiveness of ITNs in combatting malaria. In this study, mathematical models that account for asymptomatic infectious humans (through a partially immune class or a separate asymptomatic infectious class), insecticide resistance, and decay in ITN efficacy are proposed and analyzed. Analytical and numerical results of the models when ITN efficacy is constant show that there are parameter regimes for which a backward bifurcation occurs. Local and global sensitivity analyses are performed to identify parameters (some of which are potential targets for disease control) with the most significant influence on the control reproduction (R_c) and disease prevalence. These influential parameters include the maximum biting rate of resistant mosquitoes, ITN coverage, initial ITN efficacy against sensitive mosquitoes, the probability that an infectious mosquito (human) infects a susceptible human (mosquito), and the rate at which adult mosquitoes develop (lose) resistance to insecticides. Simulations of the models show that accounting for asymptomatic infectious humans through a separate class, or not accounting for the decay in ITN efficacy leads to an underestimation of disease burden. In particular, if the initial efficacy of ITNs against sensitive and resistance mosquitoes is 96%, the minimum ITN coverage required to reduce R_c below one (and hence, contain malaria) is approximately 11% (27%) lower when ITN efficacy is averaged (constant) for a model with a separate asymptomatic class. For the model with a partially immune class and decaying ITN efficacy, reducing R_c below one is impossible even if the entire populace uses ITNs. The study shows that replacing ITNs before their prescribed lifespans, or designing ITNs with longer lifespans is important for malaria control. Furthermore, the study shows that piperonyl butoxide (PBO) ITNs (which inhibit or reverse insecticide resistance) outperform regular ITNs in malaria control. Hence, prospects for effectively controlling malaria are enhanced by widespread use of high quality ITNs (e.g. PBO ITNs), especially if the useful lifespans of the ITNs are long enough and the ITNs are replaced before the end of their useful lifespans.

Countrywide insecticide resistance monitoring and first report of the presence of the L1014S knock down resistance in Niger, West Africa

BACKGROUND

Mass distribution of insecticide-treated nets (ITNs) is the principal malaria vector control strategy adopted by Niger. To better inform on the most appropriate ITN to distribute, the National Malaria Control Programme (NMCP) of Niger and its partners, conducted insecticide resistance monitoring in selected sites across the country.

METHODS

The susceptibility of Anopheles gambiae sensu lato (s.l.) to chlorfenapyr and pyrethroid insecticides was investigated in a total of sixteen sites in 2019 and 2020, using 2-5-day-old adults reared from wild collected larvae per site. The susceptibility status, pyrethroid resistance intensity at 5 and 10 times the diagnostic concentrations, and piperonyl butoxide (PBO) synergism with diagnostic concentrations of deltamethrin, permethrin and alpha-cypermethrin were assessed using WHO bioassays. Two doses (100 and 200 µg/bottle) of chlorfenapyr were tested using the CDC bottle assay method. Species composition and allele frequencies for knock-down resistance (kdr-L1014F and L1014S) and acetylcholinesterase (ace-1 G119S) mutations were further characterized using polymerase chain reaction (PCR).

RESULTS

High resistance intensity to all pyrethroids tested was observed in all sites except for alpha-cypermethrin in Gaya and Tessaoua and permethrin in Gaya in 2019 recording moderate resistance intensity. Similarly, Balleyara, Keita and Tillabery yielded moderate resistance intensity for alpha-cypermethrin and deltamethrin, and Niamey V low resistance intensity against deltamethrin and permethrin in 2020. Pre-exposure to PBO substantially increased susceptibility with average increases in mortality between 0 and 70% for tested pyrethroids. Susceptibility to chlorfenapyr (100 µg/bottle) was recorded in all sites except in Tessaoua and Magaria where susceptibility was recorded at the dose of 200 µg/bottle. Anopheles coluzzii was the predominant malaria vector species in most of the sites followed by An. gambiae sensu stricto (s.s.) and Anopheles arabiensis. The kdr-L1014S allele, investigated for the first time, was detected in the country. Both kdr-L1014F (frequencies [0.46-0.81]) and L1014S (frequencies [0.41-0.87]) were present in all sites while the ace-1 G119S was between 0.08 and 0.20.

CONCLUSION

The data collected will guide the NMCP in making evidence-based decisions to better adapt vector control strategies and insecticide resistance management in Niger, starting with mass distribution of new generation ITNs such as interceptor G2 and PBO ITNs.

Evidence of High Frequencies of Insecticide Resistance Mutations in Aedes aegypti (Culicidae) Mosquitoes in Urban Accra, Ghana: Implications for Insecticide-based Vector Control of Aedes-borne Arboviral Diseases

The most widespread arboviral diseases such as Dengue, Chikungunya, and Zika are transmitted mainly by Aedes mosquitoes. Due to the lack of effective therapeutics for most of these diseases, vector control remains the most effective preventative and control measure. This study investigated and compared the species composition, insecticide susceptibility, and resistance mechanisms in Aedes mosquito populations from a forest reserve converted to an eco-park and a peri-domestic sites in urban Accra, Ghana. Immature Aedes were sampled from the study sites, raised to adults, and exposed to deltamethrin, permethrin, DDT, fenitrothion, bendiocarb, permethrin + PBO, and deltamethrin + PBO using WHO tube assays. Melting curve analyses were performed for F1536C, V1016I, and V410L genetic mutations in surviving and dead mosquitoes following exposure to deltamethrin and permethrin. Microplate assay was used to access enzyme activity levels in adult mosquitoes from both populations. Aedes aegypti was found to be the dominant species from both study populations. The susceptibility test results revealed a high frequency of resistance to all the insecticides except fenitrothion. F1534C mutations were observed in 100% and 97% of mosquitoes from the peri-domestic and forest population, respectively but were associated with pyrethroid resistance only in the forest population (P < 0.0001). For the first time in Aedes mosquitoes in Ghana, we report the existence V410L mutations, mostly under selection only in the forest population (HWE P < 0.0001) and conclude that Aedes vectors in urban Accra have developed resistance to many commonly used insecticides. This information is important for the formulation of vector control strategies for Aedes control in Ghana.

Criteria for selecting sentinel unit locations in a surveillance system for vector-borne disease: A decision tool

Objectives

With vector-borne diseases emerging across the globe, precipitated by climate change and other anthropogenic changes, it is critical for public health authorities to have well-designed surveillance strategies in place. Sentinel surveillance has been proposed as a cost-effective approach to surveillance in this context. However, spatial design of sentinel surveillance system has important impacts on surveillance outcomes, and careful selection of sentinel unit locations is therefore an essential component of planning.

Methods

A review of the available literature, based on the realist approach, was used to identify key decision issues for sentinel surveillance planning. Outcomes of the review were used to develop a decision tool, which was subsequently validated by experts in the field.

Results

The resulting decision tool provides a list of criteria which can be used to select sentinel unit locations. We illustrate its application using the case example of designing a national sentinel surveillance system for Lyme disease in Canada.

Conclusions

The decision tool provides researchers and public health authorities with a systematic, evidence-based approach for planning the spatial design of sentinel surveillance systems, taking into account the aims of the surveillance system and disease and/or context-specific considerations.

Synergistic property of piperonyl butoxide, diethyl maleate, triphenyl phosphate and verapamil hydrochloride with deltamethrin and ivermectin against Rhipicephalus microplus ticks

The present study was taken up to evaluate the synergistic properties of piperonyl butoxide (PBO), diethyl maleate (DEM), triphenyl phosphate (TPP) and verapamil (VER) with deltamethrin (DLM) and ivermectin (IVM) against DLM and IVM resistant tick populations collected from Madhya Pradesh and Punjab states of India. The collected field tick populations were resistant to DLM (Resistance Factor [RF] in the range of 21.71-32.98) and IVM (RF in the range of 1.89-4.98). A strong synergism between DLM and, IVM with PBO and IVM with VER was noticed. The synergistic efficacy of PBO and VER with IVM in reducing the lethal concentration 50 (LC50) value (1.69-5.72 times for PBO and 3.00-10.62 times for VER) of IVM in resistant ticks suggest that a combination of these synergists with IVM can significantly enhance the effectiveness of IVM against IVM-resistant Rhipicephlaus microplus populations gradually establishing in the different parts of the country. The synergistic efficiency of PBO with DLM in reducing the LC50 value was 2.65 and 18.01 times, respectively, against DLM- resistant two R. microplus populations (KTN and LDH). The study revealed the gradual establishment of DLM and IVM resistant populations in the surveyed states suggesting the need to adopt required resistance management strategies. The use of synergists with DLM and IVM has emerged as an effective approach for controlling the acaricide-resistant ticks.

Escalating pyrethroid resistance in two major malaria vectors Anopheles funestus and Anopheles gambiae (s.l.) in Atatam, Southern Ghana

BACKGROUND

Aggravation of insecticide resistance in malaria vectors is threatening the efforts to control malaria by reducing the efficacy of insecticide-based interventions hence needs to be closely monitored. This study investigated the intensity of insecticide resistance of two major malaria vectors An. funestus sensu stricto (s.s.) and An. gambiae sensu lato (s.l.) collected in southern Ghana and assessed the bio-efficacy of several long-lasting insecticidal nets (LLINs) against these mosquito populations.

METHODS

The insecticide susceptibility profiles of Anopheles funestus s.s. and Anopheles gambiae s.l. populations from Obuasi region (Atatam), southern Ghana were characterized and the bio-efficacy of some LLINs was assessed to determine the impact of insecticide resistance on the effectiveness of these tools. Furthermore, molecular markers associated with insecticide resistance in both species were characterized in the F0 and F1 populations using PCR and qPCR methods.

RESULTS

Anopheles funestus s.s. was the predominant species and was resistant to pyrethroids, organochlorine and carbamate insecticides, but fully susceptible to organophosphates. An. gambiae s.l. was resistant to all four insecticide classes. High intensity of resistance to 5 × and 10 × the discriminating concentration (DC) of pyrethroids was observed in both species inducing a considerable loss of efficacy of long-lasting insecticidal nets (LLINs). Temporal expression analysis revealed a massive 12-fold increase in expression of the CYP6P4a cytochrome P450 gene in An. funestus s.s., initially from a fold change of 41 (2014) to 500 (2021). For both species, the expression of candidate genes did not vary according to discriminating doses. An. gambiae s.l. exhibited high frequencies of target-site resistance including Vgsc-1014F (90%) and Ace-1 (50%) while these mutations were absent in An. funestus s.s.

CONCLUSIONS

The multiple and high intensity of resistance observed in both malaria vectors highlights the need to implement resistance management strategies and the introduction of new insecticide chemistries.

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.

Insecticide Resistance Profiles and Synergism of Field Aedes aegypti from Indonesia

Information on the insecticide resistance profiles of Aedes aegypti in Indonesia is fragmentary because of the lack of wide-area insecticide resistance surveillance. We collected Ae. aegypti from 32 districts and regencies in 27 Indonesian provinces and used WHO bioassays to evaluate their resistance to deltamethrin, permethrin, bendiocarb, and pirimiphos-methyl. To determine the possible resistance mechanisms of Ae. aegypti, synergism tests were conducted using piperonyl butoxide (PBO) and S,S,S-tributylphosphorotrithioates (DEF). The Ae. aegypti from all locations exhibited various levels of resistance to pyrethroids. Their resistance ratio (RR₅₀) to permethrin and deltamethrin ranged from 4.08× to 127× and from 4.37× to 72.20×, respectively. In contrast with the findings of other studies, most strains from the highly urbanized cities on the island of Java (i.e., Banten, Jakarta, Bandung, Semarang, Yogyakarta, and Surabaya) exhibited low to moderate resistance to pyrethroids. By contrast, the strains collected from the less populated Kalimantan region exhibited very high resistance to pyrethroids. The possible reasons are discussed herein. Low levels of resistance to bendiocarb (RR₅₀, 1.24–6.46×) and pirimiphos-methyl (RR₅₀, 1.01–2.70×) were observed in all tested strains, regardless of locality. PBO and DEF synergists significantly increased the susceptibility of Ae. aegypti to permethrin and deltamethrin and reduced their resistance ratio to less than 16×. The synergism tests suggested the major involvement of cytochrome P450 monooxygenases and esterases in conferring pyrethroid resistance. On the basis of our results, we proposed a 6-month rotation of insecticides (deltamethrin + synergists ➝ bendiocarb ➝ permethrin + synergists ➝ pirimiphos-methyl) and the use of an insecticide mixture containing pyrethroid and pyrimiphos-methyl to control Ae. aegypti populations and overcome the challenge of widespread Ae. aegypti resistance to pyrethroid in Indonesia.

Insecticide resistance is a major impediment to the successful management of vector-transmitted diseases because it increases the vector’s chances of surviving under insecticide treatment. In Indonesia, the implementation of insecticide resistance management at the national level is particularly challenging due to the vast area and regional disparities in terms of population, health, and socioeconomic status. Previous studies on determining insecticide resistance of Aedes mosquito only focused on several cities in some provinces of Indonesia, making resistance monitoring results difficult to interpret and arguably reflect the generality in Indonesia. To complicate the matter, data released by the Ministry of Agriculture of Indonesia in 2022 showed that approximately 82% of insecticides registered to control Ae. aegypti in Indonesia are pyrethroid-based products. Principally, we found that the synergists PBO and DEF significantly reduce the resistance of field Ae. aegypti from Indonesia toward permethrin and deltamethrin. Bendiocarb and pirimiphos-methyl remain highly toxic to the field strains of Ae. aegypti. We suggest the feasible choice of insecticide group for Ae. aegypti vector management based on the currently registered insecticide inventory. The finding also underscores the urgent need to approve other non-pyrethroid-based insecticides as alternative tools for reducing the risk of resistance development during an outbreak.

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.

Assessing the effectiveness of malaria interventions at the regional level in Ghana using a mathematical modelling application

Supporting malaria control with interfaced applications of mathematical models that enables investigating effectiveness of various interventions as well as their cost implications could be useful. Through their usage for planning, these applications may improve the prospects of attaining various set targets such as those of the National Strategic Plan policies for malaria control in Ghana. A malaria model was adapted and used for simulating the incidence of malaria in various regions of Ghana. The model and its application were developed by the Modelling and Simulation Hub Africa and calibrated using district level data in Ghana from 2012 to 2018. Average monthly rainfall at the zonal level was fitted to trigonometric functions for each ecological zone using least squares approach. These zonal functions were then used as forcing functions. Subsequently, various intervention packages were investigated to observe their impact on averting malaria incidence by 2030. Increased usage of bednets but not only coverage levels, predicted a significant proportion of cases of malaria averted in all regions. Whereas, improvements in the health system by way of health seeking, testing and treatment predicted a decline in incidence largely in all regions. With an increased coverage of SMC, to include higher age groups, a modest proportion of cases could be averted in populations of the Guinea savannah. Indoor residual spraying could also benefit populations of the Transitional forest and Coastal savannah as its impact is significant in averting incidence. Enhancing bednet usage to at least a doubling of the current usage levels and deployed in combination with various interventions across regions predicted significant reductions, in malaria incidence. Regions of the Transitional forest and Coastal savannah could also benefit from a drastic decline in incidence following a gradual introduction of indoor residual spraying on a sustained basis.

Metabolic Resistance in Permethrin-Resistant Florida Aedes aegypti (Diptera: Culicidae)

Simple Summary

Pyrethroid-oriented vector control programs have increased worldwide to control adult Aedes aegypti mosquitoes and quell Aedes-borne disease epidemics. Due to years of pyrethroid use, resistance to pyrethroids in Ae. aegypti has become a global issue. In Florida, permethrin is the most common pyrethroid adulticide active ingredient used to control mosquito populations. Thus far, all wild Florida Ae. aegypti populations tested against permethrin have been found to be resistant. Metabolic resistance is a major mechanism of resistance in insects in which enzyme-mediated reactions cause the degradation or sequestration of insecticides. We performed assays to investigate the presence of metabolic resistance in 20 Florida Ae. aegypti populations and found that 11 populations (55%) exhibited metabolic resistance due to the action of at least one of three classes of metabolizing enzymes: oxidases, esterases, and glutathione transferases. Additionally, we identified two metabolic enzyme inhibitors: S.S.S-tributyl phosphorotrithioate (DEF; inhibits esterase activity) and diethyl maleate (DM; inhibits glutathione transferase activity), in addition to the commonly used piperonyl butoxide (PBO; inhibits oxidase activity), which were able to increase the efficacy of permethrin against resistant Ae. aegypti populations. Pre-exposure to DEF, PBO, and DM resulted in increased mortality after permethrin exposure in eight (73%), seven (64%), and six (55%) of the Ae. aegypti populations, respectively. Increasing the effectiveness of pyrethroids is important for mosquito control, as it is the primary method used for adult control during mosquito-borne disease outbreaks. Considering that DEF and DM performed similarly to PBO, they may be good candidates for inclusion in formulated pyrethroid products to increase their efficacy against resistant mosquitoes.

Abstract

Aedes aegypti is the principal mosquito vector for many arthropod-borne viruses (arboviruses) including dengue, chikungunya, and Zika. In the United States, excessive permethrin use has led to a high frequency of resistance in mosquitoes. Insecticide resistance is a significant obstacle in the struggle against vector-borne diseases. To help overcome metabolic resistance, synergists that inhibit specific metabolic enzymes can be added to formulated pyrethroid products. Using modified CDC bottle bioassays, we assessed the effect of three inhibitors (piperonyl butoxide (PBO), which inhibits oxidase activity; S.S.S-tributyl phosphorotrithioate (DEF), which inhibits esterase activity; and diethyl maleate (DM), which inhibits glutathione transferase activity) + permethrin. We performed these against 20 Florida Ae. aegypti populations, all of which were resistant to permethrin. Our data indicated that 11 out of 20 populations (55%) exhibited metabolic resistance. Results revealed 73% of these populations had significant increases in mortality attributed to DEF + permethrin, 64% to PBO + permethrin, and 55% to DM + permethrin compared to permethrin alone. Currently, PBO is the only metabolic enzyme inhibitor added to formulated pyrethroid products used for adult mosquito control. Our results suggest that the DEF and DM inhibitors could also be useful additives in permethrin products, especially against metabolically resistant Ae. aegypti mosquitoes. Moreover, metabolic assays should be conducted to better inform mosquito control programs for designing and implementing integrated vector management strategies.

Widespread multiple insecticide resistance in the major dengue vector Aedes albopictus in Hainan Province, China

BACKGROUND

Aedes albopictus is a highly invasive mosquito and has become a potential vector of dengue, chikungunya and Zika viruses. Insecticide-based mosquito interventions are the main tools for vector-borne disease control. However, mosquito resistance to insecticides is a major threat to effective prevention and control. Five Ae. albopictus populations across Hainan Province, China were investigated for susceptibility to multiple insecticide and resistance mechanisms.

RESULTS

Larval bioassays indicated that resistance to pyrethroids was common in all larval populations. Adult bioassays revealed all populations were either resistant or highly resistant to at least four of the six synthetic insecticides (deltamethrin, permethrin, cyfluthrin, propoxur, malathion, and DDT) tested. Pre-exposure of mosquitoes to the synergistic agent piperonyl butoxide (PBO) increased mosquito mortality by 2.4-43.3% in bioassays to DDT, malathion, and permethrin and rendered mosquito sensitive to deltamethrin, cyfluthrin, and propoxur. The frequency of knockdown resistance (kdr) mutations (F1534S and F1534C) ranged from 69.8% to 89.3% and from 38.1% to 87.0% in field-resistant and sensitive populations, respectively. F1534S mutation was significantly associated with pyrethroid resistance. No mutation was detected in the acetylcholinesterase (ace-1) gene in the two examined populations.

CONCLUSION

This study provides evidence of widespread resistance to multiple insecticides in Ae. albopictus in Hainan Province, China. Both kdr mutations and metabolic detoxification were potential causes of insecticide resistance for Ae. albopictus. Our findings highlight the need for insecticide resistance management and mosquito control measures that do not entirely depend on synthetic insecticides. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

20-Hydroxyecdysone (20E) signaling as a promising target for the chemical control of malaria vectors

With the rapid development and spread of resistance to insecticides among anopheline malaria vectors, the efficacy of current World Health Organization (WHO)-approved insecticides targeting these vectors is under threat. This has led to the development of novel interventions, including improved and enhanced insecticide formulations with new targets or synergists or with added sterilants and/or antimalarials, among others. To date, several studies in mosquitoes have revealed that the 20-hydroxyecdysone (20E) signaling pathway regulates both vector abundance and competence, two parameters that influence malaria transmission. Therefore, insecticides which target 20E signaling (e.g. methoxyfenozide and halofenozide) may be an asset for malaria vector control. While such insecticides are already commercially available for lepidopteran and coleopteran pests, they still need to be approved by the WHO for malaria vector control programs. Until recently, chemicals targeting 20E signaling were considered to be insect growth regulators, and their effect was mostly studied against immature mosquito stages. However, in the last few years, promising results have been obtained by applying methoxyfenozide or halofenozide (two compounds that boost 20E signaling) to Anopheles populations at different phases of their life-cycle. In addition, preliminary studies suggest that methoxyfenozide resistance is unstable, causing the insects substantial fitness costs, thereby potentially circumventing one of the biggest challenges faced by current vector control efforts. In this review, we first describe the 20E signaling pathway in mosquitoes and then summarize the mechanisms whereby 20E signaling regulates the physiological processes associated with vector competence and vector abundance. Finally, we discuss the potential of using chemicals targeting 20E signaling to control malaria vectors.

Susceptibility of Anopheles gambiae from Côte d'Ivoire to insecticides used on insecticide-treated nets: evaluating the additional entomological impact of piperonyl butoxide and chlorfenapyr

Background

Pyrethroid-treated mosquito nets are currently the mainstay of vector control in Côte d’Ivoire. However, resistance to pyrethroids has been reported across the country, limiting options for insecticide resistance management due to the paucity of alternative insecticides. Two types of insecticide-treated nets (ITNs), ITNs with pyrethroids and the synergist piperonyl butoxide (PBO), and Interceptor®G2 nets, a net treated with a combination of chlorfenapyr and alpha-cypermethrin, are believed to help in the control of pyrethroid-resistant mosquitoes.

Methods

The susceptibility of Anopheles gambiae sensu lato (s.l.) to pyrethroid insecticides with and without pre-exposure to PBO as well as to chlorfenapyr was investigated in fifteen sites across the country. Susceptibility tests were conducted on 2- to 4-day old adult female An. gambiae s.l. reared from larval collections. The resistance status, intensity, and effects of PBO on mortality after exposure to different concentrations of deltamethrin, permethrin and alpha-cypermethrin were determined using WHO susceptibility test kits. In the absence of a WHO-recommended standard protocol for chlorfenapyr, two interim doses (100 and 200 µg/bottle) were used to test the susceptibility of mosquitoes using the CDC bottle assay method.

Results

Pre-exposure to PBO did not result in full restoration of susceptibility to any of the three pyrethroids for the An. gambiae s.l. populations from any of the sites surveyed. However, PBO pre-exposure did increase mortality for all three pyrethroids, particularly deltamethrin (from 4.4 to 48.9%). Anopheles gambiae s.l. from only one site (Bettie) were susceptible to chlorfenapyr at the dose of 100 µg active ingredient (a.i.)/bottle. At the dose of 200 µg (a.i.)/bottle, susceptibility was only recorded in 10 of the 15 sites.

Conclusion

Low mosquito mortality was found for pyrethroids alone, and while PBO increased mortality, it did not restore full susceptibility. The vector was not fully susceptible to chlorfenapyr in one third of the sites tested. However, vector susceptibility to chlorfenapyr seems to be considerably higher than for pyrethroids alone or with PBO. These data should be used cautiously when making ITN procurement decisions, noting that bioassays are conducted in controlled conditions and may not fully represent field efficacy where the host-seeking behaviours, which include free-flying activity are known to enhance pro-insecticide chlorfenapyr intoxication to mosquitoes.

Assessment of Resistance to Organophosphates and Pyrethroids in Aedes aegypti (Diptera: Culicidae): Do Synergists Affect Mortality?

Aedes aegypti (L.) is the primary vector of Zika, dengue, yellow fever, and chikungunya viruses. Insecticides used in mosquito control can help prevent the spread of vector-borne diseases. However, it is essential to determine insecticide resistance (IR) status before control measures are undertaken. Only the most effective insecticides should be used to avoid ineffective control and/or promotion of IR. Pyrethroids and organophosphates are the most commonly used insecticides for mosquito control. Here, the efficacy of two active ingredients (AIs; permethrin [pyrethroid], chlorpyrifos [organophosphate]), two formulated products (FPs; Biomist [AI: permethrin]) and (Mosquitomist [AI: chlorpyrifos]), and three synergists (piperonyl butoxide, diethyl maleate, S-S-S-tributyl phosphorotrithioate) was evaluated in two Ae. aegypti colonies (pyrethroid resistant and susceptible). Mosquitomist was most effective against the pyrethroid-resistant colony (100% mortality at diagnostic time). Pre-exposure to synergists did not increase the efficacy of AIs against the pyrethroid-resistant colony. Further research is needed to discover how synergists may affect the efficacy of insecticides when used on pyrethroid-resistant mosquitoes.

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.

Evaluation of DawaPlus 3.0 and DawaPlus 4.0, deltamethrin-PBO combination nets against pyrethroid-resistant Anopheles culicifacies in experimental huts in India

Background

The development of resistance in vectors is one of the major impediments for malaria control. Adding synergists to insecticides has proven to be an alternative choice for controlling resistant mosquitoes. DawaPlus 3.0 and DawaPlus 4.0 are new long-lasting insecticidal nets (LLINs) in which deltamethrin and a synergist, piperonyl butoxide (PBO) are added into filaments and their efficacy was tested against resistant malaria vector, Anopheles culicifacies in experimental huts in India.

Methods

The performance of two trial nets in terms of deterrence induced exiting, blood-feeding inhibition and mortality of An. culicifacies was compared with DawaPlus 2.0 and untreated net.

Results

There was a significant reduction in entry, blood feeding and mortality (p < 0.05) and increase in exit rates of An. culicifacies in the treatment arms compared to untreated arm. But, both candidate LNs washed 20 times could not perform better than the washed reference net (DawaPlus 2.0). Cone bioassay results showed that all the treatment arms (both washed and unwashed) produced < 80% mortality of An. culicifacies before and after hut evaluation.

Conclusions

DawaPlus 3.0 and DawaPlus 4.0 with their current specification may not be as effective as required to control the resistant vector, An. culicifacies, in east-central India.

High insecticide resistance intensity of Anopheles gambiae (s.l.) and low efficacy of pyrethroid LLINs in Accra, Ghana

Background

Insecticide resistance of Anopheles gambiae (s.l.) against public health insecticides is increasingly reported in Ghana and need to be closely monitored. This study investigated the intensity of insecticide resistance of An. gambiae (s.l.) found in a vegetable growing area in Accra, Ghana, where insecticides, herbicides and fertilizers are massively used for plant protection. The bioefficacy of long-lasting insecticidal nets (LLINs) currently distributed in the country was also assessed to delimitate the impact of the insecticide resistance intensity on the effectiveness of those nets.

Methods

Three- to five-day-old adult mosquitoes that emerged from collected larvae from Opeibea, Accra (Ghana), were assayed using CDC bottle and WHO tube intensity assays against different insecticides. The Vgsc-L1014F and ace-1 mutations within the population were also characterized using PCR methods. Furthermore, cone bioassays against different types of LLINs were conducted to evaluate the extent and impact of the resistance of An. gambiae (s.l.) from Opeibea.

Results

Anopheles gambiae (s.l.) from Opeibea were resistant to all the insecticides tested with very low mortality observed against organochlorine, carbamates and pyrethroid insecticides using WHO susceptibility tests at diagnostic doses during three consecutive years of monitoring. The average frequencies of Vgsc-1014F and ace-1 in the An. gambiae (s.l.) population tested were 0.99 and 0.76, respectively. The intensity assays using both CDC bottle and WHO tubes showed high resistance intensity to pyrethroids and carbamates with survivals at 10× the diagnostic doses of the insecticides tested. Only pirimiphos methyl recorded a low resistance intensity with 100% mortality at 5× the diagnostic dose. The bioefficacy of pyrethroid LLINs ranged from 2.2 to 16.2% mortality while the PBO LLIN, PermaNet^® 3.0, was 73%.

Conclusions

WHO susceptibility tests using the diagnostic doses described the susceptibility status of the mosquito colony while CDC bottle and WHO tube intensity assays showed varying degrees of resistance intensity. Although both methods are not directly comparable, the indication of the resistance intensity showed the alarming insecticide resistance intensity in Opeibea and its surroundings, which could have an operational impact on the efficacy of vector control tools and particularly on pyrethroid LLINs.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3556-y) contains supplementary material, which is available to authorized users.

High Plasmodium infection and multiple insecticide resistance in a major malaria vector Anopheles coluzzii from Sahel of Niger Republic

Background

Information on insecticide resistance and the mechanisms driving it in the major malaria vectors is grossly lacking in Niger Republic, thus hindering control efforts. To facilitate evidence-based malaria control, the role of Anopheles coluzzii population from southern Niger, in malaria transmission, its insecticides resistance profile and the molecular mechanisms driving the resistance were characterized.

Methods

Blood fed female Anopheles gambiae sensu lato resting indoor were collected at Tessaoua, Niger. Source of blood was established using PCR and infection with Plasmodium determined using TaqMan assay. Resistance profile was established with the major public health insecticides, and resistance intensity determined with deltamethrin. Synergist assays were conducted with piperonyl butoxide and diethyl maleate. Presence of L1014F and L1014S knockdown resistance (kdr) mutations in the voltage-gated sodium channel (VGSC) was investigated using TaqMan genotyping, and strength of selection pressure acting on the Anopheles populations determined by assessing the genetic diversity of a fragment spanning exon-20 of the VGSC from alive and dead females.

Results

High human blood index (96%) and high Plasmodium falciparum infection (~ 13%) was observed in the An. coluzzii population. Also, a single mosquito was found infected with Plasmodium vivax. High pyrethroid and organochloride resistance was observed with mortalities of less than 20% for deltamethrin, permethrin, α-cypermethrin, and DDT. A high LD₅₀ (156.65 min) was obtained for deltamethrin, with a resistance ratio of ~ 47.18 compared to the susceptible Ngoussou colony. Moderate carbamate resistance was observed, and a full susceptibility to organophosphates recorded. Synergist bioassays with piperonyl butoxide and diethyl maleate significantly recovered deltamethrin and DDT susceptibility, respectively implicating CYP450 s (mortality = 82%, χ² = 84.51, p < 0.0001) and glutathione S-transferases (mortality = 58%, χ² = 33.96, p < 0.001) in resistance. A high frequency of 1014F kdr mutation (82%) was established, with significant difference in genotype distribution associated with permethrin resistance [odds ratio = 7.71 (95% CI 2.43–14.53, χ² = 13.67, p = 0.001]. Sequencing of intron-1 of the voltage-gated sodium channel (VGSC) revealed a low genetic diversity.

Conclusion

High pyrethroid resistance highlight the challenges to the effectiveness of the pyrethroids-based ITNs and indoor residual spraying (IRS) against An. coluzzii in Niger. The pyrethroids-synergists LLINs and organophosphate-based IRS maybe the alternatives for malaria control in southern Niger.

Temporal escalation of Pyrethroid Resistance in the major malaria vector Anopheles coluzzii from Sahelo-Sudanian Region of northern Nigeria

Despite the highest global burden of malaria, information on bionomics and insecticide resistance status of malaria vectors is grossly lacking in the densely populated Sahelo-Sudanian region of Nigeria. To support evidence-based vector control we characterised transmission and resistance profiles of Anopheles coluzzii populations from three sites in northern Nigeria. High sporozoite infection (~19.51%) was found in the An. coluzzii populations. A high pyrethroid resistance was observed with only 1% mortality against deltamethrin, a high LD₅₀ (96.57 µg/ml), and a high LT₅₀ (170.27 min, resistance ratio of ~51 compared with the fully susceptible Ngoussou colony). Moderate carbamate resistance was observed. Synergist bioassays significantly recovered deltamethrin susceptibility implicating CYP450s (mortality = 85%, χ² = 134.04, p < 0.0001) and esterases (mortality = 56%, χ² = 47.31, p < 0.0001). Reduced bed net efficacy was also observed, with mortalities on exposure to the roof of PermaNet3.0 (PBO + deltamethrin) more than 22 times compared to the side panel (deltamethrin). TaqMan genotyping revealed a high frequency of 1014F kdr mutation (82%) with significant difference in genotype distribution associated with permethrin resistance [OR = 4.69 (CI:1.53–14.35, χ² = 8.22 p = 0.004]. Sequencing of exons 18–21 of the VGSC led to detection of two additional nonsynonymous mutations, Ile10148Asn and Ser1156Gly. These findings highlight the threats posed by the highly resistant An. coluzzii to malaria control in Nigeria.

Cytochrome P450 Mono-Oxygenase and Resistance Phenotype in DDT and Deltamethrin-Resistant Anopheles gambiae (Diptera: Culicidae) and Culex quinquefasciatus in Kosofe, Lagos, Nigeria

Pyrethroids and DDT are key insecticides in the control of malaria, yellow fever, and lymphatic filariasis vectors. Knockdown and metabolic resistance mechanisms have been proven to be important in determining the efficacy of insecticides. Here we investigated cytochrome P450 as a resistance mechanism in Anopheles gambiae Giles and Culex quinquefasciatus Say exposed to deltamethrin and DDT. Two- to three-days-old adult female mosquitoes were used for insecticide exposures and PBO synergistic assays using WHO standard guidelines, kits and test papers (DDT 4%, deltamethrin 0.05%, and PBO 4%). Polymerase chain reaction (PCR) assays were used for the identification of the species and for characterization of the kdr allele. Mortality at 24 h post-exposure was 18 and 17% in An. gambiae s.s. exposed to DDT and deltamethrin, respectively; 1 and 5% in Cx. quinquefasciatus exposed to DDT and deltamethrin respectively. Significant (P < 0.01) levels of susceptibility was recorded in mosquitoes pre-exposed to PBO, as KDT50 and 24 h of exposure ranged from 37.6 min to 663.4 min and 27 to 80%, respectively. Presence of a knockdown resistance allele was recorded in An. gambiae s.s., 22.5% for homozygote resistance and 7.5% for heterozygotes, while Cx. quinquefasciatus populations showed no kdr allele despite the high level of resistance to DDT and deltamethrin. Findings from this study indicated that cytochrome P450 mono-oxygenase expression is highly implicated in the resistance phenotype to DDT and pyrethroids in An. gambiae and Cx. quinquefasciatus in the study area.

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

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

Detoxification mechanisms involved in ivermectin resistance in the cattle tick, Rhipicephalus (Boophilus) microplus

The cattle tick Rhipicephalus microplus is one of the most important ectoparasites with great sanitary and economic impact for cattle rearing worldwide. Ivermectin is commonly used to control tick populations, but its use over the last 30 years has led to the development of resistant populations of R. microplus, and a concomitant loss of efficacy. In this context, we aimed to determine the metabolic mechanisms that contribute to ivermectin resistance in a resistant strain of this species. We performed lethal time bioassays with inhibitors of detoxifying enzymes and xenobiotic transporters (four detoxification pathways) using two strains of ticks: a susceptible strain, Mozo, and a resistant strain, Juarez. We used four inhibitors to test the involvement of different families of proteins responsible for detoxification of ivermectin, namely cytochrome P450, esterases, glutathione-S-transferase, and ATP Binding Cassette Transporters. We calculated the synergistic factor for each inhibitor and strain. To different degrees, all tested inhibitors altered the mortality rates in the strain Juarez, indicating that multiple mechanisms are responsible for the resistant phenotype. Detoxification mechanisms mediated by ABC transporters were observed to be the most important. Esterases, glutathione-S-transferases, and cytochrome-oxidases played less important roles in detoxification.

QSAR modelling of synergists to increase the efficacy of deltamethrin against pyrethroid-resistant Aedes aegypti mosquitoes$

Space spraying of deltamethrin allows the control of adult Aedes (Stegomyia) aegypti mosquitoes. Unfortunately, many vector control programs are threatened by the development of resistances that decrease the efficacy of this adulticide. Faced with this situation, we can either try to use another insecticide presenting a different mechanism of action or find a strategy that brings back the efficacy of the insecticide at a satisfying level to pursue its use in vector control. Restoration of the efficacy of an insecticide can be obtained by means of a synergist. In this context, QSAR modelling was used to find synergists to combine with deltamethrin for increasing its efficacy against resistant strains of Ae. aegypti. Seventy-four structurally diverse chemicals with their 24-hour LD₅₀ values, obtained under the same experimental conditions on Ae. aegypti females, were used. Molecules were described by means of autocorrelation vectors encoding lipophilicity, molar refractivity, H-bonding acceptor and donor ability. A three-layer perceptron (TLP) was employed as statistical tool. The performances of the models were evaluated through the analysis of the prediction results obtained on the different training and test sets (80%/20%) as well as from an out-sample test set. A 6/4/1 TLP computed with the Broyden-Fletcher-Goldfarb-Shanno second-order training algorithm led to the best prediction results. The convergence was obtained in 132 cycles. The sum of squares was used as error function. The hidden and output activation functions were tanh and exponential, respectively. Various chemical structures were identified as potential synergists and searched for their commercial availability. Molecules of interest were tested in vivo on Ae. aegypti by using the susceptible reference Bora Bora strain and two resistant strains from Martinique island. This led to the identification of the PSM-05 molecule that shows interesting synergistic activity.