Bluetongue Virus Surveillance in Yunnan, China, 2025: Isolation of Multiple Serotypes From Culicoides and Seasonal Seroepidemiology in Cattle.

Bluetongue (BT) virus (BTV) is an arthropod-borne pathogen that causes substantial economic losses in ruminants globally. Yunnan Province, located in China's tropical and subtropical border region, faces a high risk of BTV circulation. This study monitored BTV vectors, genetically characterized circulating BTV strains, and assessed seroprevalence to inform early warning efforts. In 2025, 46,000 Culicoides midges were collected from three counties, identified, and grouped into 460 pools. Additionally, 5934 cattle serum samples from 17 border counties were tested using C-ELISA. Midge pools were screened by RT-qPCR for BTV nucleic acids and then inoculated into C6/ 36 cells for virus isolation. Whole-genome sequencing and phylogenetic analysis of VP2 and VP5 genes were performed on the isolates. C. oxystoma and C. trithecoides were the dominant midge species at all sites. RT-qPCR identified 11 BTV-positive midge pools, primarily involving C. oxystoma, C. imicola, C. tainanus, and C. jacobsoni. Four BTV isolates were identified as serotypes 1, 4, 5, and 16. Phylogenetic analysis of VP2 and VP5 showed all four strains likely belong to the Eastern topotype and are closely related to historical BTVs from China, Japan, India, and Australia. The overall BTV seroprevalence in cattle was 41.37% (95% confidence interval [CI], 40.12%-42.62%), with significantly higher rates in autumn (58.07%, 95% CI, 56.29%-59.85%) than in spring (24.87%, 95% CI, 23.32%-26.42%). Several BTV serotypes have been detected and isolated from diverse Culicoides in the border regions of Yunnan. Additionally, BTV seroprevalence has been observed in local cattle populations, indicating that this area may represent a significant BTV endemic hotspot. These findings underscore the need for ongoing vector surveillance and early warning systems to prevent BT outbreaks in susceptible livestock.