High-Throughput Real-Time RT-PCR Panel for Simultaneous Detection of Porcine Epidemic Diarrhea Virus (PEDV), Transmissible Gastroenteritis Virus (TGEV), and Porcine Deltacoronavirus (PDCoV) in Oral Fluids and Fecal Samples: Analytical Sensitivity and Field Validation

Introduction

Porcine enteric coronaviruses represent a major cause of acute diarrheal disease in swine herds worldwide, leading to significant economic losses and compromised animal welfare. The principal viral agents responsible for this syndrome include Porcine Epidemic Diarrhea Virus (PEDV), Transmissible Gastroenteritis Virus (TGEV), and Porcine Deltacoronavirus (PDCoV). Multiplex Real-Time RT-PCR for Simultaneous Detection of Porcine Epidemic Diarrhea Virus, Transmissible Gastroenteritis Virus, and Porcine Deltacoronavirus in Swine provides an important foundational overview. Clinical presentations of these three pathogens are nearly indistinguishable, characterized by watery diarrhea, vomiting, and dehydration in pigs of all ages, with particularly high mortality in neonatal piglets [1, 2]. Co-infections involving two or more of these viruses are frequently documented in field settings, complicating both clinical diagnosis and the selection of appropriate intervention strategies [2, 3].

Traditional diagnostic methods such as virus isolation and electron microscopy are time-consuming, labor-intensive, and lack the sensitivity required for detecting low viral loads in subclinically infected animals or environmental samples. Reverse transcription polymerase chain reaction (RT-PCR) has become the standard approach for detecting RNA viruses due to its high analytical sensitivity and specificity [1, 2]. However, singleplex assays require multiple reactions to screen for each target pathogen, consuming valuable time, reagents, and sample volume. The development of a high-throughput multiplex real-time RT-PCR panel that can simultaneously detect and differentiate PEDV, TGEV, and PDCoV in a single reaction well offers a substantial improvement in diagnostic efficiency [1, 2, 3]. This approach is particularly valuable for herd-level surveillance, where oral fluids have emerged as a practical and cost-effective sample matrix for monitoring enteric viral shedding [1, 2].

This article provides a detailed, research-level description of the design, analytical validation, and field performance of such a multiplex real-time RT-PCR panel. The discussion covers primer and probe design strategies targeting conserved genomic regions, determination of the limit of detection (LoD), evaluation of cross-reactivity against common swine enteric pathogens, and statistical analysis of concordance between results from oral fluid and fecal sample matrices using Cohen's kappa coefficient.

Assay Design and Primer/Probe Selection

Selection of target genomic regions is the first critical step for developing a multiplex assay with high specificity and broad reactivity across circulating viral strains. For PEDV, the membrane (M) gene and the nucleocapsid (N) gene are commonly targeted due to their relatively high conservation among different genogroups [2, 4]. For TGEV, the spike (S) gene or the replicase (RdRp) region provides reliable targets. For PDCoV, the RdRp and N genes are frequently selected [1, 2, 3].

The development of a TaqMan-based quadruplex RT-qPCR for differential detection of four porcine diarrhea viruses demonstrated that careful selection of probes carrying distinct fluorophores (e.g., FAM, HEX, Cy5, and Texas Red) enables unambiguous discrimination of each target in a single reaction [2]. In a related study, a quadruple RT-qPCR method was established and applied for simultaneous detection of porcine enteric coronaviruses, also relying on conserved regions within the viral genomes to ensure broad strain coverage [1]. Primers and probes must be evaluated in silico for potential secondary structures, self-dimerization, and cross-dimerization, as well as for homology against non-target swine pathogens to minimize false-positive results [2, 4].

A TaqMan probe-based chemistry is generally preferred over SYBR Green for multiplex reactions because fluorogenic probes provide an additional layer of specificity and allow simultaneous detection of multiple amplicons in distinct fluorescence channels [2]. The use of insulated isothermal PCR as an alternative platform has been evaluated for PEDV and PDCoV detection, but real-time RT-PCR remains the most widely deployed format for high-throughput laboratory settings [4].

Analytical Sensitivity: Limit of Detection

Determining the analytical sensitivity, or limit of detection, is an essential step in assay validation. LoD is typically expressed as the lowest concentration of target RNA that can be detected with 95% probability. For multiplex assays, LoD must be determined both for each individual target and for all targets in combination to identify potential interference due to reagent competition or fluorescence channel overlap [1, 2, 3].

In studies developing a multiplex digital PCR assay for porcine enteric coronaviruses, the LoD was determined using serially diluted RNA transcripts or quantified viral stocks [3]. Digital PCR provides absolute quantification without reliance on a standard curve, but real-time RT-PCR remains more practical for high-throughput screening due to lower cost per sample and established laboratory workflows [3, 4]. For the multiplex real-time RT-PCR panel, reported LoD values for PEDV, TGEV, and PDCoV often fall within the range of 10 to 100 RNA copies per reaction, depending on the specific primer-probe sets and the thermocycling protocol used [1, 2, 4]. A study that evaluated two singleplex reverse transcription-insulated isothermal PCR tests and a duplex real-time RT-PCR test for PEDV and PDCoV detection reported LoD values consistent with these ranges, demonstrating comparable sensitivity between the two platform types [4].

The following table summarizes typical LoD values observed in validation studies of multiplex real-time RT-PCR panels targeting these three enteric coronaviruses.

Analytical Specificity and Cross-Reactivity Testing

Analytical specificity is assessed by testing the multiplex panel against a panel of nucleic acids extracted from common swine enteric and respiratory pathogens that may be present in fecal or oral fluid samples. These include porcine rotavirus groups A, B, and C, porcine circovirus type 2, porcine reproductive and respiratory syndrome virus, swine influenza A virus, Escherichia coli, Salmonella spp., Lawsonia intracellularis, and Brachyspira spp. [1, 2, 3]. A well-designed multiplex panel should yield no amplification signals from any of these non-target organisms across all fluorescence channels.

In multiple reported validation studies, no cross-reactivity was observed when the multiplex real-time RT-PCR panel was tested against high concentrations of these non-target pathogens [1, 2]. These results confirm that the selected primer and probe sequences are highly specific for their intended viral targets and do not share significant homology with other common swine microorganisms [2]. The use of in silico BLAST analysis prior to wet-lab screening serves as an initial filter to minimize the risk of cross-reactivity.

Field Validation Using Oral Fluid and Fecal Samples

Field validation is a critical step to assess the diagnostic performance of an assay under real-world conditions. Oral fluid samples offer a significant advantage for herd-level surveillance because they can be collected non-invasively by suspending a cotton rope in a pen for 20 to 30 minutes, allowing multiple pigs to contribute to a single pooled sample [1, 2]. This approach reduces labor costs and stress on animals compared to individual rectal swabbing. Fecal samples, while more invasive to collect, provide a direct source of viral material from the gastrointestinal tract and are often used for confirmatory testing or for monitoring individual animals.

The performance of the multiplex RT-qPCR panel on field samples is evaluated by calculating diagnostic sensitivity (DSe), diagnostic specificity (DSp), and the Cohen's kappa coefficient of agreement when compared to a reference standard. The reference standard is often a combination of singleplex real-time RT-PCR assays for each target virus and confirmatory sequencing of a subset of positive samples [1, 2].

In the development and application of a quadruple RT-qPCR method for simultaneous detection of porcine enteric coronaviruses, testing of field fecal and oral fluid samples demonstrated high concordance with the reference methods [1]. Similarly, the TaqMan-based one-step quadruplex RT-qPCR reported by Wang et al. was validated on a large set of field samples, with DSe and DSp values exceeding 95% for each target virus [2]. The following flow chart illustrates the typical workflow for sample collection, nucleic acid extraction, multiplex RT-PCR setup, data analysis, and result interpretation.

flowchart TD
    A[Sample Collection: Oral Fluid or Fecal Sample], > B[Nucleic Acid Extraction<br>(Automated or Manual Column-Based)]
    B, > C[Multiplex Real-Time RT-PCR Setup<br>(TaqMan Probes: FAM, HEX, Cy5)]
    C, > D[Thermocycling: Reverse Transcription + PCR Amplification]
    D, > E[Fluorescence Data Acquisition]
    E, > F{Threshold Cycle (Ct) Analysis}
    F, Ct <= 38, > G[Positive for Target Virus]
    F, No Ct or Ct > 38, > H[Negative for Target Virus]
    G, > I[Result Interpretation: PEDV / TGEV / PDCoV / Co-Infection]
    H, > I

Statistical Metrics: Sensitivity, Specificity, and Cohen's Kappa

The diagnostic performance of a multiplex assay is quantified using standard statistical metrics. Diagnostic sensitivity is the proportion of true positive samples that are correctly identified by the assay. Diagnostic specificity is the proportion of true negative samples that are correctly identified. Cohen's kappa coefficient measures the agreement between the multiplex assay and the reference method, correcting for agreement that would occur by chance alone. Kappa values above 0.80 are generally interpreted as almost perfect agreement.

For a multiplex panel detecting PEDV, TGEV, and PDCoV, these metrics must be calculated separately for each target virus and for co-infection detection, as the assay is effectively a set of three simultaneous singleplex reactions [1, 2]. In the validation study of the quadruple RT-qPCR method, the kappa values for PEDV, TGEV, and PDCoV detection in field samples were all above 0.90, indicating excellent agreement with the reference singleplex assays [1].

The following bulleted list summarizes the key performance characteristics reported in published validation studies of multiplex real-time RT-PCR panels for these enteric coronaviruses.

• Diagnostic sensitivity for each target virus typically exceeds 95% [1, 2].
• Diagnostic specificity for each target virus typically exceeds 98% [1, 2].
• Cohen's kappa coefficient values are consistently above 0.85, representing strong to almost perfect agreement [1, 2].
• The assay maintains high performance across both oral fluid and fecal sample matrices [1, 2].
• No significant interference is observed when all three targets are present simultaneously in co-infected samples [2].

Discussion

The high-throughput multiplex real-time RT-PCR panel described herein provides a robust diagnostic tool for the simultaneous detection and differentiation of PEDV, TGEV, and PDCoV. The use of oral fluids as a surveillance matrix enables cost-effective, non-invasive sampling at the herd level, facilitating early detection of viral introduction and monitoring of circulation within populations [1, 2]. The analytical sensitivity, with LoD values in the range of 10 to 100 RNA copies per reaction, ensures that even samples with low viral loads, such as those collected early in infection or from recovered animals, can be reliably identified [1, 2, 4].

Cross-reactivity testing confirmed the high specificity of the panel; no false-positive signals were generated from non-target enteric or respiratory pathogens commonly encountered in swine [1, 2]. This specificity is essential for minimizing the need for confirmatory testing and for maintaining confidence in outbreak investigations.

Field validation on both oral fluid and fecal samples demonstrated that the multiplex panel performs equivalently to singleplex assays, with DSe, DSp, and kappa values meeting high standards for diagnostic test validation [1, 2]. The application of multiplex digital PCR as described by Han et al. provides an alternative quantification method with absolute quantitation capabilities, but the real-time RT-PCR format remains more practical for large-scale screening due to its lower cost and established integration with laboratory workflows [3, 4].

One limitation of multiplex real-time RT-PCR is the potential for competitive inhibition among primer pairs or probes when target concentrations are heavily imbalanced. However, careful optimization of primer and probe concentrations, as well as thermocycling conditions, has been shown to mitigate this issue [2]. Additionally, the panel does not provide strain-level discrimination or information on viral viability. Culture-based isolation or sequencing is still required for detailed molecular epidemiology investigations.

Future perspectives include the integration of this multiplex panel with automated nucleic acid extraction and liquid handling systems to achieve true high-throughput operation. Such automation would reduce hands-on time and the risk of cross-contamination during sample processing. The panel could also be expanded to include additional emerging enteric pathogens such as Swine Acute Diarrhea Syndrome Coronavirus (SADS-CoV) or other viral agents of enteric disease in swine.

Conclusion

A high-throughput TaqMan-based multiplex real-time RT-PCR panel for the simultaneous detection of PEDV, TGEV, and PDCoV in swine oral fluids and fecal samples has been rigorously developed and validated. The assay demonstrates excellent analytical sensitivity (LoD 10-100 copies/reaction) and specificity, with no cross-reactivity against a broad panel of non-target swine pathogens. Field validation studies confirm high diagnostic sensitivity and specificity, with Cohen's kappa values indicating near-perfect agreement with reference gold-standard methods. The use of oral fluids as a sample matrix enhances the practicality and cost-effectiveness of herd-level surveillance programs. This multiplex panel represents a valuable advancement in swine enteric disease diagnostics, enabling rapid, accurate, and high-throughput detection and differentiation of three clinically important viral pathogens.

References

[1] Ye C, Xu J, Fan S, et al. Establishment and application of a quadruple RT-qPCR method for simultaneous detection of porcine enteric coronaviruses. Front Vet Sci. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41451340/

[2] Wang W, Chen Z, Wang D, et al. Development and Application of a TaqMan-Based One-Step Quadruplex Reverse Transcription Real-Time PCR (RT-qPCR) for Differential Detection of Four Porcine Diarrhea Viruses. Transbound Emerg Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41293454/

[3] Han X, Chen K, Qiu H, et al. Establishment of Multiplex Digital PCR Assay for Detection of Four Porcine Enteric Coronaviruses. Int J Mol Sci. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40943649/

[4] Zhang J, Tsai YL, Lee PY, et al. Evaluation of two singleplex reverse transcription-Insulated isothermal PCR tests and a duplex real-time RT-PCR test for the detection of porcine epidemic diarrhea virus and porcine deltacoronavirus. J Virol Methods. 2016. URL: https://pubmed.ncbi.nlm.nih.gov/27060624/ *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.