Recent advances in the study of NADC34-like porcine reproductive and respiratory syndrome virus in China

THAP11-mediated K48- and K63-linked ubiquitination is essential for the degradation of porcine reproductive and respiratory syndrome virus nonstructural protein 1β

Porcine reproductive and respiratory syndrome virus (PRRSV) is a highly infectious pathogen in the global pig industry that causes significant economic losses. Owing to its rapid mutation, effective antiviral treatments or vaccines are still lacking. Therefore, it is essential to identify potential host factors that interact with PRRSV-encoded proteins. In this study, a porcine alveolar macrophage cDNA library was used to identify host proteins that interact with PRRSV nonstructural protein 1β (Nsp1β) via a yeast two-hybrid system. A total of 34 potential host factors were identified, with Thanatos-associated protein 11 (THAP11) strongly interacting with Nsp1β. These interactions were further analyzed via Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses. Co-localization of Nsp1β with THAP11, poly(rC)-binding protein 1 (PCBP1), thioredoxin-interacting protein (TXNIP), and cathepsin D (CTSD) was observed, and co-IP assays confirmed the Nsp1β-THAP11 interaction. The overexpression of THAP11 reduced PRRSV N protein accumulation, indicating an antiviral effect, whereas the silencing of THAP11 increased PRRSV replication. Furthermore, THAP11 promoted the degradation of Nsp1β by increasing K48- and K63-linked ubiquitination, thereby restricting PRRSV replication. These findings suggest that THAP11 exerts an antiviral effect by interacting with and degrading Nsp1β via the ubiquitin-proteasome system, providing insights for future PRRSV defence strategies.

The DEAD-box RNA helicase 27 negatively regulates the replication of porcine reproductive and respiratory syndrome virus by mediating GP2a autophagy degradation and inducing interferon-β production

Porcine reproductive and respiratory syndrome virus (PRRSV) causes severe economic losses to the swine industry, with its replication and immune evasion mechanisms remaining incompletely understood. While DEAD-box helicases (DDXs) are known to either promote or inhibit viral infections, no prior studies have explored the role of DDX27 in viral pathogenesis. Here, we investigated the role of DDX27 in PRRSV infection. PRRSV infection induced the upregulation of endogenous DDX27 mRNA without affecting protein levels in Marc-145 cells. Functional studies revealed that overexpression of DDX27 significantly inhibited PRRSV N protein and mRNA accumulations, while silencing DDX27 enhanced viral replication. Using yeast two-hybrid and co-immunoprecipitation assays, we identified a specific interaction between DDX27 and the viral structural protein GP2a, but not with GP3, M, or non-structural proteins. Mechanistically, DDX27 promoted GP2a degradation via mediating selective autophagy pathway and activated IFN-β production, thereby suppressing PRRSV replication and enhancing host immune responses. These findings reveal DDX27 as a novel antiviral factor that targets PRRSV through dual mechanisms. This study broadens our understanding of the DDX family's role in PRRSV infection and highlights DDX27 as a potential therapeutic target for controlling PRRSV.

Efficacy Evaluation of a VR-2332-Based Modified Live Vaccine Against NADC30-like PRRSV in China

Background: Porcine reproductive and respiratory syndrome is caused by PRRSV. Modified live vaccines (MLVs) are widely used to control PRRSV infection, but their efficacy against the emerging NADC30-like variant remains unclear. This study aimed to evaluate the efficacy of a VR-2332-based MLV against the NADC30-like PRRSV strain HNjz15. Methods: Forty piglets were randomized into a vaccination group (MLV group), negative control group (NC group), and sentinel group. MLV group piglets were immunized with a commercial MLV at 3 weeks of age and challenged with HNjz15 (106.6 TCID50/mL) at 21 days post-immunization. Clinical symptoms, viral load, antibody responses, cytokine levels, and lung lesions were monitored for 14 days post-challenge. Results: Although fever and respiratory symptoms were more pronounced in the NC group pigs than those of the MLV group (average percent occurrence: 65.2% vs. 52.9%), there was no statistical difference (p > 0.05) in the occurrence of respiratory symptoms between the two groups from 5 dpc. Reduced weight gains (by 40-53%) were also observed in the MLV and NC groups compared with the sentinels. The MLV and NC groups exhibited severe lung lesions, while there was no marked difference in viral RNA loads in serum and tissue samples between the MLV and NC groups (p > 0.05). The MLV vaccine induced a significant high level of N protein-specific antibodies compared to the NC group. There was also no significant difference in IFN-γ or TNF-α response to the HNjz15 challenge in both groups (p > 0.05). Conclusions: The VR-2332-based MLV does not provide adequate protection against challenge with the PRRSV-2 NADC30-like strain HNjz15.

Porcine Reproductive and Respiratory Syndrome Virus Prevalence and Pathogenicity of One NADC34-like Virus Isolate Circulating in China

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is one of the most significant infectious agents threatening the global pig industry. Due to its high mutation and recombination rates, the prevalence of PRRSV in domestic pig populations is complex. To better understand the epidemiology of PRRSV, we conducted a large-scale investigation in eastern China, focusing on pig farms with a history of high abortion rates. A total of 14,934 pig samples were collected from 11 sow farms and 53 fattening farms across three provinces. Among these, 13.0% of the collected samples tested positive for PRRSV, with specific prevalence rates of 19.7% in sows and 12.4% in piglets. Genetic evolution analysis of the GP5 gene from 43 PRRSV strains identified in this study revealed that NADC30-like, NADC34-like, and HP-PRRSV were the predominant lineages in domestic pig farms. The NADC30-like genotype was the most dominant and had evolved into three subgenotypes, while the NADC34-like strains had diverged into two subgenotypes. Further analysis of the Nsp2 gene from 18 strains indicated that the NSP2 gene of multiple NADC34-like strains was closely related to that of the NADC30-like, suggesting that the NADC34-like strains are primarily recombinant viruses. Sequence comparison of the Nsp2 gene showed that both NADC30-like and NADC34-like viruses share 111 amino acid deletions at positions 322-433 and 21 amino acid deletions at positions 539-558 in the Nsp2 gene coding region. For the first time, the pathogenicity of a representative NADC34-like virus isolated in China was evaluated in pregnant sow. The results showed that infected sows exhibited an increased body temperature, ear cyanosis, and typical edema and cyanosis of the external genitalia. Moreover, all infected sows experienced miscarriage, with 100% of the aborted piglets being stillbirths exhibiting a high virus load. These findings indicate that this NADC34-like virus is highly virulent to sows.

A Live-Attenuated Chimeric Vaccine Candidate Against the Emerging NADC34-Like PRRSV

NADC34-like porcine reproductive and respiratory syndrome virus (PRRSV) has been circulating in China for several years, causing substantial economic losses to the local pig industry. Current commercial vaccines have failed to provide complete protection against NADC34-like PRRSV infection. Additionally, the poor adaptation of NADC34-like strains to Marc-145 cells presents a considerable challenge for developing effective vaccines against these strains. This study addresses these challenges by developing a novel vaccine candidate against NADC34-like PRRSV. We engineered a recombinant PRRSV, rNADC34-CHSps, by replacing the structural protein region of the JS2021NADC34 strain with that of the CHR6 strain to improve its adaptation to Marc-145 cells. The rescued strain could proliferate well in Marc-145 cells, maintaining high titers and stable growth kinetics even at high passage numbers. Piglets were vaccinated with rNADC34-CHSps at passage 80 and then challenged with the virulent NADC34-like PRRSV strain, JS2021NADC34, at 28 days post-vaccination. All vaccinated piglets developed specific antibodies against PRRSV at 14 dpv and showed no significant clinical symptoms, even after exposure to PRRSV JS2021NADC34. Furthermore, the vaccinated piglets gained significantly more weight, displayed much less severe pathological lesions, and reduced viremia compared to the challenge control piglets. These results indicate that rNADC34-CHSps is a promising vaccine candidate against NADC34-like PRRSV infection, highlighting the potential of targeted genomic modifications to enhance vaccine efficacy.

First detection of NADC34-like porcine reproductive and respiratory syndrome virus strains in Japan

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen in the swine industry. We report for the first time the detection of NADC34-like PRRSV strains in Japan. Serum samples from 18 piglets in Okinawa Prefecture were analyzed for the open reading frame 5 (ORF5) gene encoding glycoprotein 5, which revealed that all samples belonged to sublineage 1.5 (L1A) of Lineage 1. Phylogenetic analysis, restriction fragment length polymorphism patterns, and glycosylation site characteristics confirmed the presence of NADC34-like strains that caused severe outbreaks in the U.S. and Asia. Despite its known pathogenicity, the farm showed only a transient increase in mortality. These findings highlight the need for further investigation into the pathogenicity of PRRSV strains and their potential for the emergence of new variants in Japan.

Efficacy of a reduced-dosage PRRS MLV vaccine against a NADC34-like strain of porcine reproductive and respiratory syndrome virus

Introduction

After being discovered for the first time in China in 2017, porcine reproductive and respiratory syndrome virus (PRRSV) NADC34-like strains have become the prevalent strain of PRRSV in certain regions of China. Our previous study showed that reduced Ingelvac PRRS MLV vaccination dosages against NADC30-like CF PRRSV had a better protection effect than the normal dosage. However, the protective effect of reduced dosages vaccination of Ingelvac PRRS MLV against NADC34-like PRRSV is unclear. Therefore, this study compared the effectiveness of 0.1 and 1 dosages against a NADC34-like PRRSV infection using commercial PRRSV vaccines, Ingelvac PRRS MLV, which have been widely utilized in China.

Methods

In this study, we immunized piglets with two different dosages of the MLV vaccine and infected piglets within a nasal way with NADC34-like CF PRRSV at 42 days post-vaccination. We observed the changes in growth performance before and after the NADC34-like PRRSV DX strain challenge and the protective effect of different vaccine dosages through multiple assays.

Results

After the challenge, the piglets from the challenge control group displayed clinical signs typical of PRRSV infection, including transient fever, high viremia, mild clinical symptoms, and histopathological changes in the lungs and lymph nodes, which indicates DX is a virulent virus. Without the challenge, the average daily gain of the non-immunized group at 5 weeks after the vaccination is greater than that of the 0.01 dosage group than that of the 1 dosage group, which proved that the commercial MLV vaccine has a negative effect on the growth performance of pigs and this effect may be dose-dependent. After the NADC34-like PRRSV challenge, there was no difference in average daily gain between the immunized pigs and pigs from the challenge control group. From the perspective of clinical score, gross lung lesions, and microscopic lesions, immunization with MLV vaccine can indeed relieve symptoms and lesions caused by the virus, and 0.1 dosage vaccination has a better effect in these aspects. Also, both dosages of MLV immunization shortened viremia with similar effects.

Discussion

Our research suggests that the MLV vaccine can provide piglets with some protection against NADC34-like PRRSV and the 0.1 dosage Ingelvac PRRS MLV vaccination showed greater benefits in our study. Therefore, considering the cost, side effects, and subsequent protective effects, we can adjust the immune dosage appropriately after further investigation to ensure safety, improve production efficiency, and reduce immunization costs.

A Universal Multi-Epitope Vaccine Design Against Porcine Reproductive and Respiratory Syndrome Virus via Bioinformatics and Immunoinformatics Approaches

Porcine reproductive and respiratory syndrome virus (PRRSV) causes reproductive disorders in sows and severe pneumonia in piglets, alongside immunosuppressive effects on the host. It poses a significant global threat to the swine industry, with no effective control measures currently available due to its complex pathogenesis and high variability. Conventional inactivated and attenuated vaccines provide inadequate protection and carry biosafety risks. In this study, we designed a universal multi-epitope peptide vaccine against PRRSV using bioinformatics and immunoinformatics approaches to address these limitations. By selecting sequences from seven representative PRRSV strains, we predicted highly conserved and immunogenic T cell (Th and CTL) epitopes across all encoded proteins. These were rationally concatenated with reported B cell neutralizing epitopes into a multi-epitope vaccine construct. We performed comprehensive assessments of the construct's physicochemical and biochemical properties, along with predictions and refinements of its secondary and tertiary structures. Molecular docking simulations with TLR2 and TLR4 revealed strong potential binding interactions. Immune simulations indicated that the multi-epitope vaccine could induce robust humoral and cellular immune responses. This study provides a scientific foundation for the development of safe and effective PRRSV subunit vaccines and offers new perspectives for designing vaccines against other viral diseases.

Current Status of Porcine Reproductive and Respiratory Syndrome Vaccines

Porcine reproductive and respiratory syndrome (PRRS), characterized by reproductive failures in breeding pigs and respiratory diseases in growing pigs, is a widespread and challenging disease. The agent, PRRSV, is a single-strand RNA virus that is undergoing continuous mutation and evolution, resulting in the global spread of multiple strains with different genetic characteristics and variable antigens. There are currently no effective measures to eradicate PRRS, and vaccination is crucial for controlling the disease. At present, various types of vaccine are available or being studied, including inactivated vaccines, modified live virus (MLV) vaccines, vector vaccines, subunit vaccines, DNA vaccines, RNA vaccines, etc. MLV vaccines have been widely used to control PRRSV infection for more than 30 years since they were first introduced in North America in 1994, and have shown a certain efficacy. However, there are safety and efficacy issues such as virulence reversion, recombination with field strains, and a lack of protection against heterologous strains, while other types of vaccine have their own advantages and disadvantages, making the eradication of PRRS a challenge. This article reviews the latest progress of these vaccines in the prevention and control of PRRS and provides scientific inspiration for developing new strategies for the next generation of PRRS vaccines.

Epidemiologic Investigation and Genetic Variation Analysis of PRRSV, PCV2, and PCV3 in Guangdong Province, China from 2020 to 2022

Recently, the emergence of HP-PRRSV (Highly Pathogenic porcine reproductive and respiratory syndrome virus) and the exacerbation of mixed infections of PRRSV and PCV have resulted in significant economic losses for the Chinese pig industry. This study collected a total of 226 samples suspected of infection with the aforementioned viruses from diverse pig farms in seven urban districts of central and northern Guangdong Province between 2020 and 2022. The positive rates of PRRSV, PCV2, and PCV3 in the samples were 33.2%, 37.6%, and 7.5%, respectively, and there were various mixed-infection scenarios present in the samples. This study successfully isolated multiple strains of PRRSV2 and PCV2 from their positive samples, and obtained the gene sequences of six PCV3 (ORF1 + ORF2) from samples. The associated sequences obtained were subjected to bioinformatic analysis and revealed the following:Predominantly prevalent strains of PRRSV in Guangdong Province include HP-PRRSV and NADC30-like variants, whereas PCV2 is primarily represented by the 2b and 2d subtypes. Specifically, the amino acid variation patterns exhibited by the PRRSV GP5 and NSP2 proteins of the strains sg_2108, qy_2008, and fs_2108 under environmental selective pressure are remarkably similar to the characteristics of Highly Pathogenic PRRSV; thus, it is inferred that they may possess higher virulence. The detected PCV3 strains were predominantly concentrated within the PCV3a-IM branch. All PRRSV strains involved in this study are wild-type-PRRSV (wt-PRRSV), comprising three recombinant strains and seven highly virulent strains. Among these strains, the ORF1a gene exhibited the highest variability in their genomes. Environmental selective pressure may enhance the virulence and immune evasion capabilities of PRRSV and drive mutations in the Cap proteins of PCV2 and PCV3. Conversely, PCV2 and PCV3 strains demonstrated greater stability in genetic evolution. In conclusion, this study enhances the epidemiological data regarding PRRSV, PCV2, and PCV3 in Guangdong Province, China, and is significant for the surveillance, prevention, and active control of these three diseases.

Boosting PRRSV-Specific Cellular Immunity: The Immunological Profiling of an Fc-Fused Multi-CTL Epitope Vaccine in Mice

The continuously evolving PRRSV has been plaguing pig farms worldwide for over 30 years, with conventional vaccines suffering from insufficient protection and biosecurity risks. To address these challenges, we identified 10 PRRSV-specific CTL epitopes through enzyme-linked immunospot assay (ELISPOT) and constructed a multi-epitope peptide (PTE) by linking them in tandem. This PTE was then fused with a modified porcine Fc molecule to create the recombinant protein pFc-PTE. Our findings indicate that pFc-PTE effectively stimulates PRRSV-infected specific splenic lymphocytes to secrete high levels of interferon-gamma (IFN-γ) and is predicted to be non-toxic and non-allergenic. Compared to PTE alone, pFc-PTE not only induced a comparable cellular immune response in mice but also extended the duration of the immune response to at least 10 weeks post-immunization. Additionally, pFc-PTE predominantly induced a Th1 immune response, suggesting its potential advantage in enhancing cellular immunity. Consequently, pFc-PTE holds promise as a novel, safe, and potent candidate vaccine for PRRSV and may also provide new perspectives for vaccine design against other viral diseases.

Development and biological characterization of an infectious cDNA clone of NADC34-like PRRSV

Introduction

Porcine Reproductive and Respiratory Syndrome virus (PRRSV) causes high abortion rates in gestating sows and stillbirths, as well as high piglet mortality, seriously jeopardizing the pig industry in China and worldwide.

Methods

In this study, an infectious clone containing the full-length genome of NADC34-like PRRSV was constructed for the first time using reverse genetic techniques. The gene was amplified segmentally onto a plasmid, transfected into BHK-21 cells, and the transfected supernatant was harvested and transfected into PAM cells, which showed classical cytopathic effects (CPE).

Results

The virus rJS-KS/2021 was successfully rescued which could be demonstrated by Western Blot and indirect immunofluorescence assays. Its growth curve was similar to the original strain. Replace the 5'UTR and 3'UTR of rJS-KS/2021 with 5'UTR and 3'UTR of HP-PRRSV (strain SH1) also failed to propagate on MARC-145.

Discussion

In this study, an infectious clone of NADC34-like was constructed by reverse genetics, replacing the UTR and changing the cellular tropism of the virus. These findings provide a solid foundation for studying the recombination of different PRRSVs and the adaption of PRRSVs on MARC-145 in the future.

Isolation and pathogenicity comparison of two novel natural recombinant porcine reproductive and respiratory syndrome viruses with different recombination patterns in Southwest China

Porcine reproductive and respiratory syndrome virus (PRRSV) causes significant economic losses in the swine industry. Frequent mutations and recombinations account for PRRSV immune evasion and the emergence of novel strains. In this study, we isolated and characterized two novel PRRSV-2 strains from Southwest China exhibiting distinct recombination patterns. They were designated SCABTC-202305 and SCABTC-202309. Phylogenetic results indicated that SCABTC-202305 was classified as lineage 8, and SCABTC-202309 was classified as lineage 1.8. Amino acid mutation analysis identified unique amino acid substitutions and deletions in ORF5 and Nsp2 genes. The results of the recombination analysis revealed that SCABTC-202305 is a recombinant with JXA1 as the major parental strain and NADC30 as the minor parental strain. At the same time, SCABTC-202309 is identified as a recombinant with NADC30 as the major parental strain and JXA1 as the minor parental strain. In this study, we infected piglets with SCABTC-202305, SCABTC-202309, or mock inoculum (control) to study the pathogenicity of these isolates. Although both isolated strains were pathogenic, SCABTC-202305-infected piglets exhibited more severe clinical signs and higher mortality, viral load, and antibody response than SCABTC-202309-infected piglets. SCABTC-202305 also caused more extensive lung lesions based on histopathology. Our findings suggest that the divergent pathogenicity observed between the two novel PRRSV isolates may be attributed to variations in the genetic information encoded by specific genomic regions. Elucidating the genetic determinants governing PRRSV virulence and transmissibility will inform efforts to control this devastating swine pathogen.IMPORTANCEPorcine reproductive and respiratory syndrome virus (PRRSV) is one of the most critical pathogens impacting the global swine industry. Frequent mutations and recombinations have made the control of PRRSV increasingly difficult. Following the NADC30-like PRRSV pandemic, recombination events involving PRRSV strains have further increased. We isolated two novel field PRRSV recombinant strains, SCABTC-202305 and SCABTC-202309, exhibiting different recombination patterns and compared their pathogenicity in animal experiments. The isolates caused higher viral loads, persistent fever, marked weight loss, moderate respiratory clinical signs, and severe histopathologic lung lesions in piglets. Elucidating correlations between recombinant regions and pathogenicity in these isolates can inform epidemiologic tracking of emerging strains and investigations into viral adaptive mechanisms underlying PRRSV immunity evasion. Our findings underscore the importance of continued genomic surveillance to curb this economically damaging pathogen.

Evolution Characterization and Pathogenicity of an NADC34-like PRRSV Isolated from Inner Mongolia, China

Porcine reproductive and respiratory syndrome virus (PRRSV) is a pathogen that causes severe abortions in sows and high piglet mortality, resulting in huge economic losses to the pig industry worldwide. The emerging and novel PRRSV isolates are clinically and biologically important, as there are likely recombination and pathogenic differences among PRRSV genomes. Furthermore, the NADC34-like strain has become a major epidemic strain in some parts of China, but the characterization and pathogenicity of the latest strain in Inner Mongolia have not been reported in detail. In this study, an NADC34-like strain (CHNMGKL1-2304) from Tongliao City, Inner Mongolia was successfully isolated and characterized, and confirmed the pathogenicity in pigs. The phylogenetic tree showed that this strain belonged to sublineage 1.5 and had high homology with the strain JS2021NADC34. There is no recombination between CHNMGKL1-2304 and any other domestic strains. Animal experiments show that the CHNMGKL1-2304 strain is moderately virulent to piglets, which show persistent fever, weight loss and high morbidity but no mortality. The presence of PRRSV nucleic acids was detected in both blood, tissues, nasal and fecal swabs. In addition, obvious pathological changes and positive signals were observed in lung, lymph node, liver and spleen tissues when subjected to hematoxylin-eosin (HE) staining and immunohistochemistry (IHC). This report can provide a basis for epidemiological investigations and subsequent studies of PRRSV.

Pathogenicity Studies of NADC34-like Porcine Reproductive and Respiratory Syndrome Virus LNSY-GY and NADC30-like Porcine Reproductive and Respiratory Syndrome Virus GXGG-8011 in Piglets

The porcine reproductive and respiratory syndrome virus (PRRSV) has caused significant economic losses to the swine industry. The U.S., China, and Peru have reported NADC30-like or NADC34-like PRRSV-infected piglets, which have been identified as the cause of a significant number of abortions in clinics. Although the pathogenicity of NADC30-like PRRSV and NADC34-like PRRSV in piglets exhibits significant variability globally, studies on their pathogenicity in China are limited. In this study, the animal experiments showed that within 8-14 days post-infection, both piglets infected with NADC30-like PRRSV GXGG-8011 and those infected with NADC34-like PRRSV LNSY-GY exhibited significant weight loss compared to the control piglets. Additionally, the viremia of the LNSY-GY persisted for 28 days, while the viremia of piglets infected with the GXGG-8011 lasted for 17 days. Similarly, the duration of viral shedding through the fecal-oral route after the LNSY-GY infection was longer than that observed after the GXGG-8011 infection. Furthermore, post-infection, both the LNSY-GY and GXGG-8011 led to pronounced histopathological lesions in the lungs of piglets, including interstitial pneumonia and notable viral colonization. However, the antibody production in the LNSY-GY-infected group occurred earlier than that in the GXGG-8011-infected group. Our research findings indicate that LNSY-GY is a mildly pathogenic strain in piglets, whereas we speculate that the GXGG-8011 might be a highly pathogenic strain.

Protective Efficacy of a Candidate Live-Attenuated Vaccine Derived from the SD-R Strain against NADC34-like Porcine Reproductive and Respiratory Syndrome Virus

NADC34-like porcine reproductive and respiratory syndrome virus (PRRSV) strains were first detected in China in 2017 and became major circulating strains in 2021. Our previous study showed that the live-attenuated vaccine candidate SD-R strain could provide broad cross-protection against different NADC30-like PRRSVs (sublineage 1.8). However, the protective effect of SD-R against NADC34-like PRRSV is unclear. Here, a novel NADC34-like PRRSV, LNTZJ1341-2012, was isolated from a pig farm experiencing disease in 2020. Sequence analysis revealed that LNTZJ1341-2012 belonged to PRRSV-2 sublineage 1.5, exhibited the same Nsp2 amino-acid deletion characteristics as IA/2014/NADC34, and had not recombined with other strains. Additionally, a good challenge model was established to evaluate the protection afforded by the candidate SD-R vaccine against infection with a representative NADC34-like strain (LNTZJ1341-2012). The control piglets in the challenge experiment displayed clinical signs typical of PRRSV infection, including transient fever, high viremia, mild clinical symptoms, and histopathological changes in the lungs and submaxillary lymph nodes. In contrast, SD-R vaccination significantly reduced serum and lung tissue viral loads, and vaccinated piglets did not show any clinical symptoms or histopathological changes. Our results demonstrated that LNTZJ1341-2012 is a mildly virulent NADC34-like PRRSV and that the live-attenuated vaccine SD-R can prevent the onset of clinical signs upon challenge with the NADC34-like PRRSV LNTZJ1341-2012 strain, indicating that SD-R is a promising vaccine candidate for the swine industry.

PRRSV Elimination in a Farrow-to-Finish Pig Herd Using Herd Closure and Rollover Approach

It is well established that PRRSV elimination is an effective strategy for PRRS control, but published reports concerning successful PRRSV elimination cases in farrow-to-finishing herds are rare. Here, we have reported a successful PRRSV elimination case in a farrow-to-finish herd by employing a "herd closure and rollover" approach with some modifications. Briefly, the introduction of pigs to the herd was stopped and normal production processes were maintained until the herd reached a PRRSV provisional negative status. During the herd closure, strict biosecurity protocols were implemented to prevent transmission between nursery pigs and sows. In the current case, introducing gilts before herd closure and live PRRSV exposure were skipped. In the 23rd week post-outbreak, the pre-weaning piglets started to show 100% PRRSV negativity in qPCR tests. In the 27th week, nursery and fattening barns fully launched depopulation. In the 28th week, nursery and fattening houses reopened and sentinel gilts were introduced into gestation barns. Sixty days post-sentinel gilt introduction, the sentinel pigs maintained being PRRSV antibody negative, manifesting that the herd matched the standard of the provisional negative status. The production performance of the herd took 5 months to bounce back to normal. Overall, the current study provided additional information for PRRSV elimination in farrow-to-finish pig herds.

First Identification and Genomic Characterization of NADC34-Like PRRSV Strains Isolated from MLV-Vaccinated Pigs in Korea

NADC34-like porcine reproductive and respiratory syndrome viruses (PRRSVs) are emerging globally. Although lineage 1 PRRSVs are emerging in Korea, NADC34-like PRRSV strains have not been detected to date. This study reports the isolation of the first NADC34-like PRRSV strains from samples of MLV-vaccinated animals on a Korean farm with a severe PRRS outbreak causing high mortality and abortion rates in July 2022. Three PRRSV strains with a whole genome length of 15,088 bp were isolated from the outbreaks, and their phylogenetic, recombination properties, and in vitro growth kinetics were evaluated comprehensively. These Korean strains clustered with those of American and Chinese sublineage 1.5 (NADC34-like strains), according to phylogenetic analyses based on the ORF5 gene and the whole genome. All three complete genome sequences originated from recombination between the RespPRRS MLV and NADC34-like strains, according to recombination analysis. Full-length Nsp2 sequence alignment indicated that all three isolates had a continuous deletion in 100 aa, which was similar to that of the IA/2014/NADC34. Based on phylogenetic analysis and the Nsp2 deletion pattern, the isolates are identified as NADC34-like PRRSV. Growth kinetic analysis based on primary porcine alveolar macrophages (PAMs) and MARC-145 cells has revealed that Korean NADC34-like PRRSV strains had better replication ability in primary PAMs than other Korean lineage 1 viruses or the reference strain VR2332, implying higher pathogenicity in pigs. Considering the fact that NADC34-like PRRSV strains were detected in MLV-vaccinated animals on a farm in the area of the highest pig population density in Korea, additional vaccine failure cases and the spread of the virus could be detected in the near future. Collectively, these results aid in comprehending the current epidemic state of PRRSVs in Korea and highlight the need for continuous surveillance.

The Genetic Variation of Porcine Reproductive and Respiratory Syndrome Virus Replicase Protein nsp2 Modulates Viral Virulence and Persistence

Fast evolution in the field of the replicase nsp2 represents a most prominent feature of porcine reproductive and respiratory syndrome virus (PRRSV). Here, we determined its biological significance in viral pathogenesis by constructing interlineage chimeric mutants between the Chinese highly pathogenic PRRSV (HP-PRRSV) strain JXwn06 (lineage 8) and the low-virulent NADC30-like strain CHsx1401 (lineage 1). Replacement with nsp2 from JXwn06 was surprisingly lethal to the backbone virus CHsx1401, but combined substitution with the structural protein-coding region (SP) gave rise to viable virus CHsx1401-SPnsp2JX. Meanwhile, a derivative carrying only the SP region (CHsx1401-SPJX) served as a control. Subsequent animal experiments revealed that acquisition of SP alone (CHsx1401-SPJX) did not allow CHsx1401 to gain much virulence, but additional swapping of HP-PRRSV nsp2 (CHsx1401-SPnsp2JX) enabled CHsx1401 to acquire some properties of HP-PRRSV, exemplified by prolonged high fever, microscopic lung hemorrhage, and a significant increase in proinflammatory cytokines in the acute stage. Consistent with this was the transcriptomic analysis of persistently infected secondary lymphoid tissues that revealed a much stronger induction of host cellular immune responses in this group and identified several core immune genes (e.g., TLR4, IL-1β, MPO, etc.) regulated by HP-PRRSV nsp2. Interestingly, immune activation status in the individual groups correlated well with the rate of viremia clearance and viral tissue load reduction. Overall, the above results suggest that the Chinese HP-PRRSV nsp2 is a critical virulence regulator and highlight the importance of nsp2 genetic variation in modulating PRRSV virulence and persistence via immune modulation. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) has been a major threat to the world swine industry. In the field, rapid genetic variations (e.g., deletion, mutation, recombination, etc.) within the nsp2 region present an intriguing conundrum to PRRSV biology and pathogenesis. By making chimeric mutants, here, we show that the Chinese highly pathogenic PRRSV (HP-PRRSV) nsp2 is a virulence factor and a much stronger inducer of host immune responses (e.g., inflammation) than its counterpart, currently epidemic, NADC30-like strains. Differences in the ability to modulate host immunity provide insight into the mechanisms of why NADC30-like strains and their derivatives are rising to be the dominant viruses, whereas the Chinese HP-PRRSV strains gradually give away center stage in the field. Our results have important implications in understanding PRRSV evolution, interlineage recombination, and persistence.

Research Progress on Glycoprotein 5 of Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome (PRRS) is an acute, febrile, and highly contagious disease caused by the porcine reproductive and respiratory syndrome virus (PRRSV). Glycoprotein 5 (GP5) is a glycosylated envelope protein encoded by the PRRSV ORF5, which has good immunogenicity and can induce the body to produce neutralizing antibodies. Therefore, study of GP5 protein is of great significance in the diagnosis, prevention, and control of PRRSV and the development of new vaccines. We reviewed GP5 protein genetic variation, immune function, interaction with viral protein and host proteins, induction of cell apoptosis, and stimulation of neutralizing antibodies. GP5 protein's influence on virus replication and virulence, as well as its use as a target for viral detection and immunization are reviewed.

Development of a Multiplex Crystal Digital RT-PCR for Differential Detection of Classical, Highly Pathogenic, and NADC30-like Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) type 1 (European genotype) and PRRSV type 2 (North American genotype) are prevalent all over the world. Nowadays, the North American genotype PRRSV (NA-PRRSV) has been widely circulating in China and has caused huge economic losses to the pig industry. In recent years, classical PRRSV (C-PRRSV), highly pathogenic PRRSV (HP-PRRSV), and NADC30-like PRRSV (NL-PRRSV) have been the most common circulating strains in China. In order to accurately differentiate the circulating strains of NA-PRRSV, three pairs of specific primers and corresponding probes were designed for the Nsp2 region of C-PRRSV, HP-PRRSV, and NL-PRRSV. After optimizing the annealing temperature, primer concentration, and probe concentration, a multiplex real-time quantitative RT-PCR (qRT-PCR) and a multiplex Crystal digital RT-PCR (cdRT-PCR) for the differential detection of C-PRRSV, HP-PRRSV, and NL-PRRSV were developed. The results showed that the two assays illustrated high sensitivity, with a limit of detection (LOD) of 3.20 × 10⁰ copies/μL for the multiplex qRT-PCR and 3.20 × 10^-1 copies/μL for the multiplex cdRT-PCR. Both assays specifically detected the targeted viruses, without cross-reaction with other swine viruses, and indicated excellent repeatability, with coefficients of variation (CVs) of less than 1.26% for the multiplex qRT-PCR and 2.68% for the multiplex cdRT-PCR. Then, a total of 320 clinical samples were used to evaluate the application of these assays, and the positive rates of C-PRRSV, HP-PRRSV, and NL-PRRSV by the multiplex qRT-PCR were 1.88%, 21.56%, and 9.69%, respectively, while the positive rates by the multiplex cdRT-PCR were 2.19%, 25.31%, and 11.56%, respectively. The high sensitivity, strong specificity, excellent repeatability, and reliability of these assays indicate that they could provide useful tools for the simultaneous and differential detection of the circulating strains of C-PRRSV, HP-PRRSV, and NL-PRRSV in the field.

Epidemiology and Genetic Characteristics of Porcine Reproductive and Respiratory Syndrome Virus in the Hunan and Hebei Provinces of China

Porcine reproductive and respiratory syndrome virus (PRRSV) is a significant threat to the pig industry in China. However, the epidemiological characteristics of PRRSV after the outbreak of African swine fever in China were not thoroughly investigated. In the present study, the serological and epidemiological investigations of PRRSV in pigs from the Hunan and Hebei provinces of China were assessed. The results showed that 73.12% (95% CI 71.74-74.49) of pigs were positive for PRRSV-special antibody by enzyme-linked immunosorbent assay. Out of 5799 samples, 482 (8.31%, 95% CI 7.60-9.02) samples were positive for PRRSV nucleic acids. The positive rates of PRRSV in healthy pigs from farms and slaughterhouses were 2.27% (47/2072) and 7.70% (217/2818), which were lower than that in diseased pigs (23.98%, 218/909). Furthermore, the full-length OFR5 gene sequences of 43 PRRSV strains were sequenced and analysed. Phylogenetic analysis revealed that 43 isolates were classified into three lineages, namely lineage 1 (n = 24), lineage 8 (n = 15), and lineage 3 (n = 4). Lineage 1 could be further divided into sublineage 1.5 (n = 2) and sublineage 1.8 (n = 22), and lineage 8 was classified into sublineage 8.1 (n = 3) and sublineage 8.7 (n = 12). Collectively, our findings revealed the severe prevalence of PRRSV in the Hunan and Hebei provinces, where sublineage 1.8 and sublineage 8.7 predominated. The present study provides the update information of the epidemiological and genetic characteristics of PRRSV in the investigated regions, which will be beneficial for PRRS control.

Epidemiological survey of PRRS and genetic variation analysis of the ORF5 gene in Shandong Province, 2020–2021

Since the rise of porcine reproductive and respiratory syndrome virus (PRRSV) in China, mutations have occurred regularly. In particular, the emergence of HP-PRRSV has significantly improved the pathogenicity of PRRSV. It has brought huge economic losses to the Chinese pig farming industry. To understand the current prevalence and evolution of PRRSV in Shandong Province, 1,344 samples suspected of having PRRSV were collected from local hog farms of different sizes. Genetic variation in the isolated PRRSV ORF5 gene was analyzed using the RT-PCR method. The results showed that the detection rate of PRRSV in the collected samples was 25.44%. The predominant strain of PRRSV in Shandong Province is still NADC30-like. However, it cannot be ignored that NADC34-like is also starting to become a prevalent strain. Mutations in ORF5 amino acids 13, 151 and neutralizing epitope (aa36-aa52) in some isolates can cause changes in virulence and ability to escape immunity. This study enriches the epidemiological data on PRRSV in Shandong Province, China. It provides an important reference for the development of new vaccines and for the prevention and control of PRRSV.