Accelerated evolution of PRRSV during recent outbreaks in China.

Porcine reproductive and respiratory syndrome virus (PRRSV) is reported to have evolved at a higher evolutionary rate than other RNA viruses. However, whether this virus is capable of evolutionary acceleration during outbreaks remains unknown. In this study, we analyze the data based on ORFs of eight newly obtained epidemic PRRSVs from Hebei province with other viral genomes from GenBank. Phylogenetic analysis suggested that all isolates during recent outbreaks (2006-2008) are grouped together. We also find that ORF5 genes of this viral group are positively selected, suggesting their higher evolutionary rates and coinciding with that period of large-scale outbreaks in China. The evolutionary rate of 3.29 x 10(-3) substitutions per nucleotide site per year also suggests the higher evolutionary rate of these viruses. We concluded that PRRSVs isolated during 2006-2008 in China underwent accelerated evolution, and predicted that this accelerated evolution equip these viruses more adapted to their primary hosts.