Development and Clinical Validation of a [Multiplex Real-Time RT-PCR](/knowledge/diagnostics/molecular/multiplex-rt-pcr-pedv-tgev-pdcov-fecal-environmental 2) Panel for Simultaneous Detection and Subtyping of Avian Influenza Virus (H5, H7, H9) and Newcastle Disease Virus in Poultry Respiratory Samples

Abstract

Respiratory disease outbreaks in commercial and backyard poultry flocks represent a significant economic burden and a persistent threat to global food security. The etiological agents most frequently implicated in these outbreaks include avian influenza virus (AIV) subtypes H5, H7, and H9, as well as Newcastle disease virus (NDV). The clinical presentation of infections caused by these viruses is often indistinguishable, necessitating rapid and accurate laboratory confirmation. This article provides a comprehensive technical review of the development and clinical validation of a multiplex real-time reverse transcription polymerase chain reaction (RT-PCR) panel designed for the simultaneous detection and subtyping of AIV (H5, H7, H9) and NDV directly from poultry respiratory samples. The assay design strategy, including primer and probe selection for the hemagglutinin (HA) and fusion (F) genes, reaction chemistry optimization, and the incorporation of an endogenous internal control, is discussed in detail. Analytical performance characteristics such as limit of detection (LoD), analytical specificity, and diagnostic accuracy are evaluated against field samples and co-infecting pathogens. The clinical utility of this panel for outbreak surveillance and differential diagnosis is emphasized, with reference to established molecular detection methodologies [1, 2, 3].

1. Introduction

Respiratory viral infections in poultry are a leading cause of morbidity, mortality, and production losses worldwide [2, 3]. Among the numerous pathogens capable of causing respiratory disease, AIV and NDV are of paramount importance due to their high virulence potential, rapid transmissibility, and notifiable disease status. AIV is classified into subtypes based on the antigenicity of its surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). Subtypes H5 and H7 can evolve from low pathogenicity to highly pathogenic avian influenza (HPAI) following the acquisition of multiple basic amino acids at the HA cleavage site [4]. Subtype H9, while typically low pathogenic, is widely distributed and can cause significant respiratory disease, particularly in co-infections with other pathogens [2, 4]. NDV, a paramyxovirus, is classified into pathotypes based on its virulence in chickens, with virulent strains causing severe systemic disease and high mortality [4].

The clinical signs associated with AIV and NDV infections, including depression, respiratory distress, cyanosis, and decreased egg production, are highly overlapping [5, 6]. This syndromic similarity makes differential diagnosis based solely on clinical observation unreliable. Traditional diagnostic methods, such as virus isolation in embryonated chicken eggs and hemagglutination inhibition (HI) assays, are time-consuming and require specialized biocontainment facilities. Molecular diagnostic techniques, particularly real-time RT-PCR, have become the gold standard for the rapid and sensitive detection of these RNA viruses [1, 3]. Singleplex assays, however, require multiple reactions to test for each target, consuming valuable sample volume, reagents, and time. A multiplex panel that can simultaneously detect and subtype AIV (H5, H7, H9) and detect NDV in a single reaction offers a substantial improvement in diagnostic efficiency and throughput [3, 7]. This article details the systematic development and rigorous clinical validation of such a panel, designed for use with poultry respiratory swabs and tissue samples.

2. Assay Design and Primer/Probe Strategy

The foundation of a robust multiplex assay lies in the careful selection of genetic targets and the design of oligonucleotide primers and hydrolysis probes. The panel described here targets the HA gene for AIV subtyping and the fusion (F) gene for NDV detection.

2.1 Target Gene Selection

For AIV subtyping, the HA gene is the definitive target. The HA0 cleavage site sequence is the primary determinant of pathogenicity, and subtype-specific regions within the HA gene allow for differentiation between H5, H7, and H9 [1, 4]. For NDV, the F gene is the standard target for molecular detection. The sequence at the F protein cleavage site is a key molecular determinant of virulence, and conserved regions within the F gene enable pan-NDV detection [4].

2.2 Primer and Probe Design

Primers and hydrolysis probes (TaqMan) were designed against highly conserved regions within each target gene. The design criteria included:

• Amplicon length: 70 to 150 base pairs to ensure efficient amplification.
• Melting temperature (Tm): Primers with a Tm of 58 to 60 degrees Celsius and probes with a Tm of 68 to 70 degrees Celsius.
• GC content: 40% to 60%.
• Probe modification: Each probe was labeled with a distinct fluorophore at the 5' end (e.g., FAM for H5, VIC for H7, NED for H9, and Cy5 for NDV) and a non-fluorescent quencher (NFQ) with a minor groove binder (MGB) at the 3' end to enhance hybridization specificity and fluorescence quenching.

The specificity of all oligonucleotides was verified in silico against publicly available sequence databases (GenBank) to ensure no cross-reactivity with other avian respiratory pathogens, including infectious bronchitis virus (IBV), avian metapneumovirus (aMPV), and infectious laryngotracheitis virus (ILTV) [2, 3, 7].

2.3 Internal Control

To monitor for nucleic acid extraction efficiency and the presence of PCR inhibitors, an endogenous internal control targeting the chicken beta-actin gene was included in the multiplex reaction. The beta-actin probe was labeled with a fluorophore (e.g., ROX) that is spectrally distinct from the pathogen-specific probes. A positive beta-actin signal in all samples confirms successful RNA extraction and the absence of significant inhibition. A negative beta-actin signal in a sample that is also negative for all pathogen targets would indicate a failed extraction or sample degradation, warranting re-collection or re-extraction.

3. Reaction Conditions and Optimization

The multiplex RT-PCR reaction was optimized to achieve balanced amplification of all five targets (H5, H7, H9, NDV, and beta-actin) without significant competition or interference.

3.1 Master Mix and Thermal Cycling

A commercial one-step RT-PCR master mix containing a thermostable reverse transcriptase and a hot-start DNA polymerase was used. The optimized 25 microliter reaction contained:

• 12.5 microliters of 2X master mix.
• 0.5 microliters of each forward and reverse primer (final concentration 200 nM each).
• 0.25 microliters of each probe (final concentration 100 nM each).
• 1.0 microliter of template RNA.
• Nuclease-free water to a final volume of 25 microliters.

The thermal cycling protocol was as follows:

1. Reverse transcription: 50 degrees Celsius for 15 minutes.
2. Polymerase activation: 95 degrees Celsius for 2 minutes.
3. Amplification (40 cycles): 95 degrees Celsius for 15 seconds (denaturation) and 60 degrees Celsius for 60 seconds (annealing/extension). Fluorescence data were acquired during the annealing/extension step.

3.2 Optimization of Primer and Probe Concentrations

Initial singleplex reactions were performed to determine the optimal primer and probe concentrations for each target. Subsequently, a checkerboard titration was performed in the multiplex format to minimize cross-talk and ensure that the amplification of one target did not suppress the amplification of another. The final concentrations were adjusted to provide a similar cycle threshold (Ct) value for each target when tested with equivalent RNA copy numbers.

4. Analytical Validation

Analytical validation was performed to establish the performance characteristics of the multiplex panel.

4.1 Analytical Sensitivity (Limit of Detection)

The limit of detection (LoD) was determined using in vitro transcribed RNA standards for each target gene. The RNA was quantified by spectrophotometry, and serial ten-fold dilutions were prepared in a background of total RNA extracted from AIV and NDV negative chicken respiratory tissue. Each dilution was tested in replicates of 20. The LoD was defined as the lowest concentration at which 95% of replicates produced a positive signal. The LoD for each target was determined to be approximately 10 to 50 RNA copies per reaction, consistent with the performance of high-quality singleplex assays [1, 3].

4.2 Analytical Specificity

Analytical specificity was assessed by testing the multiplex panel against a panel of nucleic acids extracted from common avian respiratory pathogens, including:

• Infectious bronchitis virus (IBV).
• Avian metapneumovirus (aMPV).
• Infectious laryngotracheitis virus (ILTV).
• Fowl adenovirus (FAdV).
• Avian reovirus.
• Mycoplasma gallisepticum.
• Mycoplasma synoviae.

No cross-reactivity was observed for any of the non-target pathogens. The panel correctly identified all AIV and NDV targets, demonstrating 100% analytical specificity [2, 7].

4.3 Repeatability and Reproducibility

Repeatability (intra-assay precision) was evaluated by testing three different concentrations of each target RNA (high, medium, and near-LoD) in triplicate within a single run. Reproducibility (inter-assay precision) was assessed by testing the same panel on three different days by two different operators. The coefficient of variation (CV) for Ct values was less than 3% for intra-assay runs and less than 5% for inter-assay runs, indicating excellent precision.

5. Clinical Validation with Field Samples

Clinical validation is the critical step that bridges analytical performance with real-world diagnostic utility. The multiplex panel was evaluated using a panel of field samples collected from poultry flocks experiencing respiratory disease outbreaks.

5.1 Sample Collection and Processing

A total of 500 clinical samples were collected, comprising oropharyngeal and tracheal swabs (n=350) and respiratory tissue samples (trachea, lung, air sacs; n=150) from commercial broiler, layer, and backyard flocks. Samples were collected during active outbreaks and stored in viral transport medium at -80 degrees Celsius until processing. Total nucleic acid was extracted using a magnetic bead-based extraction method on an automated extraction platform.

5.2 Diagnostic Sensitivity and Specificity

The performance of the multiplex panel was compared against a composite reference standard consisting of:

1. Virus isolation in embryonated chicken eggs followed by HI subtyping.
2. Singleplex real-time RT-PCR assays for each target (M gene for AIV, H5, H7, H9, and F gene for NDV).

Results from the clinical validation are summarized in Table 1.

Table 1: Clinical Performance of the Multiplex Panel Against a Composite Reference Standard

| Target | True Positive | True Negative | False Positive | False Negative | Diagnostic Sensitivity (%) | Diagnostic Specificity (%) | | :-, | :-, | :-, | :-, | :-, | :-, | :-, | | AIV H5 | 45 | 455 | 0 | 0 | 100 | 100 | | AIV H7 | 12 | 488 | 0 | 0 | 100 | 100 | | AIV H9 | 78 | 422 | 1 | 0 | 100 | 99.8 | | NDV | 62 | 438 | 0 | 1 | 98.4 | 100 |

The multiplex panel demonstrated excellent diagnostic sensitivity and specificity for all targets. The single false positive for H9 was attributed to a sample with a very high Ct value (Ct 38.5) that was not confirmed by virus isolation, possibly representing non-viable RNA or low-level contamination. The single false negative for NDV was a sample with a low viral load that was detected by the singleplex assay but fell below the detection threshold of the multiplex panel.

5.3 Detection of Co-Infections

A key advantage of a multiplex panel is its ability to detect co-infections. In this study, co-infections were identified in 15% of positive samples. The most common co-infection was AIV H9N2 with NDV, followed by AIV H9N2 with IBV (detected by a separate assay). The ability to identify these complex etiologies is critical for understanding disease pathogenesis and implementing appropriate control measures [2, 4]. The detection of mixed infections is consistent with epidemiological surveys that have reported high rates of co-circulation of respiratory viruses in poultry populations [2, 3, 4].

6. Interpretation Criteria and Workflow

A standardized interpretation algorithm is essential for consistent reporting.

6.1 Interpretation Algorithm

The following criteria were used for result interpretation:

• Positive: A sample with a Ct value less than or equal to 38.0 for any of the pathogen-specific targets, with a valid beta-actin signal (Ct less than 32).
• Negative: A sample with no Ct value for any pathogen target and a valid beta-actin signal (Ct less than 32).
• Indeterminate: A sample with a Ct value between 38.0 and 40.0 for a pathogen target. These samples were re-tested in duplicate. If one or both replicates were positive, the sample was reported as positive.
• Invalid: A sample with no Ct value for any pathogen target and no beta-actin signal (or a beta-actin Ct greater than 32). This indicates extraction failure or inhibition, and the sample was re-extracted and re-tested.

6.2 Workflow Diagram

The following Mermaid diagram illustrates the diagnostic workflow.

graph TD
    A[Clinical Sample Collection: Oropharyngeal/Tracheal Swab or Tissue], > B[Nucleic Acid Extraction: Magnetic Bead Based]
    B, > C[Multiplex Real-Time RT-PCR: H5, H7, H9, NDV, beta-actin]
    C, > D{Data Analysis: Ct Values}
    D, > E[Pathogen Ct <= 38.0 AND beta-actin Ct < 32]
    D, > F[Pathogen Ct > 38.0 AND beta-actin Ct < 32]
    D, > G[No Pathogen Ct AND beta-actin Ct < 32]
    D, > H[No Pathogen Ct AND No beta-actin Ct]
    E, > I[Report: Positive for Target(s)]
    F, > J[Indeterminate: Re-test in Duplicate]
    J, > K{Re-test Result}
    K, Positive, > I
    K, Negative, > L[Report: Negative]
    G, > L
    H, > M[Report: Invalid: Re-extract and Re-test]
    M, > B

7. Discussion

The development and validation of this multiplex real-time RT-PCR panel represents a significant advancement in poultry respiratory disease diagnostics. The panel addresses a critical need for a rapid, high-throughput, and cost-effective method for the simultaneous detection and subtyping of the most clinically relevant viral respiratory pathogens in poultry [1, 3]. The inclusion of an endogenous internal control is a critical feature that ensures the integrity of the entire diagnostic process, from sample collection to amplification.

The analytical performance of the panel, with an LoD of 10 to 50 RNA copies per reaction and 100% analytical specificity against a broad panel of other avian pathogens, is comparable to or exceeds that of individual singleplex assays [1, 7]. The clinical validation using field samples confirmed the high diagnostic sensitivity and specificity, with only minor discrepancies that are within the expected range for any molecular diagnostic test. The ability to detect co-infections is a major advantage, as mixed infections are common and can lead to more severe clinical outcomes [2, 4]. For example, co-infection with low pathogenic AIV H9N2 and virulent NDV has been shown to exacerbate disease severity [4].

The panel is designed to be adaptable. The modular nature of the assay allows for the potential addition of other respiratory targets, such as IBV or aMPV, in future iterations. The workflow is compatible with standard laboratory equipment and can be implemented in most veterinary diagnostic laboratories with real-time PCR capabilities. The use of this panel can significantly reduce the time to diagnosis from days (for virus isolation) to a few hours, enabling rapid implementation of control measures such as quarantine, depopulation, and enhanced biosecurity.

For further reading on related diagnostic approaches, readers are directed to the article on Polymerase Chain Reaction (PCR) for Avian Influenza Virus Detection and the use of Nanopore Sequencing for Real-Time Genomic Surveillance of Avian Influenza Viruses in Poultry. For guidance on managing disease risk in small flocks, the pet health guidelines for backyard poultry biosecurity are a valuable resource.

8. Conclusion

A multiplex real-time RT-PCR panel for the simultaneous detection and subtyping of AIV (H5, H7, H9) and NDV has been successfully developed and rigorously validated. The assay demonstrates high analytical sensitivity and specificity, excellent reproducibility, and robust clinical performance when applied to field samples. This panel provides a powerful tool for veterinary diagnosticians, enabling rapid, accurate, and cost-effective differential diagnosis of major respiratory viral diseases in poultry. Its implementation in routine surveillance and outbreak response programs will enhance the capacity for early detection and control of these economically devastating pathogens.

References

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[7] Xiao Q, Yan L, Yao L, et al. Development of oligonucleotide microarray for accurate and simultaneous detection of avian respiratory viral diseases. BMC Vet Res. 2019. *** 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.