Bionomics, seasonal dynamics and insecticide susceptibility of Anopheles mosquitoes in low and high malaria transmission settings of Ethiopia
Authors: Woyessa Delelegn , Yewhalaw Delenasaw 
Published: September 2023

Background
In Ethiopia, malaria is highly seasonal with varying intensity of transmission due to altitudinal and climatic variations. Information on principal, secondary and suspected malaria vector species composition, their seasonal dynamics and/or abundance, bionomics and insecticide susceptibility status at locality levels are important to understand malaria transmission in a particular eco-epidemiological setting in order to design effective vector control strategy.
Methods
Adult female Anopheles mosquitoes were collected from Lare district of Gambella Regional State and Jimma Zone of Oromia Regional State, Ethiopia. Mosquitoes were collected using CDC light traps and human landing catches (HLC) for 17 months between June 2017 to October 2020. All the collected Anopheles mosquitoes were morphologically identified and properly preserved for further analysis. Larvae and pupae of Anopheles gambiae complex, An. pharoensis and An. coustani were collected from breeding sites and reared to adult under field condition in the field insectary. Bioassays were conducted on non-blood-fed, 3–5 days old adult female mosquitoes emerged from larvae and pupae using discriminating concentrations of WHO insecticide impregnated papers. Randomly selected sub-sample of the morphologically identified An. coustani was molecularly identified via Sanger sequencing (COI and COII mitochondrial gene determination). TaqMan qPCR assay was employed to investigate Plasmodium parasite infection in An. pharoensis and An. coustani. Differences in mean Anopheles mosquito density between outdoor and indoor catches by HLC were compared using t-test. Mean difference among trapping methods and mosquito density were evaluated using analysis of variance (ANOVA) and Tukey’s Post-hoc test. Pearson correlation was used to assess the association between the overall Anopheles mosquito density from Lare district with climatic factors such as rain fall and temperature. Duncan post-hoc test was employed to compare mean mortality rate of mosquitoes exposed to different insecticides. Data were analyzed using IBM SPSS statistics for windows v.20.0 and values were considered significant at p < 0.05.
Results
A total of 11,876 Anopheles mosquitoes belonging to four species were collected using CDC and HLC methods during the study period. An. coustani was by far the most abundant species representing 47.84% (5681/11876) of the total collections and 96.2% of it was sampled using CDC traps. Other Anopheles mosquitoes collected were An. phraoensis (26.90%, n = 3195), An. gambiae complex (16.07%, n = 1909) and An. funestus (9.19%, n = 1091). While all the four Anopheles species were identified from Lare district, An. funestus was not recorded from Jimma zone study sites. The majority (74.22%, n = 8814) of Anopheles were collected using CDC light traps (73 trap nights) while the remaining (25.78%, n = 3062) were captured by HLC (288 person-nights). The mean monthly density of female Anopheles collected by HLC was 5.17 (95% CI 0.67–9.66) mosquitoes per-person-night while the corresponding mean monthly mosquito density by CDC LT was 17.37 mosquitoes (95% CI 3.15–31.59) per trap-night. Of all collected Anopheles mosquitoes by HLC, the number of Anopheles mosquitoes captured outdoor was higher by 1.36 times than the corresponding indoor capture. There was no direct association between Anopheles abundance and rain fall and or mean monthly maximum and minimum temperatures in Lare district. Anopheles pharoensis, An. gambiae s.l and An. coustani complex were susceptible to Primiphose methyl (organophosphate) and Propoxur and bendiocarb (carbamate). However, Anopheles pharoensis and An. gambiae s.l were resistant to DDT, deltamethrin, permethrin and malathion. On the other hand, An. coustani complex were resistant to only DDT. Sub-samples of morphologically identified as An. coustani were molecularly confirmed to belong to the An. coustani. No Plasmodium infection was detected in An. coustani and An. pharoensis.
Conclusion
Anopheles coustani was the most abundant species in this study. High vector density was recorded following rainy seasons. Relatively higher density of the principal vector, An. gambiae s.l. was captured using CDC during October-November 2017, June 2017 and September 2018 following the heavy rainy months (June-August) and/or beginning of the long rains (June). There was higher mean mosquito density outdoor than indoor using HLC. Multiple insecticides resistance was observed in both the principal and secondary malaria vector populations in both Lare and Jimma Zone study sites of Ethiopia. Sub-samples of all the morphologically identified Anopheles coustani mosquitoes were also confirmed by molecular methods.