Abstract: In order to analyze the molecular genetic evolution of Porcine reproductive and respiratory syndrome virus（PRRSV）in Tianjin from 2019 to 2022, ORF5 sequencing was performed on twenty PRRSV positive samples to carry out genetic variation research and phylogenetic analysis. The results showed that eleven of the twenty strains belonged to lineage 1.8, three strains belonged to lineage 1.5, five strains belonged to lineage 8, and one strain belonged to lineage 5, indicating that the prevalence of PRRSV strains showed diversity. Subsequently, the ORF5 gene and GP5 protein homology and amino acid sites were analyzed, and the comparison results with the reference strains of each lineage showed that the nucleotide homology between the 1.8 strain of the eleven lineage and the NADC30 strain was 91.2%~93.0%, and the amino acid homology was 89.6%~93.5%. The nucleotide homology between the three 1.5 lineage strains and the NADC34 strain was 93.4%~96.5%, and the amino acid homology was 92.5%~95.5%. The four isolates had the highest homology with HP-PRRSV strains, with nucleotide homology of 90.0%~95.5% and amino acid homology of 86.6%~91.0%. One isolate had the highest homology to the classical strain CH-1a, with nucleotide and amino acid homology of 90.0% and 89.1%, respectively, and the nucleotide and amino acid homology of one lineage 5 strain and VR2332 strain were 97.8% and 96.0%, respectively. Some key mutations were found and were likely to cause differences in viral virulence and lead to viral escape. This study may provide data supporting for the prevention and control of PRRS in Tianjin.

Key words: PRRSV, ORF5, genetic evolution