Measuring How Recombination Re-shapes the Evolutionary History of PRRSV-2: A Genome-Based Phylodynamic Analysis of the Emergence of a Novel PRRSV-2 Variant

Refining genetic classification of global porcine reproductive and respiratory syndrome virus type 1 (PRRSV-1) and investigating their geographic and temporal distributions

Porcine reproductive and respiratory syndrome virus type 1 (PRRSV-1) primarily circulates in Europe but is also detected in North America and Asia. Based on ORF5 sequences, previous studies classified PRRSV-1 into four subtypes. Subtype 1 was further classified into 12 clades (A-L) or into three lineages with lineage 1 including clades 1A-1G and lineage 3 including clades 3A-3G, but the systems are inconsistent and have not been adopted. In this study, we proposed a statistically supported PRRSV-1 genetic classification system based on 10,446 global PRRSV-1 ORF5 sequences spanning 1991-2023. We replaced the colloquial "subtype" designation with "lineage" to reflect evolutionary history and, subsequently, PRRSV-1 was classified into four lineages (L1-L4) with L1 including 18 sublineages (L1.1 to L1.18). The proposed classification system is flexible and may be amended if additional lineages, sublineages, or more granular classifications are needed to reflect contemporary PRRSV-1 detections and evolution. Geographic distributions of PRRSV-1 at lineage and sublineage levels were distinct, with L1 globally distributed and L2, L3 and L4 more restricted. Temporal dynamic changes in some countries were quantified. Classification and ORF5 nucleotide identity of six commercial PRRSV-1 vaccines to each lineage and sublineage and detection frequency of vaccine-like viruses were determined. The phylogenies based on whole-genome and ORF5 sequences demonstrated slightly different tree topologies. Recombination of PRRSV-1 was observed at within-sublineage and between-sublineage levels. A set of ORF5 reference sequences representing the refined classification is available for future diagnostic and epidemiological applications. This study provides a benchmark delineating the current genetic diversity of PRRSV-1 and introduces a refined classification system to support the global standardization and application of ORF5-based genetic classification for PRRSV-1.

PRRSV-2 variant classification: a dynamic nomenclature for enhanced monitoring and surveillance

Existing genetic classification systems for porcine reproductive and respiratory syndrome virus type 2 (PRRSV-2), such as restriction fragment length polymorphisms and sub-lineages, are unreliable indicators of close genetic relatedness or lack sufficient resolution for epidemiological monitoring routinely conducted by veterinarians. Here, we outline a fine-scale classification system for PRRSV-2 genetic variants in the United States. Based on >25,000 U.S. open reading frame 5 (ORF5) sequences, sub-lineages were divided into genetic variants using a clustering algorithm. Through classifying new sequences every 3 months and systematically identifying new variants across 8 years, we demonstrated that prospective implementation of the variant classification system produced robust, reproducible results across time and can dynamically accommodate new genetic diversity arising from virus evolution. From 2015 to 2023, 118 variants were identified, with ~48 active variants per year, of which 26 were common (detected >50 times). Mean within-variant genetic distance was 2.4% (max: 4.8%). The mean distance to the closest related variant was 4.9%. A routinely updated webtool (https://stemma.shinyapps.io/PRRSLoom-variants/) was developed and is publicly available for end users to assign newly generated sequences to a variant ID. This classification system relies on U.S. sequences from 2015 onward; further efforts are required to extend this system to older or international sequences. Finally, we demonstrate how variant classification can better discriminate between previous and new strains on a farm, determine possible sources of new introductions into a farm/system, and track emerging variants regionally. Adoption of this classification system will enhance PRRSV-2 epidemiological monitoring, research, and communication, and improve industry responses to emerging genetic variants.IMPORTANCEThe development and implementation of a fine-scale classification system for PRRSV-2 genetic variants represent a significant advancement for monitoring PRRSV-2 occurrence in the swine industry. Based on systematically applied criteria for variant identification using national-scale sequence data, this system addresses the shortcomings of existing classification methods by offering higher resolution and adaptability to capture emerging variants. This system provides a stable and reproducible method for classifying PRRSV-2 variants, facilitated by a freely available and regularly updated webtool for use by veterinarians and diagnostic labs. Although currently based on U.S. PRRSV-2 ORF5 sequences, this system can be expanded to include sequences from other countries, paving the way for a standardized global classification system. By enabling accurate and improved discrimination of PRRSV-2 genetic variants, this classification system significantly enhances the ability to monitor, research, and respond to PRRSV-2 outbreaks, ultimately supporting better management and control strategies in the swine industry.

Metagenomic detection and genome assembly of novel PRRSV-2 strain using Oxford Nanopore Flongle flow cell

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of a syndrome characterized by reproductive failure and respiratory complications (PRRS). Early detection and classification of PRRSV strains are vital for appropriate management strategies to minimize loss following outbreaks. The most widely used classification method for PRRSV is based on open reading frame 5 (ORF5) sequences. However, the effectiveness of the ORF5-based classification system in accurately representing genetic variation is under scrutiny because ORF5 constitutes less than 5% of the 15kb-long genome. In this study, a single Oxford Nanopore Flongle flow cell was used to identify and assemble the genome of a strain sampled in May of 2022 from a Midwest research farm. Based on comparisons with available PRRSV genomes, the assembled genome was determined to be a novel PRRSV-2 strain belonging to the 1-4-4 L1C.5 ORF5-based lineage. Phylogenetic analyses of ORF5 and whole-genome sequences demonstrated differences in clustering between PRRSV strains, supporting the inability of ORF5 to capture genome-wide variation. For example, high levels of variation were observed within ORF1a, which encodes the hypervariable nsp2 protein. Comparison of the newly assembled genome with the genome of a highly characterized strain (VR2332 PRRSV-2) identified a 100 amino acid deletion within nsp2 characteristic of NADC34-like PRRSV. Oxford Nanopore Technologies' Flongle flow cell has been proven in this study to provide a rapid, cost-effective and accessible approach for whole-genome sequencing of PRRSV strains present within clinical samples necessary for strain-specific genome-wide characterization.

Recombinant characterization and pathogenicity of a novel L1C RFLP-1-4-4 variant of porcine reproductive and respiratory syndrome virus in China

Porcine reproductive and respiratory syndrome (PRRS) is one of the most significant diseases affecting the pig industry worldwide and is caused by the PRRS virus (PRRSV), which has complex genetic variation due to frequent mutations, indels, and recombination. The emergence of PRRSV L1C.5 in 2020 in the United States has raised worldwide concerns about PRRSV with the RFLP 1-4-4 pattern and lineage 1C. However, studies on the pathogenic characteristics, epidemiological distribution, and effectiveness of vaccines against PRRSV with L1C and RFLP1-4-4 pattern in China are still insufficient. In this study, a novel recombinant variant of PRRSV with RFLP 1-4-4 and lineage 1C features, different from L1C.5 in the United States, was isolated in China in 2021. In pathogenicity experiments in specific pathogen-free piglets or farm piglets, 60-100% of artificially infected experimental piglets died with high fever and respiratory symptoms. Inflammatory cytokine and chemokine levels were upregulated in infected piglets. A commercially modified live vaccine against highly pathogenic PRRSV did not provide effective protection when the vaccinated piglets were challenged with the novel L1C-1-4-4 variant. Therefore, this strain merits special attention when devising control and vaccine strategies. These findings suggest that extensive joint surveillance is urgently needed and that vaccine strategies should be updated to prevent the disease from spreading further.

Circulating Lineage 3 Recombination with NADC30-Like and NADC34-Like Betaarterivirus suid 2 in Taiwan

Porcine respiratory and reproductive syndrome (PRRS) caused by Betaarterivirus suid leads to severe economic losses. The emergence of highly pathogenic Betaarterivirus suid 2 (PRRSV-II), such as NADC30 and NADC34, has been reported in the USA and several Asian countries. NADC30-like PRRSV-II was first reported in 2018 in Taiwan. To investigate the PRRSV variants currently circulating in Taiwan, sequences covering ORF2-5 of Taiwan PRRSV isolates collected between 2020 and 2023 were analyzed. Phylogenetic analysis of the ORF5 nucleotide sequence indicated that most of the Taiwan isolates were clustered in lineage 3 and three isolates were grouped in lineage 1 and were closely related to the NADC34 strain. Interestingly, these three NADC34-like Taiwan PRRSV isolates carried amino acid deletions similar to NADC30 and were more closely related to NADC30 strains than the NADC34 strains in the Nsp2 gene. Next-generation sequencing and recombination detection program showed potential recombination of lineage 3 with NADC30- and NADC34-like PRRSV-II. Our results suggest the presence of circulating mosaic recombinants and lineage 3 PRRSV-II in Taiwan during 2020 and 2023.

Recombination of Porcine Reproductive and Respiratory Syndrome Virus: Features, Possible Mechanisms, and Future Directions

Recombination is a pervasive phenomenon in RNA viruses and an important strategy for accelerating the evolution of RNA virus populations. Recombination in the porcine reproductive and respiratory syndrome virus (PRRSV) was first reported in 1999, and many case reports have been published in recent years. In this review, all the existing reports on PRRSV recombination events were collected, and the genotypes, parental strains, and locations of the recombination breakpoints have been summarized and analyzed. The results showed that the recombination pattern constantly changes; whether inter- or intra-lineage recombination, the recombination hotspots vary in different recombination patterns. The virulence of recombinant PRRSVs was higher than that of the parental strains, and the emergence of virulence reversion was caused by recombination after using MLV vaccines. This could be attributed to the enhanced adaptability of recombinant PRRSV for entry and replication, facilitating their rapid propagation. The aim of this paper was to identify common features of recombinant PRRSV strains, reduce the recombination risk, and provide a foundation for future research into the mechanism of PRRSV recombination.

Genomic Characterization of HLJDZD55: The First L1B PRRSV in China

Porcine reproductive and respiratory syndrome virus (PRRSV) critically threatens the pig industry in China. Lineage 1 PRRSV, which is divided into L1A-L1F and L1H-L1J, is widely recognized as the most extensively genetically diverse lineage globally. L1A (NADC34-like) and L1C (NADC30-like) PRRSVs have become the current major circulating strains in China. Notably, strains from other branches of L1 have not been reported in China. During our epidemiological investigation of PRRSV, we identified a new strain named HLJDZD55. Phylogenetic analysis of the ORF5 gene revealed that HLJDZD55 belongs to the L1B branch. Alignment of deduced amino acid sequences based on the Nsp2 gene indicated that HLJDZD55 has a discontinuous deletion of 131 amino acids (111 + 1 + 19). We further sequenced the whole genome of HLJDZD55, and phylogenetic analysis based on the whole-genome sequence revealed that HLJDZD55 belongs to the L1C branch. Recombination analysis of the whole genome demonstrated that HLJDZD55 is a recombinant strain of TJZH-1607 (L1C, identified in China) and Minnesota 14 (L1B, identified in the USA). These findings suggested that HLJDZD55 is a newly emerged lineage 1 PRRSV in China and is closely related to L1B PRRSV in the US, which may have been introduced from the U.S. strain and subsequently recombined with the local Chinese strain and underwent evolution. Taken together, these results demonstrated the emergence of L1B PRRSV in China for the first time.

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.

Predicting Potential PRRSV-2 Variant Emergence through Phylogenetic Inference

Porcine reproductive and respiratory syndrome (PRRS) is a significant pig disease causing substantial annual losses exceeding half a billion dollars to the United States pork industry. The cocirculation and emergence of genetically distinct PRRSV-2 viruses hinder PRRS control, especially vaccine development. Similar to other viral infections like seasonal flu and SARS-CoV-2, predictive tools for identifying potential emerging viral variants may prospectively aid in preemptive disease mitigation. However, such predictions have not been made for PRRSV-2, despite the abundance of relevant data. In this study, we analyzed a decade's worth of virus ORF5 sequences (n = 20,700) and corresponding metadata to identify phylogenetic-based early indicators for short-term (12 months) and long-term (24 months) variant emergence. Our analysis focuses on PRRSV-2 Lineage 1, which was the predominant lineage within the U.S. during this period. We evaluated population expansion, spatial distribution, and genetic diversity as key success metrics for variant emergence. Our findings indicate that successful variants were best characterized as those that underwent population expansion alongside widespread geographical spread but had limited genetic diversification. Conditional logistic regression revealed the local branching index as the sole informative indicator for predicting population expansion (balanced accuracy (BA) = 0.75), while ancestral branch length was strongly linked to future genetic diversity (BA = 0.79). Predicting spatial dispersion relied on the branch length and putative antigenic difference (BA = 0.67), but their causal relationships remain unclear. Although the predictive models effectively captured most emerging variants (sensitivity = 0.58-0.81), they exhibited relatively low positive predictive value (PPV = 0.09-0.16). This initial step in PRRSV-2 prediction is a crucial step for more precise prevention strategies against PRRS in the future.

Refining PRRSV-2 genetic classification based on global ORF5 sequences and investigation of their geographic distributions and temporal changes

IMPORTANCE

In this study, comprehensive analysis of 82,237 global porcine reproductive and respiratory syndrome virus type 2 (PRRSV-2) open reading frame 5 sequences spanning from 1989 to 2021 refined PRRSV-2 genetic classification system, which defines 11 lineages and 21 sublineages and provides flexibility for growth if additional lineages, sublineages, or more granular classifications are needed in the future. Geographic distribution and temporal changes of PRRSV-2 were investigated in detail. This is a thorough study describing the molecular epidemiology of global PRRSV-2. In addition, the reference sequences based on the refined genetic classification system are made available to the public for future epidemiological and diagnostic applications worldwide. The data from this study will facilitate global standardization and application of PRRSV-2 genetic classification.

Development, Evaluation, and Clinical Application of PRRSV-2 Vaccine-like Real-Time RT-PCR Assays

In this study, we developed and validated (1) singleplex real-time RT-PCR assays for specific detection of five PRRSV-2 MLV vaccine viruses (Ingelvac MLV, Ingelvac ATP, Fostera, Prime Pac, and Prevacent) and (2) a four-plex real-time RT-PCR assay (IngelvacMLV/Fostera/Prevacent/XIPC) including the internal positive control XIPC for detecting and distinguishing the three most commonly used vaccines in the USA (Prevacent, Ingelvac MLV, and Fostera). The singleplex and 4-plex vaccine-like PCRs and the reference PCR (VetMAXTM PRRSV NA&EU, Thermo Fisher Scientific, Waltham, MA, USA) did not cross-react with non-PRRSV swine viral and bacterial pathogens. The limits of detection of vaccine-like PCRs ranged from 25 to 50 genomic copies/reactions. The vaccine-like PCRs all had excellent intra-assay and inter-assay repeatability. Based on the testing of 531 clinical samples and in comparison to the reference PCR, the diagnostic sensitivity, specificity, and agreement were in the respective range of 94.67-100%, 100%, and 97.78-100% for singleplex PCRs and 94.94-100%, 100%, and 97.78-100% for the 4-plex PCR, with a CT cutoff of 37. In addition, 45 PRRSV-2 isolates representing different genetic lineages/sublineages were tested with the vaccine-like PCRs and the results were verified with sequencing. In summary, the vaccine-like PCRs specifically detect the respective vaccine-like viruses with comparable performances to the reference PCR, and the 4-plex PCR allows to simultaneously detect and differentiate the three most commonly used vaccine viruses in the same sample. PRRSV-2 vaccine-like PCRs provide an additional tool for detecting and characterizing PRRSV-2.

Isolation, Pathogenicity, and Comparative Phylogenetic Characteristics of an Intralineage Recombinant NADC34-Like PRRSV in China

Porcine reproductive and respiratory syndrome (PRRS), which causes reproductive failure in sows and respiratory symptoms in piglets, poses a significant threat to the global pig industry. PRRS virus (PRRSV) variants continue to emerge and spread among pigs. NADC34-like PRRSV has been imported into China in recent years and has shown potential as an endemic strain, which is of great concern. In this study, a NADC34-like PRRSV, named HLJ13 strain, was isolated from a farm where pigs experienced respiratory symptoms and abortions. Genomic analysis revealed that the HLJ13 strain was a potential recombinant of NADC34-like and NADC30-like strains, and the restriction fragment length polymorphism of HLJ13 was a novel pattern that was not yet listed. In the PRRSV HLJ13-inoculated group, the piglets showed mild clinical symptoms, such as persistent fever, and showed histopathological lesions in the lungs, and the virus was detectable at 3 and 7 days postinoculation in anal and nasal swabs, respectively. Recombination analysis revealed that interlineage recombinant events were detected in 8 out of 27 Chinese NADC34-like PRRSVs. Phylogenetic analysis showed that Chinese NADC34-like PRRSVs were distributed in two clades of lineage 1, and Chinese NADC34-like PRRSVs showed different N-glycosylation modifications in glycoproteins, especially in GP3 and GP5. These findings shed light on the genomic characteristics and pathogenicity of the NADC34-like PRRSV in China.

A recombinant porcine reproductive and respiratory syndrome virus type 2 field strain derived from two PRRSV-2-modified live virus vaccines

A porcine reproductive and respiratory syndrome virus (PRRSV) type 2 (PRRSV-2) isolate was obtained from lung samples collected from a 4.5-month-old pig at a wean-to-finish site in Indiana, USA, although no gross or microscopic lesions suggestive of PRRSV infection were observed in the lung tissue. Phylogenetic and molecular evolutionary analyses based on the obtained virus sequences indicated that PRRSV USA/IN105404/2021 was a natural recombinant isolate from Ingelvac PRRS® MLV and Prevacent® PRRS, which are PRRSV-2-modified live virus vaccines commercially available in the United States. This study is the first to report the detection of a PRRSV-2 recombinant strain consisting entirely of two modified live virus vaccine strains under field conditions. Based on clinical data and the absence of lung lesions, this PRRSV-2 recombinant strain was not virulent in swine, although its pathogenicity needs to be confirmed by clinical trials.