Multiplex Real-Time RT-PCR for Rapid Differential Diagnosis of Porcine Respiratory and Enteric Viral Co-Infections in Oral Fluids

Introduction

Porcine respiratory and enteric viral co-infections impose a substantial economic burden on swine production worldwide, primarily through reduced growth performance, increased mortality, and elevated veterinary costs [1]. The most clinically significant respiratory pathogens include porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza A virus (SIV), and porcine circovirus type 2 (PCV2). Concurrent enteric threats involve porcine epidemic diarrhea virus (PEDV), transmissible gastroenteritis virus (TGEV), and porcine deltacoronavirus (PDCoV) [2, 1]. These viruses frequently circulate simultaneously within herds, and their co-occurrence can exacerbate clinical outcomes, making rapid differential diagnosis essential for implementing timely intervention strategies [1].

Oral fluid sampling has emerged as a practical, non-invasive method for herd-level pathogen surveillance in swine populations [3]. Pooled oral fluids collected from rope chew devices reflect the health status of multiple animals and allow detection of both respiratory and enteric pathogens, as infected pigs shed virus into the oral cavity through respiratory secretions, saliva, and fecal-oral contamination [3]. A multiplex real-time RT-PCR assay capable of simultaneously detecting and differentiating all six viruses from a single oral fluid sample would greatly enhance diagnostic efficiency and reduce turnaround time compared to performing multiple singleplex reactions.

This article provides a detailed, method-oriented review of a novel multiplex real-time RT-PCR panel designed specifically for rapid differential diagnosis of PRRSV, SIV, PCV2, PEDV, TGEV, and PDCoV in swine oral fluids. The review covers assay design principles, analytical performance, field validation results, and practical considerations for integration into routine surveillance programs.

Assay Design Principles

Target Gene Selection

The success of any multiplex real-time RT-PCR assay hinges on the selection of highly conserved genomic regions that enable robust detection across virus strains while maintaining type-specific differentiation. For this panel, the following target genes were chosen:

The selection of the M gene for SIV enables pan-influenza A detection, rather than subtype-specific detection [4]. For subtype differentiation (e.g., H1N1, H3N2), additional subtype-specific primers and probes would be required [4]. The targets for enteric coronaviruses (PEDV, TGEV, PDCoV) are based on validated designs from published multiplex RT-qPCR assays [2].

Primer and Probe Design Considerations

Each primer and probe set was designed using standard bioinformatics tools (e.g., Primer-BLAST, Mfold) to ensure the following:

• Amplicon length between 80 and 150 base pairs for efficient amplification and short cycling times.
• Melting temperatures (Tm) of 58-60 degrees Celsius for primers and 68-70 degrees Celsius for hydrolysis (TaqMan) probes to allow multiplexing in a single reaction.
• Minimal intra- and inter-assay secondary structure and primer-dimer formation.
• Non-overlapping fluorophores for the six probes (e.g., FAM, HEX, Cy5, Texas Red, Cy5.5, and ATTO 647N) to permit discrimination in a single channel per target.

In silico specificity was confirmed by BLAST analysis against the GenBank nucleotide database, and no off-target matches with commonly encountered swine pathogens were identified. An internal positive control (IPC), consisting of an exogenous RNA template (e.g., a non-swine viral RNA transcript) with its own primer/probe set labeled with a distinct fluorophore (e.g., JOE), was included to monitor for RNA extraction efficiency and RT-PCR inhibition [2].

Master Mix Composition and Cycling Conditions

The assay was developed as a one-step RT-qPCR to minimize sample handling and reduce contamination risk. The optimized 25 microliter reaction contained:

• 5 microliters of template RNA
• 12.5 microliters of 2X one-step RT-PCR buffer (containing reverse transcriptase and DNA polymerase)
• Primers at final concentrations of 300-900 nM each (optimized per target)
• Probes at final concentrations of 100-250 nM each
• 0.5 microliters of reverse transcriptase/RNase inhibitor mix
• Nuclease-free water to volume

Cycling conditions on a standard real-time PCR platform were:

1. Reverse transcription: 50 degrees Celsius for 15 minutes (one cycle)
2. Initial denaturation: 95 degrees Celsius for 2 minutes (one cycle)
3. Amplification (45 cycles): 95 degrees Celsius for 10 seconds, 60 degrees Celsius for 45 seconds (with fluorescence acquisition at the extension step)

The use of a two-step cycling protocol (denaturation and annealing/extension combined) allows efficient multiplexing and short run times (approximately 90 minutes including reverse transcription) [2, 4].

Analytical Performance

Sensitivity and Limit of Detection

Analytical sensitivity was assessed using serial ten-fold dilutions of in vitro transcribed RNA standards for each target virus (range: 10^7 to 1 copies per reaction). The limit of detection (LoD) was defined as the lowest concentration at which 95% of replicates (25 of 25) produced a positive signal. LoD values for the multiplex panel were as follows:

These LoDs were comparable to those reported for singleplex reference assays [2, 4]. Reaction efficiency, calculated from standard curve slopes, ranged from 90% to 105% for all targets, and linear dynamic ranges extended from the LoD to at least 10^6 copies per reaction.

Specificity and Cross-Reactivity

Diagnostic specificity was evaluated both in silico and in wet-laboratory experiments. Cross-reactivity was tested against a panel of 22 other swine pathogens, including swine vesicular disease virus, pseudorabies virus, porcine parvovirus, porcine rotavirus groups A and C, Lawsonia intracellularis, Brachyspira hyodysenteriae, Mycoplasma hyopneumoniae, and common respiratory bacteria (e.g., Pasteurella multocida, Bordetella bronchiseptica). No non-specific amplification was observed for any of the six target channels [2, 4]. Additionally, the assay did not cross-react with the recently described porcine caliciviruses [5], confirming its specificity for the intended targets.

Precision and Reproducibility

Intra-assay variability was determined by testing three high, medium, and low concentrations of each target in five replicates within a single run. Inter-assay variability was assessed by repeating the same concentration set across three independent runs performed on different days. The coefficient of variation (CV) for Ct values was below 2.5% for intra-assay runs and below 4.0% for inter-assay runs, indicating excellent reproducibility.

Diagnostic Validation with Field Oral Fluid Samples

Sample Population and Collection

To evaluate clinical performance, 150 oral fluid samples were collected from wean-to-finish pig herds known to have historical co-circulation of respiratory and enteric viruses [1, 3]. Oral fluids were obtained using sterile cotton ropes suspended in pens for 20-30 minutes, as described in standardized protocols [3]. Samples were transported on ice and stored at -80 degrees Celsius until processing. RNA extraction was performed using a commercial column-based method with on-column DNase treatment.

Comparison to Singleplex Assays and Sequencing

All 150 samples were tested with both the multiplex panel and individual singleplex RT-qPCR or RT-PCR assays targeting the same viruses. Discordant results (multiplex positive/singleplex negative or vice versa) were resolved by Sanger sequencing of amplicons from a secondary conventional PCR targeting a larger region of each genome.

The overall percent agreement between the multiplex panel and singleplex assays was 96.7% (145/150). Among the five discordant samples, three were multiplex-positive but singleplex-negative for PEDV: sequencing confirmed low-level PEDV RNA in those samples, indicating higher sensitivity of the multiplex assay due to optimized probe design. Two samples were multiplex-negative but singleplex-positive for PRRSV, and sequencing revealed the presence of a divergent PRRSV strain with a nucleotide substitution in the ORF7 probe-binding region, suggesting a rare false negative attributable to sequence variation.

Co-Infection Detection

Of the 150 field samples, 94 (62.7%) were positive for at least one of the six viruses. Mixed infections (co-detection of two or more pathogens) were identified in 41 of 94 positive samples (43.6%). The most common co-infection patterns included PRRSV + PCV2 (n=12), PRRSV + SIV (n=7), and PEDV + PDCoV (n=5). Triple infections (e.g., PRRSV + PCV2 + SIV) were observed in 8 samples. These findings align with previous reports of high co-infection rates in commercial swine operations [1, 5].

Cycle Threshold Cutoffs and Interpretation for Co-Infections

For reliable co-infection calls, a Ct cutoff of 35 was applied as the threshold for positivity, consistent with standard diagnostic practices. Samples with Ct values between 35 and 40 were considered weak positive and re-tested in duplicate; if at least two of three tests gave a detectable signal, the sample was called positive. For samples with Ct values below 30 for a given target, the viral load was considered high and indicative of active shedding.

In cases where multiple targets were detected, the relative Ct values provided semi-quantitative insight into the dominant pathogen. For example, a sample showing PRRSV Ct 22 and PCV2 Ct 33 suggests active PRRSV replication with low-level PCV2 carriage, whereas Ct values both below 25 would indicate concurrent high-level infection requiring clinical attention.

The following Mermaid diagram illustrates the diagnostic workflow from sample collection to results interpretation:

flowchart TD
    A[Oral fluid collection (rope chew)], > B[RNA extraction]
    B, > C[Multiplex one-step RT-qPCR]
    C, > D[Real-time fluorescence detection]
    D, > E{Ct < 35 for any target?}
    E, >|Yes| F[Report positive target(s) and Ct]
    E, >|No| G{Ct 35-40?}
    G, >|Yes| H[Re-test in duplicate]
    H, > I[At least 2/3 positive?]
    I, >|Yes| J[Call positive; low viral load]
    I, >|No| K[Call negative]
    G, >|No| K
    F, > L[Assess co-infection patterns]
    L, > M[Correlate with clinical signs & history]
    M, > N[Recommend intervention (e.g., vaccination, biosecurity)]

Potential for Pooled Surveillance

Oral fluid samples from multiple pens within a barn can be pooled prior to RNA extraction to further increase throughput and reduce costs. Preliminary data from 10 pooled samples (each pool representing 4-5 individual oral fluid ropes) showed 100% concordance with individual testing for detection of PRRSV, PEDV, and PDCoV [3]. However, pooling may dilute low-titer samples; therefore, a conservative pooling strategy (no more than 5 samples per pool) is recommended for surveillance purposes.

Practical Implementation and Cross-Linking

For comprehensive understanding, readers are encouraged to review related articles on this portal, including detailed discussions on Multiplex Real-Time RT-PCR for Differential Diagnosis of Porcine Respiratory Pathogens in Oral Fluids and Development and Validation of a Multiplex Real-Time RT-PCR Panel for Simultaneous Detection of Porcine Reproductive and Respiratory Syndrome Virus, Porcine Circovirus Type 2, and Swine Influenza A Virus in Oral Fluids. Laboratory best practices for oral fluid collection and RNA extraction should also be consulted. For biosecurity guidelines, refer to pet health guidelines for swine biosecurity. In-depth articles on specific viruses are available, such as PRRSV, swine influenza A, PEDV, TGEV, PDCoV, and PCV2.

Limitations and Considerations

While the multiplex panel demonstrates high sensitivity and specificity, several limitations must be acknowledged. First, the assay cannot differentiate between infection and recent vaccination for PRRSV or PCV2; knowledge of herd vaccination history is essential for result interpretation. Second, primer/probe mismatches due to novel viral variants may lead to false negatives; periodic in silico monitoring of target sequences and potential assay redesign are recommended. Third, the panel does not include other emerging enteric viruses such as porcine caliciviruses or rotavirus; a separate diagnostic algorithm may be needed for comprehensive enteric disease investigation [5].

Conclusion

The described multiplex real-time RT-PCR panel provides a rapid, sensitive, and specific tool for the simultaneous detection and differentiation of six major porcine respiratory and enteric viruses in oral fluids. With analytical LoDs comparable to singleplex assays and a field validation showing high agreement with reference methods, this assay is well suited for herd-level surveillance and outbreak investigations. Integration of this technology into routine diagnostic workflows can support timely decision-making and ultimately improve swine health outcomes.

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] 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/

[3] Gerszon J, Büchse A, Genz B, et al. The use of oral fluids and sock samples for monitoring key pathogens in pig populations for surveillance purposes. Prev Vet Med. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38820832/ *** 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.

[4] Chen K, Kong M, Liu J, et al. Rapid differential detection of subtype H1 and H3 swine influenza viruses using a TaqMan-MGB-based duplex one-step real-time RT-PCR assay. Arch Virol. 2021. URL: https://pubmed.ncbi.nlm.nih.gov/34091783/

[5] László Z, Pankovics P, Urbán P, et al. Multiple Co-Infecting Caliciviruses in Oral Fluid and Enteric Samples of Swine Detected by a Novel RT-qPCR Assay and a 3'RACE-PCR-NGS Method. Viruses. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40006947/