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Latin American Symposium III

Latin American Symposium III

187 - Insecticide susceptibility and resistance mechanisms in Aedes aegypti populations exposed to pirimiphos-methyl by targeted indoor residual spraying (TIRS) in Merida, Mexico.

Wednesday, March 5, 2025
4:20 PM – 4:30 PM AST
Location: 202
Abstract: We report results of longitudinal monitoring of insecticide susceptibility of Aedes aegypti from a clinical trial quantifying the epidemiological impact of targeted indoor residual spraying (TIRS) in the city of Mérida, Yucatán, Mexico. Aedes aegypti eggs were collected using ovitraps from 16 clusters of the TIRS trial, which is a two-arm randomized trial conducted in 50 clusters randomly assigned 1:1 to either receive TIRS using Actellic 300CS (pirimiphos-methyl) or serve as an untreated control. The susceptibility of Ae. aegypti females to permethrin, deltamethrin, malathion and pirimiphos-methyl was determined using the CDC bottle bioassay. Mosquito populations characterized as possibly developing resistance or resistant were exposed to enzyme inhibitors (PBO, DEF, EA) to determine which enzymatic mechanisms might be involved. Phenotypic resistance to permethrin was detected in 2021 (15 out of 16 clusters, 93.7 %), 2022 (81%) and 2023 (37.5%). For deltamethrin, phenotypic resistance was detected in 6.25% of clusters in 2021, 12.5% in 2022 and 0% in 2023. For permethrin and deltamethrin, between 7% and 76% mosquitoes recovered 24 hours post-exposure. For pirimiphos-methyl, the populations showed no evidence of resistance in 2021 and 2023; in 2022 one cluster (6.25 %) had mortality below 90%. Populations showed complete susceptibility to malathion in all years. When exposing mosquitoes to enzyme inhibitors prior to exposure to permethrin or deltamethrin, the contributions of oxidases, esterases and GSTs to the resistance was estimated. Our results suggest a reduction in pyrethroid resistance in the trial area and indicate that three years of TIRS with pirimiphos-methyl have not led to generalized resistance.