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.