High Level of DDT Resistance and Reduced Susceptibility to Deltamethrin in Anopheles gambiae , Anopheles coluzzi , and Anopheles arabiensis from Urban Communities in Oyo State, South-West Nigeria

Selection of vector populations capable of withstanding doses of insecticides is a major threat facing insecticide based vector control interventions. In view of the rapidly evolving insecticide resistance in the primary vector, Anopheles, there is need to provide information on the status of insecticide resistance in Oyo State where Long Lasting Insecticide Treated Nets (LLIN) is the only intervention that has been deployed over the years. Morphologically identified Anopheles mosquito larva were collected from six localities (Ojoo, Iwo road, Bodija, Oluyole, Oyo town and Eruwa) in Oyo State. The larval samples were reared to adults and exposed to DDT (4%) and Deltamethrin (0.05%), according to WHO criteria. Knockdown was recorded every ten minutes and mortality scored at 24 hrs post exposure. The KDT 50 and KDT 95 were determined using probit regression analysis and species identification was by adult morphology and Polymerase Chain Reaction techniques. A total of 1,800 female anopheline mosquitoes were exposed to DDT and Deltamethrin insecticides. All the Anopheles mosquito populations from the six localities were resistant to DDT with 13-84% mortality. The anopheline mosquito populations from Iwo road and Oyo town were susceptible to Deltamethrin, while those from Ojoo and Eruwa showed reduced susceptibility to the insecticide. However,mosquito populations from Oluyole and Bodija were resistant to Deltamethrin. The knock down values for DDT and Deltamethrin across all sites was comparatively high indicative of some degree of selection within the populations. Of the 1,764 positively identified anophelines, 639 (36.2%), 1,034 (58.6%) and 91 (5.2%) were Anopheles coluzzi , Anopheles arabiensis and Anopheles gambiae s.s. respectively. This data indicates the widespread resistance of Anopheles coluzzi , Anopheles arabiensis and Anopheles gambiae s.s. to Dichlotrichloroethane (DDT) in Oyo State. It also showed an ongoing selection against pyrethroid which poses a major threat to the ongoing LLIN intervention in the State.