Bionomics, seasonal abundance and insecticide susceptibility of Anopheles (Diptera: Culicidae) in low and high malaria transmission settings in Ethiopia

Background : In Ethiopia, malaria is highly seasonal with varying intensities of transmission due to altitudinal and climatic variations. Information on bionomics of principal, secondary and suspected malaria vectors, species composition, seasonal dynamics and/or abundance and insecticide susceptibility status at locality level is important to understand malaria transmission in a particular eco- epidemiological setting to design effective vector control strategies. Methods : Mosquitoes were collected using CDC light traps and human landing catches (HLC) for 17 months between June 2018 and September 2020 from Lare district of Gambella Regional State, Ethiopia. The Anopheles mosquitoes were morphologically identified and preserved for further analysis. Larvae and pupae of Anopheline mosquitoes were collected from breeding sites of Lare district and Jimma zone and reared to adult. Bioassays were conducted on non-blood-fed, 3-5-day-old adult female mosquitoes emerged from larvae and pupae using discriminating concentrations of WHO insecticide-impregnated papers. Deltamethrin-survived samples of An. coustani were molecularly identified via Sanger sequencing (COI and COII mitochondrial gene determination). Plasmodium parasite infection in An . pharoensis and An. coustani was investigated using TaqMan qPCR assay. Mean monthly density differences among mosquito species were evaluated by analysis of variance (ANOVA). Differences in mean Anopheles mosquito density between outdoor and indoor catches by HLC were compared using a t test. Data were analyzed using IBM SPSS statistics for Windows v.20.0. Results : Of 11,048 Anopheles mosquitoes collected during the 17-months survey, An. coustani was the most abundant species, representing 46.20% (5103/11048), followed by An. phraoensis (27.89%, n= 3082), An. gambiae complex (16.04%, n= 1772) and An. funestus (9.88%, n= 1091). About 73.17% (n=8084) the mosquitoes were collected using CDC and while 26.83% (n=2964) were captured using HLC method The mean monthly density of female Anopheles collected by HLC was 5.17 mosquitoes per-person-night, while the corresponding mean monthly mosquito density by CDC LT was 17.37 mosquitoes per trap-night. The number of Anopheles mosquitoes captured outdoors by HLC was 1.36 times higher than the indoor capture. There was positive but weak (p > 0.05) association between Anopheles abundance and mean monthly rainfall in the Lare district. An. pharoensis , An. gambiae s.l. and An. coustani complex were susceptible to pirimiphos-methyl (organophosphate), propoxur and bendiocarb (carbamate). However, An. pharoensis and An. gambiae s.l. were resistant to DDT, deltamethrin, permethrin and malathion. On the other hand, An. coustani complex were resistant to DDT and deltamethrin (pyrethroid). Species identification of deltamethrin survivor An. coustani (morphological) was also molecularly confirmed. No Plasmodium infection was detected in An. coustani and An. pharoensis . Conclusion : Temporal variation in mean density of Anopheles mosquito species was recorded in relatively large number (except An. funestus ) during June-November. The higher mean mosquito density from outdoors coupled with multiple insecticide resistance of malaria vector populations and occurrence of An. funestus in dry season call for regular monitoring and intervention considering the behavioral dynamics of Anopheles species across seasons. Additional information on mosquito fauna and abundance in relation to metrological factors in a particular locality is essential for the development of efficient vector control interventions.