Habitat heterogeneity and green filamentous algae influence the larval ecology of Anopheles stephensi during the dry season in Eastern Ethiopia

Abstract

BACKGROUND: The recent invasion of Anopheles stephensi, an urban-adapted malaria vector, poses a threat to malaria elimination efforts in Africa. Understanding the larval ecology of this mosquito during the dry season, which represents a potential population bottleneck due to limited larval habitats and harsher environmental conditions, is critical for informing targeted interventions. METHODS: We conducted systematic surveys in three climatically distinct Ethiopian cities-Semera, Logiya and Jigjiga-during the dry season of 2023. A total of 523 water-holding habitats were identified and characterized for the presence and productivity of An. stephensi immature stages. Habitat characteristics, including container type, floating algal mass presence, cover status and water chemistry, were recorded. A Lefkovitch matrix model was used to project habitat-specific productivity. RESULTS: Overall, 40.9% of the habitats surveyed were positive for An. stephensi. Larval positivity and productivity were significantly higher in Semera and Logiya, coinciding with warmer temperatures and lower elevations. Three habitat types, namely construction pits, residential cisterns and ground-level water tanks, accounted for 87% of positive habitats and 81% of all larvae. Complete stage structures observed in key habitats indicated ongoing oviposition and larval development throughout the dry season. The Lefkovitch model identified construction pits as the most productive habitat type across all three cities. Anopheles stephensi larval presence and density were strongly associated with the presence of green filamentous algal aggregates (odds ratio [OR] 6.00, 95% confidence interval [CI] 2.76-13.04). Secondary predictors were lack of cover (OR 0.98, 95% CI 0.96-0.98) and specific water chemistry parameters (OR 1.21, 95% CI 1.03-1.42). CONCLUSIONS: Urban infrastructure and water storage practices during the dry season support discrete ecological niches that facilitate the persistence of An. stephensi populations in eastern Ethiopia. These findings underscore the importance of targeted larval source management focused on key habitat types, particularly construction-related and domestic water storage containers, to reduce urban malaria transmission risk in the Horn of Africa.