Molecular epidemiological characteristics and pathogenicity of Chinese PRRSV-2 L1B sublineage strains in piglets.

PRRSV evolves through mutation and recombination during circulation, contributing to the emergence and spread of genetically diverse lineages. Among these, the PRRSV-2 L1B sublineage has been detected in multiple countries, but its molecular epidemiology and pathogenicity remain poorly characterized. We investigated the molecular epidemiology of L1B PRRSV in China and assessed its pathogenicity in piglets. L1B strains were detected in multiple Chinese provinces. Phylogenetic analysis based on the ORF5 sequences revealed that all Chinese L1B PRRSV strains cluster within the L1B sublineage of PRRSV-2. These strains harbor a 131-amino-acid discontinuous deletion (111+1+19) in the Nsp2 protein and carry cluster-associated amino acid substitutions at positions 137 and 178 of the GP5 protein. Whole-genome analysis revealed that Chinese L1B strains possess complex genetic backgrounds, likely arising from recombination between indigenous circulating strains (NADC30-like and HP-PRRSV) and the introduced L1B strains. Animal challenge experiments demonstrated that inoculation with the representative isolate TZJ2547 induced clinical symptoms including fever, coughing, and decreased feed intake, accompanied by persistent viremia, high viral RNA loads in multiple tissues, and pathological lesions in the lungs and lymphoid tissues. These findings indicate that TZJ2547 exhibits moderate pathogenicity in the challenged piglets. This study characterizes the epidemiological distribution, genomic characteristics, and recombination profile of L1B PRRSV in China. To the best of our knowledge, this study reports the first systematic pathogenicity evaluation of a Chinese L1B PRRSV isolate in piglets. Collectively, these findings provide fundamental data to inform the surveillance and control of this viral sublineage.