LLIN bioassay and knockdown rates against resistant and
susceptible colonies
Prior to the semi field trials, the efficacy of the treated bed net was
evaluated. The knockdown response of the resistant females exposed to
DawaPlus2.0 for 60 minutes was 7% whilst 83% for the susceptible
population. The mortality rate for the resistant colony was 13% (95%
CI=[9.1-15.9]) whilst 92% (95% CI=[ 89.4-94.9]) for
susceptible population . The mortality rate for the F1 progeny of the
wild population was 59% (95% CI=[50.3-67.9]) when exposed to
DawaPlus 2.0.
Discussion
Physiological resistance in mosquito populations to common public health
insecticides across Africa is widely reported but evidence of the actual
impact of this, the functionality and efficacy of LLINs is scarcely
discussed or documented(Ranson et
al. , 2011). Monitoring the
host-seeking behavior of
physiologically resistant mosquitoes in the presence of indoor vector
control tools is necessary to determine whether the efficacy of the
tools could be compromised with the resistance profiles or whether they
can be optimized. This study provides insights into the behavior of
pyrethroid-resistant An. gambiae when they encounter
pyrethroid-based LLIN in a free flight environment similar to the field
settings. The results demonstrate that in the presence of a treated net,
the host-seeking performance was not altered for resistant females,
unlike the susceptible females that were observed to exit the house and
remained outdoors when a treated net was used.
One of the consequences of the massive roll out of LLINs is the change
in mosquito behaviour where the interventions may select vectors with
increased exophily (feeding outdoors early in the evening or morning
hours when LLINs are not in use) because of the exposure to insecticides
(Gatton et al. , 2013). This study
observed large proportion of host seeking susceptible females exiting
the house and resting outdoors than resistant females when treated net
was present. The observed behaviour suggests that in the presence of
insecticides, susceptible mosquitoes may be pushed from indoor treated
environment and resort to search blood meal outdoors or rest outdoors
and initiate their search for a host soon after dusk, leading to
increased outdoor transmission. Examples of spatial avoidance of
insecticide-treated environment has been observed in malaria vectors in
the field, displaying increased outdoor host-seeking and resting
outdoors following the implementation of IRS and ITNs (
Moiroux et al. ,
2012;Parker et al. , 2015;
Spitzen et al. , 2017). This
indicates that, substantial part of residual malaria transmission is
occurring outdoors, raising the questions on the effectiveness of LLINs
in reducing malaria infections when susceptible indoor feeding
mosquitoes are diverted to feed outdoors when people are outside LLINs.
On the other hand, the findings suggest, physiologically resistant
malaria vectors that have developed the capacity of blood-feeding or
resting indoors in the presence of LLINs, may compromise the
effectiveness of LLINs, maintaining the indoor malaria transmission.
The strategy of LLINs in malaria prevention is to deter mosquitoes from
entering houses and reduction in blood feeding rates, achieved as a
consequence of excito-repellent effects of the pyrethroids
(HODJATI & Curtis, 1997).In this study,
a higher proportion of the resistant females were caught in the treated
bed net trap compared to the susceptible population. The resistant
females were less likely to avoid the search for a host when a treated
net was present, unlike the susceptible population that was observed to
avoid contact with the treated net. One plausible explanation for the
difference in behaviours is the pleiotropic effects on nerve function
associated with a point mutation in the voltage-gated sodium channels of
resistant mosquitoes, as it interferes with the sensitivity of the
sensory nervous system to pyrethroids resulting in reduced avoidance
behavior (Lee et al. , 1999;
Corbel et al. , 2004). This implies
that in the field, physiologically resistant mosquitoes are likely to
spend more time in search of a host in the presence of insecticides
increasing their probability of encountering a host unlike their
susceptible counterparts. In nature, pyrethroid-resistant mosquitoes
have been found resting inside holed LLINs
(Ochomo et al. , 2013). Such
behavior may compromise the efficacy of the current indoor-based vector
control tools resulting in increases in malaria transmission indoors
(Killeen et al. , 2006). Recent
studies from western Kenya observed high resistance levels, rates of
human blood index and sporozoite rates in the mosquitoes resting indoors
compared to the mosquitoes collected resting outdoors
(Degefa et al. , 2017;
Machani et al. , 2020). The study
findings are in agreement with similar studies that have observed
reduced host-seeking performance of susceptible mosquitoes in the
presence of LLIN unlike the resistant mosquitoes whose behavior was not
altered (Kawada et al. , 2014;
Diop et al. , 2015;
Porciani et al. , 2017;
Zhou et al. , 2020).
This findings of this study shows that despite the coverage of the
indoor interventions, it is evident that not all malaria transmission
can be controlled with the existing tools that are indoor based. The
population of vectors that move outdoors are not taken care of, a
situation that creates a pressing need for supplementary vector control
tools to control residual transmission.