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

Background

Zanzibar has a long history of lymphatic filariasis (LF) caused by the filarial parasite Wuchereria

bancrofti, and transmitted by the mosquito Culex quinquefasciatus Say. 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 examine Culex mosquito species

from the two main islands to detect W. bancrofti infection and to determine levels of susceptibility to the insecticides used for vector control.

Methods

Culex mosquitoes collected during routine catches in Vitongoji, Pemba Island, and Makadara, Unguja Island were tested for W. bancrofti infection using PCR. Insecticide bioassays on Culex mosquitoes 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 the kdr genotype and sequencing of the mitochondrial cytochrome oxidase I (mtCOI) subunit

performed to identify ambiguous Culex species.

Results

None of the wild-caught Culex mosquitoes analysed were found to be positive for W. bancrofti. High frequencies of resistance to all insecticides were found in Wete, Pemba Island, whereas Culex from the nearby site of Tibirinzi (Pemba) and in Kilimani, Unguja Island remained relatively susceptible. Species identification confirmed

that mosquitoes from Wete were Culex quinquefasciatus. The majority of the Culex collected from Tibirinzi and all from Kilimani could not be identified to species by molecular assays. Two alternative kdr alleles, both resulting in a

L1014F substitution were detected in Cx. quinquefasciatus from 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.