Effect of impregnated nets on mortality and behaviour of mosquitoes.

The effectiveness of impregnated fabrics on mosquitoes has been studied in the laboratory with a view to improving the means for communities to protect themselves against vectors of malaria and other diseases. Di-ethyl toluamide (deet) was found to be more effective against mosquitoes when impregnated cotton nets were placed at a distance from the bait rather than close to the bait. Deet at a dose of 6.25 ml/m1 gave complete protection against An. gambiae for 2 weeks. When various types of netting and sheeting were dipped in permethrin emulsion at normal temperature, the amount of Insecticide absorbed was generally proportional to the weight of liquid taken up, i.e. there was no evidence of selective absorption. However, selective absorption of more permethrin than expected was observed when nets were impregnated at 97°C and acid pH. Diffusion of permethrin did not occur between pieces of netting and sheeting sewn together. The LD5Q of permethrin on cotton nets was found to be about three times greater than on nylon nets. Ae. aegypti was found to be more susceptible to permethrin than An. gambiae. which was more susceptible than C. quinquefasciatus. PP321 (Icon) and cypermethrin were found to be the most effective of 9 pyrethroids tested. Hand washing with cowfat soap reduced the amount of all the pyrethroids remaining on the nets, ageing in tropical condition did not have such an effect. The effectiveness of permethrin remained constant for 30 weeks when impregnated into a thick cotton net and evaluated in a "tunnel" against An. gambiae. No clear cut effect of temperature on the toxicity of permethrin against An. gambiae was detected within the range of 16 to 28°C. Mosquitoes resistant to various insecticides (one of them to DOT) did not show cross-resistance to permethrin; only two strains showed some tolerance. A prolonged exposure of one of the tolerant strain to permethrin did not increase permethrin resistance level. When a part of the same strain was exposed to DDT, resistance developed quickly but with no cross-resistance to permethrin. The present WHO method for detecting resistance in adult mosquitoes is not satisfactory. Short exposure of mosquitoes to impregnated surfaces may be a solution. Although for other groups of insecticides variation in time and dose have equivalent effects, for permethrin this was not found to be true, i.e. on halving the exposure time the LD50 was not doubled. Mosquitoes frequently find their way into, or bite through, untreated bednets. When mosquitoes were released in a room and a human subject sat under a permethrin impregnated bednet with an arm pressing against the net, mosquitoes failed to bite through the net. All the mosquitoes trying to bite through or entering the net through holes cut in it were knocked down within 30 minutes of release and ultimately died.