[Late impact of lambdacyhalothrin-treated nets on kdr allelic frequency in Anopheles gambiae s.s. (Diptera: Culicidae) from northern Côte-d'Ivoire].

Nets treated with lambdacyhalothrin 15 mg a.i./m2 were distributed in four villages of Korhogo area in northern Côte-d'Ivoire, and four other villages without nets were followed as controls. Mosquitoes were collected every three months from June 1999 to August 2000 in the villages, and then tested to determine their genotypes for kdr mutation L1014F conferring cross-resistance to pyrethroids and DDT and for M and S molecular forms of Anopheles gambiae s.s. The frequencies of the resistant allele at kdr mutation L1014F locus were very high and they varied from 0.82 to 0.96. Before the trial, the genotypic frequencies of specimens at kdr locus did not show any significant difference. However, they significantly increased in mosquitoes from treated villages (0.94) compared to those from control villages (0.87) on month 14 (P = 0.013). No significant difference was observed between mosquitoes collected outside and inside the houses (P < 0.05). In contrary, a significant difference was observed before and after the trial between mosquitoes from villages with two cycles of rice cultivation per year and those from villages without rice cultivation (P = 0.008 and P = 0.012). Nine out of twelve populations of field mosquitoes showed Hardy-Weinberg disequilibrium at kdr locus (P < 0.05). The S and M molecular forms of An. gambiae s.s. were sympatric in Korhogo area and the S form represents 97%. No hybrid between M and S forms was observed. All individuals of the M form were homozygote susceptible at kdr locus.