Introduction
Porcine reproductive and respiratory syndrome (PRRS) is a major disease causing significant economic losses in the global swine industry. The causative agent, porcine reproductive and respiratory syndrome virus (PRRSV), is an enveloped, positive-sense, single-stranded RNA virus belonging to the family Arteriviridae of the order Nidovirales (Lunney et al., 2016). The PRRSV genome is approximately 15 kb in length, encoding at least 10 open reading frames (ORFs) (Firth et al., 2011). PRRSV can be divided into type 1 (PRRSV-1) and type 2 (PRRSV-2) based on genetic and antigenic differences (Murtaugh et al., 2010). Recently, PRRSV-1 was classified as Betaarterivirus suid 1 , and PRRSV-2 was designatedBetaarterivirus suid 2 (ICTV2021). In China, PRRSV-2 strains have been circulating and predominate on farms since their initial emergence in 1996. Based on the global PRRSV classification system and ORF5 sequence, PRRSV-2 is divided into nine lineages (1-9) with several sublineages in each (Shi et al., 2010a; Shi et al., 2010b).
Before 2020, an overwhelming majority of PRRSV-2 strains in China could be divided into four lineages: sublineage 8.7 (JXA1-like and CH-1a-like), sublineage 5.1 (VR-2332-like), sublineage 3.5 (QYYZ-like) and sublineage 1.8 (NADC30-like) (Gao et al., 2017). However, in 2019, NADC34-like PPRSV was reported in China to be a potential pandemic strain (Xu et al., 2020). This kind of strain was initially reported in the United States in 2014, with dramatic abortion storms in sows and high mortality rates in piglets (van Geelen et al., 2018). This strain became endemic in Peru in 2015-2017 and had a severe impact on the local swine industry (Ramirez et al., 2019). The first NADC34-like PRRSV in China was reported in Liaoning Province in 2018 (Zhang et al., 2018); it carried a genetic marker of a 100-aa deletion corresponding to position 328-427 of the Nsp2 gene of VR2332 (Bao and Li, 2021). Since then, several studies have demonstrated spread of NADC34-like PRRSV in at least four different regions of China (Heilongjiang, Liaoning, Henan and Fujian) (Liu et al., 2019; Xie et al., 2020a; Xu et al., 2020; Zhang et al., 2018). The pathogenicity of different NADC34-like strains varies greatly. The American strain IA/2014/NADC34 exhibits high pathogenicity (van Geelen et al., 2018); in contrast, the Chinese strains PRRSV-ZDXYL-China-2018-1 and HLJDZD32-1901 have moderate and low pathogenicity, respectively (Song et al., 2020; Xie et al., 2020b).
Our lab has been monitoring PRRSV for a long time, and the detection rate of NADC34-like strains increased significantly in 2020 and 2021. On farms with cases, severe miscarriage and other clinical symptoms typically occur. To further study the complete genome characteristics of NADC34-like PRRSV in China, we selected fifteen NADC34-like strains based on large homology differences and different branches of the phylogenetic tree based on whole-genome sequencing. The prevalence and molecular characteristics of NADC34 PRRSV in China were analyzed in detail.