In Vitro Characterization of Vaccine Strain-like Porcine Reproductive and Respiratory Syndrome Virus Strains Isolated from Weaned Pigs Exhibiting Respiratory Symptoms.

Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes significant economic losses in swine production. In May 2021 and March 2023, we detected PRRSV genes in serum samples from two weaned pigs with respiratory disorders on a farm in Japan. Partial gene sequences of these strains closely resembled those of a PRRS vaccine strain. We subsequently isolated two PRRSV field strains, KU-IG21-1 and KU-IG23-1, from the 2021 and 2023 samples, respectively. The KU-IG21-1 strain exhibited more pronounced cytopathic effects and significantly higher replication efficiency in cultured cells compared to both the vaccine and KU-IG23-1 strains. Despite these phenotypic differences, complete genome sequencing revealed high genetic similarity between the field isolates and the vaccine strain, with only 16 and 24 amino acid differences in the KU-IG21-1 and KU-IG23-1 strains, respectively. These findings suggest that the field strains likely emerged through the accumulation of point mutations in the vaccine strain rather than through homologous recombination. Furthermore, we identified three amino acid substitutions that may contribute to the enhanced replication of the KU-IG21-1 strain. This study underscores the potential impact of point mutations on PRRSV phenotypes and provides new insights into the complex evolutionary dynamics of PRRSV.