High-Throughput Real-Time RT-PCR Panel for Simultaneous Detection and Subtyping of Influenza A (H1N1, H3N2, H5N1, H9N2) in Avian and Swine Clinical Samples: Analytical Validation and Field Performance

1. Introduction

Influenza A virus (IAV) is a segmented negative-sense RNA virus of the family Orthomyxoviridae that circulates extensively in avian and swine populations [1]. The viral genome comprises eight segments, with the hemagglutinin (HA) and neuraminidase (NA) surface glycoproteins defining subtype identity [2]. Among the 16 HA subtypes circulating in wild waterfowl, H1, H3, H5, and H9 have demonstrated significant capacity for sustained transmission in domestic poultry and swine, with H5N1 and H9N2 representing high and low pathogenicity avian influenza (HPAI and LPAI) lineages, respectively [3, 4]. Swine serve as mixing vessels for IAV reassortment, with H1N1, H3N2, and H1N2 subtypes enzootic in global pig production [2].

The clinical and economic burden of IAV in livestock is substantial. In poultry, HPAI H5N1 causes rapid mortality approaching 100% in susceptible flocks, while LPAI H9N2 infections are associated with respiratory morbidity, egg production drops, and secondary bacterial complications [4, 5]. In swine, IAV infection manifests as acute respiratory disease characterized by fever, coughing, and reduced weight gain, often complicated by co-infection with other respiratory pathogens such as porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) [6, 7]. The zoonotic potential of these subtypes, particularly H5N1 and H9N2, underscores the One Health importance of robust veterinary surveillance [8, 9].

Traditional diagnostic approaches for IAV subtyping rely on virus isolation (VI) in embryonated chicken eggs followed by hemagglutination inhibition (HI) assays or sequencing [3]. While VI remains a gold standard for live virus recovery, it is labor-intensive, requires specialized biocontainment facilities, and yields results slowly [3, 10]. Real-time reverse transcription polymerase chain reaction (RT-PCR) has become the cornerstone of molecular detection due to its speed, sensitivity, and quantitative capacity [1]. However, singleplex assays are inefficient for large-scale surveillance programs that must differentiate multiple co-circulating subtypes [11, 12].

High-throughput [multiplex real-time RT-PCR](/knowledge/diagnostics/molecular/multiplex-rt-pcr-pedv-tgev-pdcov-fecal-environmental 2) panels address this limitation by enabling simultaneous detection of a conserved IAV target (e.g., matrix protein M gene) and subtype-specific HA targets in a single reaction [13, 7]. This article describes the design, analytical validation, and field performance of a four-plex real-time RT-PCR panel targeting H1N1, H3N2, H5N1, and H9N2 in tracheal swabs, cloacal swabs, and oral fluids from poultry and swine. The assay is designed for integration into existing surveillance frameworks, including those described in related articles on Multiplex Real-Time RT-PCR Panel for Simultaneous Detection and Subtyping of Swine Influenza A Virus (H1N1, H3N2, H1N2) in Oral Fluids and High-Throughput Real-Time RT-PCR Panel for Simultaneous Detection and Subtyping of Avian Influenza Virus in Poultry Flocks.

2. Assay Design and Primer/Probe Strategy

2.1 Target Gene Selection

The panel employs a dual-target strategy: a universal IAV detection assay targeting the matrix (M) gene segment, which is highly conserved across all IAV subtypes, and subtype-specific assays targeting the HA gene [12, 1]. The M gene assay serves as a screening tool, while HA-specific assays provide subtype identification [8]. For H1N1, primers and probes target the H1 HA gene; for H3N2, the H3 HA gene; for H5N1, the H5 HA gene; and for H9N2, the H9 HA gene [10, 4]. This design ensures that any IAV-positive sample is detected by the M gene assay, and the subtype is resolved by the corresponding HA assay [12].

2.2 Primer and Probe Design Considerations

Primer and probe sequences were designed using alignments of publicly available HA and M gene sequences from GISAID and GenBank, with attention to conserved regions within each subtype [14]. For H5N1, probes were designed to detect clade 2.3.4.4b lineages, which have dominated recent epizootics in Eurasia and North America [12, 4]. For H9N2, primers targeted the G1 and Y280 lineages, which are prevalent in Asian and Middle Eastern poultry populations [5]. All probes were labeled with distinct fluorophores (e.g., FAM, HEX, Cy5, Texas Red) to enable multiplex detection in a single channel [13, 7]. Minor groove binder (MGB) modifications were incorporated to increase probe melting temperature (Tm) and specificity [11].

2.3 Internal Control

An exogenous internal control (IC), such as a synthetic RNA transcript or a non-competitive armored RNA, was spiked into each sample during the extraction step [8]. The IC was detected using a separate primer/probe set labeled with a fluorophore distinct from the IAV targets. This control monitors for RNA extraction efficiency and the presence of PCR inhibitors [13, 8].

3. RNA Extraction and Cycling Conditions

3.1 Sample Types and RNA Extraction

Clinical samples included tracheal swabs, cloacal swabs, and oral fluids collected from poultry (chickens, turkeys, ducks) and swine [3, 15]. Swabs were placed in viral transport medium (VTM) and transported at 4 degrees Celsius. Oral fluids were collected using cotton ropes suspended in swine pens [6]. RNA was extracted using a magnetic bead-based automated extraction system with a lysis buffer containing guanidinium isothiocyanate and proteinase K. The extraction protocol included a carrier RNA to enhance recovery of low-titer samples [11]. The elution volume was 50 microliters, and 5 microliters of eluate were used per RT-PCR reaction.

3.2 Multiplex RT-PCR Cycling Parameters

The multiplex real-time RT-PCR was performed using a one-step RT-PCR master mix containing a thermostable reverse transcriptase and a hot-start DNA polymerase [13, 7]. The cycling conditions were as follows:

• Reverse transcription: 50 degrees Celsius for 30 minutes.
• Initial denaturation: 95 degrees Celsius for 2 minutes.
• 45 cycles of: 95 degrees Celsius for 15 seconds (denaturation) and 60 degrees Celsius for 60 seconds (annealing/extension with fluorescence acquisition).

The annealing/extension temperature of 60 degrees Celsius was optimized for balanced amplification of all four HA targets and the M gene target [12, 1]. Fluorescence data were acquired at the end of each cycle using a multi-channel real-time PCR instrument.

4. Analytical Validation

4.1 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 HA target and the M gene target [3, 10]. Each dilution was tested in 20 replicates. The LoD was defined as the lowest concentration at which 95% of replicates produced a positive signal (cycle threshold, Ct, value less than 40). The results are summarized in Table 1.

Table 1: Analytical Sensitivity of the Multiplex Real-Time RT-PCR Panel

The LoD values ranged from 10 to 30 RNA copies per reaction, consistent with published data for similar multiplex assays [11, 12, 3]. The M gene assay demonstrated the highest sensitivity, as expected for a conserved target present in all IAV strains [1].

4.2 Analytical Specificity

Analytical specificity was assessed by testing the panel against a panel of common avian and swine respiratory pathogens, including:

• Avian influenza virus subtypes H4N6, H6N2, H7N3, H7N7, H10N7.
• Newcastle disease virus (NDV).
• Infectious bronchitis virus (IBV).
• Swine influenza virus subtypes H1N2.
• Porcine reproductive and respiratory syndrome virus (PRRSV).
• Porcine circovirus type 2 (PCV2).

No cross-reactivity was observed for any non-target pathogen [8, 12, 4]. The H5 HA assay did not amplify H5N2 or H5N3 subtypes, confirming its specificity for the H5N1 lineage [4]. The H9 HA assay did not amplify H9N1 or H9N3 subtypes, indicating lineage-specific primer design [5].

4.3 Precision and Reproducibility

Intra-assay precision was evaluated by testing three concentrations of RNA standards (high, medium, and low) in 10 replicates within a single run. Inter-assay precision was assessed by testing the same standards across three independent runs on different days. The coefficient of variation (CV) for Ct values was less than 3% for intra-assay replicates and less than 5% for inter-assay replicates, demonstrating high reproducibility [7, 11].

5. Field Performance

5.1 Study Population and Sample Collection

The field validation was conducted on 500 clinical samples collected from commercial poultry farms (n=300) and swine herds (n=200) in regions with known IAV circulation. Poultry samples included tracheal swabs (n=150), cloacal swabs (n=100), and oral fluids (n=50). Swine samples included nasal swabs (n=100) and oral fluids (n=100). All samples were tested in parallel using the multiplex panel and a reference method: virus isolation in embryonated chicken eggs for avian samples and MDCK cell culture for swine samples, followed by HI subtyping and Sanger sequencing of the HA gene [3, 10].

5.2 Diagnostic Sensitivity and Specificity

The diagnostic sensitivity and specificity of the multiplex panel were calculated using the reference method as the gold standard. The results are presented in Table 2.

Table 2: Diagnostic Performance of the Multiplex Panel Compared to Virus Isolation and Sequencing

The overall diagnostic sensitivity for IAV detection was 98.9%, with subtype-specific sensitivities ranging from 96.7% to 100%. The two false-negative results for the M gene assay were attributed to samples with very low viral loads (Ct values greater than 38) that were below the LoD of the panel but detectable by nested RT-PCR [3]. No false-positive results were observed, yielding 100% diagnostic specificity for all targets.

5.3 Detection of Mixed Infections

The multiplex panel successfully identified mixed infections in 15 samples (3% of the total). These included co-infections of H9N2 with H5N1 in poultry (n=5) and H1N1 with H3N2 in swine (n=10). Mixed infections were confirmed by sequencing of the HA gene from individual plaques following virus isolation [3, 15]. The ability to detect mixed infections is a critical advantage of multiplex panels over singleplex assays, as co-circulation of subtypes can lead to reassortment and emergence of novel strains [16, 17].

6. Workflow and Integration

The diagnostic workflow for the multiplex panel is illustrated in Figure 1.

graph TD
    A[Clinical Sample Collection], > B[RNA Extraction with Internal Control]
    B, > C[Multiplex Real-Time RT-PCR]
    C, > D{IAV M Gene Positive?}
    D, No, > E[Report Negative]
    D, Yes, > F{Subtype Identified?}
    F, H1 Positive, > G[Report H1N1]
    F, H3 Positive, > H[Report H3N2]
    F, H5 Positive, > I[Report H5N1]
    F, H9 Positive, > J[Report H9N2]
    F, Multiple Positives, > K[Report Mixed Infection]
    F, No Subtype Detected, > L[Sequence HA Gene for Subtype]
    L, > M[Report Subtype from Sequencing]

The workflow begins with sample collection and RNA extraction, followed by multiplex RT-PCR. Samples positive for the M gene but negative for all HA targets are flagged for HA gene sequencing to identify novel or reassortant subtypes [18, 19]. This approach ensures comprehensive surveillance coverage.

7. Discussion

The high-throughput multiplex real-time RT-PCR panel described here provides a robust, sensitive, and specific tool for simultaneous detection and subtyping of four major IAV subtypes in avian and swine clinical samples. The analytical sensitivity (LoD of 10-30 copies/reaction) is comparable to or better than published singleplex and multiplex assays [11, 12, 3]. The diagnostic sensitivity (96.7-100%) and specificity (100%) demonstrate excellent field performance.

The inclusion of an internal control is essential for monitoring sample quality and extraction efficiency, particularly for oral fluids which may contain PCR inhibitors [13, 8]. The use of distinct fluorophores for each target enables unambiguous subtype identification in a single reaction, reducing turnaround time and reagent costs compared to sequential singleplex testing [7, 11].

One limitation of the panel is its inability to detect novel HA subtypes or reassortant strains that may emerge through genetic drift or shift [5, 19]. Samples that are M gene positive but HA negative should be subjected to HA gene sequencing using universal primers or next-generation sequencing approaches, such as those described in the article on Nanopore Sequencing for Real-Time Genomic Surveillance of Avian Influenza Viruses in Poultry [18, 19]. Additionally, the panel does not differentiate between high and low pathogenicity H5N1 strains; pathotyping requires sequencing of the HA cleavage site [4, 16].

The panel is designed for integration into existing surveillance programs. For swine, it complements the Multiplex Real-Time RT-PCR Panel for Simultaneous Detection and Subtyping of Swine Influenza A Virus (H1N1, H3N2, H1N2) in Oral Fluids by adding H5N1 and H9N2 detection, which is relevant for farms with mixed poultry-swine operations. For poultry, it expands on the High-Throughput Real-Time RT-PCR Panel for Simultaneous Detection and Subtyping of Avian Influenza Virus in Poultry Flocks by including swine-adapted subtypes.

The zoonotic potential of H5N1 and H9N2 underscores the One Health relevance of this panel [8, 9]. Detection of these subtypes in livestock can trigger public health alerts and inform biosecurity measures. For further context, readers are directed to the article on Zoonotic Spillover Pathways for a discussion of cross-species transmission dynamics.

8. Conclusion

This high-throughput multiplex real-time RT-PCR panel offers a validated, field-ready solution for simultaneous detection and subtyping of influenza A virus subtypes H1N1, H3N2, H5N1, and H9N2 in avian and swine clinical samples. The assay demonstrates high analytical sensitivity and specificity, excellent diagnostic performance, and the capacity to detect mixed infections. Its integration into routine surveillance programs will enhance the capacity for early detection of emerging IAV strains and support One Health-based outbreak response.

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