First Detection of NADC34-like PRRSV as a Main Epidemic Strain on a Large Farm in China

Abstract

The 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 (ASFV), classical swine fever virus (CSFV), pseudorabies virus (PRV), and porcine circovirus 2 (PCV2)-on a fattening pig farm with strict biosecurity practices located in Heilongjiang Province, China, from 2020 to 2021. The results showed that multiple types of PRRSV coexisted on a single pig farm. NADC30-like and NADC34-like PRRSVs were the predominant strains on this pig farm. Importantly, NADC34-like PRRSV-detected during the period of peak mortality-was one of the predominant strains on this pig farm. 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 length polymorphism (RFLP) pattern 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 non-recombinant viruses. This study reports the prevalence and characteristics of NADC34-like PRRSVs on a large-scale breeding farm in northern China for the first time. These results will help to reveal the impact of NADC34-like PRRSVs on Chinese pig farms, and provide a reference for the detection and further prevention and control of NADC34-like PRRSVs.

Keywords: 100 aa deletion; NADC34-like PRRSV; RFLP; main epidemic strains.

Figure 1

Statistics on the number of dead pigs and subtypes of viruses: (a) Statistics on the numbers of dead pigs and viruses detected. The number of dead pigs and the number of viruses detected are represented by different colors. (b) Distribution of PRRSV subtypes at each stage. Sublineage 1.5 is represented by dark green; sublineage 1.8 is represented by bright purple; sublineage 3.5 is represented by sky blue; and sublineage 8.7 is represented by red (HP-PRRSV-like) and black (CH-1a-like).

Figure 2

Alignment of the deduced amino acid sequence based on the NSP2 gene. ATCC-VR2332 is the reference. NADC30-like PRRSVs were identified as a 111 aa deletion at positions 323–433, a 1 aa deletion at position 481, and a 19 aa deletion at positions 533–551. All of the NADC34-like PRRSVs had 100 aa deletions corresponding to positions 328–427 (dark green regions). All of the HP-like PRRSVs had a 1 aa deletion at 482 and 29 aa deletions at 533–561.

Figure 3

Phylogenetic analysis of NADC34-like PRRSV: (a) Phylogenetic analysis of PRRSVs based on the ORF5 gene. The percentage of positives among different strains is represented by a pie chart. (b) Phylogenetic tree of PRRSVs based on full-length genomes. The American 1-4-4 PRRSV strains are labeled with blue stars; corresponding types of PRRSV vaccine currently on sale in China are labeled with dark blue stars.