Rapid spread of double East- and West-African kdr mutations in wild Anopheles coluzzi from Côte d'Ivoire

Exploring the molecular mechanisms of increased intensity of pyrethroid resistance in Central African population of a major malaria vector Anopheles coluzzii

Molecular mechanisms driving the escalation of pyrethroid resistance in the major malaria mosquitoes of Central Africa remain largely uncharacterized, hindering effective management strategies. Here, resistance intensity and the molecular mechanisms driving it were investigated in a population of Anopheles coluzzii from northern Cameroon. High levels of pyrethroid and organochloride resistance were observed in An. coluzzii population, with no mortality for 1× permethrin; only 11% and 33% mortalities for 5× and 10× permethrin diagnostic concentrations, and <2% mortalities for deltamethrin and DDT, respectively. Moderate bendiocarb resistance (88% mortality) and full susceptibility to malathion were observed. Synergist bioassays with piperonyl butoxide recovered permethrin susceptibility, with mortalities increasing to 53.39%, and 87.30% for 5× and 10× permethrin, respectively, implicating P450 monooxygenases. Synergist bioassays with diethyl maleate (DEM) recovered permethrin and DDT susceptibilities (mortalities increasing to 34.75% and 14.88%, respectively), implicating glutathione S-transferases. RNA-seq-based genome-wide transcriptional analyses supported by quantitative PCR identified glutathione S-transferase, GSTe2 (RNA-seqFC = 2.93 and qRT-PCRFC = 8.4, p < 0.0043) and CYP450, CYP6Z2 (RNA-seqFC = 2.39 and qRT-PCRFC = 11.7, p < 0.0177) as the most overexpressed detoxification genes in the pyrethroid-resistant mosquitoes, compared to mosquitoes of the susceptible Ngousso colony. Other overexpressed genes include P450s, CYP6M2 (FC = 1.68, p < 0.0114), CYP4G16 (FC = 2.02, p < 0.0005), and CYP4G17 (FC = 1.86, p < 0.0276). While high frequency of the 1014F kdr mutation (50%) and low frequencies of 1014S (6.61%) and 1575Y (10.29%) were observed, no ace-1 mutation was detected in bendiocarb-resistant populations, suggesting the preeminent role of metabolic mechanism. Overexpression of metabolic resistance genes (including GSTe2 and CYP6Z2 known to confer resistance to multiple insecticides) in An. coluzzii from the Sudan Savannah of Cameroon highlights the need for alternative management strategies to reduce malaria burden in northern Cameroon.

Widespread occurrence of copy number variants and fixation of pyrethroid target site resistance in Anopheles gambiae (s.l.) from southern Côte d'Ivoire

Resistance to pyrethroid and organophosphate insecticides in the malaria vector Anopheles gambiae (s.l.) is conferred by a variety of genetic mutations, including single nucleotide polymorphisms (SNPs) and copy number variants (CNVs). Knowledge of the distribution of these mutations in mosquito populations is a prerequisite for establishing better strategies for their management. In this study, a total of 755 Anopheles gambiae (s.l.) from southern Côte d'Ivoire were exposed to deltamethrin or pirimiphos-methyl insecticides and were screened to assess the distribution of SNPs and CNVs known or believed to confer resistance to one or other of the insecticide classes. Most individuals from the An. gambiae (s.l.) complex were identified by molecular tests as Anopheles coluzzii. Survival to deltamethrin (from 94% to 97%) was higher than to pirimiphos-methyl (from 10% to 49%). In An. gambiae (s.s.), the SNP in the Voltage Gated Sodium Channel (Vgsc) at the 995F locus (Vgsc-995F) was fixed, while other target site mutations were rare or absent (Vgsc-402L: 0%; Vgsc-1570Y: 0%, Acetylcholinesterase Acel-280S: 14%). In An. coluzzii, Vgsc-995F was the target site SNP found at highest frequency (65%) followed by other target site mutations (Vgsc-402L: 36%; Vgsc-1570Y: 0.33%; Acel-280S: 45%). The Vgsc-995S SNP was not present. The presence of the Ace1-280S SNP was found to be significantly linked to the presence of the Ace1-CNV, Ace1_AgDup. Significant association was found between the presence of the Ace1_AgDup and pirimiphos-methyl resistance in An. gambiae (s.s.) but not in An. coluzzii. The deletion Ace1_Del97 was found in one specimen of An. gambiae (s.s.). Four CNVs in the Cyp6aa/Cyp6p gene cluster, which contains genes of known importance for resistance, were detected in An. coluzzii, the most frequent being Dup 7 (42%) and Dup 14 (26%). While none of these individual CNV alleles were significantly associated with resistance, copy number in the Cyp6aa gene region in general was associated with increased resistance to deltamethrin. Elevated expression of Cyp6p3 was nearly associated with deltamethrin resistance, although there was no association of resistance with copy number. Use of alternative insecticides and control methods to arrest resistance spread in An. coluzzii populations is merited.

Evolution of the Pyrethroids Target-Site Resistance Mechanisms in Senegal: Early Stage of the Vgsc-1014F and Vgsc-1014S Allelic Frequencies Shift

The evolution and spread of insecticide resistance mechanisms amongst malaria vectors across the sub-Saharan Africa threaten the effectiveness and sustainability of current insecticide-based vector control interventions. However, a successful insecticide resistance management plan relies strongly on evidence of historical and contemporary mechanisms circulating. This study aims to retrospectively determine the evolution and spread of pyrethroid resistance mechanisms among natural Anopheles gambiae s.l. populations in Senegal. Samples were randomly drawn from an existing mosquito sample, collected in 2013, 2017, and 2018 from 10 sentinel sites monitored by the Senegalese National Malaria Control Programme (NMCP). Molecular species of An. gambiae s.l. and the resistance mutations at the Voltage-gated Sodium Channel 1014 (Vgsc-1014) locus were characterised using PCR-based assays. The genetic diversity of the Vgsc gene was further analyzed by sequencing. The overall species composition revealed the predominance of Anopheles arabiensis (73.08%) followed by An. gambiae s.s. (14.48%), Anopheles coluzzii (10.94%) and Anopheles gambiae-coluzii hybrids (1.48%). Both Vgsc-1014F and Vgsc-1014S mutations were found in all studied populations with a spatial variation of allele frequencies from 3% to 90%; and 7% to 41%, respectively. The two mutations have been detected since 2013 across all the selected health districts, with Vgsc-L1014S frequency increasing over the years while Vgsc-1014F decreasing. At species level, the Vgsc-1014F and Vgsc-1014S alleles were more frequent amongst An. gambiae s.s. (70%) and An. arabiensis (20%). The Vgsc gene was found to be highly diversified with eight different haplotypes shared between Vgsc-1014F and Vgsc-1014S. The observed co-occurrence of Vgsc-1014F and Vgsc-1014S mutations suggest that pyrethroid resistance is becoming a widespread phenomenon amongst malaria vector populations, and the NMCP needs to address this issue to sustain the gain made in controlling malaria.

Short-term metabolic resistance inductive effect of different agrochemical groups on Anopheles gambiae mosquitoes

Background: In order to assess the impact of the different groups of agricultural pesticides used in Côte d'Ivoire on the increase of mosquitoes resistance to insecticides, the expression profiles of 7 P450 cytochromes and one GSTE2 of Anopheles gambiae involved in mosquito resistance to insecticides were studied. The goal of this study was to determine the effect of short exposure of mosquito larvae to different groups of agricultural pesticides on mosquito resistance. Methods: Three groups of pesticides were selected: (i) agricultural insecticide solutions, (ii) non-insecticide pesticide solutions (a mixture of herbicides and fungicides), and (iii) a mixture of the first two. A fourth non-pesticide solution was used as a control. Four groups of each stage 2 larvae (strain Kisumu, male and female) were exposed to 20% concentrated solution for 24 hours. Susceptibility tests for dichlorodiphenyltrichloroethane (DDT) and Deltamethrin were carried out on adults aged 2-5 days. Quantitative reverse transcription polymerase chain reaction (RT-qPCR) was performed to quantify the expression of eight metabolic genes involved in mosquito resistance to insecticides.  Results: Susceptibility to DDT showed a similar increase in the time required to knock down 50% of mosquitoes (kdt50) in l colonies exposed to insecticides and non-insecticides compared to the control colony. As for deltamethrin, kdt50 was higher in the colonies exposed to insecticides and the pesticide mixture compared to the colony exposed to non-insecticides. Of all the genes studied in all colonies, except for CYP6P1 induced only in the colony consisting of the pesticide mixture, no genes were induced. Conclusions: This study confirmed that induction is influenced by the duration, the concentration of the solution and the type of xenobiotic used as an inducer. The overexpression of CYP6P1 confirmed the inductive effect that a short exposure of mosquito larvae to agricultural pesticides could have.

Short-term metabolic resistance inductive effect of different agrochemical groups on Anopheles gambiae mosquitoes

Background: In order to assess the impact of the different groups of agricultural pesticides used in Côte d'Ivoire on the increase of mosquitoes resistance to insecticides, the expression profiles of 7 P450 cytochromes and one GSTE2 of Anopheles gambiae involved in mosquito resistance to insecticides were studied. The goal of this study was to determine the effect of short exposure of mosquito larvae to different groups of agricultural pesticides on mosquito resistance. Methods: Three groups of pesticides were selected: (i) agricultural insecticide solutions, (ii) none-insecticide pesticide solutions (a mixture of herbicides and fungicides), and (iii) a mixture of the first two. A fourth non-pesticide solution was used as a control. Four groups of each stage 2 larvae (strain Kisumu, male and female) were exposed to 20% concentrated solution for 24 hours. Susceptibility tests for dichlorodiphenyltrichloroethane (DDT) and Deltamethrin were carried out on adults aged 2-5 days. Quantitative reverse transcription polymerase chain reaction (RT-qPCR) was performed to quantify the expression of eight metabolic genes involved in mosquito resistance to insecticides.  Results: Susceptibility to DDT showed a similar increase in the time required to knock down 50% of mosquitoes (kdt50) in l colonies exposed to insecticides and none-insecticides compared to the control colony. As for deltamethrin, kdt50 was higher in the colonies exposed to insecticides and the pesticide mixture compared to the colony exposed to none-insecticides. Of all the genes studied in all colonies, except for CYP6P1 induced only in the colony consisting of the pesticide mixture, no genes were induced. Conclusions: This study confirmed that induction is influenced by the duration, the concentration of the solution and the type of xenobiotic used as an inducer. The overexpression of CYP6P1 confirmed the inductive effect that a short exposure of mosquito larvae to agricultural pesticides could have.

Rapid spread of double East- and West-African kdr mutations in wild Anopheles coluzzi from Côte d'Ivoire

Malaria morbidity and mortality rates in Sub-Saharan Africa are increasing. The scale-up of long-lasting insecticidal nets and indoor residual spraying have been the major contributors to the decrease of malaria burden. These tools are now threatened by insecticide resistance in malaria vectors, which is spreading dramatically. After two different real-time polymerase chain reaction molecular characterizations carried out on 70 mosquitoes sampled in the locality of Elibou in southern Côte d'Ivoire, results revealed that 9 mosquitoes from Anopheles coluzzi harbored the double East- and West-African knockdown resistance mutations. In the previous year, only 1 mosquito out of 150 sampled from 10 regions of the country had the same genotype. These results show the rapid spread of insecticide resistance in malaria vectors and highlight the urgent need to diversify the methods of vector control in order to avoid the failure of insecticide-based vector control tools which may favor malaria fatalities.