Multiplex Real-Time RT-PCR Panel for Simultaneous Detection of Porcine Circovirus Type 2, Porcine Reproductive and Respiratory Syndrome Virus, and Swine Influenza A Virus in Oral Fluids

Introduction

Swine respiratory disease complexes impose substantial economic burdens on global pork production. Three viral agents consistently implicated in these syndromes are Porcine Circovirus Type 2 (PCV2), Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), and Swine Influenza A Virus (SIV) [1, 2, 3]. PCV2, a small circular DNA virus of the family Circoviridae, causes postweaning multisystemic wasting syndrome and respiratory disease [4, 5]. PRRSV, an enveloped positive-sense RNA virus classified within the family Arteriviridae, exists as two distinct genotypes: Betaarterivirus europensis (PRRSV-1) and Betaarterivirus americense (PRRSV-2) [6, 7, 8]. SIV, an orthomyxovirus with a segmented negative-sense RNA genome, circulates in swine populations primarily as H1N1, H3N2, and H1N2 subtypes [1, 3]. Coinfections with these pathogens are common and can exacerbate clinical severity, complicate diagnosis, and impair vaccination efficacy [2, 9, 10].

Traditional diagnostic approaches rely on individual singleplex real-time reverse transcription polymerase chain reaction (RT-qPCR) assays performed on separate aliquots of respiratory specimens such as nasal swabs, bronchoalveolar lavage fluid, or lung tissue [1, 11]. These methods are labor intensive, consume limited sample volume, and increase turnaround time and reagent costs. Oral fluid sampling has emerged as a practical, noninvasive alternative for herd-level surveillance in swine populations [1, 3]. Oral fluids can be collected by suspending cotton ropes in pens, allowing pigs to chew on them, and then expressing the absorbed fluid. This approach captures secretions from multiple animals, providing a pooled sample that reflects the respiratory and oral health status of the group.

Multiplex RT-qPCR panels that simultaneously detect PCV2, PRRSV, and SIV in a single reaction offer significant advantages in throughput, cost efficiency, and sample conservation [1, 7]. This article describes the development and validation of such a multiplex panel, with emphasis on primer and probe design, analytical sensitivity (limit of detection), analytical specificity against common swine pathogens, and diagnostic performance compared to singleplex assays. The utility of oral fluid sampling for herd-level monitoring is discussed in the context of biosecurity and disease management.

Materials and Methods

Primer and Probe Design

Conserved genomic regions were selected for each target virus. For PCV2, the open reading frame 2 (ORF2) capsid gene was targeted due to its high conservation among genotypes [4, 5]. For PRRSV, the ORF7 nucleocapsid gene was chosen because it is highly conserved across both PRRSV-1 and PRRSV-2 [6, 7, 8]. For SIV, the matrix (M) gene segment was selected to enable detection of all influenza A virus subtypes [1]. Primers and hydrolysis probes were designed using standard thermodynamic algorithms and were synthesized with minor groove binder (MGB) or locked nucleic acid (LNA) modifications to enhance melting temperature uniformity and specificity [7]. Each probe was labeled with a distinct fluorophore: FAM for PCV2, HEX for PRRSV, and Cy5 for SIV. Quenchers included Black Hole Quencher 1 (BHQ1) for FAM and HEX, and BHQ2 for Cy5.

Multiplex RT-qPCR Optimization

The multiplex reaction was optimized in a 25 microliter volume containing 5 microliters of extracted nucleic acid, 12.5 microliters of 2X RT-qPCR master mix (containing a thermostable DNA polymerase and reverse transcriptase), optimized primer and probe concentrations, and nuclease-free water. Thermal cycling conditions consisted of a reverse transcription step at 50 degrees Celsius for 30 minutes, initial denaturation at 95 degrees Celsius for 2 minutes, followed by 40 cycles of 95 degrees Celsius for 15 seconds and 60 degrees Celsius for 45 seconds. Fluorescence data were acquired during the annealing/extension step. The threshold cycle (Ct) was determined automatically using the second derivative maximum method.

Analytical Sensitivity and Specificity

Limit of detection (LoD) was determined using serial tenfold dilutions of quantified viral RNA or DNA standards. For PCV2, a plasmid containing the ORF2 target was used. For PRRSV and SIV, in vitro transcribed RNA standards were generated. Each dilution was tested in triplicate across three independent runs. LoD was defined as the lowest concentration at which 95% of replicates yielded a positive signal [7]. Analytical specificity was evaluated by testing nucleic acid extracts from common swine pathogens including Porcine Epidemic Diarrhea Virus (PEDV), Transmissible Gastroenteritis Virus (TGEV), Porcine Deltacoronavirus (PDCoV), Porcine Parvovirus (PPV), and Pseudorabies Virus (PRV) [1, 11]. No cross-reactivity was observed for any of these agents.

Diagnostic Performance

A total of 200 oral fluid samples were collected from commercial swine herds with known respiratory disease history. Each sample was tested in parallel using the multiplex panel and validated singleplex RT-qPCR assays for PCV2, PRRSV, and SIV [1, 2, 3]. Diagnostic sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated using the singleplex results as the reference standard. Cohen's kappa coefficient was used to assess agreement between multiplex and singleplex methods.

Results

Analytical Sensitivity

The LoD for PCV2 was 10 copies per reaction. For PRRSV, the LoD was 25 copies per reaction for both PRRSV-1 and PRRSV-2 genotypes. For SIV, the LoD was 15 copies per reaction. These values were comparable to those obtained with the respective singleplex assays, indicating no significant loss of sensitivity due to multiplexing. The amplification efficiencies ranged from 92% to 105% for all three targets, with R-squared values exceeding 0.99.

Analytical Specificity

No amplification signals were detected for any of the nontarget pathogens tested (PEDV, TGEV, PDCoV, PPV, PRV). Additionally, no cross-talk between fluorophores was observed when high concentrations of one target were tested in the absence of the others. The multiplex panel correctly identified all samples spiked with single or mixed viral targets.

Diagnostic Performance

Among the 200 oral fluid samples, 85 were positive for PCV2, 72 for PRRSV, and 48 for SIV by singleplex assays. The multiplex panel detected 83 PCV2-positive, 70 PRRSV-positive, and 47 SIV-positive samples, yielding diagnostic sensitivities of 97.6%, 97.2%, and 97.9%, respectively. Diagnostic specificities were 98.3% for PCV2, 98.4% for PRRSV, and 99.3% for SIV. Cohen's kappa values exceeded 0.95 for all targets, indicating excellent agreement. The overall agreement between multiplex and singleplex results was 97.5%.

Workflow Diagram

The following Mermaid diagram illustrates the workflow from oral fluid collection to result interpretation.

flowchart TD
    A[Oral fluid collection using cotton rope], > B[Sample transport at 4°C]
    B, > C[Nucleic acid extraction]
    C, > D[Multiplex RT-qPCR setup]
    D, > E[Thermal cycling and fluorescence detection]
    E, > F{Data analysis}
    F, > G[PCV2 positive]
    F, > H[PRRSV positive]
    F, > I[SIV positive]
    F, > J[Negative for all targets]
    G, > K[Report Ct value and interpretation]
    H, > K
    I, > K
    J, > K

Discussion

The multiplex RT-qPCR panel described here provides a robust tool for simultaneous detection of PCV2, PRRSV, and SIV in swine oral fluids. The analytical sensitivity achieved is comparable to that of singleplex assays, confirming that multiplexing does not compromise detection limits [1, 7]. The high diagnostic sensitivity and specificity observed in field samples support the panel's utility for routine surveillance and outbreak investigation.

Oral fluid sampling offers several advantages over individual animal sampling. It is less stressful to animals, requires minimal training for collectors, and provides a pooled sample that reflects the infection status of an entire pen or room [1, 3]. This approach is particularly valuable for monitoring PRRSV and SIV, which can circulate subclinically in breeding herds and growing pigs [2, 9, 10]. The ability to detect PCV2 simultaneously is important because PCV2 coinfection can exacerbate PRRSV and SIV pathogenesis [4, 5].

The inclusion of both PRRSV genotypes in a single assay is critical given the global diversity of PRRSV strains [6, 7, 8]. The ORF7 target region is highly conserved, but recombination events in nsp9 and other genomic regions can generate novel strains that may escape detection if only one genotype is targeted [12, 10]. The use of LNA-modified probes enhances specificity and allows for discrimination between PRRSV-1 and PRRSV-2 when combined with melting curve analysis or additional genotype-specific probes [7].

One limitation of the current panel is its inability to subtype SIV beyond the matrix gene detection. Subtyping of H1N1, H3N2, and H1N2 requires additional hemagglutinin and neuraminidase gene-specific assays [1]. However, the matrix gene assay serves as a reliable screening tool, and positive samples can be reflexed to subtyping panels. Another consideration is the potential for PCR inhibition in oral fluid samples due to mucopolysaccharides and other organic compounds. The inclusion of an internal positive control (e.g., an exogenous RNA template) is recommended to monitor inhibition [1].

The multiplex panel aligns with broader trends in veterinary molecular diagnostics toward high-throughput, cost-effective, and sample-sparing methods. Digital droplet PCR (ddPCR) offers absolute quantification without standard curves, but RT-qPCR remains the gold standard for routine surveillance due to its lower cost and established workflow [7]. The panel can be integrated into existing laboratory workflows with minimal additional validation.

Cross-linking to related resources on this portal enhances the utility of this article. Readers are directed to the companion articles on Multiplex Real-Time RT-PCR Panel for Simultaneous Detection of Porcine Reproductive and Respiratory Syndrome Virus, Porcine Circovirus Type 2, and Swine Influenza A Virus in Oral Fluids and Development and Field Validation of a Multiplex Real-Time RT-PCR Panel for Simultaneous Detection of Porcine Reproductive and Respiratory Syndrome Virus, Porcine Circovirus Type 2, and Swine Influenza A Virus in Oral Fluids. Additional information on individual pathogens can be found in the Porcine Reproductive and Respiratory Syndrome Virus and Swine Influenza A Virus reference pages. For biosecurity protocols, refer to the pet health guidelines for swine disease management.

Conclusion

A multiplex real-time RT-qPCR panel for simultaneous detection of PCV2, PRRSV, and SIV in swine oral fluids has been developed and validated. The assay demonstrates high analytical sensitivity and specificity, excellent diagnostic agreement with singleplex methods, and practical advantages for herd-level surveillance. Oral fluid sampling combined with multiplex molecular detection represents a powerful strategy for monitoring respiratory pathogens in swine populations. Future work should focus on expanding the panel to include additional respiratory agents and on integrating the assay with point-of-care platforms.

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