Pathogenicity comparison of NADC34-like porcine reproductive and respiratory syndrome virus in 4-week-old weaned pigs versus 10-week-old growing pigs.

Porcine reproductive and respiratory syndrome virus (PRRSV) is a globally distributed and financially significant viral pathogen affecting the swine industry. Its extensive genotypic and pathotypic heterogeneity among strains makes disease control challenging. Since first reported in 2022, NADC34-like (lineage 1A, L1A) strains have become predominant in South Korea, attracting substantial attention in the domestic swine industry due to their high pathogenicity and considerable economic impact. This study aimed to explore age-dependent disease severity by comparing the pathogenicity of the Korean NADC34-like PRRSV strain GNU-2353 in 4-week-old (4WO) weaned and 10-week-old (10WO) growing pigs. The 4WO pigs exhibited most of the clinical indicators of virulence, including a high mortality rate (25 %), hyperthermia, reduced average daily weight gain (ADWG), elevated viral loads in various tissues, thymic atrophy, and extensive interstitial pneumonia. Although the 10WO challenge group showed some criteria of virulent infection, such as high fever, low ADWG, and apparent lung lesions, other indicators (viremia, nasal shedding, viral load without mortality, and thymus atrophy) were less severe than those in the 4WO challenge group. Further meta-analysis confirmed that excretion indicators were significantly higher and clinical signs tended to be more severe in 4WO pigs. GNU-2353 infection induced faster and stronger antibody responses in 10WO pigs than in 4WO pigs, while inflammatory cytokine responses were more evident in 4WO pigs. These results showed that pig age influences the outcomes of GNU-2353 infection, mirroring antibody and cytokine responses. These findings provide insights into the age-dependent pathogenesis of NADC34-like viruses for the establishment of better control measures.