Repeated Cross-Sectional Surveillance and ORF5-Based Molecular Epidemiology of Porcine Reproductive and Respiratory Syndrome Virus in Anhui Province, China, 2019–2024

Porcine reproductive and respiratory syndrome virus (PRRSV) remains a major threat to swine production, and its circulation after the African swine fever outbreak requires continued surveillance. This study investigated the temporal, regional, and genetic characteristics of PRRSV in Anhui Province from September 2019 to November 2024. Ten rounds of repeated cross-sectional surveillance were conducted at 147 slaughterhouses and 21 rendering plants. Tissue samples were collected by random, cluster, or risk-based sampling, pooled in groups of five, and tested by RT-qPCR. Representative positive samples with Ct values < 25 underwent ORF5 amplification, Sanger sequencing, and phylogenetic analysis. A total of 994 site visits yielded 18,733 tissue samples. No positive samples were detected in autumn 2020 or spring 2021, whereas PRRSV was detected again from autumn 2021 and subsequently fluctuated. The highest site-level positivity was 41.18% in autumn 2023, and the highest estimated individual-level positivity was 4.66% in spring 2023. Regional differences were statistically significant, with the highest site-level positivity in northern Anhui, and pooled-sample positivity was higher in rendering plants than in slaughterhouses. All 24 ORF5 sequences belonged to PRRSV-2 and mainly clustered with NADC30-like, NADC34-like, or MLV/classical strains. These findings demonstrate temporal fluctuations and lineage coexistence, supporting continued multisource surveillance and broader genomic and antigenic evaluation.