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.