company news about A Sichuan team discovered a novel recombinant PRRSV highly pathogenic virus strain: Recombination of vaccine and wild-ty

On July 2, 2025, a team led by Zhang Zhidong and Zhou Long from the College of Animal Science and Veterinary Medicine at Southwest University for Nationalities, in collaboration with the Sichuan Academy of Animal Sciences, published their latest research in the international journal *Frontiers in Veterinary Science*, reporting a novel porcine reproductive and respiratory syndrome virus (PRRSV) strain, named GX2024, resulting from a hybridization of a wild-type virus and a modified live virus vaccine (MLV). This strain was isolated from a pig farm in Sichuan Province, China, and exhibited extremely high pathogenicity (100% mortality) in piglets.

Research Highlights

• First discovery of the highly virulent characteristics of the L1C.5 RFLP-1-4-4 type PRRSV recombinant strain in China

• This strain was recombined from three sources: a NADC30-like wild-type strain (L1C.5) + a highly pathogenic JXA1-like strain (L8E) + a commercial live vaccine strain RespPRRS MLV (L5A)

• Infection of piglets caused high fever (40–42°C), severe pulmonary hemorrhage and edema, and all piglets died within 14 days.

• This demonstrates that current MLV vaccines pose a risk of recombination and immune escape in wild-type endemic areas.

In 2024, a research team discovered abortions and stillbirths in sows and acute mortality in piglets at a pig farm in Sichuan. Cell isolation and whole-genome sequencing analysis confirmed that the new strain GX2024 belongs to the L1C.5 type of the PRRSV-2 pedigree.

Further genetic analysis revealed that this strain is a novel multiplex recombinant virus, co-recombined from a NADC30-like wild-type strain, a highly pathogenic JXA1-like strain, and a modified live virus vaccine strain (MLV).

Animal challenge experiments showed that GX2024 is highly pathogenic; infected piglets died within a short period, exhibiting severe hemorrhagic pneumonia and pulmonary edema, significantly enlarged and hemorrhagic lymph nodes, a rapid increase in viral load, and a significant increase in serum antibody levels.

Introduction

Porcine reproductive and respiratory syndrome (PRRS) is one of the most destructive viral diseases in the global swine industry. In recent years, the NADC30-like (L1C) strain has gradually become the dominant circulating strain in Chinese pig farms, while the emergence of RFLP type 1–4-4 has attracted global attention.

Due to the coexistence of different lineages of strains and live vaccines, PRRSV gene recombination events are frequent, leading to complex viral mutations and difficulties in vaccine control.

Research Results

1. Isolation and identification of the novel strain GX2024:

· Lung tissue from piglets exhibiting severe respiratory symptoms was collected from a pig farm in Sichuan that had been vaccinated with RespPRRS MLV vaccine. The PRRSV strain GX2024 was isolated using Marc-145 cells and porcine alveolar macrophages (iPAMs), with a full-length genome of 15018 nt (GenBank accession number: PV362838).

Figure 1. Virus isolation of PRRSV GX2024

(a) iPAM and Marc-145 cells were inoculated with PRRSV GX2024 and detected 60 hours post-infection; (b) Plaque assay of the GX2024 isolate was performed using iPAM cells with a 10⁻¹⁰ dilution of the virus stock solution; (c) iPAM and Marc-145 cells were infected with sixth-generation virus culture for 48 hours, and IFA was performed using anti-N PRRSV monoclonal antibody. Cell nuclei were stained with DAPI. Scale bar = 75 μm.

• The NSP2 gene of this strain has a discontinuous deletion of 131 amino acids, consistent with the NADC30 strain (111 aa deleted at positions 322-432, 1 aa deleted at position 483, and 19 aa deleted at positions 504-522), and the GP5 protein has unique amino acid mutations (P¹⁵→L¹⁵, F²³→S²³, etc.), which are significantly different from L1C strains at home and abroad.

Figure 2. Multiple sequence amino acid alignment of NSP2 and GP5

(a) Three discontinuous deletions (blue regions) exist at amino acid positions 322–432, 483, and 504–522 in NSP2 of GX2024 and NADC30-like strains. Light red regions indicate amino acid deletions in highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV, JXA1-like strain). (b) Multiple sequence alignment of the GP5 amino acid sequences of GX2024 strain and 17 L1C PRRSV reference strains from China and the United States. Purple boxes indicate signal peptide regions, red boxes indicate hypervariable regions (HVR1 and 2), and yellow boxes indicate transmembrane domains (TM1, 2, and 3).

2. Genomic recombination analysis confirms triple recombination origin:

Analysis using RDP4 and SimPlot software revealed five recombination breakpoints in the GX2024 genome, located in the ORF1a, ORF1b, ORF3, and ORF4 regions:

· The ORF1a (1-1868nt) and ORF1b (3565-9392nt) regions showed the highest homology with the JXA1-like (L8E) strain;

· The ORF3 (12688-13172nt) region showed 90.1% homology with the RespPRRS MLV vaccine strain, higher than its parent VR-2332 strain;

· The remaining regions (1869-3564nt, 9393-12687nt, etc.) were highly homologous with the NADC30-like (L1C.5) strain, confirming it as a triple recombination strain.

Table 1. Comparison of nucleotide identity between GX2024 and 11 PRRSV reference strains

Table 2. Recombination event information detected in GX2024

Figure 3. Genomic recombination analysis of the GX2024 isolate

3. Pathogenicity experiment: 100% lethality highlights high risk

A challenge experiment was conducted on 9 four-week-old piglets without PRRSV antibodies (5 challenged, 4 controls):

· The challenged piglets developed high fever above 40℃ on day 3, and symptoms such as respiratory distress, ataxia, and diarrhea appeared on days 4-14. All piglets died within 14 days (mortality rate 100%); the control group showed no abnormalities.

Figure 4. Pathogenicity analysis of PRRSV isolate GX2024 in piglets

(a) Animal experimental design of this study. (b) Rectal temperature of piglets inoculated with GX2024 and uninfected with RPMI-1640 medium. The cutoff value for clinical fever was set at 40.0℃. (c) Clinical symptom scores on days 3, 7, 10, and 14 during the challenge experiment. Scores included clinical symptoms, respiratory symptoms, and neurological symptoms. (d) Survival and mortality curves of inoculated piglets. (e) Mean weekly weight gain of inoculated piglets during the challenge experiment.

Pathological examination showed severe consolidation, edema, and hemorrhage in the lung tissue of challenged piglets, thickening of alveolar septa, epithelial cell necrosis, blurred boundaries between the cortex and medulla of lymph nodes, decreased lymphocytes, and hemorrhage; immunohistochemistry confirmed the presence of large amounts of PRRSV antigen in the lungs and lymph nodes.

Figure 5. Observation of lung and lymph node lesions in inoculated piglets

• Viral load detection showed that the serum viral load reached its peak 7 days after challenge, and the viral load in the lung tissue was significantly higher than that in other tissues. Moreover, all challenged piglets produced PRRSV-specific antibodies within 7 days (S/P>0.4).

Figure 6. Viral load in serum and tissues and detection of porcine reproductive and respiratory syndrome virus (PRRSV) specific antibodies

Summary

This study reports for the first time a highly pathogenic L1C.5 RFLP-1-4-4 PRRSV strain recombined from a wild-type strain and a vaccine strain, which is extremely lethal to piglets. The emergence of this strain poses a serious challenge to current PRRS vaccine control strategies, highlighting the need for strengthened virus surveillance, optimized vaccine design, and emphasis on biosafety and integrated prevention and control measures.