Investigation of the Insecticide Resistance Pattern in Anopheles gambiae s.l. Mosquitoes in Selected Local Government Areas in Ekiti State, South West, Nigeria

Technical Summary: Investigation of Insecticide Resistance Patterns in Anopheles gambiae s.l. in Ekiti State, Nigeria

Problem Statement
Malaria remains a critical public health burden in Nigeria, with 97% of the population at risk and tens of millions of cases reported annually. The primary strategy for vector control relies on Long-Lasting Insecticidal Nets (LLINs) and Indoor Residual Spraying (IRS), both of which depend heavily on pyrethroids and other WHO-recommended insecticide classes. However, the efficacy of these interventions is increasingly compromised by the development of insecticide resistance in Anopheles mosquito vectors. While molecular studies and larval habitat surveys have been conducted in Ekiti State, South-West Nigeria, there was a lack of reported data regarding the susceptibility status of local Anopheles gambiae s.l. populations to standard insecticides. This study addresses this gap by evaluating the resistance patterns of An. gambiae s.l. against multiple insecticide classes across six Local Government Areas (LGAs) in Ekiti State.

Methodology
The study was conducted between April 2022 and June 2023 across six selected LGAs: Ado, Ise-Orun, Ido-Osi, Oye-Ekiti, Efon-Alaye, and Ikere-Ekiti.

• Sample Collection: Mosquito larvae were collected from various breeding habitats (both natural and man-made) using standard dipping techniques. Larvae were transported to the Entomology unit at Federal University Oye-Ekiti, reared to adulthood under controlled conditions (27°C, 82% relative humidity), and fed on yeast.
• Susceptibility Testing: Adult female mosquitoes were subjected to WHO standard bioassays. The study utilized impregnated test papers for five insecticides representing different classes:
  • Pyrethroids: Deltamethrin (0.05%).
  • Organochlorines: DDT (4%).
  • Carbamates: Bendiocarb (0.1%) and Propoxur (0.1%).
  • Organophosphates: Pirimiphos-methyl (0.25%).
• Procedure: Three replicates were conducted for each insecticide. Mosquitoes were exposed for one hour, with knockdown recorded at 10-minute intervals (10–60 minutes). Mortality was assessed 24 hours post-exposure. Control mortality remained below 5%, eliminating the need for Abbott's correction.
• Data Analysis: Mortality rates were compared against WHO criteria (Susceptible: 98–100%; Possible Resistance: 90–97%; Resistant: <90%). Knockdown times (KDT50 and KDT95) and rates were calculated using SPSS version 23.

Key Results
The study revealed a heterogeneous but concerning pattern of resistance across the six LGAs:

• Widespread Resistance to Pyrethroids and Organochlorines: Resistance to Deltamethrin and DDT was observed in all six LGAs. Mortality rates for Deltamethrin ranged from 50.67% (Ise-Orun) to 98.7% (Oye Ekiti and Efon Alaye), with several areas showing clear resistance (<90%). DDT resistance was particularly severe, with mortality rates as low as 29.33% in Ado-Ekiti.
• Variable Response to Carbamates: Susceptibility to Bendiocarb was recorded in two LGAs (Ikere-Ekiti and Ido-Osi), while resistance was observed in others. Propoxur showed susceptibility in Ikere-Ekiti and Ido-Osi but resistance in Oye-Ekiti and Efon-Alaye.
• Emerging Resistance to Organophosphates: Pirimiphos-methyl showed "possible resistance" (90–97% mortality) in several LGAs, with confirmed resistance in Ikere-Ekiti (80.0%) and Ado-Ekiti (73.33%).
• Knockdown Dynamics: Significant spatial variations in knockdown speed were noted. Bendiocarb consistently demonstrated the fastest knockdown effect (KDT50 = 25.70 minutes overall), followed by Propoxur. Deltamethrin exhibited the slowest knockdown (KDT50 = 77.67 minutes), particularly in Ado-Ekiti. Ido-Osi and Ikere-Ekiti populations generally showed faster knockdown responses compared to Ado-Ekiti.

Key Contributions

• Baseline Data for Ekiti State: This research provides the first documented report on insecticide susceptibility patterns of An. gambiae s.l. in Ekiti State, filling a critical data void in the region's vector control planning.
• Spatial Resolution: By analyzing six distinct LGAs, the study highlights that resistance is not uniform, with specific areas (e.g., Ado-Ekiti) showing more severe resistance profiles than others.
• Comparative Efficacy: The study quantifies the differential performance of five major insecticide classes, identifying Bendiocarb and Propoxur as currently more effective options in specific locations compared to the heavily compromised Deltamethrin and DDT.

Significance and Claims
The authors assert that the widespread resistance to commonly used insecticides, particularly pyrethroids (Deltamethrin) and organochlorines (DDT), represents a serious public health threat to malaria control efforts in Ekiti State. The study claims that the observed resistance patterns are likely driven by selective pressures from the overuse of agricultural pesticides, herbicides, and domestic insecticides, which may have accelerated the evolution of resistance mechanisms in local mosquito populations.

The paper concludes that the current reliance on single-class insecticides is unsustainable. It advocates for the implementation of insecticide rotation, region-specific resistance management, and Integrated Vector Management (IVM) strategies. Specifically, the authors suggest combining insecticides with different modes of action to manage resistant vectors. The findings are intended to inform local health authorities and vector control programs to adapt their strategies to the specific resistance profiles identified in each LGA, thereby sustaining the efficacy of malaria control interventions.