Isolation, Genomic Characterization and Pathogenicity of a European-Like PRRSV-1 Strain in Newborn Piglets from Southwestern China.

Porcine reproductive and respiratory syndrome virus genotype 1 (PRRSV-1), particularly the BJEU06-1-like subgroup, has shown increasing detection in China; however, the biological characteristics of newly emerging strains in southwestern regions remain insufficiently defined. Therefore, this study aimed to isolate and characterize a PRRSV-1 strain circulating in Southwestern China and to compare its biological properties and pathogenicity with those of a representative NADC30-like PRRSV-2 strain. In this study, a PRRSV-1 field strain (CDAC-SC2025) was isolated from a lung sample collected in Sichuan Province and characterized by immunofluorescence, full-genome sequencing, and maximum-likelihood phylogenetic analysis. Recombination was assessed using RDP4 and SimPlot (Baltimore, MD, USA). Pathogenicity was evaluated in newborn piglets following intranasal challenge, with monitoring of clinical signs, viremia, viral shedding, tissue viral loads, and histopathology. CDAC-SC2025 clustered within the BJEU06-1-like subgroup and showed the closest relationship to HENZMD-10 without detectable recombination. A three-amino-acid deletion (373-375 aa) was identified in nsp2. In vivo, CDAC-SC2025 induced fever, respiratory signs, growth retardation, and mortality, but the onset of death was delayed and lesion severity was lower than those caused by the NADC30-like strain DJY. Both strains exhibited predominant viral loads in the lung and tonsils, although quantitative differences were observed across tissues. Histopathology revealed moderate lesions in CDAC-SC2025-infected piglets compared with more severe multisystemic damage caused by DJY. These findings provide updated data on the biological properties of BJEU06-1-like PRRSV-1 circulating in southwestern China.