Dynamics and evolution of highly pathogenic porcine reproductive and respiratory syndrome virus following its introduction into a herd concurrently infected with both types 1 and 2

- An HP-PRRSV outbreak occurred in a herd con-currently infected with both types of PRRSV.
- The evolution of HP-PRRSV was investigated following an outbreak.
- HP-PRRSV influenced the evolution of type 2 isolates by increasing diversity.
- HP-PRRSV increasing diversity through the addition of a novel cluster.

Since its first emergence in Thailand in late 2010, highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) has caused sporadic outbreaks on Thai swine farms. The objective of this study was to investigate the dynamics and evolution of PRRSV in a herd experiencing an HP-PRRSV outbreak. Following its introduction, HP-PRRSV caused severe outbreaks and subsequently established persistent infection in the herd, resulting in the emergence of a novel cluster of type 2 (North American, NA) isolates. HP-PRRSV co-existed with type 1 (European, EU) isolates without influencing their development. In contrast, HP-PRRSV influenced the evolution of the type 2 (NA) isolates by increasing diversity through the addition of a novel cluster and influencing the evolution of other viral clusters previously existing in the herd. Recombination between the endemic and emerging isolates was observed. The recombinants, however, disappeared and were not able to survive in the herd. The results of this study suggest that the introduction of HP-PRRSV to a herd results in an increased diversity of genetically related isolates and persistent HP-PRRSV infection.