The replication efficacy of NADC34-like porcine reproductive and respiratory syndrome virus 2 is not directly associated with the pathogenicity

NADC34-like porcine reproductive and respiratory syndrome virus 2 (NADC34-like PRRSV-2) is currently a major prevalent strain in Chinese swine industry. Within which, recombination events are frequently detected. Previous studies have shown that the pathogenicity of NADC34-like PRRSV-2 isolates is highly variable. However, the characteristics between NADC34-like PRRSV-2 recombinant and non-recombinant isolates are rarely compared. In this study, two PRRSV-2 strains (BJ1805-2 and SDLY23-1742) were isolated from samples collected at 2018 and 2023 in China. ORF5-based phylogenetic analysis supported that both isolates are clustered with ORF5 RFLP 1-7-4 (NADC34-like) strains. However, genome-based phylogenetic tree showed that BJ1805-2 is still grouped with NADC34-like isolates but SDLY23-1742 is clustered with NADC30-like viruses. Furthermore, fragment comparisons and recombination detections also supported that SDLY23-1742 was recombined from NADC30-like, NADC34-like, and JXA1-like isolates while no recombination event was detected in BJ1805-2. Noticeably, BJ1805-2 had higher replication efficacy than SDLY23-1742 both in PAMs and in piglets. However, SDLY23-1742 caused longer high fever period and more severe histopathological lung lesions than BJ1805-2, indicating that SDLY23-1742 has higher pathogenicity than BJ1805-2. Overall, this study provides the first evidence that the pathogenicity of NADC34-like PRRSV-2 is not directly correlated with viral replication efficacy.