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London School of Hygiene & Tropical Medicine Malaria Centre

Malaria research in vector studies

Control of pyrethroid and DDT-resistant Anopheles gambiae by application of Indoor Residual Spraying (IRS) or mosquito nets treated with a long-lasting organophosphate insecticide, chlorpyrifos-methyl.

LSHTM investigators:
Raphael N’Guessan & Mark Rowland.
External collaborators:
P Boko, A Odjo, J Chabi, M Akogbeto (Centre de Recherche Entomologique de Cotonou, Benin).
Funding body:
The Bill & Melinda Gates Foundation through the Innovative Vector Control Consortium.

Dow Agrosciences have developed a microencapsulated formulation (CS) of the organophosphate chlorpyrifos methyl as a cost-effective, long-lasting alternative to DDT.

We tested this product as an IRS or ITN treatment in experimental huts in an area of Benin where Anopheles gambiae and Culex quinquefasiactus are resistant to pyrethroids, but susceptible to organophosphates. Efficacy and residual activity was compared to that of DDT and the pyrethroid lambdacyalothrin.

IRS with chlorpyrifos methyl killed 95% of An. gambiae that entered the hut as compared to 31% with lambdacyhalothrin and 50% with DDT. Control of Cx. quinquefasciatus showed a similar trend; although the level of mortality with chlorpyrifos methyl was lower (66%) it was still much higher than for DDT (14%) or pyrethroid (15%) treatments. Nets impregnated with lambdacyhalothrin were compromised by resistance, killing only 30% of An. gambiae and 8% of Cx. quinquefasciatus. Nets impregnated with chlorpyrifos methyl killed more (45% of An gambiae and 15% of Cx. quinquefasciatus), but its activity on netting was of short duration. Contact bioassays on the sprayed cement-sand walls over the nine months of monitoring showed no loss of activity of chlorpyrifos methyl, whereas lambdacyhalothrin and DDT lost activity within a few months of spraying.

The remarkable residual activity indicates that cost-effective alternatives to DDT are feasible through modern formulation technology