Role of invasive Aedes (Aedimorphus) vittatus in chikungunya virus transmission in the newly-invaded coastal island city of Mombasa, Kenya

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BACKGROUND: Countrywide routine entomological surveillance studies in Kenya from 2006 first identified Aedes vittatus, in small numbers (4 specimens), in Mombasa city in 2014 during a dengue outbreak investigation. Significant numbers (1,648 specimens) were collected in January 2018 during a chikungunya outbreak investigation. The presence of Ae. vittatus, and a competent vector of chikungunya virus, complicates disease epidemiology and control efforts in Mombasa underscoring the need to determine its bionomic factors. METHODS: In June 2021 and December 2021, we conducted mosquito sampling at multiple sites in Mombasa, an island city county, using CO₂-baited Biogent sentinel (BGS) traps and human landing collection (HLC) methods. The collected mosquitoes were identified morphologically, and Ae. vittatus species were confirmed further by molecular characterization based on the cytochrome c oxidase I gene (cox1). Virus isolation from mosquitoes pools was performed in Vero ccl-81 cell cultures. Cultures showing cytopathic effects were harvested and genome-sequenced using the Illumina MiSeq platform to identify the infecting virus. RESULTS: A total of 11,435 mosquitoes were collected; 7,250 by BGS traps and 4,185 by HLC. Overall, Ae. aegypti was the dominant species, accounting for 32.6% (n = 3,725), followed by Culex quinquefasciatus which accounted for 31.8% (n = 3,638) and Ae. vittatus at 24.4% (n = 2,789). Aedes aegypti (n = 2,216; 43%) predominated in HLC collections, followed by Ae. vittatus (n = 1,598; 31%). Although mosquito biting rates per person per hour (b/p/h) were higher for Ae. aegypti (3.2 b/p/h) than Ae. vittatus (2.4 b/p/h), the difference was not statistically significant (t = 6.0081, df = 1, p-value = 0.105). Chikungunya virus isolate belonging to the East Central South African genotype was isolated from a pool of female Ae. vittatus. CONCLUSION: Aedes vittatus was found widely distributed across the island city in significant numbers, suggesting that the species, which is predominant to most rural areas in Kenya, has invaded the city and successfully established. The presence of this species in the city, a confirmed vector of CHIKV, and along with Ae. aegypti which is the principle vector of CHIKV, potentially magnifies the risk of chikungunya outbreaks. This highlights the significance and need for integrated vector control strategies.

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