Emergence of Bluetongue virus serotype 5 in Sardinia- Italy, 2025.

Abstract

Southern Europe, and Italy in particular, has historically been exposed to repeated incursions of orbiviruses, including multiple bluetongue virus (BTV) serotypes and epizootic haemorrhagic disease virus serotype 8. Sardinia, Italy represents a key sentinel location due to its geographic position in the Mediterranean basin. In August 2025, fatal bluetongue cases in sheep were reported in Sardinia during a period of intense BTV circulation in Italy, characterised by widespread outbreaks associated with BTV-3 and BTV-8. These clinical samples tested positive for BTV but could not be serotyped using the available real-time RT-PCR assays. Whole-genome sequencing identified the causative agent as BTV serotype 5 (BTV-5), representing the first detection of this serotype in Europe. The virus was also successfully isolated in cell culture. Segments 2 and 6 showed the highest nucleotide identity with a BTV-5 strain identified in Nigeria in 1982, whereas the rest of the genome seems to be composed by gene segments originating from multiple BTV serotypes circulating in Africa and Europe. At the time of writing, no official reports of BTV-5 circulation were available from Northern Africa; however, during the editorial process, BTV-5 was reported in Tunisia, further supporting the Mediterranean basin as a single interconnected epidemiological ecosystem for orbivirus emergence and spread.