Introduction
Reduction in malaria morbidity and mortality over the past few years in sub-Saharan Africa is largely attributed to the effectiveness of long-lasting insecticidal nets (LLINs)(Bhatt et al. , 2015).This has been possible because the main malaria vectors primarily feed indoors at night, a behavioral pattern that coincides with the period when human hosts are indoors and asleep (Gillies & Coetzee, 1987; Killeen et al. , 2006; Bayoh et al. , 2014). However, extensive use of insecticides has subjected mosquitoes to intensive selection pressure, resulting in the development of physiological and behavioral resistance, threatening the future of existing tools, hence the need for continuous monitoring of their efficacy and development of novel LLINs(Ranson et al. , 2011).
The continued success of the current vector control interventions is dependent on the susceptibility of the target mosquito population to the insecticides used. Pyrethroids are one of the insecticide classes recommended for treating mosquito nets owing to their low mammalian toxicity, unique modes of action such as fast knockdown, and high insecticidal potency, although currently assessment of some innovative nets treated with a combination of a pyrethroid and either a non pyrethroid compound is ongoing(WHO, 2019)(Zaim et al. , 2000).Over the past two decades, the use of insecticide-treated nets has increased, exerting greater selection pressure on the malaria vector populations resulting in higher incidences of pyrethroid insecticide resistance that is likely to affect the effectiveness of vector control(Ranson & Lissenden, 2016).Some studies have reported the spread of pyrethroid resistance and the mechanisms involved including target site insensitivity caused by kdr mutations(Ranson et al. , 2011; Ochomo et al. , 2012) and detoxification enzymes that metabolize the insecticide before reaching its target site(Hemingway & Ranson, 2000). However, it is less clear how the observed resistance affects current control measures. Recent field observations in Africa suggests changes such as increased outdoor host-seeking within the principal malaria vectors, An. gambiae and An. funestus species complexes(Russell et al. , 2011; Derua et al. , 2012; Gatton et al. , 2013; Bayoh et al. , 2014). These behavioral shifts have been largely linked to the increased use of vector control measures selecting for vector species with more exophilic behavior (Bayoh et al. , 2010; Mwangangi et al. , 2013).
Existing literature on behavioral changes comes mainly from pyrethroid susceptible mosquitoes but the data on the behaviorof pyrethroid-resistant malaria vectors is sparse and, at times conflicting, highlighting the need for additional research. It is suggested that avoidance behavior in mosquitoes that have become insensitive to pyrethroids may weaken due to increased selection pressure exerted by the insecticides used(He et al. , 2019).Some authors assert that physiologically resistant mosquitoes may use the recognition of insecticides as a proxy for host presence(Ngufor et al. , 2016; Porciani et al. , 2017; Carrasco et al. , 2019). It is unclear if mechanisms related to insecticide resistance may influence the behavior of malaria vectors, as any molecular change in the insect nervous system, may have a pleiotropic effect on nerve function and insect behavior(Kliot & Ghanim, 2012).
Given the important role of the current vector control interventions as a means of reducing the burden of malaria transmission and increasing insecticide resistance, the behaviorof physiologically resistant malaria vectors should be well defined. The host-seeking behavior of the major malaria vector, Anopheles gambiae s.s . (hereafterAn.gambiae ) was assessed when an intact human-occupied insecticide-treated and untreated bed net is in place, with respect to their insecticide resistance status under semi-field conditions. We hypothesize that pyrethroid-resistant mosquitoes will seek and bite human hosts indoors despite the presence of indoor-based vector control interventions while the susceptible mosquitoes will leave the house through the windows or eaves and seek blood meal elsewhere. This study provides more information on how the behavior of physiologically resistant vectors might differ in comparison to their susceptible counterparts, an aspect that is poorly understood. There is an urgent need for evidence-based studies on the behavior of malaria vectors in the presence of vector control interventions, given the rapid development of insecticide resistance in a large number of malaria vectors, if the significant gains made in reducing malaria morbidity and mortality is to be maintained.