Introduction
Porcine reproductive and respiratory syndrome virus (PRRSV) is one of
the most important pathogens affecting swine globally (Ramirez et al.,
2019). PRRSV belongs to the genus Betaarterivirus, family Arteriviridae,
and order Nidovirales (Dokland, 2010). PRRSV is currently classified
into two distinct species, Betaarterivirus suid 1 (PRRSV-1) and
Betaarterivirus suid 2 (PRRSV-2) (Brinton et al., 2021). Both PRRSVs
have been circulating in China for decades and have caused substantial
economic losses among the Chinese pig industry. PRRSV-2 has been the
main epidemic strain in China, and is further classified into 9 lineages
based on open reading frame 5 (ORF5) sequences (Shi et al., 2010). Four
lineages (lineages 1, 3, 5 and 8) of PRRSV-2 circulate in the field (Guo
et al., 2018). In recent years, the dominant lineages of PRRSV-2 have
shifted from sublineage 8.7 (CH-1a-like and HP-PRRSV-like) to sublineage
1.8 (NADC30-like).
In 2014, the ORF5 RFLP 1-7-4 lineage was prevalent in the United States
(U.S.) and was recognized to be the cause of dramatic abortion
“storms” in sow herds and high mortality among piglets (Alkhamis et
al., 2016; van Geelen et al., 2018). Subsequently, that sublineage was
confirmed to be endemic in Peru (Ramirez et al., 2019). In China,
NADC34-like PRRSV was first detected in Liaoning Province in 2017 (Zhang
et al., 2018). Subsequently, the presence of such strains was also
detected in southern China (Liu et al., 2019). In 2020 and 2021, the
detection of sublineage 1.5 (NADC34-like) and research on its spread
were successively reported (Bao and Li, 2021; Xie et al., 2020b; Xu et
al., 2020). Unlike American NADC34-like PRRSVs, Chinese NADC34-like
PRRSVs are currently mildly or moderately pathogenic in piglets (Song et
al., 2020; Xie et al., 2020a). However, the prevalence and impact of
NADC34-like PRRSV in Chinese pig farms is unknown. During the
epidemiological investigation of seven pig farms in five provinces in
China (Heilongjiang, Xinjiang, Hebei, Henan, and Hubei), we found that
NADC34-like PRRSV was one of the main endemic strains in a large pig
farm in Heilongjiang Province. In this study, we monitored PRRSVs on
this farm for 150 days and conducted a detailed study on the epidemic
process and molecular characteristics of NADC34-like PRRSV.