Abstract
Insecticide-treated bed-nets (ITNs) control malaria by keeping mosquitoes from reaching people sleeping under a net and by killing mosquitoes. Most tests of ITNs consider their overall epidemiological outcome without considering the different behaviors underlying their effects. Here we consider one of these behaviors: that mosquitoes can bite through the net if its user is touching it. We assayed the ability of an insecticide-sensitive strain of the mosquito Anopheles gambiae to bite through a permethrin-treated or an untreated net, and their subsequent survival and fecundity. Despite the irritancy of permethrin, 71% of the mosquitoes took blood through the ITN (vs. 99% through the untreated net). The ITN reduced the time spent biting, the blood-meal size and the fecundity, and it killed about 15% of the mosquitoes within 24 hours of feeding (vs. 5% on the untreated net). However, the mosquito’s survival was much higher than what we found in WHO cone assays, suggesting that the bloodmeal increased the mosquito’s resistance to the insecticide. Thus, our results suggest that the irritancy and the toxicity of ITNs are reduced when mosquitoes contact and feed on their host, which will affect our understanding of the personal and community protection offered by the ITNs.
Highlights
Insecticide-treated bed nets (ITNs) are among the most cost-effective tools used to control malaria[1,2,3]
Permethrin may keep mosquitoes from biting through the net, a laboratory experiment suggests that complete protection requires a higher concentration than what is used in commercially available Insecticide-treated bed-nets (ITNs) like Olyset (1 g/m2)[14]; at 0.8 g/m2 more than half of the mosquitoes were able to bite through the net
Despite that irritancy, 88% of the mosquitoes tried to bite and 71% succeeded to take a blood meal through the net. This feeding through the permethrin-treated net came with fitness costs for the mosquitoes, for it reduced their chance to survive and their fecundity, corroborating the results from previous studies[14,16,17]
Summary
Insecticide-treated bed nets (ITNs) are among the most cost-effective tools used to control malaria[1,2,3]. In one study in Tanzania, for example, treating a bed net with permethrin had almost no effect on the number of mosquitoes that entered experimental houses, but reduced the probability that a mosquito passed through a net by a factor of about 810. Mosquitoes irritated by insecticides are likely to stop their blood-seeking behavior for several days[17] ( protecting the ITN-users and others from being bitten) and lay fewer eggs[17] (potentially reducing the number of mosquitoes in the population). While the efficacy of ITNs to reduce malaria prevalence is not questioned here, our goal was to understand better the properties and effects of an ITN in the scenario of mosquitoes having access to a human host by biting through the net.
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