Detection of dengue virus serotype 2 in local Aedes aegypti populations, Madeira Island, Portugal, 2025

Background Since 2010, dengue virus (DENV) has caused sporadic outbreaks across Europe, namely in Croatia, Spain, France, Italy and the Portuguese island of Madeira. Aedes aegypti mosquito is established in the Autonomous Region of Madeira, and along the eastern Black Sea coast of Cyprus. In Madeira Island, an outbreak of DENV serotype 1 occurred between 2012 and 2013, resulting in 1,080 confirmed cases. Despite ongoing entomological surveillance, no further local transmission was detected in the following decade. Methods In January 2025, following two suspected dengue cases on Madeira Island, increased entomological surveillance efforts were implemented to confirm a local event transmission of dengue virus. A network of mosquito traps was complemented by targeted surveillance using 17 BG-PRO traps positioned in the vicinity of suspected human cases. Daily collections of adult Ae. aegypti , collected from January 10 to March 31, 2025, were screened by RT-PCR for Aedes -borne viruses in the reference laboratory. Viral sequencing was performed using as target enrichment and bioinformatics with INSaFLU-TELEVIR. The climate-driven suitability for dengue transmission by Ae. aegypti was also investigated. Serological and molecular tests were conducted on samples from suspected human cases. Results Out of 80 analysed Ae. aegypti pools (N = 393 mosquitoes), one pool, with nine mosquitoes collected near the home of suspected human cases, tested positive for dengue virus. The dengue whole genome sequence from this sample was determined and classified as DENV-2 lineage 2II_F.1.1.3. The same virus was retrospectively confirmed in one of the clinical cases. Analysis of mosquito abundance and climate data confirmed the occurrence of this local transmission event during a period of low mosquito abundance and low climatic suitability. Conclusions Here, we report an in-depth analysis of a local dengue transmission event that occurred in Funchal, the capital of Madeira Island, in January 2025, with whole-genome evidence of DENV-2II_F.1.1.3 in field-caught Aedes aegypti mosquitoes. Retrospective analysis confirmed the presence of the same virus in one of the two clinical cases, establishing a direct link between human and mosquito infections, and highlighting the risk of off-season arboviral introductions.