Assessing Insecticide Susceptibility Status of Anopheles Mosquitoes in Gondar Zuria District, Northwest Ethiopia
Authors: Barasa Silesh , Jejaw Ayalew , Aemero Mulugeta , Woldesenbet Dagmawi , Abebe Wagaw 
Published: November 2023

Background
Insecticide-based vector control, which comprises the use of insecticide-treated bed nets (ITNs) and indoor residual spraying (IRS), is the key method of malaria control in Ethiopia. However, it’s effectiveness is threatened as malaria vectors become resistant to insecticides. Assessing the susceptibility status of these Anopheles mosquitoes at different time is important to complement vector control interventions. However, there is no reports on the insecticide susceptibility status of malaria vectors from Gondar zuria woreda, Northwest Ethiopia. Thus, this study investigated the susceptibility status of malaria vectors in Gondar zuria woreda, Northwest Ethiopia against various insecticides recommended for use in ITNs and/or IRS to provide evidence to suggest alternates for malaria vector control tools.
Methods
An entomological study was carried out from March 1, 2022, to August 31, 2022. 900 three-day-old adult female Anopheles mosquitoes from larva collected in the field from Gondar zuria district using dipper were used for bioassay and synergist. They were tested for susceptibility to permethrin (0.75%), deltamethrin (0.05%), alpha-cypermethrin (0.05%), propoxur (0.1%), and pirimiphos-methyl (0.25%) by using WHO susceptibility test papers. Baches of 20 and 25 adult female Anopheles mosquitoes were used for both bioassay and synergist assay, respectively. 50%KDT and 90%KDT were determined by logit-probit analysis after 1 hour. One-way ANOVA was used to compare the mean knockdown rates of the Anopheles mosquitoes by insecticides after 60 minutes. An independent sample t-test was used to compare the mean knockdown rates of the Anopheles mosquitoes by pyrethroid insecticide with and without PBO after 60 minutes. Mortality rates pos-24 hours were recorded.
Results
A total of 900 female Anopheles mosquitoes were examined, of which 878 (97.5%), 15 (1.5%), and 7 (0.7%) were found to be Anopheles Gambae(s.l), Anopheles funestus group, and Anopheles Pharoensis, respectively. They exhibited susceptibility to pirimiphos-methyl (100%) and propoxur (100%), but resistant to permethrin (88.8%), alphacypermethrin (67.5%), and deltamethrin (73.75). Although permethrin restoration was only 96% in permethrin resistant Anopheles mosquitoes, the efficacy of alphacypermethrin and deltamethrin was totally restored by pre-exposure to PBO.
Conclusion and recommendation
This study demonstrated that malaria vectors are susceptible to pirimiphos-methyl propoxur, and PBO + pyrethroid insecticides but resistant to pyrethroids insecticide-only. It also revealed that Anopheles gambae s.l., Anopheles funestus group, and Anopheles pharoensis group are malaria vectors in Gondar zuria woreda with Anopheles gambae complex predominating. Based on the findings of this study, it is better to use pyrethroids-impregnated mosquito nets with PBO and/or the use of propoxur and primiphos-methyl to control malaria. Further investigations are needed to better understand the molecular basis of pyrethroids insecticide resistant-malaria vectors.