High-Throughput Multiplex RT-qPCR Panel for Simultaneous Detection and Subtyping of Avian Influenza A Viruses (H5, H7, H9) in Poultry Clinical Samples: Validation and Field Performance

Introduction

Avian influenza A virus (AIV) remains a significant pathogen in global poultry production, with certain hemagglutinin (HA) subtypes including H5, H7, and H9 posing considerable economic and animal health burdens [1]. Subtypes H5 and H7 are classified as notifiable highly pathogenic avian influenza (HPAI) in many jurisdictions, while H9N2 viruses circulate widely as low pathogenicity strains that can cause substantial respiratory morbidity and secondary bacterial infections [1]. Rapid and accurate molecular differentiation of these subtypes is essential for outbreak response, surveillance, and implementation of control measures [1]. Traditional diagnostic workflows involving virus isolation in embryonated chicken eggs followed by serological subtyping are time consuming and labor intensive [1]. Reverse transcription quantitative polymerase chain reaction (RT-qPCR) offers superior speed, sensitivity, and the capability for multiplexing. This review describes the design, analytical validation, and field performance of a high-throughput multiplex RT-qPCR panel targeting the hemagglutinin genes of H5, H7, and H9 AIV subtypes in poultry tracheal and cloacal swabs. The panel incorporates an internal RNA extraction control to monitor sample integrity and extraction efficiency.

Assay Design and Primer Probe Selection

The multiplex panel described by Yang et al. (2022) employed a single-step real-time RT-qPCR format using subtype specific primers and TaqMan probes [1]. Target genes were selected from the hemagglutinin (HA) segment for each of the three subtypes, ensuring specific identification. Primer and probe sets were designed against conserved regions within the HA gene for H5, H7, and H9 based on sequence alignments of contemporaneous circulating strains [1]. The use of subtype specific HA gene targets avoids the necessity for an initial matrix (M) gene screening step followed by separate subtyping reactions, thereby reducing time to result and reagent consumption [1].

Each probe was labeled with a distinct fluorophore to permit simultaneous detection in a single reaction well. Fluorophores were selected to minimize spectral overlap and permit reliable multicomponent analysis on standard real-time PCR platforms. For internal control purposes, an exogenous RNA template such as MS2 bacteriophage RNA was spiked into each sample prior to nucleic acid extraction [1]. A separate primer probe set targeting the MS2 genome was included in the multiplex reaction. This control generated a signal only if RNA extraction and reverse transcription were successful and if no substantial inhibitors were present in the sample. Failure of the MS2 control in the absence of target amplification triggered a result of invalid and required reextraction or resampling.

Analytical Sensitivity and Limit of Detection

Analytical sensitivity of the multiplex panel was determined using serial dilutions of in vitro transcribed RNA standards or cultured virus preparations of known titer for each subtype [1]. The limit of detection (LoD) was defined as the lowest concentration of target RNA that could be detected in at least 95% of replicate reactions. Yang et al. (2022) reported LoD values of approximately 10 copies per reaction for H5 and H7 targets and approximately 50 copies per reaction for H9 targets, demonstrating high sensitivity across all three subtypes [1]. These values are comparable to those reported for other singleplex avian influenza detection assays and for a related multiplex panel targeting H6 subtypes [2, 1]. The inclusion of a multiplex format did not substantially compromise analytical sensitivity when compared to singleplex reactions, a finding consistent with optimized primer and probe design and balanced reaction component concentrations [1].

Analytical Specificity and Cross-Reactivity

Analytical specificity was evaluated by testing the multiplex panel against a panel of other avian respiratory pathogens and against other influenza A subtypes not targeted by the assay [1]. The specificity panel included Newcastle disease virus (NDV), infectious bronchitis virus (IBV), avian metapneumovirus, and other AIV subtypes such as H1, H3, H4, H6, and H10. No cross-reactivity was observed for any of the non-targeted pathogens or subtypes [1]. This specificity is attributable to the careful selection of primer and probe sequences targeting regions unique to each of the three HA subtypes [1]. The assay correctly identified samples containing mixed infections of H5 and H9, demonstrating that the multiplex format could resolve coinfections without loss of signal from either target [1]. This capability is particularly relevant for field situations in which multiple AIV subtypes may be circulating concurrently.

Internal Control and Nucleic Acid Extraction Monitoring

The use of MS2 bacteriophage RNA as an internal extraction control is a well established practice in molecular virology [1]. A known quantity of MS2 phage particles was added to each sample lysis buffer prior to extraction. Amplification of the MS2 target gene in the multiplex reaction confirmed that RNA extraction was successful and that no significant PCR inhibition was present [1]. In field samples, the MS2 control cycle threshold (Ct) values were monitored for consistency across runs. Significant elevation of the MS2 Ct above a predefined threshold indicated partial sample degradation, incomplete extraction, or the presence of inhibitory substances. In such cases, sample results were flagged as potentially compromised [1]. The incorporation of this control increased confidence in negative results and reduced the risk of false negative reporting due to sample quality issues.

Validation Metrics and Statistical Framework

Diagnostic validation of the multiplex RT-qPCR panel was performed using a well defined reference standard. The reference standard typically comprised virus isolation in embryonated chicken eggs combined with subtype specific serological or molecular confirmation [1]. For field validation, a total of several hundred clinical samples including tracheal and cloacal swabs were collected from poultry flocks in different geographic regions and production systems [1]. Diagnostic sensitivity, diagnostic specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated with 95% confidence intervals. The results reported by Yang et al. (2022) demonstrated diagnostic sensitivity exceeding 98.5% for H5 and H7 and approximately 95% for H9, with diagnostic specificity above 99% for all three subtypes [1]. PPV and NPV were similarly high, confirming the assay's reliability for field use.

Table 1. Summary of analytical and diagnostic validation metrics for the multiplex RT-qPCR panel targeting H5, H7, and H9 AIV subtypes based on data from Yang et al. (2022) [1].

Field Performance and Sample Type Suitability

Field validation was conducted using paired tracheal and cloacal swabs collected from poultry during routine surveillance and outbreak investigations [1]. Both sample types were processed using the same nucleic acid extraction protocol and tested with the multiplex panel. Detection rates were generally higher in tracheal swabs for respiratory subtype H9, while cloacal swabs showed comparable performance for H5 and H7 subtypes [1]. The ability to detect viral RNA in both sample matrices provided flexibility for field sampling protocols. In several flocks, the multiplex panel identified H9 subtype circulation in the absence of clinical signs, underscoring the value of molecular surveillance for low pathogenicity strains [1]. The panel also detected H5 and H7 subtypes in samples from flocks with overt respiratory disease and mortality, consistent with expectations for notifiable avian influenza outbreaks [1].

Workflow and Throughput Considerations

The high-throughput design of the panel permitted batch processing of multiple samples per run on standard 96-well or 384-well real-time PCR platforms. The single-step RT-qPCR format eliminated the need for separate reverse transcription and PCR steps, reducing hands-on time and the potential for carryover contamination [1]. A generic workflow diagram is presented in Figure 1.

graph TD
    A[Sample Collection: Tracheal or Cloacal Swab], > B[Lysis Buffer + MS2 Phage Spike]
    B, > C[Nucleic Acid Extraction]
    C, > D[Multiplex RT-qPCR Setup: H5, H7, H9, MS2 targets]
    D, > E[Real-Time PCR Amplification]
    E, > F{Data Analysis}
    F, > G[Subtype Positive: H5, H7, or H9 detected]
    F, > H[Subtype Negative: All targets below threshold]
    F, > I[Invalid: MS2 control out of range]
    G, > J[Report: Subtype identification and Ct value]
    H, > J
    I, > K[Repeat extraction or request new sample]

Figure 1. Workflow diagram for the high-throughput multiplex RT-qPCR panel for AIV H5, H7, and H9 detection and subtyping. MS2 bacteriophage is added as an internal extraction and inhibition control.

Comparison with Other Molecular Approaches

The multiplex RT-qPCR approach offers several advantages over other molecular methods for AIV subtyping. Traditional gel based RT-PCR requires post-amplification processing and is less amenable to high-throughput formats than real-time PCR. Digital droplet PCR provides absolute quantification and may offer higher precision at low target concentrations, but it entails higher per-sample costs and longer turnaround times. Sequencing based approaches, including nanopore sequencing, can provide full genome information but require more complex bioinformatics infrastructure and longer processing times. The multiplex RT-qPCR panel described here balances speed, sensitivity, cost, and throughput, making it suitable for routine surveillance and outbreak response in veterinary diagnostic laboratories [1]. Supplementary information on sequencing for avian influenza is available in the related article on Nanopore Sequencing for Real-Time Genomic Surveillance of Avian Influenza Viruses in Poultry.

Limitations and Considerations

Despite its strong performance, the multiplex RT-qPCR panel has limitations. The assay targets only H5, H7, and H9 HA subtypes. Other AIV subtypes with poultry health significance, such as H6, are not detected [2]. Samples testing positive for influenza A matrix gene but negative for H5, H7, and H9 may warrant additional subtyping using alternative methods. As with all PCR based assays, detection of viral RNA does not differentiate between infectious and non-infectious virus. False positive results from environmental contamination or carryover of amplicon can occur if strict laboratory practices are not followed. Furthermore, genetic drift of circulating AIV strains may reduce primer and probe binding efficiency over time, necessitating periodic sequence surveillance and assay redesign [1].

Conclusion

The high-throughput multiplex RT-qPCR panel targeting the hemagglutinin genes of H5, H7, and H9 AIV subtypes demonstrates excellent analytical sensitivity, specificity, and field performance for detection and subtyping in poultry clinical samples. The incorporation of an MS2 internal control ensures reliable monitoring of RNA extraction and PCR inhibition. The panel provides a rapid, cost-effective, and scalable tool for routine surveillance and outbreak investigation, supporting timely decision making for veterinary authorities and poultry producers. Continued monitoring of circulating AIV sequence diversity will be essential to maintain the assay's relevance and diagnostic accuracy over time [1]. For broader diagnostic context, readers may refer to related reviews including 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 and to general information on Avian Influenza A Virus in Wild Birds and Poultry: Etiology, Epidemiology, Clinical Signs, Pathology, Diagnostics, Treatment, and Control.

References

[1] Yang F, Dong D, Wu D, et al. A multiplex real-time RT-PCR method for detecting H5, H7 and H9 subtype avian influenza viruses in field and clinical samples. Virus Res. https://pubmed.ncbi.nlm.nih.gov/34954007/ *** 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.

[2] Li X, Tian J, Zhou W, et al. Development and validation of a sensitive fluorescence RT-qPCR assay with TaqMan-MGB probe for detection of H6 subtype avian influenza A virus. J Vet Diagn Invest. https://pubmed.ncbi.nlm.nih.gov/41562546/