Molecular Epidemiology, Phylogeography, and Recombination Dynamics of PRRSV-2 Sublineage L1C in Mainland China

Since NADC30-like PRRSV-2 was first detected in Henan Province in 2012, it has continued to spread and has become one of the predominant PRRSV-2 groups in mainland China. However, the long-term spatiotemporal dynamics, interprovincial transmission patterns, transmission drivers and recombination associated evolutionary features of sublineage L1C (L1C; NADC30-like) remain incompletely resolved. Here, we analysed 9541 quality-controlled lineage 1 ORF5 sequences from 12 countries, including ORF5 sequences from 62 laboratory-derived complete genomes assigned to L1C. Globally, 3787 sequences were classified as L1C. Among the 1648 lineage 1 sequences from China, 1362 were assigned to L1C, accounting for 82.65% of Chinese lineage 1 sequences. Phylodynamic analysis dated the global common ancestor of L1C to around 2002 and suggested that strains circulating in mainland China were likely introduced from US-related strains around 2008. Before the African swine fever (ASF) outbreak, inferred interprovincial transmission links were concentrated in a limited number of key provinces. During the early ASF period, observable links decreased, but they subsequently recovered and expanded across more provinces. Transmission-driver analysis suggested that pig inventory and pig output were associated with stronger inferred interprovincial L1C transmission links, whereas geographic distance was associated with a spatial-decay effect. Whole-genome recombination analysis revealed extensive recombination signals in L1C genomes involving other PRRSV-2 lineages. Among inter-lineage associations, L8E (HP-PRRSV/JXA1-like) was the most frequently implicated background, followed by L5 and L3. These findings provide systematic evidence for the persistent prevalence, regional transmission and recombination-driven evolution of L1C in mainland China, and support molecular surveillance, regional risk warning and optimization of PRRSV-2 control strategies.