Autochthonous dengue outbreak in Northern Italy, September 2024: epidemiological, microbiological, entomological investigation and public health response

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BACKGROUND: Dengue is an emerging public health threat in temperate regions of Europe where Aedes albopictus is established. In 2024 Italy experienced an unprecedented increase in autochthonous dengue transmission. In September 2024, a cluster of locally acquired dengue cases was detected in Cavezzo, Emilia-Romagna, Northern Italy. METHODS: An outbreak investigation was initiated following confirmation of the first autochthonous dengue case on 17 September 2024. Enhanced surveillance was activated including retrospective case finding, targeted community screening, and strengthened laboratory diagnostics. Vector control measures were implemented starting within a 200m radius of confirmed cases and expanded based on epidemiological and entomological findings. Blood and organ donation safety measures were introduced, and whole genome sequencing were performed on human and mosquito samples. RESULTS: 44 autochthonous dengue cases were identified with symptom onset between 31 July and 23 September 2024; 68.2% were detected retrospectively. No epidemiologically linked cases were detected outside Cavezzo. Most cases occurred in older adults (median age 63 years); two patients were hospitalised and recovered fully. Dengue virus serotype 2 (DENV-2), genotype II, lineage F.1.1, was detected in both human and Ae. albopictus samples. Phylogenetic analysis identified a distinct cluster separate from sequences associated with the contemporaneous outbreak reported in Fano, Italy. Following vector-control interventions, mosquito density declined and no further virus-positive pools were detected. CONCLUSION: This outbreak highlights the increasing vulnerability of temperate European regions to dengue transmission and underscores the importance of rapid case detection, community engagement, and targeted vector-control strategies in preventing wider spread.

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