Development and Validation of a Multiplex Real-Time RT-PCR Assay for Simultaneous Detection of Porcine Epidemic Diarrhea Virus, Transmissible Gastroenteritis Virus, and Porcine Deltacoronavirus in Fecal and Oral Fluid Samples

Introduction

Porcine enteric coronaviruses (PECoVs) are a major cause of acute gastroenteritis in swine herds worldwide, leading to significant economic losses due to mortality, morbidity, and reduced growth performance [1]. The three principal viral agents are Porcine Epidemic Diarrhea Virus (PEDV), Transmissible Gastroenteritis Virus (TGEV), and Porcine Deltacoronavirus (PDCoV) [1, 2]. These viruses share a common clinical presentation of watery diarrhea, vomiting, and dehydration, particularly in neonatal piglets, making differential diagnosis through molecular assays essential for appropriate management and control [1, 3]. Co-infections are frequently reported, further complicating clinical diagnosis and highlighting the need for multiplex detection platforms [4, 5].

Fecal samples and oral fluids have become the preferred diagnostic matrices for enteric virus surveillance due to their non-invasive or minimally invasive collection and high viral shedding titers during acute infection [3, 6]. However, fecal samples often contain inhibitors that can compromise reverse transcription and amplification efficiencies, while oral fluids present lower viral loads and require sensitive, robust assays [4, 5]. To address these challenges, multiple research groups have developed and validated multiplex real-time reverse transcription polymerase chain reaction (RT-qPCR) assays targeting conserved genomic regions of PEDV, TGEV, and PDCoV [4, 2, 6]. This article reviews the assay design principles, sample processing protocols, RNA extraction considerations, and validation parameters that underpin a credible multiplex RT-qPCR panel for these three viruses in fecal and oral fluid matrices. Supporting information can be found in related articles on similar diagnostic panels, such as the Development and Validation of a Multiplex Real-Time RT-PCR Panel for Simultaneous Detection of Porcine Epidemic Diarrhea Virus (PEDV), Transmissible Gastroenteritis Virus (TGEV), and Porcine Deltacoronavirus (PDCoV) in Fecal and Environmental Samples.

Assay Design: Primer and Probe Selection

The success of a multiplex RT-qPCR assay depends critically on the selection of primer and probe sequences that target highly conserved regions within each viral genome while avoiding cross-reactivity with other porcine pathogens and host nucleic acids [4, 6]. For PEDV, the nucleocapsid (N) gene and membrane (M) gene are frequently targeted because of their relatively high conservation among circulating strains [2, 5]. TGEV assays typically target the N gene or a region within the spike (S) gene that distinguishes TGEV from porcine respiratory coronavirus (PRCV), a related alphacoronavirus that shares sequence homology [4, 6]. For PDCoV, the N gene is also a common target, although some assays use the M gene or a non-structural protein gene region to differentiate PDCoV from other deltacoronaviruses [2, 5].

Lazov et al. [4] designed a multiplex RT-qPCR panel using TaqMan probes labeled with distinct fluorophores (e.g., FAM, HEX, Cy5) to enable single-tube, three-target detection. Ye et al. [2] and Zhu et al. [5] extended similar designs by incorporating additional targets such as porcine rotavirus A (PoRVA) to create quadruplex assays. The primer and probe mixes are optimized to avoid formation of primer dimers and to ensure balanced amplification efficiencies across all targets, typically evaluated by serial dilution of in vitro transcribed RNA standards or known positive clinical samples [7, 6]. The reaction conditions commonly employ a one-step RT-qPCR format combining reverse transcription and PCR amplification in a single reaction using a thermostable reverse transcriptase and a hot-start DNA polymerase [4, 5].

Table 1 summarizes typical target genes and fluorophore assignments reported in the literature.

Table 1. Representative target genes and fluorophores used in multiplex RT-qPCR assays for PEDV, TGEV, and PDCoV.

Sample Collection and Processing for Fecal and Oral Fluid Matrices

Fecal samples are typically collected from the rectum or freshly voided feces using sterile swabs or collection cups [3, 6]. The sample is then suspended in a transport medium such as phosphate-buffered saline (PBS) or a commercial viral transport medium to create a 10% to 20% (w/v) fecal slurry [5]. This slurry is vortexed thoroughly and clarified by centrifugation at low speed (e.g., 3000–5000 × g for 10 minutes) to remove large particulate debris, followed by filtration through a 0.45 μm syringe filter to further reduce inhibitors [4, 7]. Oral fluids are collected by suspending a clean cotton rope in the pen for 20–30 minutes; the rope is then placed in a plastic bag and the fluid is expressed by manual pressure [3, 8]. The oral fluid sample is centrifuged at 2000 × g for 10 minutes to pellet feed particles and cellular debris, and the supernatant is used for RNA extraction [3].

RNA Extraction Efficiency

RNA extraction from fecal and oral fluid matrices must overcome the presence of polysaccharides, humic acids, bile salts, and other inhibitors that can severely reduce RT-qPCR sensitivity [4, 6]. Most published multiplex assays use a commercial silica membrane column-based or magnetic bead-based extraction kit with a carrier RNA to improve recovery of low-abundance viral RNA [4, 5]. An important validation step is to spike known quantities of a non-competitive internal control (e.g., an exogenous RNA transcript or a synthetic RNA construct) into each sample before extraction to monitor for efficiency losses and to detect inhibition [2, 7]. Acceptable extraction efficiency is typically defined by a cycle threshold (Ct) value within a predefined window (e.g., Ct 28–32 for the internal control) [6]. Han et al. [7] reported that extraction efficiency from fecal samples ranged between 60% and 95% depending on the commercial kit used, and recommended including a negative extraction control in every batch.

Validation Parameters

Analytical Sensitivity (Limit of Detection)

The limit of detection (LOD) for a multiplex RT-qPCR assay is established using serial dilutions of quantified viral RNA transcripts or cell culture–derived virus stocks [4, 7]. LOD is defined as the lowest concentration at which the target is detected in at least 95% of replicates. Lazov et al. [4] reported LODs of 10 RNA copies per reaction for PEDV, 10 copies for TGEV, and 5 copies for PDCoV in singleplex and multiplex formats without significant loss of sensitivity. Ye et al. [2] and Zhu et al. [5] demonstrated LODs ranging from 1 to 10 copies per reaction for their quadruplex assays. The LOD may be slightly higher in fecal extracts compared to oral fluids due to matrix inhibition, but optimized reaction master mixes containing additives such as bovine serum albumin (BSA) or betaine can mitigate this effect [3, 6].

Analytical Specificity and Cross-Reactivity

Analytical specificity is assessed by testing the multiplex panel against a panel of other porcine viruses (e.g., porcine reproductive and respiratory syndrome virus, porcine circovirus type 2, swine influenza A virus, porcine rotavirus A) and host genomic DNA [4, 6]. No cross-reactivity should be observed. Additionally, the primers must discriminate between TGEV and PRCV; assays targeting the S gene deletion region allow differentiation [4]. Zhu et al. [5] and Zhou et al. [6] reported 100% specificity for their triplex and quadruplex assays when tested against 20 or more non-target viruses. In silico analysis using BLAST alignment of primer and probe sequences further supports specificity [2].

Repeatability and Reproducibility

Repeatability (intra-assay precision) is evaluated by testing multiple replicates (typically 6–10) of the same sample within a single run, while reproducibility (inter-assay precision) is assessed across runs performed on different days by different operators [4, 5]. Acceptable coefficients of variation (CV) for Ct values are generally below 5% for intra-assay and below 10% for inter-assay [7, 6]. Han et al. [7] observed intra-assay CVs of 1.2%–3.4% and inter-assay CVs of 2.1%–5.6% for their multiplex digital PCR assay, demonstrating high precision. Similar performance is expected for well-optimized RT-qPCR panels [4].

Performance Compared to Singleplex Assays

A critical validation step is to compare the multiplex assay against reference singleplex RT-qPCR assays for each virus using a panel of clinical fecal and oral fluid samples (e.g., 100–200 samples) [2, 3]. The multiplex assay should achieve high concordance (kappa value >0.8) and a correlation coefficient (R^2) >0.9 for Ct values across positive samples [4, 5]. Song et al. [3] reported a 98.5% agreement between their multiplex method and singleplex tests for 200 field fecal samples. Slight reductions in sensitivity (0.5–1 Ct shift) are sometimes observed in multiplex due to competition for reagents, but these are acceptable if the LOD remains clinically relevant [4, 6]. Alternative amplification technologies such as digital PCR [7] or isothermal methods [9, 8] offer different trade-offs in sensitivity, throughput, and instrument cost.

Application in Surveillance and Outbreak Investigation

The validated multiplex RT-qPCR assay is well suited for routine surveillance programs that monitor the circulation of enteric coronaviruses in swine populations [1, 3]. Oral fluids provide a practical herd-level sample that can be collected routinely without individual animal handling, enabling early detection of emerging strains [3, 8]. Fecal samples from diarrheic piglets remain the gold standard for individual case confirmation and for genotyping [4, 5]. By incorporating the multiplex panel into a diagnostic algorithm, laboratories can rapidly differentiate between PEDV, TGEV, and PDCoV and guide appropriate biosecurity interventions and vaccination strategies [1, 2].

The Mermaid diagram below outlines a typical workflow from sample collection to result reporting.

flowchart TD
    A[Sample Collection: Fecal swabs or Oral fluids], > B[Transport to laboratory at 4°C]
    B, > C[Sample processing: clarify by centrifugation]
    C, > D[RNA extraction with internal control spike]
    D, > E{Extraction QC: internal control Ct ok?}
    E, Yes, > F[Multiplex RT-qPCR: PEDV, TGEV, PDCoV]
    E, No, > G[Repeat extraction or report inhibition]
    F, > H[Data analysis: Ct values, baseline correction]
    H, > I{Interpretation per target}
    I, Any Ct < 40, > J[Positive result for detected virus]
    I, All Ct > 40, > K[Negative result]
    J, > L[Report to herd veterinarian / surveillance system]
    K, > L

Conclusion

The development and validation of a multiplex real-time RT-PCR assay capable of detecting PEDV, TGEV, and PDCoV in fecal and oral fluid samples represents a significant advance in swine enteric disease diagnostics. Careful primer and probe design targeting conserved genomic regions, combined with robust sample processing and RNA extraction protocols, yields an assay with high analytical sensitivity, specificity, and reproducibility. The comparison with singleplex methods demonstrates excellent concordance, supporting the clinical utility of the multiplex format for both individual diagnosis and population-level surveillance. As circulating strains continue to evolve, periodic reevaluation of primer and probe sequences will be necessary to ensure sustained detection efficacy.

References

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