Assessing the oviposition and larval ecologies of mosquitoes and Stegomyia indices along an urban-rural gradient during ongoing dengue outbreaks in the arboviral hotspots of Cocody-Bingerville, southeastern Côte d’Ivoire
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Background
Côte d’Ivoire (West Africa) has recurrently faced multiple outbreaks of Aedes mosquito-transmitted arboviral diseases (e.g., dengue (DEN) and yellow fever (YF)), mainly in the health district of Cocody-Bingerville subjected to rapid urbanization. Thus, this study assessed the ecology of mosquito immatures and Stegomyia indices along an urban-rural gradient during ongoing dengue outbreaks within arboviral hotspots in Cocody-Bingerville.
Methods
Between August 2023 and July 2024, we collected mosquito eggs and larvae in urban, suburban and rural areas of Cocody-Bingerville using standard ovitraps and larval surveys. Collections were done in the domestic and peridomestic ecozones, and during rainy and dry seasons. Species composition, oviposition indices (ovitrap positive index (OPI), mean egg counts per ovitrap (MEO) and egg density index (EDI)) and Stegomyia indices (house index (HI), container index (CI) and Breteau index (BI)) were compared across the study areas, ecozones and seasons, and with the World Health Organization (WHO) DEN and YF epidemic thresholds.
Results
Overall, all the study areas were highly infested with diversified mosquito species, with the highest Stegomyia risk indices recorded in the urban areas. We identified (individuals: species) in the all study (70,550: 15), urban (32,331: 10), suburban (19,768: 14) and rural (18,451: 15) areas. Mosquitoes were mostly abundant in the domestic ecozones and rainy seasons. Larvae bred mainly in tires in the urban and suburban, and small containers in the rural areas. Wild Aedes species ( Aedes dendrophilus , Aedes lilii , Aedes fraseri , Aedes luteocephalus , Aedes metallicus and Aedes vittatus ) were mostly restricted in the suburban and rural areas, as well as mosquito predatory larvae ( Lutzia tigripes , Toxorhynchites and Eretmapodites ). However, Aedes aegypti spread and dominated throughout, showing highest proportions in the urban (90.2%), followed by the suburban (68.6%) and rural (68.5%) areas. OPI, MEO (egg/ovitrap/week) and EDI (egg/ovitrap/week) values were higher in the urban areas (53.0%, 8.7 and 16.4) than in the suburban (43.1%, 5.56 and 12.9) and rural (33.7%, 4.4 and 13.0) areas, respectively, and correlated with Stegomyia indices. HI, CI and BI were higher in the urban (48.0%, 40.4% and 61.8) compared with the suburban (37.8%, 32.0% and 43.3) and rural (34.5%, 28.87% and 39.8) areas. Stegomyia indices corresponded to the WHO-density scales of 6-8 in the urban and 5-7 in the suburban and rural areas. Stegomyia indices were all above the WHO epidemic thresholds: YF risk was high the urban and moderate in the suburban and rural areas, while people were permanently exposed to high risk of DEN outbreaks in all the areas.
Conclusions/significance
In Cocody-Bingerville, urbanization shifts mosquito biodiversity, with restriction of wild Aedes and predatory species in the rural and suburban areas and dominance of Ae. aegypti in the urban areas. Stegomyia indices exceeded the WHO DEN and YF epidemic thresholds in all the areas, with highest risks in the urban areas. This could explain the current DEN outbreaks in urban Cocody-Bingerville. Our study provides a baseline for raising local community awareness and guiding appropriate preventive actions, including managing identified larval habitats to contain ongoing dengue outbreaks in Côte d’Ivoire.
Author summary
Côte d’Ivoire has experienced several epidemics of dengue and yellow fever transmitted by Aedes mosquitoes. Over 80-90% of dengue and yellow fever cases were reported in the urban areas of the health district of Cocody-Bingerville, southeastern Côte d’Ivoire. The increase in the frequency, magnitude and burden of these outbreaks has paralleled to the fast and uncontrolled urbanization. However, data are still lacking on how urbanization influences the ecology of the local mosquito vectors and the risks of transmission of dengue and yellow fever, thus compromising adequate planning of preventive actions. Therefore, we assessed the mosquito species composition and larval breeding sites, and Stegomyia indices in urban, suburban and rural areas within the disease hotspots during ongoing dengue outbreaks in the Cocody-Bingerville. Our results showed that mosquito species diversity altered considerably from urban to rural areas, leading to a dominance of the main vector Aedes aegypti in all the study areas. Moreover, Aedes aegypti showed the highest proportions in the urban areas that contained more larval breeding sites (e.g., tires and small containers). Stegomyia indices were very high and above the World Health Organization (WHO) epidemic thresholds, suggesting that people were exposed to high dengue risks and moderate yellow fever risks in all the study areas, but were more exposed to both diseases in the urban areas. This could explain the recurrent and present dengue outbreaks in urban Cocody-Bingerville. A community-based management of identified larval habitats might help to control the ongoing dengue outbreaks in Cocody-Bingerville, and more widely in similar settings.
