Abstract
BackgroundInsecticide-treated bed nets (ITNs) help to control malaria by mechanically impeding the biting of mosquitoes, by repelling and irritating them and by killing them. In contrast to spatial repellency, irritancy implies that mosquitoes contact the ITN and are exposed to at least a sub-lethal dose of insecticide, which impedes their further blood-seeking. This would weaken the transmission of malaria, if mosquitoes are infectious.MethodsIt was therefore tested whether sub-lethal exposure to permethrin impedes blood-feeding differently in uninfected mosquitoes and in mosquitoes carrying the non-transmissible stage (oocysts) or the infectious stage (sporozoites) of the malaria parasite Plasmodium berghei. In addition, as the degree of irritancy determines the dose of insecticide the mosquitoes may receive, the irritancy to permethrin of infected and uninfected mosquitoes was compared.ResultsIn this laboratory setting, sub-lethal exposure to permethrin inhibited the blood-seeking behaviour of Anopheles gambiae mosquitoes for almost 48 h. Although infection by malaria did not affect the irritancy of the mosquitoes to permethrin at either the developmental stage or the infectious stage, both stages of infection shortened the duration of inhibition of blood-seeking.ConclusionsThe results suggest that the impact of ITNs may be weaker for malaria-infected than for uninfected mosquitoes. This will help to understand the global impact of ITNs on the transmission of malaria and gives a more complete picture of the effectiveness of that vector control measure.
Highlights
Insecticide-treated bed nets (ITNs) help to control malaria by mechanically impeding the biting of mosquitoes, by repelling and irritating them and by killing them
The efficacy of ITNs is partly due to their dual mode of protection: they protect the community by killing mosquitoes, thereby lowering the likelihood that mosquitoes live long enough to transmit malaria, and they protect individuals with the physical barrier of the net and the insecticide’s repellency to
If the mosquitoes had been exposed to the control paper, 88.5% of them tried to bite within 24 h after exposure, and their biting rate decreased with time (Fig. 2)
Summary
Insecticide-treated bed nets (ITNs) help to control malaria by mechanically impeding the biting of mosquitoes, by repelling and irritating them and by killing them. In contrast to spatial repellency, irritancy implies that mosquitoes contact the ITN and are exposed to at least a sub-lethal dose of insecticide, which impedes their further blood-seeking. This would weaken the transmission of malaria, if mosquitoes are infectious. Spatial repellency occurs when mosquitoes detect the volatiles of the insecticide and avoid the area near the ITN. It occurs before mosquitoes have had any physical contact with the ITN. Permethrin, for example, is only slightly spatially repellent but strongly irritant, whereas DTT repels and irritates mosquitoes [3,4,5]
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