Insecticide Resistance of Xenopsylla cheopis in Madagascar: Revision of Diagnostic Doses for Bioassay and Exploration of Biochemical Mechanisms.

The Oriental rat flea, Xenopsylla cheopis, is known worldwide as an efficient plague vector, including in Madagascar, where the disease remains a public health concern. Chemical control is the primary response method against X. cheopis in Madagascar. Previous bioassays focusing on different flea populations from Madagascar showed phenotypic resistance to various insecticides, including deltamethrin and fenitrothion, which, respectively, represent the previous and current chemicals for flea vector control. Despite apparent insecticide resistance, the associated mechanisms of this resistance remain poorly known. The aims of this study were to adjust diagnostic doses of deltamethrin and fenitrothion and to investigate the metabolism-based insecticide resistance of X. cheopis in Madagascar. Five available laboratory-reared flea strains of X. cheopis were selected, and their susceptibility statuses to deltamethrin and fenitrothion were determined using the WHO standard bioassay. Diagnostic doses of each insecticide were determined by the probit method, in accordance with concentration gradients. Biochemical microplate-based assays were performed to detect overproduction of cytochrome P450, alpha-/beta-esterases, and glutathione S-transferase (GST), which are signatures of metabolic resistance. The five tested strains showed different susceptibility statuses against deltamethrin and fenitrothion. The diagnostic doses were estimated to be 0.07% for deltamethrin and 1.56% for fenitrothion. Increased activities of cytochrome P450, beta-esterase, and GST enzymes in the resistant strains were revealed in comparison with those of the susceptible strain. In conclusion, readjusted diagnostic doses will help to better understand the susceptibility status of X. cheopis to deltamethrin and fenitrothion. The overproduction of cytochrome P450, beta-esterase, and GST observed on deltamethrin-resistant flea strains suggests metabolic resistance.