Abstract:

This study aims to understand the latest infection and genetic evolution of porcine circovirus type 2 (PCV2) in various regions of Hunan Province. Using qPCR methods established in the laboratory, this study conducted viral nucleic acid detection on 1 244 samples of lymph nodes, swabs, brain tissue, and blood collected from 14 prefecture-level cities in Hunan Province, including Changsha, Yueyang, Huaihua, Changde, and Zhuzhou, from 2021 to 2023. PCV2-positive samples were then screened out, and the ORF2 gene sequences of PCV2 were analyzed through PCR amplification and sequencing. The results showed that 778 of the 1 244 samples were PCV2-positive, with an overall positive rate of 62.54%. Among them, Shaoyang City had the highest PCV2 positive rate of 92.55% (87/94), while Chenzhou City had the lowest positive rate of 18.28% (17/93). Sequencing analysis of the ORF2 gene of 57 PCV2 strains revealed that the proportions of PCV2a, PCV2b, and PCV2d were 12.28%, 7.02%, and 80.70%, respectively. The nucleotide and amino acid sequence homologies of the ORF2 gene among the sequenced strains ranged from 88.5% to 100.0% and 86.3% to 100.0%, respectively. There were 45 amino acid variant sites among 57 PCV2 strains, and 15 variant amino acid sites among PCV2d strains. Selection pressure analysis indicated that there were three positive selection sites in PCV2. This study suggests that PCV2 is currently mutating rapidly, with significant amino acid site differences among various strains. Consistent with previous reports, PCV2d has become the absolutely dominant epidemic strain in this study. These variations of PCV2 may weaken the protective effect of vaccines, which deserves attention.

Key words: porcine circovirus type 2 (PCV2), molecular epidemiological investigation, genetic variation, ORF2 gene, selection pressure