Molecular Characterization and Insecticide Susceptibility Status of Anopheles gambiae Complex (Giles, 1902) in Osun State, Southwestern Nigeria

Abstract

A major challenge facing malaria control today is the high spate of insecticide resistance in mosquito populations. The present study was designed to investigate the susceptibility of An. gambiaes.l. to Dichlorodiphenyltrichloroethane (DDT) and Deltamethrin, and determine the sibling species at selected locations in Osun State, Nigeria. Larvae of An. gambiae complex were collected from Ibokun, Ife, Ede, Osogbo, Okuku and Iwo representing the three geopolitical zones in Osun State. Two to three day old unfed female mosquitoes were subjected to bioassays using papers impregnated with 0.05% Deltametrin (pyrethroid) and 4% DDT (organochlorine) in accordance with WHO protocols. Knock down was recorded every 10 minutes for one hour and mortality measured after 24 hours. The mosquitoes were then identified using morphological keys and Polymerase Chain Reaction (PCR) assay. The results of the knockdown effects of the insecticides ranged from 40-70% in mosquitoes exposed for Deltamethrin and 15-45% for DDT across all locations. The post-exposure mortality caused by the two insecticides varied significantly; and mortality recorded in mosquitoes exposed to Deltamethrin was significantly higher than DDT (p<0.05) in all the study locations. All the mosquitoes exposed were highly resistant to DDT across all locations (0%-66.7% mortality). However, mosquitoes from Ede were susceptible to deltamethrin (100% mortality), and those from Ife and Osogbo were suspected to be resistant to deltamethrin with 95% and 90% mortalities respectively while the Anopheles mosquitoes from Okuku and Ibokun were resistant to the insecticide (with 79% and 75.6% mortalities respectively). Molecular identification showed that An. gambiae s.s was the only species found across all localities except for Osogbo and Ibokun where An. Arabiensis and An. gambiae s.s were detected. The high resistance of the mosquito populations to DDT and the suspected resistance to Deltametrin indicate that DDT may not be suitable for vector control activities in Osun State while close monitoring of deltamethrin resistance is required to inform vector control interventions in the state.