Combined pyrethroid and carbamate 'two-in-one' treated mosquito nets: field efficacy against pyrethroid-resistant Anopheles gambiae and Culex quinquefasciatus

A new approach is proposed in the treatment of mosquito nets, using a 'two- in-one' combination of pyrethroid and non-pyrethroid insecticides applied t o different parts of bednets, The objectives are mainly to overcome certain limitations of pyrethroid-impregnated bednets currently recommended for ma laria control purposes. Apart from developing alternatives to pyrethroid de pendency, we sought to counteract pyrethroid irritant effects on mosquitoes (excito-repellency) and resistance to pyrethroids. The idea takes advantag e of the presumed host-seeking behaviour of mosquitoes confronted by a net draped over a bed, whereby the mosquito may explore the net from the top do wnwards. Thus, nets could be more effective if treated on the upper part wi th residual non-irritant insecticide (carbamate or organophosphate) and wit h a pyrethroid on the lower part. Sequential exposure to different insectic ides with distinct modes of action is equivalent to the use of a mixture as a potential method of managing insecticide resistance. We also intended to improve the control of nuisance mosquitoes, especially Culex quinquefascia tus Say (Diptera: Culicidae) that often survive pyrethroids. in order to en courage public compliance with use of insecticide-treated nets (ITNs). Polyester bednets were pretreated with residual pyrethroid (bifenthrin 50 m g/m(2) or deltamethrin 25 mg/m(2)) on the lower half and with carbamate (ca rbosulfan 300 mg/m(2)) on the upper half to minimize contact with net users . Unreplicated examples of these 'two-in-one' treated nets were field-teste d against wild mosquitoes, in comparison with an untreated net and bednets treated with each insecticide alone, including PermaNet(TM) wash-resistant formulation of deltamethrin 50 mg/m(2). Overnight tests involved volunteers sleeping under the experimental bednets in verandah-trap huts at Yaokofikr o, near Bouake in Cote d'Ivoire, where the main malaria vector Anopheles ga mbiae Giles, as well as Culex quinquefasciatus Say, are highly resistant to pyrethroids. Efficacy of these ITNs was assessed in the huts by four entom ological criteria: deterrency and induced exophily (effects on hut entry an d exit), blood-feeding and mortality rates (immediate and delayed). Overall, the best impact was achieved by the bednets treated with carbosulf an alone, followed by 'two-in-one' treatments with carbosulfan plus pyrethr oid. Blood-feeding rates were 13% An. gambiae and 17% Cx, quinquefasciatus in huts with untreated nets, but only 3% with carbosulfan ITNs, 7-11% with combined ITN treatment, 6-8% An. gambiae and 12-14% Cx. quinquefasciatus wi th pyrethroid alone. Mosquitoes that entered the huts were killed sooner by nets with combined treatment than by pyrethroid alone. Mortality-rates in response to ITNs with carbosulfan (alone or combined with pyrethroid) were significantly greater for Cx. quinquefasciatus. but not for An. gambiae, co mpared to ITNs with only pyrethroid. About 20% of sleepers reported potenti al side-effects (headache and/or sneezing) from use of ITN treated with car bosulfan alone. Further development of this new 'two-in-one' ITN concept re quires a range of investigations (choice of effective products, cost-benefi t analysis, safety, etc.) leading to factory production of wash-resistant i nsecticidal nets treated with complementary insecticides.