Porcine reproductive and respiratory syndrome viruses predominant in southeastern China from 2004 to 2007 were from a common source and underwent further divergence

The full-length glycoprotein 5 (GP5) gene and a partial nonstructural protein 2 (NSP2) gene fragment of 46 porcine reproductive and respiratory syndrome viruses (PRRSV) from pig farms in southeastern China between 2004 and 2007 were sequenced for phylogenetic analysis. All of the PRRSV isolates in this study were of the North American type, and the majority of them were clustered in subgroup II and had 84.1-89.1% amino acid sequence identity to those of subgroup I including the North American strain VR-2332. Three variable regions containing epitopes A and B in the N-terminal region were identified and found to be under positive selection. Several additional mutations, which were also located in the variable regions, were seen in isolates from the years 2006 and 2007 in subgroup II, as compared with those of earlier years (2004-2005) in the same group. Further analysis revealed that the majority of the subgroup II PRRSV isolates prevalent in the region since 2004 had thirteen mutation sites that distinguished them from subgroup I strains, indicating a possible introduction of a certain strain from the same source in the region or elsewhere well before 2004. A 29-aa deletion in the NSP2 fragment was found in PRRSV isolates as early as in 2005, one year earlier than the virulent PRRSV with the same deletion became dominant in China. Taken together, this study shows that subgroup II PRRSV strains with a partial deletion of nsp2 are currently prevailing in southeastern China.