Genetic evolution and alterations in pathogenicity of highly pathogenic porcine reproductive and respiratory syndrome virus.

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) and HP-PRRSV variant strains remain prevalent in China and Southeast Asian countries. However, their epidemiological aspects, genomic characteristics, relationships with vaccine strains, and changes in pathogenicity remain unclear. In this study, 2509 global L8.7 ORF5 sequences were utilized for analysis, we classified L8.7 into 7 groups (L8.7.1-L8.7.7). L8.7.1-L8.7.3 strains corresponded to previously reported classical PRRSVs, intermediate strains, and HP-PRRSVs, respectively, whereas L8.7.4-L8.7.7 were designated HP-like PRRSVs. Statistical analysis revealed that HP-like PRRSVs were the most prevalent among L8.7 strains, and L8.7.5 and L8.7.6 strains accounted for the highest proportions in recent years. A comprehensive analysis of the genome revealed that the majority (72.15%) of L8.7 strains presented a wild-type phenotype. Evolution rate analysis revealed that the evolution rate of L8.7.3-L8.7.7 PRRSV in China was reduced by about 4.1 times after the use of HP-PRRSV MLV. Pathogenicity test results indicate that in comparison with HP-PRRSV (L8.7.3: HuN4), the HP-like PRRSV strains (L8.7.5: DLF; L8.7.6: DLW) presented reduced pathogenicity in piglets while maintaining relatively high virulence. In summary, this study systematically elucidated the epidemiological dynamics, evolutionary trends, relationships with vaccine strains, and changes in pathogenicity associated with L8.7 strains, providing crucial data to support prevention and control strategies against L8.7 PRRSV.