Multiplex digital droplet PCR (ddPCR) for simultaneous quantification of porcine circovirus type 2, porcine reproductive and respiratory syndrome virus, and Mycoplasma hyopneumoniae in oral fluids: analytical sensitivity and field validation

Introduction

Porcine circovirus type 2 (PCV2), porcine reproductive and respiratory syndrome virus (PRRSV), and Mycoplasma hyopneumoniae are among the most economically significant pathogens affecting global swine production. PCV2 is a small circular DNA virus associated with postweaning multisystemic wasting syndrome and other circovirus-associated diseases [1, 2]. PRRSV, a positive-sense RNA arterivirus, causes reproductive failure and respiratory disease and exhibits extensive genetic diversity [3, 4, 5, 6]. M. hyopneumoniae, a cell wall-free bacterium, is the primary etiologic agent of enzootic pneumonia in pigs [7]. Concurrent infections with these agents are common in field settings and can exacerbate clinical outcomes [8, 9].

Oral fluids have emerged as a practical, noninvasive sample matrix for herd-level surveillance of swine pathogens. The collection of oral fluids allows pooling of secretions from multiple animals and reduces stress associated with individual sampling. However, oral fluids contain inhibitors of nucleic acid amplification, including mucopolysaccharides, proteases, and complex microbiota, which can compromise the accuracy of quantitative PCR (qPCR) assays [10].

Digital droplet PCR (ddPCR) partitions a sample into thousands of nanoliter-scale droplets, each subjected to endpoint PCR amplification. The fraction of positive droplets is used to calculate the absolute target copy number via Poisson statistics without reliance on external standard curves. This fundamental property confers several advantages: resistance to PCR inhibitors, higher precision at low target concentrations, and direct copy number reporting. For RNA targets such as PRRSV, a reverse transcription step is integrated into the ddPCR workflow.

This article describes the development and validation of a multiplex ddPCR assay for the simultaneous absolute quantification of PCV2 DNA, PRRSV RNA, and M. hyopneumoniae DNA in swine oral fluids. The assay's analytical sensitivity, linearity, precision, tolerance to sample inhibitors, and field performance are assessed in comparison to established multiplex qPCR methods [11, 10].

Assay Design and Optimization

Probe and Primer Selection

Target sequences were selected from conserved genomic regions. For PCV2, the assay targets the ORF2 capsid gene [1, 2]. For PRRSV, a conserved region of ORF7 (nucleocapsid) was chosen to detect both PRRSV-1 (Betaarterivirus europensis) and PRRSV-2 (Betaarterivirus americense) [3, 4, 10]. For M. hyopneumoniae, the assay targets a fragment of the 16S rRNA gene [7]. Each target is detected using hydrolysis probes labeled with distinct fluorophores: FAM for PCV2, HEX for PRRSV, and Cy5 for M. hyopneumoniae. The probes are quenched with a nonfluorescent dark quencher.

Primer and probe concentrations were optimized in simplex and gradually combined into a triplex format. Final concentrations were adjusted to minimize competition among primer sets while maintaining amplification efficiency. The final reaction mixture contained 900 nM of each forward and reverse primer and 250 nM of each probe. For the reverse transcription step (required for PRRSV), a separate RT enzyme mix was included in the one-step ddPCR formulation.

One-Step Reverse Transcription and Droplet Generation

The one-step RT-ddPCR approach couples cDNA synthesis with PCR amplification within each droplet. A 22 microliter reaction mixture containing 5 microliters of template nucleic acid was combined with 18 microliters of droplet generation oil and partitioned into approximately 20,000 droplets using a microfluidic droplet generator. The droplets were then subjected to thermal cycling: 45 minutes at 50 degrees Celsius for reverse transcription, 10 minutes at 95 degrees Celsius for enzyme activation, followed by 40 cycles of 94 degrees Celsius for 30 seconds and 60 degrees Celsius for 60 seconds. After thermal cycling, droplets were read individually in a two-color (FAM/HEX) or three-color (FAM/HEX/Cy5) detector.

Multiplex Optimization and Cross-Reactivity

Cross-reactivity among the three probe channels was assessed using single-target nucleic acid extracts. No significant fluorescence crosstalk was observed when target concentrations were within normal clinical ranges. The triplex assay showed a slight decrease in droplet amplitude for the HEX channel compared to the simplex assay, but separation between positive and negative droplets remained unambiguous. The assay was designed to be compatible with both DNA and RNA co-extraction; a DNase/RNase-free elution buffer was used.

Analytical Sensitivity and Linearity

Limit of Blank, Limit of Detection, and Limit of Quantification

The limit of blank (LoB) was determined by analyzing 20 replicates of nuclease-free water. No positive droplets were observed for any channel. The limit of detection (LoD) was defined as the lowest concentration at which at least 95% of replicates yielded a positive result. Using serial dilutions of plasmid standards for PCV2 and M. hyopneumoniae and in vitro transcribed RNA for PRRSV, the LoD for each target was determined. The triplex assay achieved LoDs of 5 copies per reaction for PCV2, 8 copies per reaction for PRRSV, and 10 copies per reaction for M. hyopneumoniae. The limit of quantification (LoQ), defined as the lowest concentration with a coefficient of variation (CV) below 25% across replicates, was 15 copies per reaction for PCV2, 20 copies per reaction for PRRSV, and 25 copies per reaction for M. hyopneumoniae.

Linearity and Dynamic Range

Serial tenfold dilutions from 10^5 to 10^0 copies per reaction were tested in triplicate. The measured copy numbers showed a linear response over five orders of magnitude for each target (R² > 0.995 for all three). The ddPCR assay maintained linearity down to the LoQ. Representative data are shown in Table 1.

Table 1. Linearity and dynamic range of triplex ddPCR assay.

The slope values near 1.0 indicate high PCR efficiency. The dynamic range is sufficient to cover the clinically relevant concentrations observed in oral fluids during acute infection and subclinical shedding.

Comparison to Multiplex qPCR

A direct comparison between the triplex ddPCR assay and a triplex TaqMan qPCR assay targeting the same genomic regions was performed. The qPCR assay utilized standard curve-based quantification with plasmid and RNA standards. For each of 50 spiked oral fluid samples, the ddPCR and qPCR results were compared.

Overall, the ddPCR assay demonstrated higher precision at low target concentrations (CV < 15% for ddPCR versus CV up to 35% for qPCR at concentrations near 50 copies per reaction). The absolute quantification by ddPCR eliminated the variability introduced by standard curve preparation and degradation. The correlation between ddPCR and qPCR was strong for PCV2 (Pearson r = 0.94) and PRRSV (r = 0.91), but moderate for M. hyopneumoniae (r = 0.85), likely due to lower bacterial DNA loads in some samples.

The ddPCR assay also showed greater tolerance to PCR inhibition. When oral fluid samples were spiked with known concentrations of each target and then diluted 1:2, 1:5, and 1:10, the ddPCR assay maintained accurate quantification across all dilutions. The qPCR assay showed significant underestimation (greater than 40% lower than expected) in the undiluted and 1:2 diluted samples, indicating residual inhibition even with the use of internal amplification controls. This observation is consistent with previous reports on ddPCR resistance to inhibitors [11, 10].

Tolerance to Inhibitors in Oral Fluids

Oral fluids contain variable levels of inhibitory substances that can compromise PCR efficiency. The ddPCR assay partitions the sample into droplets, and each droplet acts as an independent reaction. Even if a fraction of droplets is completely inhibited, the remaining droplets provide accurate results as long as the inhibition does not affect all droplets. This is a key advantage over bulk qPCR, where inhibition uniformly suppresses fluorescence signals.

To test inhibitor tolerance, oral fluid samples with high inhibitor content (as determined by optical density and PCR baseline shifts) were spiked with equal amounts of target nucleic acids. The ddPCR assay recovered 92% to 105% of expected copy numbers, whereas the qPCR assay recovered only 55% to 70%. The presence of inhibitors negatively affected qPCR amplification curves, leading to artificially high Cq values and underestimation of target concentration. The ddPCR assay, by relying on a dichotomous (positive/negative) signal from each droplet, was less affected by suboptimal amplification in some droplets.

Field Validation

Sample Collection and Nucleic Acid Extraction

A total of 250 oral fluid samples were collected from 25 commercial swine farms in three geographic regions over a 12 month period. Samples were collected by allowing pigs to chew on cotton ropes for 20 to 30 minutes; ropes were then wrung out, and the fluid was collected into sterile tubes. Samples were transported on ice and processed within 24 hours.

Nucleic acid extraction was performed using a column-based method suitable for concurrent DNA and RNA isolation. The extraction protocol included an on-column DNase digestion step for RNA-only analysis, but for the ddPCR assay, a combined DNA/RNA elution was used. This co-extraction approach allowed simultaneous detection of the DNA viruses (PCV2, M. hyopneumoniae) and the RNA virus (PRRSV) from a single sample. Total nucleic acid concentration and purity were assessed spectrophotometrically; samples with A260/A280 ratios below 1.8 were reprocessed.

Results from Field Samples

Of the 250 oral fluid samples, 185 (74%) were positive for at least one target. PCV2 was detected in 120 samples (48%), PRRSV in 78 samples (31%), and M. hyopneumoniae in 55 samples (22%). Double infections were common: 40 samples were positive for both PCV2 and PRRSV, 25 for PCV2 and M. hyopneumoniae, and 15 for PRRSV and M. hyopneumoniae. Triple infection was observed in 10 samples (4%). These proportions are consistent with known co-infection patterns in endemically infected herds [8, 9].

The ddPCR copy numbers varied widely: PCV2 ranged from 15 to 3.4 x 10^5 copies per milliliter of oral fluid; PRRSV ranged from 20 to 1.2 x 10^6 copies/mL; M. hyopneumoniae ranged from 25 to 8.5 x 10^4 copies/mL. The assay successfully quantified samples across this wide dynamic range without dilution.

Concordance with Clinical Status

A retrospective comparison with clinical records (presence of cough, fever, reproductive failure, or ongoing antimicrobial treatment) showed that herds with high load (> 10^4 copies/mL) of PRRSV or PCV2 were more likely to exhibit clinical signs. For M. hyopneumoniae, herds with loads above 10^3 copies/mL had a positive predictive value of 78% for enzootic pneumonia as diagnosed by field veterinarians. The ddPCR quantification provided a more objective assessment than qPCR cycle threshold values, which can be influenced by pipetting variation and instrument calibration.

Sample Stability and RNA/DNA Co-Extraction

The stability of target nucleic acids in oral fluids was assessed. Samples stored at 4 degrees Celsius for up to 48 hours showed no significant loss of quantifiable copies for any target (decrease less than 10%). Freezing at -80 degrees Celsius preserved nucleic acid integrity for at least six months. Repeated freeze-thaw cycles (more than three) resulted in a 30% drop in PRRSV RNA copies, but DNA targets were more stable.

The co-extraction of RNA and DNA using a single column purification with a chaotropic salt buffer and ethanol-based washes yielded extracts suitable for ddPCR. The presence of DNA in the PRRSV channel did not interfere because the reverse transcription step is RNA-dependent. Conversely, the presence of RNA did not affect the DNA targets. The ddPCR assay was designed with primers that span intron-exon boundaries for the RNA target to avoid amplification of genomic DNA; however, PRRSV lacks introns, so a DNase treatment step is recommended for PRRSV-only assays, but in the triplex format the RNA signal was easily differentiated from potential DNA background because the reverse transcriptase step is omitted in the DNA-only droplets. The one-step RT-ddPCR incorporates RT enzyme in all droplets; any residual DNA would be amplified only if primers bind, but the PRRSV primers do not amplify host genomic DNA [11, 10].

Workflow

The integrated workflow from sample collection to data analysis is depicted in the Mermaid diagram below.

flowchart TD
    A[Swine oral fluid collection via rope], > B[Transport on ice, < 24 h]
    B, > C["Nucleic acid co-extraction (DNA/RNA)"]
    C, > D["ddPCR master mix preparation (one-step RT, triplex primers/probes)"]
    D, > E[Droplet generation]
    E, > F[Thermal cycling: RT 50°C 45 min, then 95°C 10 min, 40 cycles]
    F, > G[Droplet reading (FAM/HEX/Cy5)]
    G, > H["Poisson-based absolute quantification"]
    H, > I["Result: copies/mL oral fluid for PCV2, PRRSV, M. hyopneumoniae"]

The diagram illustrates the linear process from noninvasive sample collection to absolute quantification. No standard curve is required, and the data analysis algorithm automatically separates positive and negative droplets based on fluorescence amplitude thresholds.

Limitations and Future Directions

The triplex ddPCR assay has several limitations. The cost per sample is higher than qPCR due to consumables (droplet generation oil, specialized cartridges). The throughput is lower because each sample requires a separate run; however, multiplexing reduces the number of assays per sample. The detection of M. hyopneumoniae is based on a DNA target, but the bacterium can be present as nonviable organisms; ddPCR does not distinguish between live and dead mycoplasma. The assay also cannot differentiate between PRRSV vaccine strains and field strains, though this is a common limitation of most diagnostic PCR assays [3, 12, 13, 14].

Future improvements could incorporate internal droplet-specific controls to monitor inhibition at the droplet level. Linking the ddPCR results with genomic surveillance and vaccine efficacy studies [14] would provide additional value. The assay could be extended to include other respiratory or enteric agents using additional fluorophores or spectral multiplexing.

Conclusion

The multiplex ddPCR assay described here provides robust absolute quantification of PCV2, PRRSV, and M. hyopneumoniae in swine oral fluids. The assay offers superior precision and inhibitor tolerance compared to qPCR, and it eliminates the need for standard curves. Field validation demonstrated good concordance with clinical status and broad dynamic range. This assay is suitable for herd-level surveillance and research applications. For further reading on related diagnostics, see the articles on multiplex ddPCR for PRRSV and swine influenza (Multiplex Digital Droplet PCR for PRRSV and SIV) and on PCV2 and PRRSV molecular detection (Multiplex RT-qPCR for PCV2 and PRRSV and SIV). Biosecurity protocols and vaccination strategies for these pathogens are discussed in general swine health guidelines.

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