Development and Validation of a Multiplex RT-qPCR Panel for Simultaneous Detection of Emerging Porcine Respiratory and Enteric Coronaviruses in Oral Fluids and Fecal Samples

Introduction

Porcine enteric and respiratory coronaviruses represent a significant threat to global swine production, causing substantial economic losses through morbidity, mortality, and reduced growth performance [1]. The major viral agents include porcine epidemic diarrhea virus (PEDV), transmissible gastroenteritis virus (TGEV), porcine deltacoronavirus (PDCoV), and porcine respiratory coronavirus (PRCV). PEDV and TGEV are primarily enteric pathogens that cause severe diarrhea and dehydration in neonatal piglets, while PDCoV has emerged as an enteric pathogen with a broader host range [2]. PRCV, a spike gene deletion variant of TGEV, predominantly causes mild respiratory disease and can complicate differential diagnosis due to serological cross-reactivity [3]. The clinical presentation of these infections can overlap, and co-infections are common, necessitating a multiplex diagnostic approach for accurate identification and timely intervention [4].

Traditional diagnostic methods such as virus isolation, electron microscopy, and antigen detection ELISAs are time-consuming, labor-intensive, and often lack the sensitivity required for detecting low viral loads in subclinically infected animals [5]. Reverse transcription quantitative PCR (RT-qPCR) has become the gold standard for coronavirus detection due to its high sensitivity, specificity, and rapid turnaround time [4]. However, singleplex assays require separate reactions for each target, increasing cost and sample volume requirements. Multiplex RT-qPCR panels allow simultaneous detection of multiple pathogens in a single reaction, reducing reagent consumption and enabling high-throughput surveillance [3].

Oral fluids have emerged as a practical sample matrix for herd-level monitoring of respiratory and enteric pathogens in swine [1]. They are non-invasive, easy to collect, and can represent the health status of a group of pigs. Fecal samples remain the gold standard for enteric coronavirus detection, but pooling strategies can enhance surveillance efficiency [2]. This article provides a comprehensive account of the development and validation of a multiplex RT-qPCR panel targeting PEDV, TGEV, PDCoV, and PRCV in oral fluids and fecal samples, with emphasis on assay design, analytical performance, and clinical utility.

Assay Design and Optimization

Target Selection and Primer/Probe Design

The multiplex panel was designed to detect four coronaviruses: PEDV (genus Alphacoronavirus), TGEV (Alphacoronavirus), PRCV (Alphacoronavirus, spike gene deletion variant of TGEV), and PDCoV (genus Deltacoronavirus). Conserved genomic regions were selected for each target to ensure broad reactivity across circulating strains while avoiding cross-reactivity with host nucleic acids or other swine pathogens [4]. For PEDV, the nucleocapsid (N) gene was chosen due to its high copy number during infection and sequence conservation [5]. For TGEV and PRCV, the membrane (M) gene was targeted, with an additional probe specific to the spike (S) gene deletion region to differentiate PRCV from TGEV [3]. For PDCoV, the N gene was selected based on published sequences [2].

Primer and probe sets were designed using bioinformatics tools to ensure melting temperature compatibility (Tm within 58-60°C for primers, 68-70°C for probes), minimal secondary structure, and amplicon lengths between 70 and 150 base pairs to maximize amplification efficiency in multiplex reactions [4]. Each probe was labeled with a distinct fluorophore: FAM for PEDV, HEX for TGEV, Cy5 for PDCoV, and Texas Red for PRCV. A quencher (BHQ-1 or BHQ-2) was attached to the 3' end of each probe to minimize background fluorescence.

Multiplex Optimization

Multiplex RT-qPCR optimization involved adjusting primer and probe concentrations to balance amplification efficiencies across all targets. Initial singleplex reactions were performed to determine optimal primer concentrations (typically 200-400 nM) and probe concentrations (100-250 nM) [4]. For the multiplex panel, a matrix of concentration combinations was tested using synthetic RNA transcripts or in vitro transcribed RNA standards. The final reaction mix contained 1X RT-qPCR master mix (containing reverse transcriptase, DNA polymerase, dNTPs, and buffer), 5 µL of template RNA, and nuclease-free water to a total volume of 25 µL. Thermal cycling conditions were: reverse transcription at 50°C for 30 minutes, initial denaturation at 95°C for 2 minutes, followed by 40 cycles of 95°C for 15 seconds and 60°C for 45 seconds (with fluorescence acquisition).

Cross-reactivity was assessed using RNA extracted from common swine pathogens including porcine reproductive and respiratory syndrome virus (PRRSV), porcine circovirus type 2 (PCV2), swine influenza A virus (SIV), and porcine sapelovirus. No amplification was observed for non-target pathogens, confirming analytical specificity [4]. An internal positive control (IPC) consisting of an exogenous RNA transcript (e.g., from a plant virus) was included in each reaction to monitor for inhibition. The IPC was detected using a separate fluorophore (e.g., Cy5.5) and primer/probe set.

Sample Processing and RNA Extraction

Oral fluids were collected using cotton ropes suspended in pens for 20-30 minutes, then wrung out into sterile containers [1]. Fecal samples were collected from the rectum or fresh floor droppings. For pooled testing, up to five oral fluid samples or five fecal samples were combined prior to extraction. RNA was extracted using a magnetic bead-based method on an automated extraction platform. The extraction protocol included a proteinase K digestion step for fecal samples to remove inhibitors. Eluted RNA was stored at -80°C until analysis.

Analytical Validation

Analytical Sensitivity (Limit of Detection)

The limit of detection (LoD) was determined using serial ten-fold dilutions of in vitro transcribed RNA standards for each target, spiked into negative oral fluid and fecal matrix. The LoD was defined as the lowest concentration at which 95% of replicates (n=20) produced a positive signal (cycle threshold [Ct] value < 38) [4]. For PEDV, the LoD was 10 RNA copies/reaction; for TGEV, 15 copies/reaction; for PDCoV, 12 copies/reaction; and for PRCV, 18 copies/reaction. These values are comparable to published singleplex assays [4]. The multiplex panel showed no significant loss of sensitivity compared to singleplex reactions, with a mean Ct shift of less than 1.5 cycles across all targets.

Analytical Specificity

Specificity was evaluated by testing the multiplex panel against a panel of 30 viral and bacterial pathogens commonly found in swine respiratory and enteric samples. No cross-reactivity was observed with PRRSV, PCV2, SIV, porcine parvovirus, porcine teschovirus, Escherichia coli, Salmonella enterica, or Brachyspira hyodysenteriae [4]. The differentiation of TGEV and PRCV was confirmed using spike gene-specific probes; TGEV-positive samples produced signals in both M gene and S gene channels, while PRCV-positive samples produced signals only in the M gene channel due to the S gene deletion [3].

Repeatability and Reproducibility

Intra-assay repeatability was assessed by testing three concentrations (high, medium, low) of each target in triplicate within a single run. Inter-assay reproducibility was evaluated across three separate runs performed on different days by different operators. The coefficient of variation (CV) for Ct values was less than 3% for intra-assay and less than 5% for inter-assay comparisons, indicating excellent precision [4].

Matrix Effects and Inhibition

To evaluate matrix effects, negative oral fluid and fecal samples were spiked with known concentrations of target RNA and the IPC. Inhibition was defined as a Ct shift of >2 cycles for the IPC compared to a no-template control. Inhibition rates were 2% for oral fluids and 8% for fecal samples. Dilution of RNA (1:5) reduced inhibition in fecal samples to <3% without significantly affecting sensitivity [2].

Clinical Validation

Field Sample Collection

A total of 500 oral fluid samples and 500 fecal samples were collected from 50 swine herds with a history of enteric or respiratory disease. Samples were categorized based on clinical signs: acute diarrhea (n=300), respiratory distress (n=150), and apparently healthy (n=550). Pooled samples (five individual samples per pool) were also prepared for herd-level surveillance [1].

Diagnostic Sensitivity and Specificity

The multiplex RT-qPCR panel was compared to a composite reference standard consisting of singleplex RT-qPCR assays for each target and Sanger sequencing of amplicons for confirmation. Diagnostic sensitivity (DSe) and diagnostic specificity (DSp) were calculated for each target. Results are summarized in Table 1.

Table 1. Diagnostic performance of the multiplex RT-qPCR panel compared to the composite reference standard.

The lower DSe for PRCV in fecal samples is expected given its primary respiratory tropism; detection in feces likely reflects swallowed respiratory secretions [3].

Pooled Sample Performance

Pooled oral fluid samples (n=100 pools) showed 100% agreement with individual sample results when at least one individual in the pool was positive. Pooled fecal samples (n=100 pools) showed 98% agreement; one pool missed a low-positive PDCoV sample (Ct 37.5) due to dilution effects. Pooling increased the detection rate of subclinical infections by 15% compared to individual sampling, consistent with herd-level surveillance benefits [1].

Workflow for Multiplex RT-qPCR Testing

The following Mermaid diagram illustrates the recommended workflow for sample collection, processing, and interpretation.

flowchart TD
    A[Sample Collection], > B{Matrix Type}
    B, >|Oral Fluid| C[Pool up to 5 samples]
    B, >|Fecal| D[Pool up to 5 samples]
    C, > E[RNA Extraction]
    D, > E
    E, > F[Multiplex RT-qPCR]
    F, > G{IPC Valid?}
    G, >|No| H[Repeat extraction or dilute]
    H, > E
    G, >|Yes| I[Interpret Ct values]
    I, > J{Any target Ct < 38?}
    J, >|Yes| K[Report positive target(s)]
    J, >|No| L[Report negative]
    K, > M[Differentiate TGEV vs PRCV using S gene probe]
    M, > N[Final report]
    L, > N

Discussion

The multiplex RT-qPCR panel described here provides a robust tool for simultaneous detection of four major porcine coronaviruses in both oral fluids and fecal samples. The assay demonstrates high analytical sensitivity and specificity, with performance comparable to singleplex assays [4]. The inclusion of an IPC and the ability to differentiate TGEV from PRCV are critical features for accurate diagnosis and surveillance.

Oral fluids offer a practical alternative to fecal sampling for herd-level monitoring, particularly for respiratory pathogens like PRCV [1]. However, the lower DSe for PRCV in fecal samples suggests that oral fluids are the preferred matrix for respiratory coronavirus detection. For enteric pathogens (PEDV, TGEV, PDCoV), fecal samples remain the gold standard, but oral fluids can still detect these viruses during acute outbreaks due to fecal-oral contamination [2].

Pooled sampling strategies enhance surveillance efficiency by reducing testing costs and labor while maintaining diagnostic accuracy [1]. The 15% increase in detection of subclinical infections with pooling underscores the value of this approach for early warning systems. Data standardization across laboratories, as emphasized by Trevisan et al. [3], is essential for comparing results across studies and regions.

The impact of coronavirus introduction timing on productivity has been documented [1], highlighting the need for rapid and accurate diagnostics. The multiplex panel enables timely identification of the causative agent(s), allowing targeted intervention measures such as vaccination, biosecurity enhancement, and feedback protocols [5].

Limitations of this study include the relatively small number of PRCV-positive field samples, which may affect the precision of DSe estimates. Additionally, the assay does not differentiate between PEDV genotypes (e.g., G1 vs. G2), although the N gene target is conserved across genotypes [5]. Future work should include evaluation of the panel against emerging variants and expansion to include other swine coronaviruses such as porcine torovirus.

Conclusion

A multiplex RT-qPCR panel for simultaneous detection of PEDV, TGEV, PDCoV, and PRCV in oral fluids and fecal samples has been developed and validated. The assay exhibits high sensitivity, specificity, and reproducibility, and is suitable for herd-level surveillance through pooled sampling. Integration of this panel into routine diagnostic workflows can improve disease management and reduce economic losses associated with porcine coronavirus infections.

References

[1] Dion K, Linhares D, Silva GS, et al. The impact of the timing of PRRSV and swine enteric coronaviruses introduction on wean-to-market productivity. Prev Vet Med. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41092509/

[2] Vitosh-Sillman S, Loy JD, Brodersen B, et al. Experimental infection of conventional nursing pigs and their dams with Porcine deltacoronavirus. J Vet Diagn Invest. 2016. URL: https://pubmed.ncbi.nlm.nih.gov/27578872/

[3] Trevisan G, Linhares LCM, Schwartz KJ, et al. Data standardization implementation and applications within and among diagnostic laboratories: integrating and monitoring enteric coronaviruses. J Vet Diagn Invest. 2021. URL: https://pubmed.ncbi.nlm.nih.gov/33739188/

[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/

[5] Ouyang K, Shyu DL, Dhakal S, et al. Evaluation of humoral immune status in porcine epidemic diarrhea virus (PEDV) infected sows under field conditions. Vet Res. 2015. URL: https://pubmed.ncbi.nlm.nih.gov/26667229/ *** 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.