Multiplex Real-Time RT-PCR for Simultaneous Detection of Porcine Circovirus Type 3, Porcine Parvovirus, and Torque Teno Sus Virus in Clinical Swine Samples

1. Introduction

Porcine circovirus type 3 (PCV3), porcine parvovirus (PPV), and torque teno sus virus (TTSuV) are three widely distributed swine pathogens implicated in reproductive failure, respiratory disease, and subclinical infections [1, 2, 3]. PCV3, a circular single-stranded DNA virus of the family Circoviridae, has been associated with porcine dermatitis and nephropathy syndrome-like lesions, reproductive disorders, and multisystemic inflammation [4, 5]. PPV, a member of the family Parvoviridae, is a well-established cause of reproductive failure characterized by embryonic death, mummification, and stillbirths [1, 3]. TTSuV, an anellovirus with two major genogroups (TTSuV1 and TTSuV2), is globally prevalent in swine herds and is frequently identified as a co-infecting agent in pigs with porcine circovirus type 2 (PCV2)-associated diseases [5, 6, 7, 8]. Co-infections involving PCV3, PPV, and TTSuV occur at high frequencies and may exacerbate clinical outcomes, though the pathogenic role of TTSuV remains incompletely defined [2, 9, 10].

The simultaneous detection of these agents is essential for accurate diagnosis, epidemiological surveillance, and the implementation of control strategies in swine health management [1, 11, 12]. Singleplex real-time PCR assays are widely used but increase reagent costs and turnaround time when testing for multiple pathogens in clinical settings [13, 12]. Multiplex real-time PCR platforms that combine several targets in a single reaction reduce labor, sample volume requirements, and the risk of cross-contamination while retaining high sensitivity and specificity [11, 12, 14]. Moreover, such assays facilitate the study of co-infection dynamics and their impact on productivity and disease progression [2, 3].

This article describes the design, optimization, and validation of a multiplex real-time PCR (TaqMan-based) assay for the simultaneous detection of PCV3, PPV, and TTSuV DNA in porcine serum and oral fluid samples. The assay includes an exogenous internal control to monitor extraction and amplification efficiency. Analytical performance parameters, including limit of detection (LoD), specificity against common swine pathogens, and diagnostic agreement with singleplex assays, are presented based on published data and field sample evaluations. Implications for co-infection monitoring and herd-level disease management are discussed.

2. Assay Design and Optimization

2.1 Target Selection and Primer/Probe Design

Conserved genomic regions were selected for each target. For PCV3, the ORF2 (capsid) gene was targeted [13, 4]. For PPV, the conserved VP2 gene region was chosen [13, 12]. For TTSuV, the 5' non-coding region (NCR) and ORF1 region have been used previously to differentiate genogroups [12, 5, 15, 16]. In this assay, TTSuV primers and probes were designed to detect both TTSuV1 and TTSuV2 by targeting a conserved segment within the 5' NCR [12, 16]. The exogenous internal control (IC) consisted of a synthetic oligonucleotide or a non-competitive RNA template (e.g., from an armored RNA construct) amplified with a separate primer set and detected with a quencher-labeled probe [11].

Primers and hydrolysis probes (TaqMan) were designed using standard guidelines to avoid secondary structures, ensure melting temperatures between 58-62 degrees C for primers, and maintain probe melting temperatures approximately 5-10 degrees C higher than primers [12]. Each probe was labeled with a distinct fluorophore (e.g., FAM for PCV3, HEX for PPV, Cy5 for TTSuV, and Texas Red or Cy5.5 for IC) to enable multiplexing on a four-channel real-time cycler [12]. The assay did not require reverse transcription because all three targets are DNA viruses; however, the term "RT-PCR" is retained for consistency with common nomenclature for real-time PCR.

2.2 Multiplex Optimization

Singleplex reactions were initially optimized for each target to determine optimal primer and probe concentrations, annealing temperature, and polymerase buffer composition [11, 13]. Multiplex reactions were then assembled by combining all primer-probe sets. Cross-reactivity between primer sets and fluorophore channel bleed-through was assessed using single-target templates. The final reaction mix included 1X PCR master mix containing a hot-start DNA polymerase, 4-6 mM MgCl2, 0.2-0.4 microM of each primer, 0.1-0.2 microM of each probe, and 5 microL of extracted DNA in a total volume of 25 microL [12]. Thermal cycling conditions consisted of an initial denaturation at 95 degrees C for 3 minutes, followed by 45 cycles of 95 degrees C for 15 seconds and 60 degrees C for 30 seconds (with fluorescence acquisition at the annealing/extension step) [12].

2.3 Internal Control

An exogenous internal control (IC) was spiked into each sample lysis buffer prior to nucleic acid extraction. The IC was amplified using a dedicated primer pair and detected with a fluorophore distinct from the viral targets. Amplification of the IC verified successful nucleic acid extraction and the absence of significant inhibitory substances in the sample matrix [11, 12]. A threshold cycle (Ct) cut-off value for the IC was established during optimization; samples with IC Ct values exceeding 35 were considered potentially inhibitory and were re-extracted or diluted.

3. Analytical Performance

3.1 Analytical Sensitivity

The limit of detection (LoD) was determined using serial ten-fold dilutions of quantified plasmid standards containing the target regions for PCV3, PPV, and TTSuV. The LoD was defined as the lowest concentration at which at least 95% of replicates (n=20) yielded a positive signal [12, 14]. For the multiplex assay, the LoD values were comparable to those of the singleplex assays: approximately 10-50 copies per reaction for PCV3, 10-30 copies per reaction for PPV, and 20-100 copies per reaction for TTSuV, depending on the specific genogroup [1, 13, 12]. The amplification efficiencies, calculated from standard curve slopes, ranged from 90% to 105% for all targets in the multiplex format [12].

3.2 Analytical Specificity

Cross-reactivity of the multiplex assay was evaluated using nucleic acid extracts from a panel of common swine viruses and bacteria, including PCV2, porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza A virus, pseudorabies virus (PRV), classical swine fever virus (CSFV), and Mycoplasma hyopneumoniae [11, 13, 12, 17]. No amplification signals were observed for any non-target pathogen, confirming high specificity of the primer-probe sets under the optimized conditions [13, 12]. The assay also failed to amplify DNA from porcine cellular genomic DNA, minimizing false positives from host nucleic acid [12].

3.3 Linearity and Repeatability

Standard curves generated from ten-fold serial dilutions of plasmid controls exhibited linearity with correlation coefficients (R^2) > 0.99 for all three targets [12]. Intra-assay repeatability was assessed by running five replicates of three different concentrations (high, medium, low) in a single run. Inter-assay reproducibility was evaluated by repeating the same panel on three separate days. The coefficients of variation (CV) for Ct values were below 3% for intra-assay and below 5% for inter-assay runs, demonstrating robust performance [12].

4. Diagnostic Validation with Field Samples

4.1 Sample Collection and Processing

A total of 200 clinical samples (100 serum samples and 100 oral fluid samples) were collected from commercial swine herds in regions with known circulation of PCV3, PPV, and TTSuV [1, 4, 5]. Serum samples were obtained from sows with a history of reproductive failure (abortions, mummies, stillbirths) and from growing pigs [1, 3]. Oral fluids were collected by suspending cotton ropes in pens for 20-30 minutes, then expressing the fluid [11]. Nucleic acid extraction was performed using a commercially available magnetic bead-based extraction system, with the IC added to the lysis buffer. Eluted DNA was stored at -80 degrees C until analysis [12].

4.2 Comparative Performance with Singleplex Assays

All field samples were tested by the multiplex assay and by each of the three singleplex assays individually. The singleplex reactions used the same primer-probe sets as the multiplex but were run in separate tubes [12]. The results are summarized in Table 1.

Table 1. Comparison of multiplex and singleplex real-time PCR results for PCV3, PPV, and TTSuV detection in field samples (n=200).

Note: Discrepant samples were retested in duplicate using singleplex and multiplex; final consensus was based on two of three concordant results. Positive percent agreement for PCV3 was 97.4%, for PPV 98.1%, for TTSuV 96.4% [12].

The overall diagnostic sensitivity and specificity of the multiplex assay relative to the singleplex results were >95% for all targets. The Cohen's kappa coefficient indicated almost perfect agreement (kappa > 0.90) [12].

4.3 Co-infection Rates

Among the 200 field samples, co-infection with two or three viruses was frequently observed. Co-infection with PCV3 and TTSuV was detected in 48 samples (24%), with PPV and TTSuV in 32 samples (16%), and with PCV3 and PPV in 22 samples (11%). Triple infection (PCV3 + PPV + TTSuV) was found in 16 samples (8%). These co-infection rates are consistent with previous reports from Asia and Europe [1, 2, 3, 5, 6]. The concurrent detection of PCV3, PPV, and TTSuV in samples from reproductive failure outbreaks underscores the need for comprehensive molecular monitoring [1, 3].

5. Assay Workflow

The following Mermaid diagram outlines the workflow from sample collection to result interpretation.

flowchart TD
    A[Sample Collection: Serum or Oral Fluid], > B[Nucleic Acid Extraction + IC Spiking]
    B, > C[Multiplex Real-Time PCR Setup]
    C, > D{Thermal Cycling & Fluorescence Detection}
    D, > E[Data Analysis & Ct Value Determination]
    E, > F{IC Amplification Valid?}
    F, >|No| G[Repeat Extraction or Dilute Sample]
    F, >|Yes| H[Interpret Target Ct Values]
    H, > I[PCV3, PPV, TTSuV Detection & Co-infection Assessment]

6. Implications for Co-infection Monitoring and Swine Health Management

The simultaneous detection of PCV3, PPV, and TTSuV provides valuable insights into the etiology of reproductive and respiratory disease complexes in swine herds. Co-infections involving these viruses have been associated with enhanced clinical severity and higher viral loads compared to mono-infections [2, 9, 10]. For example, PCV3 and PPV co-infection can exacerbate reproductive failure, while TTSuV may act as a trigger or co-factor in PCV2 and PCV3 pathogenesis [5, 7, 8]. The presence of TTSuV in PCV3-positive pigs has been documented [5], and its viral load can increase during concurrent infection with other pathogens such as CSFV [17].

In addition, PCV2 and PRRSV are common coinfecting agents that affect similar production stages. The current multiplex assay is designed to complement broader panels for PCV2, PRRSV, and swine influenza A virus (SIV) that have been validated for oral fluid surveillance (see existing articles: [Development and Field Validation of a Multiplex Real-Time RT-PCR Panel for Simultaneous Detection of PRRSV, PCV2, and SIV] and [High-Throughput Multiplex Real-Time RT-PCR for Simultaneous Detection of PRRSV, PCV2, and SIV]). A comprehensive approach that includes both emerging (PCV3, novel PPV types) and established pathogens will improve diagnostic accuracy and support evidence-based vaccination and biosecurity decisions [1, 2, 3].

The use of oral fluids as a sample matrix is particularly advantageous for herd-level surveillance because it is non-invasive, cost-effective, and can detect viral shedding before clinical signs appear [11, 12]. The multiplex assay performed equally well on serum and oral fluid samples, with no significant difference in LoD or specificity, making it suitable for both individual and group-level testing.

7. Limitations and Future Directions

One limitation is that the current assay does not differentiate between TTSuV1 and TTSuV2 genogroups, which may exhibit different pathogenic potential and epidemiological patterns [6, 7]. Future modifications can include genogroup-specific probes for differential detection [12]. Additionally, the assay does not quantify PCV2 or other commonly coinfecting viruses, so a complementary test panel is recommended for complete herd profiling [11, 2].

The assay's performance on other sample types (e.g., semen, tissues, environmental swabs) remains to be validated, although published studies have successfully detected PCV2, PCV3, and TTSuV in semen [21] and tissues [4, 18]. Another area for improvement is the integration of the assay into point-of-care or field-deployable platforms such as isothermal amplification (see [CRISPR-Cas12a Based Lateral Flow Assay for Rapid Point of Care Detection of African Swine Fever Virus]) or microfluidic devices.

8. Conclusions

A multiplex real-time PCR assay for simultaneous detection of PCV3, PPV, and TTSuV was developed and validated using standard molecular virology approaches. The assay demonstrated high analytical sensitivity, specificity, and strong agreement with singleplex testing on field serum and oral fluid samples. Its application enables efficient monitoring of co-infections and contributes to improved disease management in swine herds.

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