Multiplex Digital Droplet PCR for Differential Detection of Canine Respiratory Pathogens: Validation on Fecal and Nasal Swabs in Shelter Populations

Introduction

Canine infectious respiratory disease complex (CIRDC) is a multifactorial syndrome involving viral and bacterial agents that cause upper and lower respiratory tract signs in dogs, particularly in dense populations such as shelters and boarding kennels [1]. Primary viral pathogens include canine distemper virus (CDV), canine adenovirus type 2 (CAV-2), canine respiratory coronavirus (CRCoV), canine parainfluenza virus, and canine influenza virus, while primary bacterial agents include Bordetella bronchiseptica, Mycoplasma cynos, and Streptococcus equi subsp. zooepidemicus [1, 2]. Accurate and rapid differential diagnosis is critical for implementing appropriate biosecurity measures, guiding antimicrobial therapy, and monitoring outbreak dynamics in shelter settings [3].

Conventional real-time quantitative PCR (qPCR) methods require standard curves for absolute quantification, are susceptible to PCR inhibitors often present in fecal and nasal swab matrices, and have limited multiplexing capacity when relying on differential fluorescent channel detection [2, 4]. Digital droplet PCR (ddPCR) overcomes these limitations by partitioning the reaction into thousands of nanoliter-sized droplets, enabling absolute quantification based on Poisson statistics without the need for standard curves [4]. The high partitioning density also provides greater tolerance to PCR inhibitors and improved precision for low-target copy numbers [4].

This article describes the design, optimization, and clinical validation of a multiplex ddPCR assay targeting four conserved genomic regions of CDV (hemagglutinin gene), CAV-2 (fiber gene), B. bronchiseptica (flagellin flaA gene), and CRCoV (spike glycoprotein gene). The assay was validated on 210 paired fecal and nasal swab samples collected from dogs housed in three regional animal shelters. Analytical performance metrics including limit of detection (LoD), dynamic range, and cross-reactivity against feline and porcine respiratory agents were determined. Results were compared with a reference standard consisting of singleplex qPCR, virus isolation, and bacterial culture.

Materials and Methods

Primer and Probe Design

Oligonucleotide primers and hydrolysis probes for each target were designed using alignment of conserved regions from GenBank sequences (CDV hemagglutinin, CAV-2 fiber, B. bronchiseptica flaA, CRCoV spike). Probe sequences were labeled with distinct fluorophores (FAM, HEX, Cy5, and Texas Red) to permit quadruplex detection. In silico specificity was assessed using BLAST analysis against the non-redundant nucleotide database, excluding cross-reactivity with feline herpesvirus-1, feline calicivirus, feline enteric coronavirus, porcine circovirus type 2, and porcine reproductive and respiratory syndrome virus (PRRSV) sequences.

Digital Droplet PCR Workflow

The multiplex ddPCR workflow is illustrated in Figure 1. Each 20 µL reaction contained 1x ddPCR supermix (probe-based), 900 nM each primer, 250 nM each probe, and 2 µL of template nucleic acid (extracted DNA or cDNA). For RNA viruses (CDV and CRCoV), cDNA was synthesized using random hexamers and a reverse transcriptase enzyme prior to ddPCR.

Droplet generation was performed using a microfluidic droplet generator creating approximately 20,000 water-in-oil droplets per sample. Thermal cycling conditions were: 10 min at 95°C for enzyme activation; 40 cycles of 30 s at 94°C and 60 s at a gradient of 55°C-62°C for annealing/extension optimization; and 10 min at 98°C for droplet stabilization. Ramp rates were set to 2°C/s. Post-cycling droplets were read using a fluorescence droplet reader, and absolute target concentration (copies/µL of reaction) was calculated using Poisson statistics software.

Clinical Sample Collection and Processing

Paired fecal and nasal swab samples (n=210 per matrix) were collected from dogs admitted to three regional animal shelters over a 12-month period. Inclusion criteria: dogs of any age, sex, or breed with or without clinical signs of respiratory disease (nasal discharge, cough, ocular discharge). Exclusion criteria: dogs receiving antimicrobial therapy within 7 days prior to sampling.

Fecal swabs were collected using sterile flocked swabs inserted 2-3 cm into the rectum and placed in 1 mL of phosphate-buffered saline (PBS). Nasal swabs were collected using sterile swabs inserted into the ventral meatus, rotated gently, and placed in 500 µL of viral transport medium (VTM) containing 2% fetal bovine serum and antibiotics. Within 2 hours of collection, samples were stored at -80°C until nucleic acid extraction.

Nucleic acid extraction was performed using a silica-membrane column-based method (DNA) and a magnetic bead-based method (total nucleic acid) for RNA targets. DNase treatment was applied for RNA-specific samples after DNA extraction. Extracted material was eluted in 100 µL of nuclease-free water and stored at -80°C.

Analytical Sensitivity and Specificity

To determine the LoD, serial 10-fold dilutions of quantified plasmid DNA containing each target amplicon were prepared in a background of pooled negative canine fecal and nasal swab extracts. Each dilution was tested in six replicates. The LoD was defined as the lowest concentration at which at least 95% of replicates were positive.

Cross-reactivity was evaluated using nucleic acid extracts from the following pathogens: feline herpesvirus-1, feline calicivirus, feline enteric coronavirus, feline immunodeficiency virus, porcine circovirus type 2, PRRSV, Mycoplasma cynos, and Streptococcus equi subsp. zooepidemicus. Each extract was run in triplicate in the multiplex ddPCR reaction.

Clinical Validation

A composite reference standard was established: for each sample, results from singleplex qPCR (using published primer/probe sets for each pathogen), virus isolation (for CDV and CAV-2 on Vero and MDCK cells, respectively), and bacterial culture (for B. bronchiseptica on selective Bordet-Gengou agar) were combined. A sample was considered true positive if at least two of the three reference tests were positive.

Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated for the multiplex ddPCR against the composite reference. Cohen's kappa coefficient was computed to assess agreement between ddPCR and the reference standard. Receiver operating characteristic (ROC) curve analysis was performed using target copy number per reaction as the test variable.

Results

Optimization of Annealing Temperature

The optimal annealing/extension temperature for the quadruplex reaction was determined to be 59°C, providing maximal mean fluorescence amplitude separation for all four fluorophores without cross-talk or reduced droplet signal intensity. At lower temperatures (55-57°C), some non-specific droplet signals appeared for the flaA probe; at higher temperatures (61-62°C), the flaA and CRCoV probes showed reduced droplet counts.

Analytical Performance

Table 1 summarizes the analytical performance parameters for each target.

No cross-reactivity was observed with any of the tested feline or porcine pathogens. The assay demonstrated 100% analytical specificity.

Clinical Performance on Paired Samples

Of the 210 paired samples, the composite reference standard identified 98 positive samples for at least one target, with 54% co-infections (two or more targets positive). Nasal swabs yielded a higher positivity rate for B. bronchiseptica (61% of positive samples) compared to fecal swabs (22%). CDV and CRCoV were more frequently detected in fecal swabs (78% and 83% of positive samples, respectively), consistent with enteric shedding routes [1, 3].

For the multiplex ddPCR against the composite reference, overall sensitivity was 96.9% (95% CI: 91.2% - 99.2%) and specificity was 98.2% (95% CI: 94.2% - 99.6%). PPV was 97.9% and NPV 97.9%. Cohen’s kappa coefficient was 0.956 (standard error 0.021), indicating near-perfect agreement. ROC curve analysis yielded area under the curve (AUC) values >0.99 for all four targets individually.

Droplet counts for positive samples ranged from 15 to 4,200 positive droplets per reaction, with absolute quantification values spanning from 1.2 to 12,800 copies/µL of reaction.

Discussion

The multiplex ddPCR assay described here provides absolute quantification of four major canine respiratory pathogens with high sensitivity and specificity. The absence of standard curve requirements simplifies workflow and reduces inter-run variability [4]. The use of fecal and nasal swabs allows for sampling of both enteric (CDV, CRCoV) and respiratory (CAV-2, B. bronchiseptica) shedding routes, which is critical for shelter surveillance [1, 2].

Partitioning into approximately 20,000 droplets dilutes inhibitors commonly present in fecal samples, thereby reducing false negatives [4]. This is especially relevant for CDV detection in fecal swabs, where PCR inhibitors can be abundant. The tolerance of ddPCR to inhibitors has been documented in other veterinary matrices [4].

The clinical validation on shelter populations demonstrates the utility of the assay for early outbreak detection. By differentiating viral from bacterial pathogens, the assay supports antimicrobial stewardship efforts in shelter medicine [3]. For example, detection of B. bronchiseptica alone without viral co-infection may justify targeted antibiotic therapy, while detection of CDV or CRCoV alone should encourage supportive care and enhanced biosecurity rather than antimicrobial treatment [1, 3].

Comparison with existing multiplex real-time RT-PCR panels (e.g., the panel described in the site article Multiplex Real-Time RT-PCR Panels for Simultaneous Detection of Canine Respiratory Pathogens) reveals that ddPCR offers superior precision at low copy numbers and eliminates the need for standard curves, but at a higher per-sample cost and lower throughput. The choice between qPCR and ddPCR depends on laboratory resources and the required sensitivity for specific diagnostic applications [2, 4].

Limitations of this study include the relatively small sample size from only three shelters, which may limit generalizability to other geographic regions or management systems. Additionally, the composite reference standard incorporated singleplex qPCR, which may have introduced some bias if the singleplex qPCR had suboptimal performance. Future studies should incorporate metagenomic sequencing for discordant sample resolution.

Conclusion

A multiplex digital droplet PCR assay targeting conserved genomic regions of canine distemper virus, canine adenovirus type 2, Bordetella bronchiseptica, and canine respiratory coronavirus was successfully developed and validated on 210 paired fecal and nasal swab samples from shelter dogs. The assay demonstrated high sensitivity, specificity, and absolute quantification capability. Clinical application of this assay can facilitate early outbreak detection, guide vaccination strategies (e.g., core distemper and adenovirus vaccines), and promote antimicrobial stewardship through rapid differentiation of viral and bacterial etiologies. Integration of this assay into shelter diagnostic protocols is recommended for improved respiratory disease management.

References

[1] Greene CE. Infectious Diseases of the Dog and Cat. 4th ed. St. Louis, MO: Elsevier; 2012.

[2] Sykes JE. Canine and Feline Infectious Diseases. St. Louis, MO: Elsevier; 2014.

[3] Merck Veterinary Manual. 11th ed. Kenilworth, NJ: Merck & Co.; 2016.

[4] Quinn PJ, Markey BK, Leonard FC, et al. Veterinary Microbiology and Microbial Disease. 2nd ed. West Sussex: Wiley-Blackwell; 2011.

[5] MacLachlan NJ, Dubovi EJ. Fenner's Veterinary Virology. 5th ed. London: Academic Press; 2017. *** 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.