company news about A novel L1A/C RFLP 1-3-4 porcine reproductive and respiratory syndrome virus has been discovered in China, causing sever

Porcine reproductive and respiratory syndrome (PRRS), caused by the PRRS virus, is one of the most devastating swine diseases worldwide. In recent years, multiple strains of lineage 1 have been widely prevalent in the United States, while L1 RFLP 1-3-4 strains have been less commonly reported in China.

Recently, researchers from Tianjin Agricultural University and Tianjin University of Traditional Chinese Medicine jointly published a study in Elsevier on a recombinant PRRSV variant with an L1A/C RFLP 1–3–4 pattern and analyzed its genome.

Introduction

Porcine reproductive and respiratory syndrome (PRRS) is a serious infectious disease caused by the PRRS virus. It is divided into two major genotypes, the European and North American types. RFLP and lineage classification systems are commonly used to analyze the genetic diversity of PRRSV-2. Lineage 1 (L1) is prevalent in the United States, with common RFLP patterns of 1-3-4, 1-4-4, and 1-7-4. In China, the 1-4-4 L1C strain predominates, but its proportion is declining annually. Although less commonly reported, 1-3-4 has unique intestinal colonization characteristics and may become a new dominant strain, posing a serious threat to the pig industry.

Research Results

1. Isolation of L1 PRRSV from Diarrheal Piglets at Tianjin Pig Farms

From May to June 2024, serum samples were collected from suckling piglets at three Tianjin pig farms after severe diarrhea. Serum samples were positive for L1 PRRSV using PAM cell isolation and negative for several common porcine viruses. The isolates exhibited typical cell aggregation and fragmentation and were purified and named TJ2401, TJ2402, and TJ2406. Immunofluorescence analysis revealed green fluorescence, with TCID₅₀ values ​​of 1×10^6.34, 1×10^6.25, and 3×10^5.0/0.1 mL, respectively.

Figure 1. Isolation and Identification of PRRSV Strains TJ2401, TJ2402, and TJ2406

(A-D) 48 hours after infection. (E-H) Labeled with anti-N protein antibody. Scale bar, 100 μm.

Conclusion: Porcine reproductive and infectious syndrome virus (PRRSV) L1 was successfully isolated from samples of suckling piglets with severe diarrhea in Tianjin between May and June 2024. Its high virulence was confirmed by cytopathic effect and immunofluorescence assays.

2. Genetic Characteristics and Subtype Analysis of Isolated PRRSV L1 Strains

Full-length genome sequencing of isolates TJ2401, TJ2402, and TJ2406 revealed a deletion pattern in NSP2 consistent with the L1 (NADC30-like) subtype. Whole-genome phylogenetic analysis revealed a close relationship with the L1.8 subgroup. RFLP analysis of the ORF5 gene assigned TJ2401 to the L1A subtype, and TJ2402 and TJ2406 to the L1C subtype.

Figure 2. NSP2 sequences and phylogenetic tree of TJ2401, TJ2402, and TJ2406

(A) The NSP2 deletion pattern of TJ2401, TJ2402, and TJ2406 is identical to that of NADC30.

(B) The three isolates are closely related to the L1.8 (NADC30-like) PRRSV.

(C) TJ2401 belongs to L1A, while TJ2402 and TJ2406 belong to L1C.

Conclusion: All three isolates, TJ2401, TJ2402, and TJ2406, are L1 (NADC30-like) PRRSV, belonging to the L1A and L1C subtypes, respectively.

3. Recombination Characteristics and Analysis of Key Gene Segments of Isolated PRRSV Strains

Recombination analysis revealed that TJ2401, TJ2402, and TJ2406 all exhibited recombination characteristics. TJ2401 and TJ2402 shared a common gene segment (1-1970 nt) from L8 (JXA1-like) viruses, potentially contributing to their similar clinical pathogenicity. TJ2406, on the other hand, was primarily homologous to the NADC30 strain and contained restricted recombinant segments.

Figure 3. Phylogenetic tree constructed based on representative PRRSV genomes

Conclusion: All three PRRSV isolates are recombinant strains. TJ2401 and TJ2402 share the L8 segment, potentially resulting in similar clinical pathogenicity.

4. Analysis of Clinical Signs and Viral Dynamics of the Three Isolates in Piglets

Piglets inoculated with TJ2401, TJ2402, or TJ2406 developed elevated body temperature, severe diarrhea, and increased clinical symptom scores, decreased daily weight gain, and seroconversion on day 11. Serum and anal swab viral loads were significantly elevated, while the control group showed no symptoms or positive viral tests.

Figure 4. Clinical indicators and virological and immunological test results in piglets

(A) Piglets inoculated with TJ2401, TJ2402, or TJ2406 all had elevated body temperatures.

(B) Daily weight gain of piglets was significantly reduced.

(C) On day 11 post-infection, piglets achieved seroconversion of PRRS antibodies (threshold 0.4).

(D) Serum viral load increased 5 days post-infection and peaked on day 11.

(E) Viral load in anal swabs was consistent with serum viral load.

(F) Virus was present in multiple tissues, most prominently in the intestine.

Conclusion: TJ2401, TJ2402, and TJ2406 all induced significant clinical signs and viral infection in piglets, while no abnormalities were observed in the control group.

5. Comparative Analysis of PRRSV Isolates on Lung and Intestinal Pathology in Piglets

Piglets in all three groups were humanely euthanized on day 21 post-infection for necropsy. The results showed that piglets infected with TJ2401, TJ2402, or TJ2406 all developed interstitial pneumonia, lung consolidation and hemorrhage, and the lesion scores were significantly higher than those of the uninfected group. The TJ2401 and TJ2402 infection groups also showed thinning of the intestinal wall, watery intestinal contents, and enlarged mesenteric lymph nodes, while the TJ2406 group showed no obvious intestinal lesions. Microscopic examination showed that the alveolar septa of piglets infected with the three virus strains were thickened and the lung microscopic lesion scores were increased. In terms of intestinal pathology, the TJ2401 and TJ2402 groups showed intestinal epithelial desquamation and lymphocytic infiltration, while the TJ2406 group and the control group showed no obvious lesions.

Figure 4. Macroscopic and microscopic histological damage in lung and intestinal samples.

(A) TJ2401, TJ2402, and TJ2406 infection resulted in significant lung lesions and prominent inflammatory cell infiltration.

(B) TJ2401 and TJ2402 caused both lung and intestinal lesions, while no significant lesions were observed in the uninfected group.

Conclusion: TJ2401 and TJ2402 caused significant lung and intestinal lesions in piglets, while TJ2406 primarily caused lung lesions with no significant intestinal abnormalities.

Summary

This study found that L1A TJ2401 and L1C TJ2402 caused severe intestinal infection, while TJ2406 only caused interstitial pneumonia. This suggests that the pathogenicity of NADC30-like PRRSV is complex and that vaccines are limited in their effectiveness. Strengthened surveillance and timely adjustment of control strategies are necessary.