Comparison of Bacterial Culture and Real-Time PCR for the Detection of Salmonella in Grow–Finish Pigs in Western Canada Using a Bayesian Approach

Authors

  • W. Wilkins,

    1.  Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, SK, Canada
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  • C. Waldner,

    1.  Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, SK, Canada
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  • A. Rajić,

    1.  Policy Advice and Effectiveness Program, Laboratory for Foodborne Zoonoses, Public Health Agency of Canada, Guelph, ON, Canada
    2.  Department of Population Medicine, Ontario Veterinary College, University of Guelph, ON, Canada
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  • M. McFall,

    1.  Food Safety and Animal Health Division, Alberta Agriculture and Rural Development, Edmonton, AB, Canada
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  • A. Muckle,

    1.  Diagnostic Services, Atlantic Ve3terinary College, University of Prince Edward Island, Charlottetown, PE, Canada
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  • R. C. Mainar-Jaime

    1.  Unidad Sanidad Animal, Centro de Investigación y Tecnología Agroalimentaria (CITA), Gobierno de Aragón, Zaragoza, Spain
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Wendy Wilkins, DVM. Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, 52 Campus Drive, Saskatoon, Saskatchewan S7N 5B4, Canada.
E-mail: wendy.wilkins@usask.ca

Summary

The study objective was to evaluate the accuracy of a real-time polymerase chain reaction (RT-PCR) and a culture protocol used to detect Salmonella in the faeces of grow–finish pigs using a Bayesian approach. The RT-PCR was invA-gene-based assay, while the culture protocol included pre-enrichment in buffered peptone water, selective enrichment in tetrathionate and Rappaport-Vassiliadis broths, and isolation on semi-solid (modified semi-solid RV) or solid (XLT4, Rambach) agar plates. Bayesian analysis was performed using a two-test, two-population model with dependence between culture and RT-PCR and compared to a second model with conditional independence between these two tests. Two hundred and ninety three individual faecal and 294 pooled pen samples from grow–finish pig collected from 10 farms were tested and results were divided into two groups according to herd size (five herds <250 sows, five herds with >400 sows). In the dependence model, RT-PCR sensitivity (Se) and specificity (Sp) were estimated to be 90% (95% probability interval 74, 97) and 99% (98, 99), respectively. Culture Se was 92% (75, 99), while culture Sp was considered 100% as all culture-positive samples were confirmed by serotyping. In the conditional independence model, RT-PCR Se and Sp, and culture Se, were 96% (93, 98), 99% (98, 100) and 97% (94, 100), respectively. The dependence model resulted in posterior estimates of Se that were lower and with broader probability intervals than the independence model, indicating that when RT-PCR and culture are evaluated relative to each other, the correlation between these tests is an important source of bias and should be adjusted for during analysis. The RT-PCR evaluated in this study performed almost comparably to culture; given the cost savings associated with using this test and more timely results, the RT-PCR may be a useful alternative to culture for screening large numbers of samples, particularly when Salmonella prevalence is low.

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