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London School of Hygiene & Tropical Medicine Malaria Centre

Malaria research in vector studies

Evaluation of the long-lasting organophosphate pirimifos methyl CS for Indoor Residual Spraying (IRS) against pyrethroid-resistant Anopheles gambiae Giles and Culex quinquefasciatus: an experimental hut trial in Southern Benin.

LSHTM investigators:
Raphael N’Guessan & Mark Rowland.
External collaborators:
Pelagie Boko, Abibatou Odjo, Estelle Vignonou, Hermione Adje, Alex Asidi & Martin Akogbeto (Centre de Recherche Entomologique de Cotonou, Benin).
Funding body:
The Bill & Melinda Gates Foundation through the Innovative Vector Control Consortium.

There is an urgent need to develop safe, long- lasting alternatives to DDT and pyrethroids to sustain malaria vector control and reduce selection pressure for pyrethroid resistance.

Two formulations of pirimifos methyl (CS) applied as an IRS treatment were evaluated in experimental huts in an area of southern Benin where Anopheles gambiae and Culex quinquefasiactus are resistant to pyrethroids but susceptible to organophosphates. Dosages tested were 1g/m2 and 0.5g/m2 against standard pyrimifos methyl EC for up to 12 months on mud and cement walled substrates.

In mud huts 1 g/m2 was clearly superior to 0.5 g/m2 and  induced >50% mortality of An. gambiae for more than 10 months.  In cement walled huts 0.5 g/m2 induced high levels of control for almost one year. Overall, the BM formulation seemed superior to B formulation. The EC formulation of pirimifos methyl sprayed at 1 g/m2 was giving inadequate control of An. gambiae within 2 months of spraying. A microencapsulated formulation of lambdacyhalothrin (Icon CS) was sprayed as a positive control and although it showed prolonged residual activity in cone tests on walls using a susceptible reference strain, it was unable to control freely entering pyrethroid resistant An. gambiae. Pirimifos methyl CS was also found to be highly effective against Culex quinquefasciatus, giving between 90-50% control for up to 10 months at 1 g/m2 on cement surfaces and for 6 months on mud surfaces.

Pirimifos methyl CS showed great promise for control of pyrethroid-resistant An gambiae in West Africa and for delaying development of pyrethroid resistance where  LLINs are already widely used. A cost effective alternative to DDT, giving year-round transmission control  in endemic rural settings in Africa is now a realistic prospect.