Pathogenicity of the rescued porcine circovirus type 3 strain in conventional weaned pigs

Abstract Porcine circovirus type 3 (PCV3) has been increasingly detected in swine, but its pathogenic profile in conventional weaned pigs remains incompletely defined. In this study, seven-week-old, PCV-negative pigs were random grouped and mock or infected with PCV3 to characterize clinical signs, viremia kinetics, tissue distribution, and lesions. No significant rectal temperature difference between mock and infection pigs were observed throughout 35 days post-infection (dpi) ( P > 0.05). Average body weight increased in both groups and relative average daily gain difference was not significant ( P > 0.05). qPCR detected the highest serum PCV3 DNA level of approximately 10 7.7 copies/mL at 1 dpi that declined to 10 6.0 copies/mL by 35 dpi, yet remained detectable throughout experiment. At 35 dpi, pulmonary hilar lymph nodes carried the 10 9.2 copies/g virus, followed by lungs and spleens, while other lymphoid tissues and parenchymal organs range within 10 6.5 –10 8.5 copies/g. Spleen and inguinal lymph node indices did not differ between groups ( P > 0.05). Histopathology revealed interstitial pneumonia characterized by widened alveolar septa and bronchial mucosal desquamation, focal myofiber disruption with mild cardiomyocyte necrosis, and mild hepatocellular vacuolar change. Immunohistochemistry detected brown granular cytoplasmic PCV3 antigen in lymphocytes and macrophage-like cells within lymph nodes and spleen. These findings indicate that PCV3 causes a lymphotropic, afebrile infection in conventional weaned pigs, which will support the development of commercial vaccine against PCV3.