Genomic surveillance and evolutionary dynamics of type 2 porcine reproductive and respiratory syndrome virus in China spanning the African swine fever outbreak.

Porcine reproductive and respiratory syndrome virus (PRRSV) poses a serious threat to the pig industry in China. Our previous study demonstrated that PRRSV persists with local circulations and overseas imports in China and has formed a relatively stable epidemic pattern. However, the sudden African swine fever (ASF) outbreak in 2018 caused serious damage to China's pig industry structure, which resulted in about 40 per cent of pigs being slaughtered. The pig yields recovered by the end of 2019. Thus, whether the ASF outbreak reframed PRRSV evolution with changes in pig populations and further posed new threats to the pig industry becomes a matter of concern. For this purpose, we conducted genomic surveillance and recombination, NSP2 polymorphism, population dynamics, and geographical spread analysis of PRRSV-2, which is dominant in China. The results showed that the prevalence of ASF had no significant effects on genetic diversities like lineage composition, recombination patterns, and NSP2 insertion and deletion patterns but was likely to lead to changes in PRRSV-2 recombination frequency. As for circulation of the two major sub-lineages of Lineage 1, there was no apparent transmission of NADC30-like among provinces, while NADC34-like had obvious signs of inter-provincial transmission and foreign importation during the ASF epidemic. In addition, two suspected vaccine recombinant epidemic strains suggest a slight safety issue of vaccine use. Herein, the interference of ASF to the PRRSV-2 evolutionary pattern was evaluated and vaccine safety was analyzed, in order to monitor the potential threat of PRRSV-2 to China's pig industry in the post-epidemic era of ASF.