Nationwide emergence and spread of highly virulent PRRSV-2 mutants in Korea.

Porcine reproductive and respiratory syndrome virus type 2 (PRRSV-2) continues to pose a major threat to global swine health. In South Korea, the co-circulation of endemic strains and repeated introductions of foreign variants has led to a complex viral landscape. This study analyzed 907 non-redundant ORF5 sequences from Korean field isolates (2018–2024), derived from 1,305 compiled sequences after removing near-duplicates at 99.9% identity using CD-HIT, together with 996 global reference ORF5 sequences, to refine phylogenetic classification and trace recent epidemiological patterns. A previously undefined Korean-specific clade (formerly referred to as Lineage KOR B or LKB) was proposed as Lineage 12 (L12), based on > 15% nucleotide divergence from existing lineages. In addition, we identified localized diversification within Lineage 1 sublineages (L1A and L1C), suggesting regional adaptation after foreign introduction. The most striking observation was the rapid emergence and spread of NADC34-like PRRSV (L1A). First detected in 2022, L1A rose from 11.7% to 23.3% of relative positive rates by 2024 and disseminated across major swine-producing provinces. A representative case in 2023 was associated with severe reproductive failure including severe abortion and high mortality in newborn piglets. Notably, the outbreak also demonstrated unusually high sow mortality (~ 10%), indicating high virulence of Korean NADC34-like PRRSV strains. Postmortem examinations from other outbreaks revealed consistent systemic lesions in deceased sow or gilts, indicating a reproducible pathological signature of Korean NADC34-like strains in old-aged pigs. These findings highlight the evolving virulence of PRRSV-2 in Korea and underscore the urgent need for updated lineage surveillance, strain-specific vaccines, and enhanced transboundary disease monitoring frameworks.