Molecular Epidemiological Characteristics and Genetic Evolution of Bluetongue Virus in Yunnan Province, China

Bluetongue is an insect-borne disease of ruminants caused by the Bluetongue virus (BTV). It is primarily transmitted through the bites of Culicoides midges and is widely distributed in tropical, subtropical, and temperate regions around the world. Due to its favorable geographical location and abundant population of BTV vector midges, Yunnan Province, China, has become a key region for research on BTV diversity. However, there are gaps in research on the genetic evolution of BTV in this region. Between January 2024 and December 2025, Culicoides midges were collected from three border counties in Yunnan Province. Researchers isolated BTV strains and sequenced the full-length segment 2 (Seg-2) gene. Subsequently, phylogenetic analysis, Bayesian evolutionary rate analysis, population dynamics analysis, and phylogeographic analysis were performed utilizing Seg-2 sequence data sourced from the NCBI database. Four BTV serotypes (BTV-1, BTV-4, BTV-5, and BTV-16) were identified. The estimated average evolutionary rate varied from 2.88 × 10−4 to 4.26 × 10−3 substitutions per site per year, with ancestral origins dating back to the period between 1872 and 1988. The effective population size displayed significant fluctuations over time. Phylogeographic reconstructions indicated that the pathogen entered Yunnan through various transcontinental transmission pathways, originating from Europe, South Asia, Oceania, and East Asia, with evidence of reciprocal transmission between Africa, Europe, and West Asia. The BTV serotypes circulating in Yunnan Province are highly diverse, and the population dynamics of different serotypes exhibit significant heterogeneity. Yunnan is simultaneously exposed to BTV introductions from multiple directions, including Europe, South Asia, Oceania, and East Asia.