Abstract
BackgroundChemical insecticides are crucial to malaria control and elimination programmes. The frontline vector control interventions depend mainly on pyrethroids; all long-lasting insecticidal nets (LLINs) and more than 80% of indoor residual spraying (IRS) campaigns use chemicals from this class. This extensive use of pyrethroids imposes a strong selection pressure for resistance in mosquito populations, and so continuous resistance monitoring and evaluation are important. As pyrethroids have also been used for many years in the Manhiça District, an area in southern Mozambique with perennial malaria transmission, an assessment of their efficacy against the local malaria vectors was conducted.MethodsFemale offspring of wild-caught Anopheles funestus s.s. females were exposed to deltamethrin, lambda-cyhalothrin and permethrin using the World Health Organization (WHO) insecticide-resistance monitoring protocols. The 3-min WHO cone bioassay was used to evaluate the effectiveness of the bed nets distributed or available for purchase in the area (Olyset, permethrin LLIN; PermaNet 2.0, deltamethrin LLIN) against An. funestus. Mosquitoes were also exposed to PermaNet 2.0 for up to 8 h in time-exposure assays.ResultsResistance to pyrethroids in An. funestus s.s. was extremely high, much higher than reported in 2002 and 2009. No exposure killed more than 25.8% of the mosquitoes tested (average mortality, deltamethrin: 6.4%; lambda-cyhalothrin: 5.1%; permethrin: 19.1%). There was no significant difference in the mortality generated by 3-min exposure to any net (Olyset: 9.3% mortality, PermaNet 2.0: 6.0%, untreated: 2.0%; p = 0.2). Six hours of exposure were required to kill 50% of the An. funestuss.s. on PermaNet 2.0.ConclusionsAnopheles funestus s.s. in Manhiça is extremely resistant to pyrethroids, and this area is clearly a pyrethroid-resistance hotspot. This could severely undermine vector control in this district if no appropriate countermeasures are undertaken. The National Malaria Control Programme (NMCP) of Mozambique is currently improving its resistance monitoring programme, to design and scale up new management strategies. These actions are urgently needed, as the goal of the NMCP and its partners is to reach elimination in southern Mozambique by 2020.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-015-0807-z) contains supplementary material, which is available to authorized users.
Highlights
Chemical insecticides are crucial to malaria control and elimination programmes
Anopheles funestus All PCR-identified individuals were An. funestus s.s. (181/200; 19 did not amplify), so it is assumed that the results pertain to this species
World Health Organization (WHO) tube tests (Table 1) revealed that An. funestus was highly resistant to deltamethrin (3.1 and 9.6% mortality, average: 6.4%) and permethrin (13.4 and 25.8% mortality, average: 19.1%)
Summary
The frontline vector control interventions depend mainly on pyrethroids; all long-lasting insecticidal nets (LLINs) and more than 80% of indoor residual spraying (IRS) campaigns use chemicals from this class. This extensive use of pyrethroids imposes a strong selection pressure for resistance in mosquito populations, and so continuous resistance monitoring and evaluation are important. The malaria community fears that this resistance will undermine effective mosquito control and elimination strategies, and erode the progress that has been made in reducing the number of malaria deaths to-date [2–5] It is not clear what level of resistance, as defined by current methods and guidelines, precipitates control failure in terms of human malaria cases in an area [6]. Even with the uncertainty underlying the association, resistance monitoring remains a recommended priority of vector control programmes, so that action can be taken before failure occurs [8, 9]
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