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.