First detection of NADC34-like PRRSV as a main epidemic strain on a large farm in Heilongjiang Province, China

Newly emerged sublineage 1.5 (NADC34-like) porcine reproductive and respiratory syndrome virus (PRRSV) has posed a direct threat to the Chinese pig industry since 2018. However, the prevalence and impact of NADC34-like PRRSV on Chinese pig farms is unclear. In the present study, we continuously monitored pathogens, including PRRSV, African swine fever virus (AFSV), classical swine fever (CSFV), pseudorabies virus (PRV) and porcine circovirus 2 (PCV2), on seven fattening pig farms with strict biosecurity practices located in five provinces of China from 2020 to 2021. The results showed that multiple types of PRRSVs commonly coexisted on a single pig farm. NADC30-like PRRSV was the predominant strain on most pig farms. Importantly, NADC34-like PRRSV, detected during the period of peak mortality, was one of the predominant strains on one pig farm in northern China. Sequence alignment suggested that these strains shared the same 100-aa deletion in the Nsp2 protein as IA/2014/NADC34 isolated from the United States (U.S.) in 2014. Phylogenetic analysis based on open reading frame 5 (ORF5) showed that the genetic diversity of NADC34-like PRRSV on this farm was relatively singular, but it had a relatively high rate of evolution. Restriction fragment polymorphism pattern (RFLP) analysis showed that almost all ORF5 RFLPs were 1-7-4, with one 1-4-4. In addition, two complete genomes of NADC34-like PRRSVs were sequenced. Recombination analysis and sequence alignment demonstrated that both viruses, with 98.9% nucleotide similarity, were nonrecombinant viruses. This study reports the prevalence and characteristics of NADC34-like PRRSV on a large-scale breeding farm in northern China for the first time. These results will help reveal the impact of NADC34-like PRRSV on Chinese pig farms and provide a reference for detection and further prevention and control of NADC34-like PRRSV.