Characterization and Pathogenicity of a Porcine Reproductive and Respiratory Syndrome Virus Strain with Strong Homology to a HP-PRRSV Vaccine Strain in the Field.

A PRRSV strain, PRRSV2/CN/FJLX06/2021, with strong homology to an MLV-like strain HeN1201 that evolved from the highly pathogenic PRRSV vaccine virus HuN4-F112, was isolated from a dying piglet in China. BLAST and phylogenetic analyses showed that PRRSV2/CN/FJLX06-2021 was most closely related to HeN1201 and HuN4 (the parental strain of MLV HuN4-F112) and clustered with Chinese HP-PRRSV strains in PRRSV-2 lineage 8.7. Importantly, 29 of the 39 characteristic amino acid mutations in the HuN4-F112 genome were found at the corresponding sites of PRRSV2/CN/FJLX06-2021. Animal studies showed that piglets infected with PRRSV2/CN/FJLX06-2021 had a persistent high fever, higher viremia, presence of interstitial pneumonia, and a higher mortality rate (40%) within 2 weeks than those vaccine-inoculated with HuN4-F112. Taken together, these data suggest that PRRSV2/CN/FJLX06-2021 is an MLV-like strain that has evolved from MLV HuN4-F112 and is highly pathogenic to piglets.