Increasing detections of the invasive mosquito Aedes albopictus in Belgium

Aedes albopictus is an invasive mosquito species expanding its territory in Europe, posing a health risk as the species is a competent vector of dengue, chikungunya and Zika virus. In European countries autochthonous transmission of these viruses are reported in localities where the species is established. In Belgium the introduction of Ae. albopictus was first monitored through active surveillance at Points of Entry (PoEs). Since 2018, the increased observation of Ae. albopictus at parking lots located along the highways suggested a rise in introduction through route traffic. Hence, in 2022, a passive surveillance based on citizen science was implemented to complement the active surveillance and expand the coverage of the monitoring countrywide. We present the current situation for Ae. albopictus in Belgium based on the results of both active and passive surveillance. Via an online platform (web/app), citizens uploaded pictures of potential Ae. albopictus specimens after answering filtering questions about morphological characteristics of the mosquito related to its size, color and stripes on the hind legs. Subsequently, pictures were then analysed to determine whether or not it is Ae. albopictus. When Ae.albopictus was confirmed on the picture, a field inspection was performed. This inspection included larval sampling and the set-up of ten oviposition traps for one or two weeks around the notification point. Additionally, in 2022 and 2023, ten oviposition traps were set-up at eight parking lots between May and October. In 2023, a longitudinal surveillance was also implemented to monitor overwintering and potential spread at two locations where the presence of Ae. albopictus was confirmed in 2022. In 2024, overwintering monitoring happened through larval sampling at four locations where Ae. albopictus was detected in 2023. DNA-based validation of all life stages of Ae. albopictus collected during field visits from several locations was performed to validate the identification of the species, and to investigate the haplotype composition of the population. We received 12 notifications of Ae. albopictus from citizens from nine locations in 2022, 29 from 15 new locations in 2023 and 47 from 12 new locations in 2024. Overall, Ae. albopictus was detected at 36 locations in Belgium over these three years. Further, the exotic species was detected in 2022 at three, and in 2023 at seven parking lots. Longitudinal surveillance in 2023 confirmed the presence of Ae. albopictus at two locations, indicating local establishment and overwintering. In 2024, overwintering was confirmed at another three locations. Prior to 2022, Aedes albopictus was in the early stage of its invasion process in Belgium, with confirmed occurrences limited to PoEs. Since 2022, the implementation of citizen surveillance has led to a steep increase in detections, including in residential areas, alongside numerus findings at parking lots. Additionally, the confirmation of overwintering at five locations, indicates that the species is being increasingly imported into Belgium via ground vehicular traffic and has become locally established in recent years.

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