Potency and implications of use of insecticide mixtures in management of resistant Anopheles gambiae s.l. (Diptera: Culicidae) mosquitoes
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Date
2021-04
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UNIVERSITY OF ILORIN
Abstract
The preponderance of insecticide resistance development in malaria vectors constitutes a major challenge in malaria management. The dire need to stem this challenge imposed by resistant mosquito populations informed this study aimed at exploring the possibility of deploying sequentially programmed insecticide mixtures. The specific objectives were to: (i) determine susceptibility status of Anopheles gambiae in Lagos state; (ii) determine the prevalence of resistant genes; (iii) determine the susceptibility status of resistant populations of Anopheles gambiae to insecticide mixtures; (iv) compare susceptibility statuses of mixtures and single insecticides; and (v) determine and compare metabolic enzymes activities in Anopheles gambiae exposed to individual insecticides and insecticide mixtures.
Susceptibility status was determined using WHO susceptibility test protocol. Twenty-five (25) female adult mosquitoes of 3 – 5 days old were exposed in four (4) replicates to insecticide treated papers impregnated with WHO diagnostic doses of DDT, Permethrin, Pirimiphos-methyl and Propoxur. Exposure to mixtures was done by slight modification of WHO method. WHO treated papers were cut into half and combined in the following format; DDT/Pirimiphos-methyl, DDT/Propoxur, Permethrin/Pirimiphos-methyl and Permethrin/Propoxur into WHO susceptibility bottles for exposure of mosquitoes to mixtures. Resistant genes were determined using Allele-Specific Polymerase Chain Reaction assay. Activities of metabolic enzymes; esterase, monooxygenase and glutathione-S-transferase were determined using naphthyl acetate end point assay, monooxygenase titration assay and glutathione-S-transferase assay respectively. Susceptibility status was analysed by calculating percentage mortality. Knockdown times (KDT50 and KDT95) were determined by probit analysis and used in calculating resistance reduction index. Prevalence of resistant genes was calculated using percentages. Paired student t-Test was employed to determine the significant difference at P<0.05. Results from metabolic assays were interpreted spectrophotometrically and analysed using standard graphs.
The findings of the study were that:
(i) Anopheles gambiae from Kosofe, Lagos mainland and Ojo were absolutely susceptible (100%) to Propoxur but resistant to DDT (44%, 46%, 29%), Permethrin (50%, 48%, 96%) and Pirimiphos-methyl (25%, 38%, 32%) respectively.
(ii) The allelic frequency of resistant gene; kdr were low in Kosofe (11.6%), Lagos Mainland (16.4%) and Ojo (6.7%). No ace-1R gene was found.
(iii) Anopheles gambiae from Kosofe, Lagos mainland and Ojo were susceptible to all propoxur mixtures and Permethrin/Pirimiphos-methyl (99%, 97%, 99%) but resistant to DDT/Pirimiphos-methyl (43%, 29%, 35%) respectively.
(iv) Resistance reduction indices in mixtures involving DDT and permethrin mixtures had higher knockdown impacts. Whereas mixtures involving pirimiphos-methyl
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mixtures had lower knockdown impact, mixtures involving propoxur mixtures had comparable knockdown impact when compared with individual insecticides.
(v) There was significant elevation (P=0.039) of esterase activities in populations exposed to DDT/Pirimiphos-methyl when compared with DDT-exposed populations. GST activities was significantly elevated (P=0.016) in Permethrin/Propoxur-exposed populations when compared with Permethrin-exposed populations. For monooxygenase, there was significant decrease in activities of DDT/Pirimiphos-methyl-exposed (P=0.031) and Permethrin/Propoxur-exposed (P=0.023) populations when compared to Pirimiphos-methyl-exposed and Permethrin-exposed populations respectively.
The study concluded that insecticide mixtures are potent in the management of resistant Anopheles gambiae populations. The study recommended that Permethrin/Propoxur, DDT/Propoxur and Permethrin/Pirimiphos-methyl can be employed in the management of multiple resistance in Anopheles gambiae.
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Potency, implications, insecticide mixtures, resistant Anopheles gambiae s.l., mosquitoes