Bluetongue virus serotype 12 in sheep and cattle in the Netherlands in 2024 - A BTV serotype reported in Europe for the first time.

Bluetongue (BT) is a viral vector borne disease primarily affecting ruminants such as sheep, cattle, and goats. On 3 September 2023, the Netherlands reported the first case of bluetongue virus serotype 3 (BTV-3/NET2023)), after being BTV free for eleven years. Vaccination with inactivated BT vaccines for serotype 3 has been applied in the Netherlands since May 2024. Nonetheless, in late June/July 2024, BTV-3 re-emerged and spread over large parts of Europe. In October 2024, BTV-12 was identified by follow-up diagnostics after a BTV-3 vaccinated sheep with signs of BT was tested positive for BTV but negative for serotype 3. This marks a significant event, as BTV-12 had never been reported in Europe. Screening of farms in close proximity to the sheep farm and retrospective analysis of samples from clinically affected animals that were panBTV PCR positive resulted in the detection of nine BTV-12 affected farms in total. The emergence of BTV-12 in the Netherlands raises important questions about the route of introduction of BT in the Netherlands and mechanisms of viral spread of this specific serotype. Possible adaptation of new BTV serotypes to the European climatic and husbandry conditions prompts reconsideration of prevention, surveillance, and control strategies in relation to changing ecological conditions and vector dynamics. The initial findings, respective studies as well as the initial attempts to trace the origin of BTV-12/NET2024 are described.