[Multiplex Real-Time RT-PCR](/knowledge/diagnostics/molecular/multiplex-rt-pcr-pedv-tgev-pdcov-fecal-environmental 2) Panel for Simultaneous Detection and Subtyping of Porcine Respiratory Coronavirus, PRRSV, and Swine Influenza A Virus in Oral Fluids: Analytical Validation and Field Performance

Introduction

Swine respiratory disease complexes present a substantial diagnostic challenge due to overlapping clinical signs and frequent coinfections [1, 2]. Porcine respiratory coronavirus (PRCV), a deletion mutant of transmissible gastroenteritis virus (TGEV), causes mild to subclinical respiratory infections in pigs but can predispose animals to secondary bacterial infections [1]. Porcine reproductive and respiratory syndrome virus (PRRSV) and swine influenza A virus (SIV) are major primary pathogens associated with severe respiratory disease and reproductive failure [1, 2]. The differential diagnosis of these three viruses is essential for implementing appropriate management and control measures [1, 2].

Oral fluid sampling has emerged as a practical, noninvasive method for herd-level surveillance in swine populations, allowing the detection of multiple pathogens from a single pen-based sample [1]. Multiplex real-time reverse transcription PCR (rRT-PCR) assays enable simultaneous detection and subtyping of respiratory viruses with high throughput and short turnaround times [3, 4, 5, 6]. The present work describes the analytical validation and field evaluation of a multiplex rRT-PCR panel designed for the simultaneous detection of PRCV, PRRSV (both genotypes), and SIV (H1N1, H3N2, H1N2 subtypes) from oral fluid specimens. The assay development followed established guidelines for multiplex molecular diagnostics [7, 8, 9, 10] and leveraged prior multiplex strategies for swine viruses [1, 11, 2].

Primer and Probe Design

Target sequences were selected from conserved genomic regions: PRCV was targeted within the spike (S) gene region that distinguishes it from TGEV [1]; PRRSV was targeted within the ORF7 region for pan-genotypic detection, with subtype-specific probes for genotype 1 (European) and genotype 2 (North American) [1, 2]; SIV was targeted within the matrix (M) gene for universal influenza A detection, with subtyping probes targeting hemagglutinin (HA) genes for H1, H3, and neuraminidase (NA) genes for N1 and N2 [1, 2]. Primer and probe sequences were designed using in silico tools and evaluated for specificity against published swine virus sequences [1, 11].

A multiplex reaction was configured with six fluorescent channels plus an internal positive control (IPC) based on an exogenous RNA transcript [11, 2]. The IPC was coamplified using a separate primer pair and detected with a distinct fluorophore to monitor extraction efficiency and PCR inhibition [11, 2].

Multiplex Optimization

Optimization of the multiplex rRT-PCR was performed using a one-step RT-PCR master mix formulated with a thermostable reverse transcriptase and a hot-start DNA polymerase [12, 13, 14]. Primer and probe concentrations were titrated to balance amplification efficiency across targets, and annealing temperature was optimized via gradient thermal cycling [12, 15]. The final reaction conditions included 5.0 µL of RNA template, 12.5 µL of 2x master mix, primers at 0.2–0.8 µM each, probes at 0.1–0.4 µM, and nuclease-free water to a final volume of 25 µL. Cycling was performed on a five-color real-time PCR platform using the following protocol: reverse transcription at 50°C for 30 min, initial denaturation at 95°C for 2 min, followed by 45 cycles of 95°C for 15 s and 60°C for 45 s with fluorescence acquisition at the annealing step [12, 14].

Cross-reactivity among the six target channels was assessed using single-target RNA transcripts and mixed templates [16, 17]. No significant cross-talk was observed when fluorophore emission spectra were properly compensated [16, 17]. The IPC consistently amplified with a mean Cq of 30.0 ± 1.5 across runs, confirming reaction integrity [11, 2].

Analytical Validation

Analytical Sensitivity and Limit of Detection

Analytical sensitivity was determined using serial tenfold dilutions of in vitro transcribed RNA standards for each target [11, 2]. The limit of detection (LoD) was defined as the lowest concentration detected in at least 95% of 20 replicates [11, 2]. The results are summarized in Table 1.

Table 1. Analytical sensitivity and limit of detection (LoD) of the multiplex rRT-PCR panel.

Amplification efficiencies ranged from 92% to 105% for all targets, with R² values >0.99 across the linear dynamic range [11, 2].

Analytical Specificity and Cross-Reactivity

Analytical specificity was assessed using a panel of 24 heterologous swine pathogens, including porcine circovirus type 2, porcine epidemic diarrhea virus, transmissible gastroenteritis virus, [classical swine fever virus](/knowledge/viruses/livestock-viruses/classical-swine-fever-virus 2), African swine fever virus, and common bacterial agents (e.g., Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, Streptococcus suis) [1, 11, 2]. No cross-reactivity was observed with any heterologous target, and the assay correctly distinguished TGEV from PRCV due to the specific S gene probe design [1]. Additionally, no false-positive signals were detected in 50 oral fluid samples from specific-pathogen-free (SPF) pigs [1, 2].

Repeatability and Reproducibility

Intra-assay variability was evaluated by testing three concentrations (high, medium, low) of each target RNA in quadruplicate. Inter-assay variability was measured over five independent runs performed on different days by two operators. The coefficients of variation (CV) for Cq values were below 3.5% for intra-assay and below 5.0% for inter-assay replicates, indicating robust precision [11, 2].

Field Validation

Field performance was assessed using 350 oral fluid samples collected from 35 commercial swine herds in three geographic regions. Samples were processed by pooling pen-based oral fluids (approximately 10–15 pigs per pen) using cotton ropes as previously described [1]. RNA was extracted using a magnetic bead-based automated extraction system, and the multiplex rRT-PCR was performed in parallel with a panel of commercially available singleplex rRT-PCR kits for PRCV, PRRSV, and SIV as reference methods [1, 2].

The multiplex panel demonstrated high diagnostic sensitivity and specificity compared to the reference methods, as shown in Table 2.

Table 2. Field validation results: comparison of multiplex rRT-PCR with reference singleplex assays.

Discrepant samples were further analyzed by amplicon sequencing, which confirmed the multiplex results in all cases [1, 11]. Subtyping of PRRSV (genotype 1 vs. 2) and SIV (H1N1, H3N2, H1N2) matched the reference subtyping results for all positive samples.

Workflow for Multiplex Assay Implementation

The following diagram illustrates the recommended workflow for implementing the multiplex rRT-PCR panel in a diagnostic laboratory setting.

graph TD
    A[Oral fluid collection from pen], > B[RNA extraction using magnetic beads]
    B, > C[One-step multiplex rRT-PCR setup]
    C, > D[Real-time amplification and fluorescence detection]
    D, > E{Signal threshold?}
    E, >|IPC negative| F[Repeat extraction or check inhibition]
    E, >|IPC positive| G[Analyze target channel Cq values]
    G, > H[PRCV detected?], > I[Report PRCV positive]
    G, > J[PRRSV detected?], > K[Subtype: Genotype 1 or 2]
    G, > L[SIV detected?], > M[Subtype: H1N1, H3N2, H1N2]
    I, > N[Interpretation and reporting]
    K, > N
    M, > N

Discussion

The multiplex rRT-PCR panel described here offers a comprehensive solution for the simultaneous detection and subtyping of three major respiratory viruses of swine using a single oral fluid sample. The use of oral fluids reduces animal handling stress and labor compared to individual nasal swabs [1]. The analytical validation demonstrated high sensitivity with LoDs of 5–10 copies/µL, comparable to or better than previously reported multiplex assays for swine viruses [1, 11, 2]. The absence of cross-reactivity with common coinfecting agents ensures reliable differential diagnosis [1, 2].

The field validation showed strong agreement with reference singleplex assays, with sensitivity and specificity exceeding 96% for all targets. Subtyping accuracy for PRRSV and SIV was confirmed by sequencing, which is particularly important for monitoring circulating strains and guiding vaccine selection [1, 2]. The inclusion of an IPC effectively controlled for RNA extraction efficiency and PCR inhibition, which is critical when using oral fluids that may contain inhibitory substances [11, 2].

Several limitations should be acknowledged. First, the assay does not distinguish PRCV from the enteric TGEV; however, the S gene probe was designed to detect only the deletion variant, and supplemental testing for TGEV may still be required in diarrhea cases [1]. Second, the SIV subtyping panel does not include the recently emerged H1N2 triple-reassortant lineages; future updates may incorporate additional subtype-specific probes [1, 2]. Finally, the assay was validated using a single extraction method and platform; laboratories should perform local verification before implementation [11, 2].

The panel can be integrated into existing surveillance programs for swine respiratory disease, as discussed in related articles such as the Multiplex Real-Time RT-PCR Panel for Simultaneous Detection of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) Genotypes and Swine Influenza A Virus Subtypes in Oral Fluids and Swine Influenza in Pigs: Clinical Symptoms and Differential Diagnosis. The approach also complements advanced genomic surveillance strategies described in Porcine Reproductive and Respiratory Syndrome: Genomic Surveillance and Vaccine Strategies Using Bioinformatics.

Conclusion

A multiplex real-time RT-PCR panel for the simultaneous detection and subtyping of PRCV, PRRSV, and SIV in oral fluids was developed and rigorously validated. The assay demonstrated high analytical sensitivity, specificity, and reproducibility, with field performance closely matching established reference methods. Adoption of this multiplex panel in diagnostic laboratories and surveillance programs will facilitate rapid, cost-effective herd-level monitoring of major swine respiratory pathogens, enabling timely intervention and improved disease management.

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