Modifications of pyrethroid effects associated with kdr mutation in Anopheles gambiae

Effects of knockdown resistance (kdr) were investigated in three pyrethroid‐resistant (RR) strains of the Afrotropical mosquito Anopheles gambiae Giles (Diptera: Culicidae): Kou from Burkina Faso, Tola and Yao from Côte d'Ivoire; compared with a standard susceptible (SS) strain from Kisumu, Kenya. The kdr factor was incompletely recessive, conferring 43‐fold resistance ratio at LD₅₀ level and 29‐fold at LD₉₅ level, as determined by topical application tests with Kou strain. When adult mosquitoes were exposed to 0.25% permethrin‐impregnated papers, the 50% and 95% knockdown times (KdT) were 23 and 42 min for SS females, compared with 40 and 62 min for RS (F₁ Kou × Kisumu) females. On 1% permethrin the KdT₅₀ and KdT₉₅ were 11 and 21 min for SS compared with 18 and 33 min for RS females. Following 1 h exposure to permethrin (0.25% or 1%), no significant knockdown of Kou RR females occurred within 24 h. Permethrin irritancy to An. gambiae was assessed by comparing ‘time to first take‐off’ (TO) for females. The standard TO₅₀ and TO₉₅ values for Kisumu SS on untreated paper were 58 and 1044 s, respectively, vs. 3.7 and 16.5 s on 1% permethrin. For Kou RR females the comparable values were 27.3 s for TO₅₀ and 294 s for TO₉₅, with intermediate RS values of 10.1 s for TO₅₀ and 71.9 s for TO₉₅. Thus, TO values for RS were 2.7–4.4 times more than for SS, and those for RR were 7–18 times longer than for SS. Experiments with pyrethroid‐impregnated nets were designed to induce hungry female mosquitoes to pass through holes cut in the netting. Laboratory ‘tunnel tests’ used a bait guinea‐pig to attract mosquitoes through circular holes (5 × 1 cm) in a net screen. With untreated netting, 75–83% of laboratory‐reared females passed through the holes overnight, 63–69% blood‐fed successfully and 9–17% died, with no significant differences between SS and RR genotypes. When the netting was treated with permethrin 250 mg ai/m² the proportions that passed through the holes overnight were only 10% of SS vs. 40–46% of RR (Tola & Kou); mortality rates were 100% of SS compared with 59–82% of RR; bloodmeals were obtained by 9% of Kou RR and 17% of Tola RR, but none of the Kisumu SS females. When the net was treated with deltamethrin 25 mg ai/m² the proportions of An. gambiae that went through the holes and blood‐fed successfully were 3.9% of Kisumu SS and 3.5% of Yaokoffikro field population (94% R). Mortality rates were 97% of Kisumu SS vs. 47% of Yaokoffikro R. Evidently this deltamethrin treatment was sufficient to kill nearly all SS and half of the Yaokoffikro R An. gambiae population despite its high kdr frequency. Experimental huts at Yaokoffikro were used for overnight evaluation of bednets against An. gambiae females. The huts were sealed to prevent egress of mosquitoes released at 20.00 hours and collected at 05.00 hours. Each net was perforated with 225 square holes (2 × 2 cm). A man slept under the net as bait. With untreated nets, only 4–6% of mosquitoes died overnight and bloodmeals were taken by 17% of SS vs. 29% of Yaokoffikro R (P < 0.05). Nets treated with permethrin 500 mg/m² caused mortality rates of 95% Kisumu SS and 45% Yao R (P < 0.001) and blood‐feeding rates were reduced to 1.3% of SS vs. 8.1% of Yao R (P < 0.05). Nets treated with deltamethrin 25 mg/m² caused mortality rates of 91% Kisumu SS and 54% Yao R (P < 0.001) and reduced blood‐feeding rates to zero for SS vs. 2.5% for Yao R (P > 0.05). Pyrethroid‐impregnated bednets in experimental huts and ‘tunnel tests’ gave equivalent results, showing that nets impregnated with permethrin or deltamethrin provided good levels of protection against kdr homozygous strains of An. gambiae (Kou and Tola), and against the field population at Yaokoffikro with 94% kdr frequency. The explanation seems to be that (a) high proportions of kdr females are killed by prolonged contact with pyrethroids through diminished sensitivity to the usual irritant and repellent effects, and (b) relatively few kdr females take advantage of this prolonged contact to ingest a bloodmeal.