Evaluation of pathogenicity of a Korean NADC34-like Porcine reproductive and respiratory syndrome virus strain in pregnant gilts

Porcine reproductive and respiratory syndrome virus (PRRSV) remains a major cause of reproductive failure in swine, with emerging NADC34-like strains drawing increasing attention due to their enhanced virulence and association with late-term abortion. In this study, we evaluated the pathogenicity of a Korean NADC34-like isolate, SNUVP231106, in eight PRRSV-free pregnant gilts. Infected animals exhibited fever, anorexia, lethargy, and one case requiring euthanasia, despite the absence of prominent respiratory signs. Serum TNF-α and IFN-α levels were significantly elevated compared with the control group. All four inoculated gilts delivered prematurely at gestation days 110–112, and all 51 fetuses were stillborn, including autolytic, decomposed, and mummified forms. Real-time qPCR confirmed viremia between dpi 4–7 and seroconversion by dpi 7. Notably, PRRSV RNA was detected in 67 % of fetal serum and 95 % of thymus samples, with higher viral loads in the thymus. Although histopathological changes were not apparent, PRRSV antigens were detected in thymic macrophages via IHC. The Korean NADC34-like strain SNUVP231106 was shown in this study to cause severe systemic illness and reproductive failure in pregnant gilts. These findings will contribute to the development of diagnostic and preventive strategies for NADC34-like strains, and potentially for other highly virulent PRRSV variants.