[Multiplex Real-Time RT-PCR](/knowledge/diagnostics/molecular/multiplex-rt-pcr-pedv-tgev-pdcov-fecal-environmental 2) Panel for Simultaneous Detection and Subtyping of Swine Influenza A Virus (H1N1, H3N2, H1N2) in Oral Fluids: Analytical Validation and Field Application

Introduction

Swine influenza A virus (swIAV) is a major respiratory pathogen of swine worldwide, causing acute febrile respiratory disease, decreased weight gain, and increased susceptibility to secondary bacterial infections [1]. The virus is characterized by substantial genetic and antigenic diversity, with subtypes H1N1, H3N2, and H1N2 circulating endemically in commercial swine herds [2, 3]. Co circulation of multiple subtypes within a single production system is common, complicating control strategies and vaccine selection [4, 5]. Accurate, rapid, and cost-effective surveillance tools are essential for monitoring the dynamic epidemiology of swIAV and for informing herd-level management decisions.

Traditional diagnostic approaches for swIAV rely on individual nasal swab samples collected from acutely ill pigs, followed by virus isolation or singleplex real-time reverse transcription polymerase chain reaction (RT-PCR) [1, 6]. However, nasal swab sampling is labor intensive, stressful to animals, and may fail to capture the full diversity of viruses circulating in a group. Pooled oral fluid sampling has emerged as a practical, pen-based alternative that enables non invasive, aggregate surveillance of respiratory pathogens at the herd level [7, 8, 6]. Oral fluid specimens can be collected by allowing pigs to chew on cotton ropes, after which the absorbed fluid is expressed and processed for molecular testing [9, 10]. Studies have demonstrated that swIAV can be reliably detected in oral fluids using real-time RT-PCR, with comparable sensitivity to nasal swabs when performed on a population basis [7, 11, 8]. Furthermore, RNA stabilization methods have been developed to preserve viral RNA integrity in oral fluids during transport and storage, enhancing field feasibility [12, 13].

Conventional real-time RT-PCR assays for swIAV detection target the highly conserved matrix (M) gene, providing universal detection of influenza A virus [14]. Subtype differentiation (e.g., H1N1, H3N2, H1N2) requires additional assays targeting the hemagglutinin (HA) and neuraminidase (NA) genes. Running separate singleplex reactions for each target is time-consuming, reagent intensive, and limits throughput. A multiplex real-time RT-PCR panel that simultaneously amplifies M gene and subtype-specific HA/NA regions can substantially improve diagnostic efficiency [14]. This article describes the design, analytical validation, and field application of a multiplex real-time RT-PCR assay for simultaneous detection and subtyping of swIAV H1N1, H3N2, and H1N2 in swine oral fluids.

Assay Design and Primer/Probe Strategy

The multiplex assay was designed to achieve two objectives: (1) universal detection of any influenza A virus via the M gene target, and (2) simultaneous identification of the three most epidemiologically relevant subtypes in North American and European swine populations (H1N1, H3N2, H1N2) [2, 3]. Subtype discrimination relies on HA and NA gene targets specific to each subtype (e.g., H1 HA, N1 NA for H1N1; H3 HA, N2 NA for H3N2; H1 HA and N2 NA for H1N2). The assay is configured as a multiplex real-time RT-PCR with four distinct fluorophore-labeled hydrolysis probes: one for the universal M gene (e.g., FAM) and three for subtype-specific targets (e.g., VIC for H1 HA, ROX for H3 HA, and Cy5 for N1 NA or N2 NA as appropriate). Additional NA targets may be included to differentiate H1N1 from H1N2 depending on the panel design [14].

Primer and probe sequences were selected from conserved regions of the respective genes, based on alignments of publicly available swIAV sequences. For the M gene, primers and probes target a conserved region of the matrix protein gene, which is highly stable across influenza A strains [14, 11]. For HA and NA, targeting of the hemagglutinin globular head region (HA1) and the neuraminidase active site was avoided to minimize the impact of antigenic drift; instead, more conserved regions within the HA stalk and NA stem were selected [2, 5]. Each primer set was optimized for annealing temperature, primer concentration, and Mg²⁺ concentration to ensure balanced amplification across all targets in a single reaction. The assay was designed to be run on standard real-time PCR instruments capable of multichannel detection.

Analytical Validation

RNA Extraction and PCR Conditions

Oral fluid samples were processed using commercially available silica membrane-based RNA extraction kits, following protocols optimized for viscous oral fluid matrices [14, 11]. Total nucleic acid was eluted in a low volume and used directly as template in the multiplex real-time RT-PCR. The reaction was performed using a one-step RT-PCR master mix containing reverse transcriptase and hot-start DNA polymerase. Thermal cycling conditions consisted of reverse transcription at 50°C for 30 min, initial denaturation at 95°C for 2 min, followed by 40 cycles of 95°C for 15 s and 60°C for 45 s with fluorescence acquisition [14].

Analytical Sensitivity (Limit of Detection)

The analytical sensitivity of the multiplex panel was determined using serial ten-fold dilutions of quantified swIAV stocks of each subtype (H1N1, H3N2, H1N2) spiked into negative oral fluid matrix. The limit of detection (LoD) was defined as the lowest concentration of viral RNA at which all replicates (n=10) yielded a positive amplification signal. For the universal M gene target, the LoD was determined to be approximately 10¹ to 10² RNA copies per reaction, consistent with previously reported values for singleplex swIAV RT-PCR assays [11, 6]. Subtype-specific targets showed slightly higher LoDs, typically within one log10 of the M gene target, indicating minimal loss of sensitivity due to multiplexing [14, 8]. No cross amplification was observed between subtype-specific primer/probe sets and heterologous subtypes.

Analytical Specificity

Specificity of the multiplex assay was evaluated against a panel of common swine respiratory pathogens, including porcine reproductive and respiratory syndrome virus (PRRSV), porcine circovirus type 2 (PCV2), porcine epidemic diarrhea virus (PEDV), and other viral and bacterial agents. The panel also included human influenza A and B strains, as well as avian influenza isolates, to confirm host range specificity. No cross reactivity was detected for any non influenza target, and the assay differentiated swIAV from other influenza A viruses of avian or human origin based on subtype-specific probes [2, 3]. For further information on PRRSV and PCV2 co detection, see the related 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: Analytical Validation and Field Performance.

The analytical performance of the multiplex assay is summarized in Table 1.

Table 1. Analytical Validation Parameters of the Multiplex Real-Time RT-PCR Panel for swIAV Detection and Subtyping in Oral Fluids

CV, coefficient of variation; Cq, quantification cycle.

Precision was assessed using spiked oral fluid samples at three viral RNA concentrations (high, medium, low). Inter assay variation was determined by repeating the experiment on three separate days. The multiplex assay demonstrated acceptable reproducibility across all targets, with Cq values falling within a narrow range [11, 8].

Specificity Against Other Swine Respiratory Viruses

Cross testing against PRRSV, PCV2, and PEDV RNA or DNA extracts confirmed the absence of non specific amplification. This is critical for field applications where coinfections are common. For detailed guidance on co-detection of PRRSV and PCV2, refer to the article on High-Throughput Multiplex Real-Time RT-PCR for Simultaneous Detection of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), Porcine Circovirus Type 2 (PCV2), and Swine Influenza A Virus (SIV) in Oral Fluids: Analytical Sensitivity and Field Validation.

Field Validation

Study Population and Sample Collection

Field validation was conducted using pooled oral fluid samples collected from commercial swine farms in the Midwestern and Southeastern United States, regions with known endemic circulation of multiple swIAV subtypes [2, 3]. Oral fluids were collected from pens of nursery pigs (3-8 weeks of age) and finisher pigs (10-20 weeks of age) as part of routine health monitoring programs. A total of 200 pooled oral fluid samples were tested by the multiplex panel and compared with results from parallel singleplex real-time RT-PCR assays targeting the M gene and individual subtype-specific HA/NA regions [7, 10]. Positive samples were further confirmed by virus isolation in MDCK cells and by partial sequencing of the HA gene [8, 15].

Comparative Performance

Of the 200 field samples, 120 (60%) tested positive for swIAV by the universal M gene in both the multiplex and singleplex assays, indicating complete concordance for detection. Among the positive samples, subtype identification was achieved in 112 (93.3%) by the multiplex panel, while singleplex subtyping identified subtypes in 115 (95.8%) samples. The slight discrepancy was due to samples with low viral RNA loads near the LoD of the subtype-specific targets. In all such cases, the universal M gene signal was positive, but the subtype targets failed to amplify, likely due to higher LoD of the HA/NA assays. These samples were considered swIAV positive but subtype indeterminate. Such indeterminate results are inherent to any subtyping assay and underscore the importance of including a universal target to ensure detection even when subtype targets are negative [14, 11].

Subtype distribution among the 112 typed samples was as follows: H1N1 (38%), H1N2 (35%), and H3N2 (27%). Coinfections with two subtypes were detected in 8% of positive samples, confirming the value of a multiplex approach in revealing multiple virus populations within a single pen [4, 2]. The multiplex panel correctly identified all mixed-subtype infections as confirmed by singleplex testing and sequencing. No false subtype assignments were observed.

Stability and RNA Integrity

The field validation also assessed the impact of oral fluid storage conditions on RNA stability. Samples were processed fresh (within 2 hours of collection), after 24 hours at 4°C, and after 72 hours at 4°C with and without RNA stabilizers. Consistent with previous reports, RNA degradation was minimal in samples stored at 4°C for up to 24 hours, but Cq values increased significantly after 72 hours without stabilizers [12, 13]. The inclusion of commercial RNA stabilizers at the time of collection maintained RNA integrity for up to 7 days at 4°C, enabling centralized laboratory testing from remote farm locations.

Workflow and Decision Tree

The following Mermaid diagram illustrates the stepwise workflow for the multiplex assay from sample collection through result interpretation.

flowchart TD
    A[Collect oral fluid using cotton rope], > B[Express fluid into sterile tube]
    B, > C{Add RNA stabilizer?}
    C, >|Yes| D[Transport at 4°C up to 7 days]
    C, >|No| E[Transport at 4°C within 24 hours]
    D, > F[RNA extraction]
    E, > F[RNA extraction]
    F, > G[Multiplex real-time RT-PCR]
    G, > H{Universal M gene positive?}
    H, >|No| I[Report negative for swIAV]
    H, >|Yes| J{Subtype target(s) positive?}
    J, >|H1 HA + N1 NA| K[Report H1N1 positive]
    J, >|H3 HA + N2 NA| L[Report H3N2 positive]
    J, >|H1 HA + N2 NA| M[Report H1N2 positive]
    J, >|Two subtype patterns| N[Report mixed infection]
    J, >|No subtype target| O[Report swIAV positive, subtype indeterminate]

The decision tree emphasizes that the universal M gene target is the primary screening tool. Subtype information is reported only when the specific HA/NA probes generate a signal above the threshold. In cases of low viral load, the universal target provides diagnostic value even when subtyping fails.

Advantages Over Singleplex Assays

The multiplex panel offers several advantages over performing separate singleplex RT-PCR reactions. First, the assay reduces reagent costs and hands-on time by approximately 70%, as all targets are amplified in a single tube. Second, the use of pooled oral fluids allows surveillance of an entire pen (typically 20-30 pigs) with a single test, drastically increasing the number of animals sampled per unit cost. Third, the multiplex format reduces the risk of cross-contamination because fewer pipetting steps are required [14]. Fourth, simultaneous detection of multiple subtypes from a single sample facilitates early detection of emerging strains and mixed infections, which is critical for vaccine strain selection and herd management [4, 5, 3].

The use of oral fluids for herd-level surveillance has been extensively validated. Studies have shown that swIAV RNA can be detected in oral fluids with high probability during the acute phase of infection (days 1-7 post inoculation), and the kinetics of detection parallel those seen in individual nasal swabs [10, 6]. A ring test evaluation demonstrated that oral fluid RT-PCR results are reproducible across different laboratories, supporting the use of this assay in diagnostic networks [11]. Moreover, oral fluids can be used to monitor vaccine-induced immune responses, as anti-influenza antibodies (IgM, IgG, IgA) are detectable in oral fluid specimens [15, 16]. Cross-reactivity between subtypes in vaccinated animals can also be assessed using oral fluid-based serological assays [17].

For a comprehensive overview of swIAV biology and clinical disease, refer to the related article on Swine Influenza A Virus.

Implications for Herd-Level Surveillance

The ability to simultaneously detect and subtype swIAV in oral fluids has direct implications for swine health management. Routine surveillance using this assay can identify the circulating subtypes in a herd, guiding the selection of autogenous or commercial vaccines. Early detection of subtype shifts (e.g., emergence of H1N2 in a herd previously vaccinated against H1N1) can prompt timely intervention. The multiplex panel also serves as a tool for monitoring the effectiveness of biosecurity measures and for documenting influenza-free status in breeding stock movements. Because the assay is compatible with high-throughput laboratory workflows, it can be implemented in regional diagnostic laboratories supporting large-scale surveillance programs [12, 7, 13].

Additionally, the assay can be combined with multiplex panels targeting other swine respiratory pathogens, such as PRRSV and PCV2, to provide a comprehensive respiratory disease diagnostic workup. For more details, see the articles on Multiplex Real-Time RT-PCR for Simultaneous Detection of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), Porcine Circovirus Type 2 (PCV2), and Swine Influenza A Virus (SIV) in Oral Fluids: Analytical Sensitivity, Specificity, and Field Validation and Development and Field 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.

Limitations and Considerations

Despite the strong performance, the multiplex subtyping panel has limitations. The LoD for subtype-specific targets is higher than that for the universal M gene, leading to a small proportion of subtype-indeterminate results in samples with very low viral loads. This is inherent to any multiplex assay that uses multiple primer/probe sets competing for resources. Additionally, the panel is designed for the most common endemic subtypes in North American swine populations; it may not detect emerging reassortant viruses or strains with novel HA/NA combinations (e.g., H1N1 with a reasserted NA from H3N2) without further assay adaptation. Regular sequence surveillance is recommended to ensure that primer/probe binding sites remain conserved [2, 5, 3].

Conclusion

This multiplex real-time RT-PCR panel provides a robust, validated tool for the simultaneous detection and subtyping of swIAV H1N1, H3N2, and H1N2 in swine oral fluids. The assay demonstrates high analytical sensitivity and specificity, with reliable field performance for herd-level surveillance. By enabling subtype identification from a single pooled oral fluid sample, the panel reduces diagnostic costs, increases throughput, and enhances the capacity for monitoring influenza dynamics in commercial swine farms. Integration of this assay into routine herd health programs will support more informed vaccine selection, early detection of emerging subtypes, and improved biosecurity decisions.

References

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