Characterization of insecticide resistance in Anopheles gambiae s.l, the principal malaria vectors in Northern Tanzania

Introduction

The decline of malaria in many parts of Sub-Saharan Africa has been ascribed to a combination of interventions including ITNs, IRS, ACT. Unfortunately, intensive use of insecticides has led to insecticide resistance in many parts including East Africa. The study is being conducted in Moshi and Muleba districts to investigate resistance mechanisms and alternative insecticides for controlling and preventing further selection of insecticide resistant malaria vectors in Northern Tanzania.

Methodology

Indoor resting collections of semi-gravid mosquitoes from several villages were exposed to diagnostic concentrations of pyrethroids, carbamates, organochlorines and DDT using WHO resistance test kits. Mosquitoes were identified and kdr mutation detected with real time PCR using TaqMan assays. The synergist study followed CDC bottle bioassays. Biochemical assays were used to quantify the levels of detoxification enzymes in individual mosquitoes. Detox chip microarrays were used for the gene expression studies; where over-transcription of detoxification genes in the resistant strain will be confirmed by using real-time quantitative RT-PCR. However, analysis of biochemical assays and microarrays data and kdr genotyping for Moshi population have not been completed.

Results

More than 98% of malaria vectors in Moshi was found to be Anopheles arabiensis while 80% of the Anopheles gambiae s.l in Muleba was found to be An.gambiae s.s. and 20% An. arabiensis. The An. arabiensis of Moshi has been found to be resistant to permethrin, deltamethrin and lambdacyhalothrin but susceptible to DDT, carbamates and organophosphates. High phenotypic resistance to DDT and pyrethroids was observed in Muleba An.gambiae s.s. and kdr frequency has nearly reached fixation where the East African kdr mutation was present in the homozygous form in 97% of the An.gambiae s.s. but was absent in An.arabiensis. CDC Bottle bioassays indicate significant synergism for both populations when exposed to permethrin with PBO, DEF or both. PBO and DEF inhibit oxidase and esterase enzymes respectively.

Conclusion

Persistence of An. gambiae s.s in Muleba despite several years of pyrethroid based IRS and increase in LLINs coverage suggests a reduced efficacy of the current vector control strategies against An. gambiae s.s. The presence of multiple resistance mechanisms including metabolic resistance threatens sustainability of both ITNs and IRS scaling-up programmes in the area. Further investigation will be needed to understand the operational impact of the detected resistance on vector control programmes, especially ITNs.

Supervisors:

Franklin Mosha (KCMC) & Mark Rowland (LSHTM)

Advisors:

Reginald Kavishe (KCMC) & Hilary Ranson (LSTM)