Evaluation of the Pathogenicity of a Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Variant in Piglets

Since May 2006, a highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) variant characterized by 30 amino acids deletion within its NSP2-coding region emerged and caused extensive economic losses to China's pig industry. To investigate the in vivo pathogenicity and immune responses of the newly emerging PRRSV, 3 groups of 60-d-old conventional piglets were inoculated intranasally with a representative strain of the HP-PRRSV variant HuN4 with 3 different infection doses (3x10(3)-3x10(5) TCID(50)). The results revealed that the virus variant caused severe disease in piglets and the significant clinical characteristics consisted of persistently high fever (41.0-41.9 degrees C) and high morbidity and mortality (60-100%), the marked clinical signs of PRRS and severe histopathogenic damages in multiple organs. It induced rapid and intense humoral immune responses and seroconversion was detected in most infected pigs at 7 d post-infection (DPI). The virus vigorously replicated in vivo and the highest virus average titer was 9.7 log copies mL(-1) serum at 7 DPI. Elevated levels of IFN-gamma and IL-10 cytokine production in serum in this study were also observed. Taken together, our results demonstrated that the HP-PRRSV variant HuN4 strain is highly pathogenic for piglets and suitable to be a reference strain of highly virulent PRRSV for evaluating the efficacy of the new vaccines.