Autochthonous chikungunya virus (CHIKV) outbreak in the province of Modena, Emilia-Romagna region, Italy, August to October 2025: epidemiology, clinical features and virological findings.

Between 10 August and 31 October 2025, 343 cases (306 confirmed and 37 probable) of symptomatic autochthonous chikungunya virus (CHIKV) infection were notified in eight municipalities in the province of Modena, Emilia-Romagna region, northern Italy. The infection was diagnosed by detection of CHIKV RNA in blood and urine samples, and by detection of IgM antibodies against CHIKV. Most common symptoms were arthralgia (n = 326) and fever (n = 317). No fatalities were reported. Chikungunya virus RNA was detected in 39 (14.8%) of 263 pools of Aedes albopictus mosquitoes, thereby confirming local vector-borne transmission. Sequences from 11 case samples and two mosquito pools were whole genome sequenced. The viral strain belonged to the East/Central/South African (ECSA) lineage 2 and was closely related to the strains circulating in Reunion Island in 2024-25. The public health response included rapid vector control measures, reinforced epidemiological surveillance and multidisciplinary coordination between public health authorities and clinicians, laboratories and entomologists. This was one of the largest autochthonous CHIKV outbreaks ever recorded in a temperate European region. It highlights the critical importance of integrated, multidisciplinary preparedness and response for arboviral threats in non-endemic areas.