title: Effects of Bed Nets on Host Seeking Behaviour of African Malaria Vectors
Creators: name: Parker, J
Publisher: name: University of Liverpool Repository
Year: 2016

description: Long lasting insecticide treated bed nets (LLINs) are a key tool in malaria control in sub-Saharan Africa, and their widespread distribution has contributed significantly to recent reductions in malaria prevalence. Sustaining this impact will require thorough understanding of anopheline host seeking behaviour and LLIN mode of action. However, the behaviour of anopheline mosquitoes during interactions with LLINs, and how insecticides affect that behaviour, is poorly investigated. To pursue this, novel video systems, scaled to record and track nocturnally active free-flying mosquitoes at different levels of detail, were developed and evaluated in a series of behavioural studies, primarily with insecticide susceptible Anopheles gambiae s.s.. The spatial repellent properties of deltamethrin and DDT were investigated using two small-scale cage assays that presented mosquitoes with a human thumb bait and LLIN with (larger choice test) or without (smaller single test) an untreated control alternative. Results from single tests indicated repellency (in deltamethrin only) but the larger choice tests, (and subsequent large-scale tracking), did not. The results highlighted the limitations of such assays, and the caution required when otherwise convenient laboratory behavioural assays are used. The flight behaviour of host-seeking mosquitoes as they navigated through an open window was investigated in a laboratory environment using a novel 3D tracking system. The study proved the principle of this 3D tracking concept, which uses a retro-reflective material to identify a mosquito’s position during flight using a single camera. Analyses of tracks showed that mosquitoes approached windows from higher flight elevations, consistently descending to low levels following passage from the window into the room. Large scale tracking experiments used Fresnel lenses to illuminate a large field of view, and record activity of free flying An. gambiae s.s. at a human-baited bed net. These laboratory tests characterised mosquito flight into four behavioural modes, showing that insecticide treatment rapidly reduced mosquito activity around the net, and provoked a shift in flight behaviour resulting in less net contact. Highest levels of net contact were centred on the net roof above the volunteer’s torso. Insecticide treatment reduced the time a mosquito spent in contact with the net, and an individual mosquito was estimated to accumulate less than 100 seconds of direct physical contact with the LLIN during a 60-minute test. Velocity measurements showed that mosquitoes detected nets, including unbaited untreated nets, prior to contact. The large scale tracking system was transported to, and operated successfully at an experimental hut in Tanzania, to investigate the behaviour of a wild mosquito population consisting predominantly of An. arabiensis. Experimental outcomes were similar in both settings, though field tests did not show such pronounced activity decay as was observed in laboratory tests. The large-scale system was used to explore the host seeking flight behaviour of An. gambiae s.s. in the absence of a bed net. Flight activity at a supine human host was separated into approaching or departing tracks. Flight elevation and speed were similar in both, but tortuosity was higher in tracks approaching the host. Mosquitoes showed no preferences for feeding on any part of the host’s body and bites were distributed evenly across the volunteer’s exposed skin. This study delivers the most complete characterisations of mosquito-LLIN interactions to date. The tracking systems provide a new platform for a range of further studies and the findings contribute to evidence base required for vector control tool design, research on basic host seeking behaviour and the behavioural mechanisms of insecticide resistance.