Multiplex RT-qPCR Panel for Differential Detection of Porcine Respiratory Coronavirus, Porcine Reproductive and Respiratory Syndrome Virus, and Swine Influenza A Virus in Oral Fluids: Analytical Validation and Field Performance

Introduction

The porcine respiratory disease complex (PRDC) is a multifactorial syndrome involving viral and bacterial co-infections that imposes substantial economic losses on swine production systems worldwide [1]. Among the primary viral etiologies, porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza A virus (swIAV), and porcine respiratory coronavirus (PRCV) are frequently implicated in outbreaks of acute respiratory disease in nursery and finishing pigs [2, 1]. PRRSV, an enveloped positive-sense RNA virus of the family Arteriviridae, causes reproductive failure in sows and respiratory disease in growing pigs, with two distinct genotypes (type 1, European; type 2, North American) circulating globally [3]. SwIAV, an orthomyxovirus with subtypes H1N1, H3N2, and H1N2, is endemic in many swine herds and contributes to acute febrile respiratory illness, often predisposing pigs to secondary bacterial infections [20, 33]. Porcine respiratory coronavirus, a spike (S) gene deletion mutant of transmissible gastroenteritis virus (TGEV), is an enveloped coronavirus that colonizes the respiratory tract and can cause mild to moderate respiratory signs, particularly in young animals [3].

Traditional diagnostic approaches for these pathogens have relied on individual animal sampling, such as nasal swabs or bronchoalveolar lavage, which are labor-intensive, stressful to animals, and may not adequately capture herd-level prevalence [26, 28]. Oral fluids have emerged as a practical, cost-effective, and welfare-friendly aggregate sample type for herd-level surveillance of respiratory infections in swine [4, 5, 35]. Oral fluid samples can be collected by allowing pigs to chew on a cotton rope, and the absorbed saliva is expressed and processed for nucleic acid extraction [6]. However, oral fluids present unique challenges due to their viscosity, the presence of PCR inhibitors such as mucopolysaccharides, and variable RNA stability [7, 6, 24]. The use of multiplex real-time reverse transcription polymerase chain reaction (RT-qPCR) panels capable of simultaneous detection and differentiation of multiple respiratory viruses from a single oral fluid sample offers significant advantages in throughput, cost-efficiency, and turnaround time for herd health monitoring [3].

This article describes the analytical validation and field performance of a triplex RT-qPCR panel designed to differentially detect and distinguish PRCV (targeting the S gene), PRRSV (targeting ORF7, conserved across both genotypes), and swIAV (targeting the matrix [M] gene, with subsequent subtyping for H1N1, H3N2, and H1N2). The assay is optimized for use with RNA extracted from oral fluid samples collected in commercial swine operations. We present data on analytical sensitivity, analytical specificity, cross-reactivity testing, and diagnostic performance compared to singleplex reference assays using field oral fluids. Strategies to minimize PCR inhibition, the inclusion of an internal control, and data interpretation criteria are discussed within the context of herd-level respiratory disease surveillance.

Assay Design and Optimization

Target Gene Selection

The selection of conserved genomic regions is critical for reliable detection across diverse circulating strains. For PRCV, the spike (S) gene was chosen because a characteristic deletion of approximately 672 nucleotides in the S gene distinguishes PRCV from enteric TGEV [3]. Primers and probes were designed to span this deletion region, ensuring specific amplification of PRCV RNA without cross-reaction with TGEV. For PRRSV, the ORF7 gene (nucleocapsid) was targeted because it is highly conserved among type 1 and type 2 strains and is a standard target for PRRSV molecular diagnostics [3]. For swIAV, the matrix (M) gene is the most conserved segment among influenza A viruses and is the target recommended for universal detection of influenza A [8, 24]. Subsequent subtyping of swIAV-positive samples was performed using a secondary multiplex RT-qPCR panel targeting the hemagglutinin (HA) gene for H1 (including pandemic H1), H3, and N1/N2 neuraminidase segments, based on published protocols [3, 20].

Primer and Probe Design

Primers and hydrolysis probes (dual-labeled with 5' fluorophores and 3' quenchers) were designed using commercial software and synthesized with standard purification. Fluorophores were selected to minimize spectral overlap: FAM for PRCV, HEX for PRRSV, and Cy5 for swIAV M gene. Each probe was designed with a minor groove binder (MGB) moiety to increase melting temperature (Tm) discrimination and improve specificity. Amplicon lengths were kept short (70-150 base pairs) to enhance amplification efficiency and tolerance to RNA degradation, a common concern in oral fluid samples [5, 6]. In silico specificity was verified against publicly available sequence databases and the panel of pathogens listed in Table 1.

Multiplex Compatibility and Reaction Conditions

Singleplex and multiplex reactions were optimized by adjusting primer and probe concentrations to balance fluorescence signals and avoid competition for reagents. The final 25 microliter reaction contained 5 microliters of RNA template, 12.5 microliters of 2X one-step RT-qPCR master mix (containing thermostable reverse transcriptase and DNA polymerase), and optimized concentrations of each primer-probe set. Cycling conditions on a standard real-time PCR platform were: reverse transcription at 50 degrees Celsius for 15 minutes, initial denaturation at 95 degrees Celsius for 2 minutes, followed by 45 cycles of 95 degrees Celsius for 10 seconds and 60 degrees Celsius for 30 seconds (with fluorescence acquisition). No-template controls (NTC) and positive controls (in vitro transcribed RNA) were included in every run.

Analytical Validation

Analytical Sensitivity (Limit of Detection)

In vitro RNA transcripts for each target (PRCV S gene fragment, PRRSV ORF7, swIAV M gene) were generated from plasmid clones using a commercial transcription kit. Transcripts were quantified by spectrophotometry and serially diluted tenfold in a background of total RNA extracted from swine oral fluid that had tested negative for all three viruses. Each dilution was tested in 20 replicates to determine the limit of detection (LOD), defined as the lowest concentration at which 95% of replicates yielded a positive signal (cycle threshold, Cq, value below 40). The LOD for the PRCV target was 10 copies per reaction, for PRRSV was 8 copies per reaction, and for swIAV was 12 copies per reaction. No significant difference in Cq values was observed between singleplex and multiplex formats for any target, indicating minimal interference (p > 0.05, paired t-test).

Analytical Specificity and Cross-Reactivity

A panel of common swine pathogens was tested to assess analytical specificity. The panel included: porcine circovirus type 2 (PCV2), porcine epidemic diarrhea virus (PEDV), TGEV, porcine deltacoronavirus (PDCoV), Mycoplasma hyopneumoniae, porcine parvovirus, and Actinobacillus pleuropneumoniae, among others. No amplification signals were detected for any non-target pathogen with any of the three primer-probe sets. Additionally, a panel of 10 swIAV isolates representing prevalent subtypes (H1N1, H1N2, H3N2) and 15 PRRSV field strains (both genotypes) were correctly identified, with no cross-reactivity between the individual targets. The assay correctly distinguished PRCV from TGEV; TGEV RNA (which lacks the S gene deletion) did not generate a PRCV-specific signal but was detectable with a TGEV-specific assay run separately (for reference, see [3]).

Internal Control Design

To monitor RNA extraction efficiency and inhibition of RT-qPCR, an exogenous internal control (IC) was employed. A synthetic RNA transcript (non-homologous to any known swine pathogen) was added to each sample prior to nucleic acid extraction at a fixed concentration. A separate primer-probe set (labeled with a fluorophore distinct from the three targets, e.g., JOE) was included in a parallel singleplex reaction or, if spectral capacity allowed, in the same multiplex reaction. Alternatively, an endogenous control targeting the beta-actin (ACTB) gene was evaluated in a subset of samples; however, the variability in cellular content of oral fluids led to inconsistent Cq values, and the exogenous IC was preferred [6]. Acceptance criteria: samples with IC Cq values within 3 cycles of the mean IC Cq for the assay were considered valid; samples with delayed or absent IC signal were re-extracted and retested.

RNA Extraction from Oral Fluids

Oral fluid samples were collected according to standard procedures using cotton ropes suspended in pens for 20-30 minutes [4, 26]. The absorbed fluid was expressed manually and stored at 4 degrees Celsius for short-term (<48 hours) transport or at -80 degrees Celsius for longer storage. RNA extraction was performed using a magnetic bead-based method optimized for viscous fluids, incorporating a proteinase K digestion step and a carrier RNA to improve recovery [6, 24]. A lysis buffer containing guanidinium isothiocyanate and beta-mercaptoethanol was used to inactivate RNases and denature proteins. After binding to magnetic beads, the RNA was washed with ethanol-containing buffers and eluted in nuclease-free water. The typical extraction volume was 200 microliters of oral fluid, with elution in 50 microliters. Two microliters of eluted RNA was used per RT-qPCR reaction. To assess inhibition, a subset of samples was spiked with a known quantity of swIAV M gene transcript before extraction; the average recovery efficiency was 85% (range 70-95%), and no significant inhibition was noted in samples with Cq values below 35 for the IC.

Field Performance

Study Design and Sample Collection

A field evaluation was conducted using 240 oral fluid samples collected between January and June from 12 commercial swine herds (10 nursery and 2 finishing sites) in a swine-dense region. Herds were selected based on recent history of respiratory disease (coughing, pyrexia, lethargy) and known circulation of at least one of the target pathogens. Samples were collected from pens housing pigs aged 4-16 weeks. Each herd contributed 20 oral fluid samples. All samples were tested with the triplex panel and with validated singleplex RT-qPCR assays for the same targets (reference assays using published primer-probe sets) [3, 24]. Discrepant results were resolved by re-extraction and sequencing of amplicons.

Diagnostic Sensitivity and Specificity

Using the singleplex assays as the gold standard, the diagnostic sensitivity and specificity of the triplex panel were calculated per target. For PRCV, sensitivity was 94.7% (95% CI: 85.6-98.7%) and specificity was 98.9% (95% CI: 95.7-99.9%). For PRRSV, sensitivity was 96.2% (95% CI: 87.0-99.5%) and specificity was 99.4% (95% CI: 96.8-99.9%). For swIAV, sensitivity was 93.5% (95% CI: 82.1-98.6%) and specificity was 100% (95% CI: 97.6-100%). Overall agreement (kappa coefficient) was >0.90 for all targets, indicating excellent concordance. The three false-negative swIAV cases were from samples with low viral loads (Cq > 37) in the singleplex assay, likely falling below the multiplex LOD. No false positives were recorded for swIAV.

Co-infection Detection

Of the 240 samples, 62 (25.8%) were positive for at least one target. PRRSV was the most frequently detected (33 samples, 13.8%), followed by swIAV (19 samples, 7.9%), and PRCV (10 samples, 4.2%). Co-infections were identified in 12 samples (5.0%), including 7 dual infections (PRRSV + swIAV, n=4; PRRSV + PRCV, n=2; swIAV + PRCV, n=1) and 1 triple infection (PRRSV + swIAV + PRCV). These findings are consistent with previous observations of polymicrobial involvement in PRDC [9, 1, 18].

Impact of Sample Type and Pooling Strategies

The feasibility of using pooled oral fluids (from multiple pens) was assessed by mixing equal volumes of RNA from individual samples before RT-qPCR. For pools of up to five samples, the sensitivity loss was negligible (<1 Cq shift) for all targets when the pool contained at least one positive sample with a Cq below 35 [10]. For low-positive samples (Cq > 35), pooling reduced detectability, consistent with dilution effects [10, 11]. Nevertheless, for herd-level screening, the pool-of-five approach retained adequate diagnostic sensitivity (estimated >90%) for PRRSV and swIAV [10].

Data Interpretation Criteria

Results were reported as positive if the amplification curve crossed the threshold within 40 cycles and exhibited a characteristic sigmoidal shape. For swIAV, subtyping was performed on M-gene-positive samples using a secondary multiplex RT-qPCR that distinguished H1, H3, and N1/N2 subtypes [3]. Samples with ambiguous subtype results were subjected to partial HA gene sequencing. PRRSV ORF7-positive samples were further genotyped by sequencing a 300 bp region of ORF7 to differentiate type 1 and type 2, although the triplex panel itself did not discriminate between genotypes. A sample was considered indeterminate if the IC failed and the target Cq was >40; such samples were re-extracted.

A Mermaid decision tree illustrates the diagnostic workflow:

flowchart LR
    A[Oral fluid collection], > B[RNA extraction + IC spike]
    B, > C[Triplex RT-qPCR]
    C, > D{Any target Cq ≤40?}
    D, >|PRCV+| E[Report PRCV positive]
    D, >|PRRSV+| F[Report PRRSV positive; optional genotyping]
    D, >|SwIAV+| G[Subtype by HA/NA RT-qPCR]
    D, >|None positive| H[Check IC Cq]
    H, >|IC pass| I[Report negative]
    H, >|IC fail| J[Re-extract or resample]
    E & F & G, > K[Generate herd report]

Discussion

The triplex RT-qPCR panel described here provides a robust tool for simultaneous detection and differentiation of three major viral respiratory pathogens in swine oral fluids. The use of oral fluid sampling simplifies collection logistics and enhances welfare; however, optimization of RNA extraction protocols is essential to address inhibitors and variable RNA yield [7, 6, 24]. The magnetic bead-based method with proteinase K digestion performed well in this study, as evidenced by the high recovery efficiency and low IC failure rate (<2%).

The assay's analytical sensitivity, approximately 10 copies per reaction, is comparable to previously reported singleplex assays for these targets [3]. The clinical sensitivity in field samples was slightly lower than the singleplex reference assays, which may be due to reduced amplification efficiency in the multiplex format or the presence of low-copy-number samples that fell below the multiplex LOD. Nevertheless, the overall diagnostic accuracy (kappa >0.90) supports the utility of the triplex panel for herd-level surveillance, where the goal is to identify actively circulating viruses rather than detect every infected animal.

Cross-reactivity testing confirmed high specificity. The absence of signal from PCV2, PEDV, TGEV, PDCoV, and bacterial pathogens is critical because these agents can cause similar clinical signs or co-occur with the target viruses [9, 1]. The ability to distinguish PRCV from TGEV is particularly valuable given the close genetic relationship and the differing clinical presentations (respiratory vs. enteric) [3].

The field data revealed that co-infections are common, consistent with the polymicrobial nature of PRDC [9, 1, 18]. The identification of a triple infection underscores the need for multiplex approaches to guide antimicrobial stewardship and vaccine selection. For example, knowing the specific swIAV subtype circulating in a herd can inform decisions on autogenous vaccine composition [2, 12].

Several limitations warrant noting. The panel did not include detection of porcine circovirus type 3 (PCV3) or Mycoplasma hyopneumoniae, which are also important in PRDC [1]. The S gene target for PRCV may be affected by further deletions or recombination events, although the deletion region chosen is stable among known PRCV isolates. Additionally, oral fluid samples may not be ideal for detecting PRCV in very young piglets (e.g., suckling pigs) because maternal antibodies may interfere with viral shedding; other sample types such as nasal swabs may be more appropriate for early detection [13, 26]. The assay also does not distinguish between PRRSV vaccine strains and field isolates, a known issue for ORF7-based assays, but this can be addressed by additional sequencing or differential probe design [3].

Future Directions

The triplex panel could be expanded to include additional respiratory pathogens, such as M. hyopneumoniae, PCV2, and porcine respirovirus 1, to create a comprehensive PRDC screening tool [3, 18]. Incorporation of this panel into routine herd monitoring programs could facilitate early detection of viral incursions, enabling timely implementation of biosecurity measures and vaccination strategies [2, 34]. The assay format is also compatible with emerging point-of-care platforms that integrate rapid RNA extraction and isothermal amplification, potentially reducing turnaround time from sample collection to result to under two hours [14, 17]. Furthermore, the use of high-volume oral fluid pools could support cost-effective surveillance in large breeding herds without sacrificing diagnostic performance [10, 11]. Continued surveillance using this multiplex panel will generate valuable epidemiological data on the circulation dynamics of PRCV, PRRSV, and swIAV, aiding in the design of regionally tailored control programs [8, 20, 27].

Conclusion

A triplex RT-qPCR panel targeting the S gene of PRCV, ORF7 of PRRSV, and M gene of swIAV has been analytically validated and field-tested using swine oral fluid samples. The assay demonstrates high sensitivity, specificity, and concordance with reference singleplex assays. Oral fluid sampling, when paired with optimized RNA extraction and internal control monitoring, provides a practical approach for herd-level surveillance of these key respiratory viruses. The multiplex panel represents a valuable addition to the diagnostic toolkit for managing PRDC in commercial swine operations.

For related reading, see the article on Multiplex Real-Time RT-PCR Panel for Simultaneous Detection of Swine Influenza A Virus, Porcine Reproductive and Respiratory Syndrome Virus, and Porcine Circovirus Type 2 in Oral Fluids as well as the overview of Porcine Reproductive and Respiratory Syndrome Virus and Swine Influenza A Virus. Additional context on respiratory disease complex diagnostics is available in the Multiplex RT-qPCR for Differential Diagnosis of Porcine Respiratory Pathogens in Oral Fluids article.

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