Silent circulation of Bluetongue and Schmallenberg viruses in Culicoides biting midges in northwestern Italy: implications for the 2024 BTV outbreak.

Vector-borne viruses like Bluetongue (BTV) and Schmallenberg virus (SBV) pose an ongoing threat to livestock health across Europe. Transmitted by Culicoides biting midges, these pathogens can circulate silently, escaping early detection. In this study, we investigated the presence and infection status of Culicoides on 2 cattle farms in northwestern Italy, where congenital malformations and abortions had previously been reported. Between June and October 2023, over 4,000 Culicoides were captured using UV-light suction traps; over 95% belonged to the C. obsoletus complex. Molecular screening of pooled insects revealed a low viral prevalence (0.5%) for both BTV and SBV. Notably, no official outbreaks were recorded in the area during 2023, suggesting a silent viral circulation without clinical signs in animals. The persistence of BTV in vectors and possible animal reservoirs, including wildlife, may have contributed to its reemergence in 2024 when a new BTV outbreak occurred in northern Italy. Our findings highlight the critical role of continuous entomological and virological surveillance in anticipating and mitigating future outbreaks.