Abstract
BackgroundThe emergence and spread of insecticide resistance in the major African malaria vectors Anopheles gambiae s.s. and Anopheles arabiensis may compromise control initiatives based on insecticide-treated nets (ITNs) or indoor residual spraying (IRS), and thus threaten the global malaria elimination strategy.MethodsWe investigated pyrethroid resistance in four populations of An. arabiensis from south-western Ethiopia and then assessed the bio-efficacy of six World Health Organization recommended long lasting insecticidal nets (LLINs) using these populations.ResultsFor all four populations of An. arabiensis, bottle bioassays indicated low to moderate susceptibility to deltamethrin (mortality at 30 minutes ranged between 43 and 80%) and permethrin (mortality ranged between 16 and 76%). Pre-exposure to the synergist piperonylbutoxide (PBO) significantly increased the susceptibility of all four populations to both deltamethrin (mortality increased between 15.3 and 56.8%) and permethrin (mortality increased between 11.6 and 58.1%), indicating the possible involvement of metabolic resistance in addition to the previously identified kdr mutations. There was reduced susceptibility of all four An. arabiensis populations to the five standard LLINs tested (maximum mortality 81.1%; minimum mortality 13.9%). Bio-efficacy against the four populations varied by net type, with the largest margin of difference observed with the Jimma population (67.2% difference). Moreover, there were differences in the bio-efficacy of each individual standard LLIN against the four mosquito populations; for example there was a difference of 40% in mortality of Yorkool against two populations. Results from standard LLINs indicated reduced susceptibility to new, unused nets that was likely due to observed pyrethroid resistance. The roof of the combination LLIN performed optimally (100% mortality) against all the four populations of An. arabiensis, indicating that observed reductions in susceptibility could be ameliorated with the combination of PBO with deltamethrin, as used in PermaNet® 3.0.ConclusionOur results suggest that bio-efficacy evaluations using local mosquito populations should be conducted where possible to make evidence-based decisions on the most suitable control products, and that those combining multiple chemicals such as PBO and deltamethrin should be considered for maintaining a high level of efficacy in vector control programmes.
Highlights
The emergence and spread of insecticide resistance in the major African malaria vectors Anopheles gambiae s.s. and Anopheles arabiensis may compromise control initiatives based on insecticide-treated nets (ITNs) or indoor residual spraying (IRS), and threaten the global malaria elimination strategy
At the 30 minute diagnostic period, all four populations showed low to moderate susceptibility to deltamethrin and permethrin
The synergist PBO reduced the expression of deltamethrin and permethrin resistance in the Overall, there was a significant relationship between % knockdown and % mortality (R2 = 0.53, n = 959, p < 0.001), noting that one data point was missing from the bio-efficacy data set
Summary
The emergence and spread of insecticide resistance in the major African malaria vectors Anopheles gambiae s.s. and Anopheles arabiensis may compromise control initiatives based on insecticide-treated nets (ITNs) or indoor residual spraying (IRS), and threaten the global malaria elimination strategy. Wide-scale deployment of ITNs protects users as well as non-users through personal and community level protection gained with high coverage rates [1,2] In this way, ITNs have been shown to reduce the burden of malaria in pregnant women and young children [3] and reduce the incidence of uncomplicated malarial episodes by around 40% in areas of both stable and unstable malaria relative to untreated nets [4]. There are increasing reports of malaria vectors that have developed resistance to the pyrethroids commonly used in LLINs and pyrethroid resistance is firmly established throughout Africa [7,8,9] This resistance to pyrethroids may compromise malaria control as LLINs may lose efficacy, at present there are no studies linking insecticide resistance to LLIN control failure
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