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.