Abstract
BackgroundHoles in netting provide potential routes for mosquitoes to enter ITNs. Despite this, there is little information on how mosquitoes respond to holes in bed nets and how their responses are affected by hole size, shape and orientation or by ambient conditions around the net.MethodsFemale Anopheles gambiae (G3) were recorded in a simulated bed net consisting of two sizes of untreated netting-covered behavioural arenas placed above and beside (to simulate the bed net roof and sides respectively) the experimenter who was a source of host cues from ‘inside’ the net. A round hole of 9 mm or 13 mm diameter was cut into the centre of the netting of each arena. Videos of unfed female mosquitoes in arenas were analysed for time spent flying, walking and standing still and for exit through the hole. The effects of the experimenter on temperature and relative humidity around the simulated net were also measured.ResultsMosquitoes were significantly more active in overhead arenas than in arenas to the side. Hole passage was significantly more likely in smaller arenas than larger ones and for larger holes than smaller ones. In arenas to the side, hole passage rate through small holes was about 50 % less likely than what could be explained by area alone. Passage rate through holes in overhead arenas was consistent with hole area. Temperature in arenas did not strongly reflect the experimenter’s presence in the simulated net. Relative humidity and absolute humidity in overhead arenas, but not in arenas to the side, were immediately affected by experimenter presence.ConclusionsHigher levels of activity in overhead arenas than in arenas to the side were likely due to the rising heat and humidity plume from the experimenter. Lower than expected passage rates through smaller vertically oriented holes may have been be due to an edge effect that does not apply to horizontally oriented holes. Results suggest that current methods of assessing the importance of physical damage to ITNs may not accurately reflect mosquito entry risk in all cases.
Highlights
Holes in netting provide potential routes for mosquitoes to enter Insecticide-treated bed net (ITN)
Insecticide-treated bed nets (ITNs) have been credited with reducing the global burden of malaria since mass distribution programmes began in the early 2000s [1]
Source colonies Mosquitoes used for the videos were drawn from stock colonies of Anopheles gambiae s.s. (G3 strain) maintained by the Malaria Branch at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia
Summary
Holes in netting provide potential routes for mosquitoes to enter ITNs. Despite this, there is little information on how mosquitoes respond to holes in bed nets and how their responses are affected by hole size, shape and orientation or by ambient conditions around the net. Holes in the netting of bed nets provide a potential route for malaria-infected mosquitoes to enter the net and bite. This could result in malaria infection for the net occupants or, if the occupants are already infected, it could result in infection of mosquitoes and the possibility of subsequent transmission to other human victims. ITNs can sustain significant levels of physical damage and still provide the equivalent protection of an undamaged untreated net because of the insecticide ‘back up’ [2, 3]. This is an essential element of the ITN strategy. Smith et al [10] showed that over 50 % of 2023 holes (not including holes 0.5 cm across or less) in 50 nets taken from the field after 38 months of use had long dimensions of 3 cm or less while 31 holes greater than 10 cm were found
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