Olyset Net((R)) efficacy against pyrethroid-resistant Anopheles gambiae and Culex quinquefasciatus after 3 years' field use in Cote d'Ivoire

Pyrethroid-impregnated bednets are advocated for personal protection agains t malaria vectors. To avoid the need for periodic re-treatment, it would be advantageous to have nets that retain insecticidal efficacy for years and withstand repeated washing. Such a type of commercially produced bednet wit h permethrin 2% incorporated in polyethylene fibres (trademark Olyset Net(R ) supplied by Sumika Life-Tech Co., Osaka, Japan) was evaluated against mos quitoes in veranda-trap huts at Yaokoffikro, near Bouake, Gate d'Ivoire, by standard WHOPES phase II procedures. Four Olyset Nets were compared with a standard untreated polyester net as control. They comprised three examples previously used in a village for over 3 years (one washed, one dirty, one very dirty) and a previously unused Olyset Net, newly unwrapped, from the s ame original batch. Bioassays with 3 min exposure of susceptible Anopheles gambiae Giles (Diptera: Culicidae) gave >99% mortality of female mosquitoes tested on the 'new' Olyset Net. The used Olyset Nets gave mortality rates averaging 83% for the washed net, 85% for the dirty net and 55% for the ver y dirty net (within 24-h following 3 min exposure). Thus, Olyset Nets were found to remain remarkably effective against susceptible Art. gambiae for a t least 3 years under field conditions. Wild pyrethroid-resistant populations of Culex quinquefasciatus Say and An. gambiae (savanna cytotype with 96% kdr) were assessed during June-August 1 999 for their responses to sleepers protected by nets in the experimental h uts. With regard to hut entry by foraging female mosquitoes, Olyset Nets sh owed some deterrency against An. gambiae (44% reduction by the new net, sim ilar to 20% by the dirty nets, none by the washed net), but not against Cx. quinquefasciatus. Among mosquitoes entering the hut with untreated control net, 30-34% tried to leave (exophily) but were caught in the verandah trap . The permethrin repellency of Olyset Nets increased exophily by 19% for An . gambiae and 14% for Cx. quinquefasciatus. Blood-feeding rates were 16% An . gambiae and 35% Cx. quinquefasciatus in the hut with sleeper under the un treated net (showing considerable prevention of biting), 22-26% of both spe cies in huts with washed or dirty used Olyset Nets (not significantly diffe rent from control), while the biting success rate of Cx. quinquefasciatus ( but not kdr An. gambiae) was more than halved by the 'new' Olyset Net. Mort ality rates of pyrethroid-resistant An. gambiae and Cx. quinquefasciatus fr om the huts were, respectively, 3% and 8% with the untreated polyester net, 27.5% and 17% with the 'new' Olyset, 15% and 17.5% with the washed Olyset, 16-25% and 17-20% with dirty old Olyset Nets. Kill differences between net s are significantly different for both An. gambiae and Cx. quinquefasciatus . Unfortunately the washed used Olyset Net showed least activity against re sistant mosquitoes, despite its greatest activity against susceptible An. g ambiae. In each case there was evidence that a high proportion of mosquitoe s failed to feed through the net (many of them dying from starvation when t hey could not leave the closed hut), with indications that dirty Olyset net s enhanced this protective value.