Multiplex Real-Time RT-PCR for Simultaneous Detection of Porcine Epidemic Diarrhea Virus, Transmissible Gastroenteritis Virus, and Porcine Deltacoronavirus in Field Fecal Samples

Introduction

Porcine enteric coronaviruses (PECoVs) are a major cause of acute gastroenteritis, vomiting, dehydration, and high mortality in neonatal piglets, resulting in substantial economic losses to the global swine industry [1, 2]. The three primary viral agents responsible for this clinical syndrome are Porcine Epidemic Diarrhea Virus (PEDV), Transmissible Gastroenteritis Virus (TGEV), and Porcine Deltacoronavirus (PDCoV) [3, 4]. These viruses belong to distinct coronavirus genera: PEDV and TGEV are classified within the genus Alphacoronavirus, while PDCoV is a member of the genus Deltacoronavirus [5, 6]. Despite their taxonomic differences, they produce nearly indistinguishable clinical signs in affected herds, making differential diagnosis based solely on clinical observation unreliable [7, 8].

Co-infections involving two or all three of these viruses are frequently documented in field settings, complicating both diagnosis and disease management [9, 10]. The simultaneous circulation of PEDV, TGEV, and PDCoV necessitates a diagnostic tool capable of rapid, sensitive, and specific detection of each pathogen in a single reaction [11]. Singleplex real-time reverse transcription polymerase chain reaction (RT-PCR) assays, while sensitive, require separate reactions for each target, increasing reagent costs, sample volume requirements, and turnaround time [5, 8]. Multiplex real-time RT-PCR assays address these limitations by incorporating multiple primer-probe sets in a single reaction vessel, enabling the concurrent detection and differentiation of viral targets through the use of fluorophore-labeled probes with distinct emission spectra [3, 4].

This article provides a detailed technical review of the development and validation of a multiplex real-time RT-PCR assay targeting conserved regions of the PEDV, TGEV, and PDCoV genomes for use with field fecal samples. The discussion encompasses primer and probe design strategies, RNA extraction protocols, one-step RT-PCR reaction conditions, analytical performance metrics including limit of detection and specificity, and field evaluation data demonstrating diagnostic sensitivity and specificity.

Primer and Probe Design Strategy

The selection of target genes is a critical determinant of assay specificity and inclusivity. For PEDV, the membrane (M) gene and the nucleocapsid (N) gene are commonly targeted due to their high conservation among circulating strains [9, 10]. The TGEV spike (S) gene is frequently selected because it provides adequate sequence diversity to differentiate TGEV from other alphacoronaviruses, while the PDCoV N gene or M gene offers a conserved target for reliable detection [3, 10]. In the multiplex assay described by Yan Li et al., specific primers and TaqMan probes were designed based on the PEDV M gene, the TGEV S gene, and the PDCoV M gene [9]. In contrast, the assay developed by Jianpeng Chen et al. utilized the PEDV M gene, TGEV S gene, and PDCoV N gene, incorporating an internal control targeting the porcine beta-actin gene [10].

Each TaqMan probe is conjugated with a distinct reporter fluorophore at the 5' end and a quencher molecule at the 3' end. Common fluorophore choices include FAM, HEX, ROX, and Cy5, selected to minimize spectral overlap and enable unambiguous channel discrimination on real-time PCR instruments [4, 8]. The probes are designed to anneal to the target amplicon between the forward and reverse primers. During the extension phase of PCR, the 5' to 3' exonuclease activity of the DNA polymerase cleaves the probe, separating the reporter from the quencher and generating a fluorescence signal proportional to the accumulating amplicon [5].

Primer and probe sequences must be evaluated for potential secondary structure formation, self-dimerization, and heterodimerization with other oligonucleotides in the multiplex pool. Thermodynamic parameters such as melting temperature (Tm) and guanine-cytosine (GC) content are optimized to ensure uniform annealing conditions across all targets, typically targeting a Tm of 58-60 degrees Celsius for primers and 68-70 degrees Celsius for probes [3, 6]. In silico analysis using BLAST against publicly available sequence databases is performed to confirm that the selected oligonucleotides do not cross-react with other swine pathogens, including Porcine Circovirus 2, Porcine Reproductive and Respiratory Syndrome Virus, and Swine Influenza A Virus [10].

RNA Extraction from Swine Fecal Samples

Fecal samples are a practical and non-invasive specimen type for diagnosing enteric viral infections in swine. The extraction of high-quality viral RNA from fecal material is challenging due to the presence of PCR inhibitors such as bile salts, polysaccharides, and complex organic compounds [2, 7]. A standardized RNA extraction protocol is essential for consistent assay performance.

Approximately 0.1 to 0.5 grams of fecal sample is homogenized in a suitable volume of phosphate-buffered saline (PBS) or a commercial lysis buffer. The homogenate is clarified by centrifugation at high speed (e.g., 12,000 x g for 5 minutes) to remove particulate debris [9]. The supernatant is then subjected to RNA extraction using a silica membrane-based column method or magnetic bead-based technology. These methods rely on the binding of nucleic acids to a solid phase in the presence of chaotropic salts, followed by washing steps to remove contaminants and a final elution in nuclease-free water [7].

The inclusion of a carrier RNA in the lysis buffer can improve recovery yields when viral loads are low. An internal control, such as an exogenous RNA transcript or a cellular housekeeping gene (e.g., beta-actin), is added to the lysis buffer to monitor extraction efficiency and detect the presence of PCR inhibitors [10]. The extracted RNA is stored at -80 degrees Celsius to preserve integrity until amplification.

One-Step Multiplex Real-Time RT-PCR Conditions

The multiplex assay is typically performed using a one-step RT-PCR format, in which reverse transcription and cDNA amplification occur sequentially in a single closed tube. This approach minimizes handling steps, reduces the risk of contamination, and shortens overall assay time [5, 8]. The reaction mixture contains a commercial one-step RT-PCR master mix that includes a thermostable reverse transcriptase, a DNA polymerase with 5' to 3' exonuclease activity, deoxynucleotide triphosphates (dNTPs), and optimized buffer components.

A representative reaction volume of 25 microliters includes 5 microliters of template RNA, 12.5 microliters of 2X master mix, and a cocktail of primers and probes at optimized final concentrations. For the triplex assay described by Yan Li et al., the final primer concentrations ranged from 0.2 to 0.4 micromolar per target, and probe concentrations ranged from 0.1 to 0.2 micromolar [9]. The thermal cycling protocol typically begins with a reverse transcription step at 50 degrees Celsius for 30 minutes, followed by an initial denaturation at 95 degrees Celsius for 2 to 5 minutes. Amplification proceeds for 40 to 45 cycles of denaturation at 95 degrees Celsius for 15 seconds and combined annealing/extension at 60 degrees Celsius for 30 to 60 seconds, during which fluorescence data are collected [4, 6].

The use of a one-step format reduces the risk of amplicon carryover contamination because the tube remains sealed throughout the process. Fluorescence signals are measured at the end of each annealing/extension step, and the cycle threshold (Ct) value for each target is determined by the instrument software. A sample is considered positive if the amplification curve crosses the threshold within 40 cycles [3, 10].

Analytical Sensitivity: Limit of Detection

The limit of detection (LoD) is the lowest concentration of target RNA that can be reliably detected with a defined probability, typically 95%. LoD is determined by testing serial dilutions of quantified viral RNA transcripts or cell culture-derived virus stocks of known titer [4, 9].

For the multiplex assay developed by Yan Li et al., the LoD for each virus was determined to be 2.95 x 10^0 copies per microliter of reaction [9]. Jianpeng Chen et al. reported an LoD of 10 copies per microliter for all three targets [10]. These values are comparable to or better than those reported for singleplex assays, indicating that the multiplex format does not substantially compromise sensitivity [5, 8]. The high analytical sensitivity is attributable to the efficient amplification of short amplicons (typically 80 to 150 base pairs) and the use of highly specific TaqMan probes [3].

The linear dynamic range of the assay is assessed by testing a 10-fold dilution series of target RNA. A strong linear correlation (R^2 > 0.99) between the log of the input RNA concentration and the Ct value is expected across at least six orders of magnitude [6, 7]. This linearity allows for semi-quantitative estimation of viral load in clinical samples, which can be useful for monitoring disease progression or response to intervention.

Analytical Specificity and Cross-Reactivity Testing

Analytical specificity is evaluated by testing the multiplex assay against a panel of nucleic acids extracted from other swine pathogens that may be present in fecal samples. This panel typically includes Porcine Circovirus 2, Porcine Reproductive and Respiratory Syndrome Virus, Swine Influenza A Virus, Porcine Rotavirus, Porcine Kobuvirus, Porcine Sapelovirus, and Porcine Teschovirus [2, 10].

In the studies reviewed, no cross-reactivity was observed for any of the non-target pathogens [9, 10]. The absence of non-specific amplification confirms that the primer and probe sets are highly specific for their intended targets. Additionally, the assay is tested against a panel of genetically diverse strains of PEDV, TGEV, and PDCoV to ensure inclusivity. The use of conserved target regions (M gene for PEDV and PDCoV, S gene for TGEV) facilitates detection of variant strains that may emerge over time [3, 4].

Field Evaluation: Diagnostic Sensitivity and Specificity

The clinical performance of a multiplex assay must be validated using a large set of field fecal samples collected from pigs with clinical signs of diarrhea. The diagnostic sensitivity and specificity are calculated by comparing the multiplex assay results to those obtained using reference singleplex real-time RT-PCR assays or sequencing [9, 10].

In a field evaluation of 160 clinical samples, Yan Li et al. reported positive rates of 38.13% for PEDV, 1.88% for TGEV, and 5.00% for PDCoV using the multiplex assay [9]. Co-infection rates were 1.25% for PEDV+TGEV, 1.25% for PEDV+PDCoV, and 0.63% for triple infection (PEDV+TGEV+PDCoV). The positive coincidence rate between the multiplex assay and single-reaction qRT-PCR was 100% [9].

In a larger study involving 462 clinical samples from five Chinese provinces, Jianpeng Chen et al. found discrete positive rates of 19.70% for PEDV, 0.87% for TGEV, and 10.17% for PDCoV [10]. Mixed infection rates were notably higher in this cohort: PEDV/PDCoV co-infection was detected in 23.16% of samples, and triple infection in 11.90% of samples [10]. These data underscore the high prevalence of co-infections in field settings and the necessity of a multiplex diagnostic approach.

The diagnostic sensitivity of the multiplex assay is typically reported as greater than 95%, and diagnostic specificity approaches 100% when compared to sequencing-based confirmation [3, 6]. The intra-assay and inter-assay coefficients of variation (CV) for Ct values are generally below 3%, demonstrating excellent reproducibility [10].

Workflow Diagram

The following Mermaid diagram illustrates the stepwise workflow for the multiplex real-time RT-PCR assay from sample collection to result interpretation.

flowchart TD
    A[Collect fecal sample from diarrheic pig], > B[Homogenize in PBS or lysis buffer]
    B, > C[Centrifuge to clarify supernatant]
    C, > D[Extract viral RNA using silica column or magnetic beads]
    D, > E[Add internal control RNA to monitor inhibition]
    E, > F[Prepare one-step RT-PCR master mix with multiplex primer/probe cocktail]
    F, > G[Add extracted RNA template]
    G, > H[Run thermal cycling: RT at 50°C, denature at 95°C, anneal/extend at 60°C]
    H, > I[Collect fluorescence data in real time]
    I, > J{Analyze amplification curves}
    J, > K[Determine Ct values for each fluorophore channel]
    K, > L[Interpret results: PEDV, TGEV, PDCoV positive or negative]
    L, > M[Report co-infection status if multiple targets detected]

Advantages of Multiplexing Over Singleplex Assays

The adoption of a multiplex real-time RT-PCR assay offers several practical advantages over running three separate singleplex reactions. First, reagent costs are reduced by approximately two-thirds because a single master mix is used for all targets [5, 8]. Second, the required sample volume is lower, which is particularly beneficial when sample quantity is limited. Third, turnaround time is shortened because only one thermal cycling run is needed instead of three [3, 4]. Fourth, the risk of pipetting errors and cross-contamination is minimized due to fewer handling steps. Finally, the simultaneous detection of co-infections provides a more complete picture of the etiological agents involved in a disease outbreak, informing more targeted intervention strategies [9, 10].

Limitations and Considerations

Despite its advantages, the multiplex format presents certain challenges. Competition among primer sets for reagents and polymerase can lead to reduced amplification efficiency for targets present at very low concentrations, especially in the presence of a high-abundance target [6]. Careful optimization of primer and probe concentrations is required to balance amplification across all channels. Additionally, the dynamic range of detection may be narrower in multiplex compared to singleplex formats [4]. The use of an internal control is essential to rule out false negatives due to PCR inhibition, which is common in fecal samples [10].

Conclusion

Multiplex real-time RT-PCR assays targeting the N or M genes of PEDV, TGEV, and PDCoV provide a robust, sensitive, and specific method for the simultaneous detection and differentiation of these three major porcine enteric coronaviruses in field fecal samples. The assay demonstrates a limit of detection as low as 1 to 10 copies per reaction, no cross-reactivity with other common swine pathogens, and excellent concordance with reference singleplex assays. Field evaluations confirm high rates of co-infection, underscoring the clinical utility of the multiplex format. By reducing cost, sample volume, and turnaround time, this assay is well suited for routine diagnostic surveillance and outbreak management in swine populations. For further reading on related diagnostic approaches, see the articles on Porcine Epidemic Diarrhea Virus, Transmissible Gastroenteritis Virus, and Porcine Deltacoronavirus: Veterinary Reference.

References

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