Evaluation of Reversion to Virulence on a Modified Live Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Vaccine Strain in Pigs

Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus (HP-PRRSV) has spread most provinces of China. An attenuated vaccine strain, TJM-F92, was obtained by passaging HP-PRRSV strain TJ on Marc-145 cells for 92 passages and showed a continuous 120 amino acids deletion in nsp2 gene. The purpose of this study was to evaluate the virulence of TJM-F92 vaccine strain by serial passages in pigs and to analyze the genetic changes of the nsp2 region.TJM-F92 vaccine virus was passaged continuously five times in pigs. At the end of each passage, pigs were euthanized and necropsied, and tissue samples were collected and examined. The lungs were harvested for histopathological examination. The Nsp2 gene of isolates from the fifth passage (P5) was sequenced and compared to the Nsp2 gene sequences of TJM-F92 vaccine virus and its parent strain TJ. All pigs showed no clinical symptoms related to PRRS each passage. The viremia in pigs from P1 to P5 showed no significantly difference. No gross-lung lesions were observed in pigs from P1 to P5. Sequence analysis showed the continuous 120 aa deletion in nsp2 in TJM-F92 vaccine virus existed persistently in vivo. In conclusion, the TJM-F92 stain did not convert its pathogenicity to pigs after continuously passaging in pigs.