Evolutionary analysis of six isolates of porcine reproductive and respiratory syndrome virus from a single pig farm: MLV-evolved and recombinant viruses.

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important pathogens with economic significance for swine industry globally. The virus is characterized by its genetically extraordinary variability and diversified strains due to high mutation frequency and recombination rates. In the current study, we analyzed the genomes of six PRRSV strains isolated in different years from a single pig farm practicing HP-PRRSV-derived modified-live virus (MLV) vaccination in China, and in detail dissected their evolutionary relationship to MLV vaccines. The results show that all the six isolates belong to type 2 PRRSV, and are clustered into two lineages (lineage 1 and 8) based on their ORF5 genes. Further comparative analyses of genomes and the characteristic amino acid sites show that both HeN1201 and HeN1502 are MLV-like strains evolved from the MLV HuN4-F112. Recombination analyses reveal that HeN1301 is a recombinant virus emerged from the recombination event between two MLV-like strains evolved from the MLV TJM-F92 and HuN4-F112, and HeN1501 is a recombinant virus between two MLV-like strains evolved from HuN4-F112, while HeN1401 and HeN1601 are recombinant viruses derived from the recombination between NADC30-like and a MLV-like strain evolved from TJM-F92. Our findings provide more evidence for the fact that MLV-derived from the Chinese HP-PRRSV can be evolved and reversed in the field and enrich the data about the recombination between the viruses evolved from MLV and newly infected strain of PRRSV. This study stresses the necessity for pig producers to restrain the use of HP-PRRSV MLV vaccines in the field.