Isolation and Pathogenicity of an Emerging Highly Virulent CSFV 2.1c Strain in South China

Classical swine fever (CSF) is an infectious disease caused by classical swine fever virus (CSFV), which is endemic in many areas of China, causing serious economic losses to pig farms. Currently, 2.1 subgenotype strains are predominantly prevalent in China. Although abundant information is available on 2.1 subgenotype isolates, limited data are available on pathogenicity analysis. In this study, a CSFV strain was isolated from a pig farm in Guangdong Province, China. Whole genome sequencing showed that the strain had a genome length of 12,296 bp, and it was named GD-2024. Based on genetic evolutionary analysis, the strain was categorized into subgenotype 2.1c, and the nucleotide and amino acid homology of the strain with the representative strains of each subgenotype was in the range of 83.1-97.6% and 90.8-99.4%, respectively. Further mutation analysis revealed that the strain had three nucleotide site mutations in the 5'UTR and 3'UTR regions and two amino acid site mutations in the E2 region. The clinical pathogenicity of this strain was investigated. Infection with GD-2024 led to persistent fever and high viremia in pigs as well as inflammatory damage in multivisceral tissues. The mortality rate of infected pigs reached as high as 60%, contradicting the currently reported virulence of 2.1 strains. In summary, we have isolated and reported a subgenotype 2.1c strain with high virulence. Its genomic variation provides a basis for further analysis of virulence determinants and serves as a clinical reference and guide for the prevention and control of CSF.