Evolutionary Dinamics of Bluetongue Virus Strains circulating in Italy, 2024-2025

In 2024, Italy experienced a major resurgence of Bluetongue (BT), a viral disease of ruminants, caused by Bluetongue virus (BTV), an RNA virus circulating in multiple serotypes transmitted by Culicoides midges. By April 2025, over 7,000 confirmed cases were reported, mainly involving BTV-3, BTV-4, and BTV-8. We analyzed the full genomes of BTV strains collected from different Italian regions in 2024 through WGS and phylogenetic analysis. At least three distinct genomic constellations of BTV-3 were identified, two resulting from multiple reassortments involving European strains, and one closely related to the Northern European BTV-3. BTV-4 strains also displayed multiple genomic profiles, with gene segments from different origins. BTV-8 showed high genetic identity (up to 100% in Seg-2) with strains from France, suggesting viral spread through the trade of viremic animals. Notably, BTV-8 genome segments were found within BTV-3 and BTV-4 strains, highlighting the ongoing role of reassortment in shaping BTV evolution. These findings reveal the complex genomic landscape of BTV in Italy and emphasize the importance of WGS in tracking viral dynamics.