Abstract
BackgroundMillions of pyrethroid LLINs have been distributed in Mali during the past 20 years which, along with agricultural use, has increased the selection pressure on malaria vector populations. This study investigated pyrethroid resistance intensity and susceptible status of malaria vectors to alternative insecticides to guide choice of insecticides for LLINs and IRS for effective control of malaria vectors.MethodsFor 3 years between 2016 and 2018, susceptibility testing was conducted annually in 14–16 sites covering southern and central Mali. Anopheles gambiae (s.l.) were collected from larval sites and adult mosquitoes exposed in WHO tube tests to diagnostic doses of bendiocarb (0.1%) and pirimiphos-methyl (0.25%). Resistance intensity tests were conducted using CDC bottle bioassays (2016–2017) and WHO tube tests (2018) at 1×, 2×, 5×, and 10× the diagnostic concentration of permethrin, deltamethrin and alpha-cypermethrin. WHO tube tests were conducted with pre-exposure to the synergist PBO followed by permethrin or deltamethrin. Chlorfenapyr was tested in CDC bottle bioassays at 100 µg active ingredient per bottle and clothianidin at 2% in WHO tube tests. PCR was performed to identify species within the An. gambiae complex.ResultsIn all sites An. gambiae (s.l.) showed high intensity resistance to permethrin and deltamethrin in CDC bottle bioassay tests in 2016 and 2017. In 2018, the WHO intensity tests resulted in survivors at all sites for permethrin, deltamethrin and alpha-cypermethrin when tested at 10× the diagnostic dose. Across all sites mean mortality was 33.7% with permethrin (0.75%) compared with 71.8% when pre-exposed to PBO (4%), representing a 2.13-fold increase in mortality. A similar trend was recorded for deltamethrin. There was susceptibility to pirimiphos-methyl, chlorfenapyr and clothianidin in all surveyed sites, including current IRS sites in Mopti Region. An. coluzzii was the primary species in 4 of 6 regions.ConclusionsWidespread high intensity pyrethroid resistance was recorded during 2016–2018 and is likely to compromise the effectiveness of pyrethroid LLINs in Mali. PBO or chlorfenapyr LLINs should provide improved control of An. gambiae (s.l.). Clothianidin and pirimiphos-methyl insecticides are currently being used for IRS as part of a rotation strategy based on susceptibility being confirmed in this study.
Highlights
Millions of pyrethroid Long-lasting insecticidal net (LLIN) have been distributed in Mali during the past 20 years which, along with agricultural use, has increased the selection pressure on malaria vector populations
Clothianidin and pirimiphos-methyl insecticides are currently being used for indoor residual spraying (IRS) as part of a rotation strategy based on susceptibility being confirmed in this study
A study by Protopopoff et al [32] in Tanzania showed that piperonyl butoxide (PBO) LLINs improved control of malaria transmission compared to standard pyrethroid LLINs, but there was no additional benefit of non-pyrethroid IRS combined with PBO LLINs when compared to PBO LLINs alone
Summary
Millions of pyrethroid LLINs have been distributed in Mali during the past 20 years which, along with agricultural use, has increased the selection pressure on malaria vector populations. Malaria remains an important disease in Mali, with the Demographic and Health Survey (DHS) of 2018 estimating the prevalence in children aged 6–59 months to be highest in the regions of Sikasso (30%), Segou (26%), Mopti (25%) and Koulikoro (22%), with Bamako having the lowest prevalence (1%) [1]. Millions of pyrethroid LLINs have been distributed in Mali during the past 20 years which, along with agricultural use of pyrethroids, has increased the selection pressure on malaria vector populations. Questionnaires as part of the Demographic Health Survey in 2018 showed that 56% of the population in Bamako slept under an LLIN the previous night, while the highest rate was in Mopti Region at 82% [1]
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