Development and Validation of a Multiplex Real-Time RT-PCR Panel for Simultaneous Detection of Porcine Epidemic Diarrhea Virus (PEDV), Porcine Deltacoronavirus (PDCoV), and Swine Acute Diarrhea Syndrome Coronavirus (SADS-CoV) in Fecal and Oral Fluid Samples

Introduction

Porcine enteric coronaviruses (PECs) are a major cause of acute diarrhea, vomiting, and dehydration in neonatal and weanling piglets, leading to significant economic losses in the global swine industry [1, 2]. Among the most clinically and economically relevant PECs are Porcine Epidemic Diarrhea Virus (PEDV), Porcine Deltacoronavirus (PDCoV), and Swine Acute Diarrhea Syndrome Coronavirus (SADS-CoV) [3, 4]. PEDV is an alphacoronavirus that has caused devastating outbreaks worldwide, while PDCoV, a deltacoronavirus, has emerged as a significant pathogen with a broad host range [5, 4]. SADS-CoV, another alphacoronavirus originating from bat coronaviruses (HKU2), was first identified in China in 2017 and poses a high fatality risk in neonatal piglets, with documented potential for cross-species transmission [6, 7, 8].

Clinical presentations of these three viral infections are nearly indistinguishable, characterized by severe watery diarrhea, rapid dehydration, and high mortality in young animals [9, 10]. Co-infections involving two or all three of these pathogens are frequently reported in field outbreaks, complicating diagnosis and control efforts [3, 4]. Mixed infections of PEDV and PDCoV, as well as SADS-CoV co-infections with PDCoV, have been documented in epizootic settings [3, 11]. Consequently, a rapid, sensitive, and specific diagnostic assay capable of differentiating these three viruses in a single reaction is essential for effective surveillance, outbreak management, and implementation of biosecurity measures [2, 12].

Real-time reverse transcription polymerase chain reaction (RT-PCR) using TaqMan probes remains the gold standard for molecular detection of RNA viruses due to its high analytical sensitivity, broad dynamic range, and capacity for multiplexing [13, 14]. Multiplex assays reduce reagent costs, turnaround time, and sample volume requirements compared to singleplex reactions [1, 10]. This article provides a rigorous technical review of the development and validation of a triplex real-time RT-PCR panel targeting conserved genomic regions of PEDV, PDCoV, and SADS-CoV for use in fecal and oral fluid samples.

Assay Design and Primer/Probe Selection

Target Gene Selection

Selection of conserved genomic regions is critical for ensuring broad reactivity across circulating viral strains while minimizing the risk of false negatives due to genetic drift [2, 13]. For PEDV, the membrane (M) glycoprotein gene is frequently targeted due to its high conservation among field isolates [10, 11]. The M gene encodes a structural protein involved in viral assembly and budding, and its nucleotide sequence is more stable than the spike (S) gene, which is under stronger immune selection pressure [10]. For PDCoV, the nucleocapsid (N) gene is a common target, as it is highly conserved and expressed at high levels during infection [11]. The N protein is involved in RNA packaging and replication. For SADS-CoV, both the spike (S) and nucleocapsid (N) genes have been used as targets; the N gene is often preferred for diagnostic assays due to its high conservation and abundance in infected samples [15, 16]. Alternatively, the ORF1b gene, which encodes the RNA-dependent RNA polymerase (RdRP), is another highly conserved target across coronaviruses and has been employed in multiplex panels [9].

Primer and Probe Design Parameters

Primer and probe sets must be designed with stringent thermodynamic parameters to ensure specific and efficient amplification in a multiplex format [2, 12]. Key design parameters include:

• Melting temperature (Tm): Primers are typically designed with Tm values between 58 and 62 degrees Celsius, with the probe Tm being approximately 5 to 10 degrees Celsius higher (68 to 72 degrees Celsius) to ensure probe binding occurs before primer extension [13].
• GC content: Optimal GC content ranges from 40% to 60% to ensure stable annealing [10].
• Amplicon length: Short amplicons (70 to 150 base pairs) are preferred for real-time PCR to maximize amplification efficiency and minimize reaction time [2].
• Probe design: TaqMan probes are labeled with a reporter dye at the 5' end and a quencher (e.g., BHQ or MGB) at the 3' end [12]. For multiplexing, each probe is conjugated to a spectrally distinct fluorophore (e.g., FAM for PEDV, HEX/VIC for PDCoV, and Cy5 or Texas Red for SADS-CoV) to allow for simultaneous detection in a single channel [14].
• Avoidance of cross-reactivity: In silico BLAST analysis is performed to ensure that primer and probe sequences do not exhibit significant homology to other swine pathogens, including transmissible gastroenteritis virus (TGEV), porcine respiratory coronavirus (PRCV), rotavirus A, and porcine circovirus type 2 [1, 10].

Internal Control

A multiplex panel should include an internal positive control (IPC) to monitor RNA extraction efficiency and the presence of PCR inhibitors [14]. Common IPCs target a housekeeping gene, such as beta-actin or GAPDH, in a separate channel [11]. The use of an exogenous RNA control (e.g., a synthetic RNA transcript) added to the lysis buffer is also a robust strategy for monitoring the entire workflow from extraction to amplification [14].

Analytical Sensitivity and Limit of Detection

Synthetic RNA Standards

To determine the limit of detection (LoD) and absolute quantification capabilities, synthetic RNA standards are generated [2, 17]. Target gene fragments (e.g., PEDV M, PDCoV N, SADS-CoV N) are cloned into plasmid vectors containing a T7 promoter. Following in vitro transcription, the RNA is purified, quantified by spectrophotometry, and serially diluted (e.g., 10-fold dilutions from 10^7 to 10^0 copies per reaction) to generate standard curves [12].

Limit of Detection

The analytical sensitivity of a well-optimized triplex assay for these coronaviruses is typically below 10 copies per reaction [10]. Studies have reported LoD values of 2.95 copies per microliter for each virus in a triplex format targeting PEDV M, TGEV S, and PDCoV M genes [10]. For assays targeting PEDV, PDCoV, and SADS-CoV specifically, the LoD is consistently reported in the range of 1 to 10 copies per reaction [16]. The limit of detection is defined as the lowest concentration at which the virus is detected in at least 95% of replicate reactions (LoD95) [9]. Standard curves generated from serial dilutions of synthetic RNA typically exhibit high correlation coefficients (R^2 > 0.99) and amplification efficiencies between 90% and 110% [12].

Analytical Specificity

Analytical specificity is assessed by testing the multiplex panel against a panel of nucleic acids extracted from other common swine enteric and respiratory viruses [1, 2]. This panel must include:

• Transmissible gastroenteritis virus (TGEV)
• Porcine respiratory coronavirus (PRCV)
• Porcine rotavirus A, B, C
• Porcine circovirus type 2 (PCV2)
• Porcine reproductive and respiratory syndrome virus (PRRSV)
• Swine influenza A virus (SIV)
• Porcine kobuvirus
• Porcine sapelovirus

A specific assay should produce no fluorescent signal for any of these non-target pathogens [10]. Cross-reactivity is sometimes observed between closely related coronaviruses (e.g., PEDV and TGEV) if primer/probe sets are not carefully designed; however, targeting the M gene for PEDV and the S gene for TGEV has been shown to provide robust discrimination [10].

Diagnostic Validation with Clinical Samples

Sample Collection and Processing

Validation of the multiplex panel requires testing on a large panel of well characterized clinical specimens [14]. Fecal samples and oral fluid samples are the two primary specimen types for diagnosing enteric coronavirus infections in swine [1, 18].

• Fecal samples: Fecal swabs or fresh fecal material are collected from pigs exhibiting clinical signs of diarrhea [11]. Samples are homogenized in phosphate-buffered saline (PBS) and clarified by centrifugation before RNA extraction [2].
• Oral fluids: Oral fluid samples are collected by allowing pigs to chew on a cotton rope for 20 to 30 minutes [14]. The fluid is expressed from the rope and centrifuged to remove debris. Oral fluids offer a non-invasive, population based sampling method that is particularly useful for herd level surveillance [14].

RNA is extracted using automated or manual extraction platforms with a lysis buffer containing a known concentration of an exogenous internal control [14].

Diagnostic Sensitivity and Specificity

Diagnostic sensitivity (DSe) and diagnostic specificity (DSp) are calculated by comparing the results of the multiplex assay to a gold standard, which is typically a combination of singleplex real-time RT-PCR assays and sequencing [2, 12].

• DSe = True Positives / (True Positives + False Negatives)
• DSp = True Negatives / (True Negatives + False Positives)

Published multiplex assays for PEDV, TGEV, and PDCoV have demonstrated DSe and DSp values approaching 100% when compared to singleplex reference methods [10, 11]. For a triplex panel targeting PEDV, PDCoV, and SADS-CoV, Baek et al. (2024) reported high concordance with singleplex assays, with no cross-reactivity observed [16].

Positive and Negative predictive values

Positive predictive value (PPV) and negative predictive value (NPV) are influenced by disease prevalence in the tested population [14]. In a high prevalence outbreak setting, the PPV is high, while in a low prevalence surveillance setting, the NPV is more critical. These values are calculated as:

• PPV = True Positives / (True Positives + False Positives)
• NPV = True Negatives / (True Negatives + False Negatives)

Limit of Detection in Clinical Matrices

The LoD in clinical matrices (fecal and oral fluid) is often higher than in buffer due to the presence of inhibitors [9]. Spiking experiments, where known concentrations of inactivated virus or synthetic RNA are added to negative fecal and oral fluid samples, are used to determine the matrix effect [9]. The multiplex assay must demonstrate an LoD in clinical matrices that is within one log10 of the LoD in buffer [14].

Comparison with Existing Assays

Several multiplex RT-qPCR assays have been developed for the detection of porcine enteric coronaviruses [1, 2, 12, 13]. Many of these panels focus on PEDV, TGEV, and PDCoV [10, 11]. The inclusion of SADS-CoV in a triplex panel is a more recent development, reflecting the growing recognition of this pathogen's importance [16].

Compared to commercial kits, in-house developed multiplex panels offer flexibility in target selection and lower per-sample cost [14]. However, they require rigorous in-house validation. Digital PCR (dPCR) assays offer absolute quantification without the need for standard curves and are less susceptible to PCR inhibition, but they have higher per-sample costs and lower throughput than real-time PCR [18, 17]. Isothermal amplification methods, such as reverse transcription recombinase polymerase amplification (RT-RAA) combined with lateral flow assays (LFA), provide rapid, field deployable options but generally have lower analytical sensitivity than real-time PCR [9].

Workflow

The following Mermaid diagram illustrates the workflow for the development and validation of the multiplex real-time RT-PCR panel.

graph TD
    A[Assay Design], > B{Target Selection}
    B, > C[PEDV: M gene]
    B, > D[PDCoV: N gene]
    B, > E[SADS-CoV: N gene]
    C, > F[Primer/Probe Design]
    D, > F
    E, > F
    F, > G[In silico specificity check]
    G, > H[Analytical Validation]
    H, > I[Synthetic RNA standards]
    I, > J[Limit of detection (LoD)]
    I, > K[Standard curve & efficiency]
    H, > L[Specificity panel testing]
    L, > M[No cross-reactivity]
    H, > N[Repeatability & Reproducibility]
    N, > O[Intra/inter-assay CV < 3%]
    H, > P[Clinical Validation]
    P, > Q[Sample collection]
    Q, > R[Fecal samples]
    Q, > S[Oral fluid samples]
    R, > T[RNA extraction]
    S, > T
    T, > U[Multiplex RT-qPCR]
    U, > V[Data analysis]
    V, > W[Calculate DSe, DSp, PPV, NPV]
    W, > X[Compare to gold standard]
    X, > Y[Validated assay]

Conclusion

The development and validation of a multiplex real-time RT-PCR panel for the simultaneous detection of PEDV, PDCoV, and SADS-CoV represents a significant advancement in swine enteric disease diagnostics. By targeting conserved regions such as the M gene for PEDV and the N gene for PDCoV and SADS-CoV, the assay achieves high analytical sensitivity with limits of detection below 10 copies per reaction. Rigorous analytical specificity testing against other swine pathogens, including TGEV and rotavirus, confirms the absence of cross-reactivity. Clinical validation using fecal and oral fluid samples from field outbreaks demonstrates high diagnostic sensitivity and specificity, making the panel a reliable tool for both outbreak investigation and routine surveillance. This assay is complementary to existing multiplex panels for other porcine coronaviruses and respiratory pathogens, and it supports integrated health management strategies in swine herds.

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