Abstract
Anopheles gambiae, An. coluzzii and An. arabiensis are the three major vectors of malaria in Nigeria. These mosquitoes have developed resistance to different insecticides. Insecticides resistance intensity assay was recently introduced to provide insight into the potential operational significance of insecticide resistance. Here, we present data on pyrethroids resistance intensity and resistance mechanisms from six vector surveillance sites (Lagos, Ogun, Edo, Anambra, Kwara and Niger) in Nigeria. Adult Anopheles reared from larval collections were tested using WHO insecticides susceptibility protocol with 1x concentration of permethrin and deltamethrin followed with intensity assays with 5x and 10x concentrations of both insecticides. Synergistic and biochemical assays were carried out and underlying resistance mechanisms determined following standard protocols. Anopheles gambiae constituted >50% samples tested in five sites. Permethrin and deltamethrin resistance was observed at all the sites. The Kdt50 varied from 15 minutes (CI = 13.6–17.2) in deltamethrin to 42.1 minutes (CI = 39.4–44.1) in permethrin. For both insecticides, Kdt95 was >30 minutes with 25% to 87% post exposure mortality at the different sites. The West Africa knock down resistance (kdr-w) mechanism was found at each site. Resistant An. gambiae from Lagos, Ogun and Niger synergized prior to permethrin or deltamethrin exposure showed significant mortality (89–100%) compared to unsynergized mosquitoes (Lagos, p = 0.031; Ogun, p = 0.025; Niger, p = 0.018). Biochemical analyses revealed significant increased levels of P450 enzymes in resistant Anopheles gambiae from Lagos (p = 0.038); Ogun (p = 0.042) and Niger (p = 0.028) in addition to GST in Lagos (p = 0.028) and Ogun (p = 0.033). Overall, the results revealed high pyrethroid resistance associated with increased activities of metabolic enzymes (P450 + GST) in An. gambiae and An. coluzzii from Lagos and Ogun. The presence of kdr + P450 conferred moderate resistance whereas low resistance was the case where kdr was the sole resistance mechanism. Findings thus suggests that elevated levels of cytochrome P450 enzymes together with GST were responsible for high or severe pyrethroid resistance.
Highlights
The use of long-lasting insecticidal nets (LLINs) is one of the most effective measures for malaria control [1,2]
Resistance of Anopheles to pyrethroid insecticides is a much more recent development, first reported in Anopheles gambiae from Cote d’Ivoire [6] and wide- spread in West, Central and East Africa [7,8]
The Kisumu strain used as control was fully susceptible to the standard (1x) diagnostic concentration of permethrin (0.75%) and deltamethrin (0.05%) with 50% knock down recorded within 10 minutes’ exposure to both insecticides and 100% mortality 24-hours’ post exposure
Summary
The use of long-lasting insecticidal nets (LLINs) is one of the most effective measures for malaria control [1,2]. The basis of this intervention lies in the continuing susceptibility of Anopheles mosquito vectors to limited numbers of insecticides. Resistance of Anopheles to pyrethroid insecticides is a much more recent development, first reported in Anopheles gambiae from Cote d’Ivoire [6] and wide- spread in West, Central and East Africa [7,8]. Metabolic resistance mechanisms which are principally associated with three enzyme families (cytochrome p450 mono-oxygenaeses (p450s), carboxylesterases (CEOEs) and glutathione-S-transferases (GSTs), have been implicated in pyrethroid resistance in many sites in Africa [4,12]
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