Insecticide resistance in Culex quinquefasciatus from Zanzibar: implications for vector control programmes

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Abstract
Background: Zanzibar has a long history of lymphatic filariasis (LF) caused by the filarial parasiteWuchereria bancrofti, and transmitted by the mosquitoCulex quinquefasciatusSay. The LF Programme in Zanzibar has successfully implemented mass drug administration (MDA) to interrupt transmission, and is now in the elimination phase. Monitoring infections in mosquitoes, and assessing the potential role of interventions such as vector control, is important in case the disease re-emerges as a public health problem. Here, we examineCulexmosquito species from the two main islands to detectW. bancroftiinfection and to determine levels of susceptibility to the insecticides used for vector control.Methods: Culexmosquitoes collected during routine catches in Vitongoji, Pemba Island, and Makadara, Unguja Island were tested forW. bancroftiinfection using PCR. Insecticide bioassays onCulexmosquitoes were performed to determine susceptibility to permethrin, deltamethrin, lambda-cyhalothrin, DDT and bendiocarb. Additional synergism assays with piperonyl butoxide (PBO) were used for lambda-cyhalothrin. Pyrosequencing was used to determine thekdrgenotype and sequencing of the mitochondrial cytochrome oxidase I (mtCOI) subunit performed to identify ambiguousCulexspecies.Results: None of the wild-caughtCulexmosquitoes analysed were found to be positive forW. bancrofti. High frequencies of resistance to all insecticides were found in Wete, Pemba Island, whereasCulexfrom the nearby site of Tibirinzi (Pemba) and in Kilimani, Unguja Island remained relatively susceptible. Species identification confirmed that mosquitoes from Wete wereCulex quinquefasciatus. The majority of theCulexcollected from Tibirinzi and all from Kilimani could not be identified to species by molecular assays. Two alternativekdralleles, both resulting in a L1014F substitution were detected inCx. quinquefasciatusfrom Wete with no homozygote susceptible detected. Metabolic resistance to pyrethroids was also implicated by PBO synergism assays.Conclusions: Results from the xenomonitoring are encouraging for the LF programme in Zanzibar. However, the high levels of pyrethroid resistance found in the principle LF vector in Pemba Island will need to be taken into consideration if vector control is to be implemented as part of the elimination programme.

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