West Nile Virus Pilot Screening in Field-Collected Aedes japonicus (Theobald, 1901): An Update of Species Distribution in Poland, 2025.

(1) Background: The Asian bush mosquito Aedes japonicus is an invasive species in Europe, including Poland. Given its laboratory-confirmed competence for West Nile virus (WNV) transmission and its detection as a WNV vector in field-collected mosquitoes, this study investigated whether Polish Aedes japonicus harbor WNV and aimed to update knowledge on its distribution in Poland. (2) Methods: In September 2024, 137 adult Aedes japonicus were collected from three suburban sites in Poland (Kielce, Mikołów, Kraków). Specimens were screened for WNV using RT-PCR and rRT-PCR. Additionally, unpublished records of Aedes japonicus were compiled to update the species' distribution. (3) Results: No WNV genetic material was detected in field-collected mosquitoes in Poland. By 2025, Aedes japonicus had been recorded in half of Polish voivodeships, with most observations in Małopolskie, Śląskie, and Łódzkie. The largest adult populations occurred in Kielce. Ecological traits in Poland matched European and US data, including larval development in artificial containers, preference for suburban and forested habitats, and peak adult activity in late summer. (4) Conclusions: Although WNV was not detected, the rapid spread of Aedes japonicus in Poland underlines the need for continued monitoring of its distribution, population dynamics, and potential role in WNV transmission.