Emergence of novel recombination lineage 3 of porcine reproductive and respiratory syndrome viruses in Southern China.

Lineage 3 of porcine reproductive and respiratory syndrome viruses, which belong to North America type 2, has a long epidemic history in China. The novel lineage 3 viruses constantly emerging in recent years are characterized by a high detection rate and significant pathogenicity. In this study, we investigated the prevalence of lineage 3 in southern China and selected two isolated strains for genome and virulence analyses. A cross-sectional epidemiology investigation indicated that the prevalence of lineage 3 antigens was 35.68% (95% CI: 27.6-44.3%) among 227 samples collected from over 100 infected farms from January 2016 to July 2017 in southern China. Two novel isolates of lineage 3 were selected. After 20 passages, Marc-145 cells were not susceptible to those viruses. Full-length genome analysis indicated that the two strains share 95.2% homology with each other and 95.7%-96.2% with highly pathogenic porcine reproductive and respiratory syndrome viruses (HP-PRRSVs; JXA1-like strain, lineage 8.7). Phylogenetic and molecular evolutionary results showed that for the two isolates, HP-PRRSV provides most of the ORF1 gene. Animal experiment revealed discrepancies in virulence between the strains. Although challenge resulted in 100% morbidity, the isolate carrying most of the HP-PRRSV ORF1 caused severe clinical symptoms and 40% mortality, whereas the other isolate containing part of the ORF1 gene caused no mortality. Overall, these findings suggest that lineage 3 viruses might be commonly circulating in most of southern China. Frequent recombination events within HP-PRRSVs of this lineage with changing virulence could represent potential threats to the pig industry.