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Title: Impact of exposure to long lasting insecticide treated nets on mosquito survival and behaviour at the net interface in insecticide susceptible and resistant strains of the Afrotropical Anopheles mosquito
Author: Hughes, Angela Jane
ISNI:       0000 0004 7656 5972
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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The central role of long-lasting insecticidal nets (LLINs) in malaria prevention in Africa is threatened by insecticide resistance. Little is known about mosquito behaviour near, or of the importance of sub-lethal or delayed impacts of, the most widely-used LLIN types, and yet these effects are fundamental for developing new net treatments and for understanding the nature of resistance and managing its emergence. Effects of insecticide exposure on longevity and reproductive output of pyrethroid-resistant and susceptible adult female Anopheles gambiae mosquitoes were investigated using standard tests and newly developed video assays. To investigate effects on longevity, mosquitoes were repeatedly exposed to LLINs (PermaNet® 2.0, Olyset®) or untreated netting using WHO cone bioassays, at various intervals between 4- and 16-days post-emergence, to simulate the natural exposure of repeat blood feeding attempts. Effects on reproductive fitness were investigated by quantifying egg production (and hatch rate) in mosquitoes that had bloodfed through an LLIN or bloodfed after repeated LLIN exposure. On average, over all experiments, longevity of moderate and highly resistant mosquitoes was reduced significantly from 11.3 to 6 days, and 14.6 to 9.2 days, respectively, following exposure to PermaNet. Egg production by mosquitoes exposed to LLINs was not significantly different to those on untreated net. In extremely resistant mosquitoes, this delayed mortality effect was reduced, which would ensure that pyrethroids continued to reduce the transmission potential of insecticide resistant mosquitoes but could eventually be eroded by continued, intense selection for resistance. Precisely how insecticides affect mosquito behaviour has never been fully elucidated or quantified. Using two simple bench top tests, the Cone Video Test and the Thumb Test, detailed behavioural events associated with insecticide exposure were compared in insecticide susceptible and resistant strains of Anopheles gambiae and Anopheles funestus at different LLIN types (PermaNet® 2.0, Olyset®, Olyset Duo® and Duranet®). The Cone Video test employs scan-sampling to quantify a video record of behaviour during a standard 3-min WHO cone test, classifying activity as resting on net, on cone, or in-flight. Throughout all tests, susceptible and resistant strain mosquitoes rested preferentially on untreated and treated netting, except for susceptible mosquitoes on Olyset®, where net resting was significantly lower than the other LLIN types. Notably, this effect was not seen in pyrethroid resistant mosquito strains. In a variant of the same bioassay, when direct LLIN contact was prevented, resting activity was restored, suggesting that the different response to Olyset was a contact-irritancy response rather than non-contact repellency. In Cone Video tests that incorporated a human host, increased net contact occurred in all tests, suggesting that host attraction exceeds any deterrent properties on these LLINs. The Thumb tests enable quantification of the detailed behavioural events during feeding, or attempts to feed, by a single mosquito at a human-baited LLIN interface. Here, all mosquito strains readily fed through all LLINs tested, but the duration of blood feeding was significantly lower than on untreated net, in all susceptible and resistant mosquito strains. The study findings also highlight the importance of investigating the impacts of resistance beyond immediate mortality. It is possible that delayed mortality effects, or other fitness costs from LLIN exposure, may be reducing the impact of resistance on LLIN performance. However, as the degree of resistance increases, over the lifetime of resistant mosquitoes, the magnitude of these fitness costs may diminish and eventually disappear.
Supervisor: Ranson, Hilary ; Paine, Mark Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral