A Chlorfenapyr Mixture Net Interceptor® G2 Shows High Efficacy and Wash Durability against Resistant Mosquitoes in West Africa

Raphael N'Guessan, Abibatou Odjo, Corine Ngufor, David Malone, Mark Rowland, Raphael N'Guessan, Abibatou Odjo, Corine Ngufor, David Malone, Mark Rowland

Abstract

Background: Malaria control through use of long-lasting insecticidal nets (LN) is threatened by the selection of anopheline mosquitoes strongly resistant to pyrethroid insecticides. To sustain future effectiveness it is essential to identify and evaluate novel insecticides suitable for nets. Mixtures of two insecticides with contrasting mode of action have the potential to kill resistant vectors and restore transmission control provided the formulation can withstand regular washing over the net's life span.

Method: The efficacy of a novel mixture LN, Interceptor® G2, that combines the pyrrole chlorfenapyr and pyrethroid alpha-cypermethrin was evaluated under controlled household conditions (experimental hut trial) and by laboratory bioassay against pyrethroid resistant An. gambiae in Benin before and after standardized washing. Comparison arms included standard alpha-cypermethrin LN, nets hand-treated with chlorfenapyr-only and untreated nets.

Results: The chlorfenapyr-alphacypermethrin LN demonstrated improved efficacy and wash resistance compared to a standard alpha-cypermethrin LN against pyrethroid resistant mosquitoes (resistance ratio 207). In experimental hut trial alpha-cypermethrin LN killed only 20% (95% CI 15-26%) of host-seeking An. gambiae whilst mixture LN killed 71% (95% CI 65-77%). Mixture LN washed 20 times killed 65% (95% CI 58-71%), and thus intensive washing reduced efficacy by only 6% (95% CI 1.3-11%). The chlorfenapyr net killed 76% (95% CI 70-81%). Personal protection and blood feeding inhibition did not differ between mixture and pyrethroid LN; however, the mixture LN was 2.5 (95% CI: 2.1-3.1) times more protective than untreated nets. Standard WHO cone bioassays conducted during day time hours failed to anticipate field efficacy but overnight tunnel tests successfully predicted mixture LN and chlorfenapyr net efficacy in field trials.

Conclusion: Interceptor® G2 LN demonstrates the potential to control transmission and provide community protection over the normal lifespan of long lasting nets where standard pyrethroid LN show signs of failing due to resistance.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1. Experimental hut trials against wild…
Fig 1. Experimental hut trials against wild Anopheles gambiae s.l. with Interceptor® G2 and other nets.
(A) Percentage blood-feeding, (B) Percentage mortality.
Fig 2. Insecticide content of Interceptor ®…
Fig 2. Insecticide content of Interceptor® G2 LN and other nets washed up to 20 times used in the hut trial.
(A) Chemical analysis (mean ± standard error). (B) Scanning electron microscope images of Interceptor® G2 (courtesy of BASF SE) showing chlorfenapyr (elongate) and alphacypermethrin (short) crystals on netting fibres.
Fig 3. Mortality rates of Anopheles gambiae…
Fig 3. Mortality rates of Anopheles gambiae in cone tests with 3 min exposure to Interceptor® G2 and other nets.
(A) Susceptible Kisumu strain. (B) Resistant Cové strain.
Fig 4. Response of Anopheles gambiae in…
Fig 4. Response of Anopheles gambiae in tunnel tests with Interceptor® G2 and other nets.
(A) Mortality of susceptible strain. (B) Mortality of resistant strain. (C) Blood-feeding rate of susceptible strain. (D) Blood-feeding rate of resistant strain.

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