Genetic recombination and pathogenicity assessment of porcine reproductive and respiratory syndrome virus 2 strains in China

Porcine reproductive and respiratory syndrome virus type 2 (PRRSV-2) has been endemic in China for more than three decades; however, comprehensive epidemiological investigations under field conditions remain limited in recent years. To address this gap, a total of 21,413 clinical samples were collected from 6,974 pig farms across 31 provinces in China between 2023 and 2024 to investigate the epidemiological dynamics of PRRSV-2. Phylogenetic analysis of 1,528 ORF5 sequences identified five major circulating lineages: lineage 1.8 (NADC30-like), lineage 1.5 (NADC34-like), lineage 3.5 (QYYZ-like), lineage 5.1 (VR2332-like), and lineage 8.7 (HP-PRRSV). Among these, lineage 1.8 predominated in both 2023 (47.29%) and 2024 (49.08%). To further characterize the genomic features and pathogenicity of the dominant clinical strains, five recombinant lineage 1.8 PRRSV isolates harboring a characteristic discontinuous 131-amino-acid deletion in nsp2 were successfully isolated. Pathogenicity assessments in piglets revealed distinct differences in virulence among three representative isolates, as reflected by elevated rectal temperatures, reduced average daily weight gain, and high levels of viremia and viral loads in serum, nasal swabs, spleen, lungs, and lymph nodes. Notably, strains with a JXA1-like major parental backbone and an NADC30-like minor parental contribution exhibited higher pathogenicity than those with an NADC30-like major parent and a WUH4-like minor parent. Collectively, this study demonstrates that PRRSV-2 circulation in China is characterized by pronounced seasonality, extensive genetic recombination, and the dominance of genetically diverse lineage 1.8 strains, including recombinants, providing critical insights for targeted surveillance and effective control strategies.