Abstract
BackgroundTo control malaria in Tanzania, two primary vector control interventions are being scaled up: long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS). The main threat to effective malaria control is the selection of insecticide resistance. While resistance to pyrethroids, the primary insecticide used for LLINs and IRS, has been reported among mosquito vectors in only a few sites in Tanzania, neighbouring East African countries are recording increasing levels of resistance. To monitor the rapidly evolving situation, the resistance status of the malaria vector Anopheles gambiae s.l to different insecticides and the prevalence of the kdr resistance allele involved in pyrethroid resistance were investigated in north-western Tanzania, an area that has been subject to several rounds of pyrethroid IRS since 2006.MethodsHousehold collections of anopheline mosquitoes were exposed to diagnostic dosages of pyrethroid, DDT, and bendiocarb using WHO resistance test kits. The relative proportions of An. gambiae s.s and Anopheles arabiensis were also investigated among mosquitoes sampled using indoor CDC light traps. Anophelines were identified to species and the kdr mutation was detected using real time PCR TaqMan assays.ResultsFrom the light trap collections 80% of An. gambiae s.l were identified as An. gambiae s.s and 20% as An. arabiensis. There was cross-resistance between pyrethroids and DDT with mortality no higher than 40% reported in any of the resistance tests. The kdr-eastern variant was present in homozygous form in 97% of An. gambiae s.s but was absent in An. arabiensis. Anopheles gambiae s.s showed reduced susceptibility to the carbamate insecticide, bendiocarb, the proportion surviving WHO tests ranging from 0% to 30% depending on season and location.ConclusionAnopheles gambiae s.s has developed phenotypic resistance to pyrethroids and DDT and kdr frequency has almost reached fixation. Unlike in coastal Tanzania, where the ratio of An. gambiae s.s to An. arabiensis has decreased in response to vector control, An. gambiae s.s persists at high frequency in north-western Tanzania, probably due to selection of pyrethroid resistance, and this trend is likely to arise in other areas as resistance spreads or is subject to local selection from IRS or LLINs.
Highlights
To control malaria in Tanzania, two primary vector control interventions are being scaled up: longlasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS)
Kdr is caused by mutations to the sodium channel, a leucine to phenylalanine change first observed in West Africa [5] and a leucine to serine mutation observed in East Africa [6]
WHO resistance tests Anopheles gambiae s.s accounted for 96.0% and An. arabiensis for 3.7% of the An. gambiae s.l tested for resistance (N = 901)
Summary
To control malaria in Tanzania, two primary vector control interventions are being scaled up: longlasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS). The primary insecticide used for LLINs and IRS, has been reported among mosquito vectors in only a few sites in Tanzania, neighbouring East African countries are recording increasing levels of resistance. Strong commitment from international agencies and home governments to reduce the burden of malaria in sub-Saharan Africa has led to a major scale-up of vector control measures and increased access to effective antimalarial treatment, and it is reported that malaria is on the wane in several African countries [1]. Resistance to pyrethroid insecticide is spreading rapidly across Africa and could reduce the impact of our two most successful malaria prevention interventions - indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) [2,3,4]. A new mutation in the sodium channel conferring additional resistance to DDT and permethrin as been reported associated with the kdrwest mutation [7]
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