Effects of sample storage and delayed secondary enrichment on detection of Salmonella spp in swine feces

Julia M. O'Carroll From the Departments of Food Animal and Equine Medicine (O'Carroll, Davies, Slenning) and Microbiology, Pathology and Parasitology (Correa), Population Medicine Program, College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27606.

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Peter R. Davies From the Departments of Food Animal and Equine Medicine (O'Carroll, Davies, Slenning) and Microbiology, Pathology and Parasitology (Correa), Population Medicine Program, College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27606.

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Maria T. Correa From the Departments of Food Animal and Equine Medicine (O'Carroll, Davies, Slenning) and Microbiology, Pathology and Parasitology (Correa), Population Medicine Program, College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27606.

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Barrett D. Slenning From the Departments of Food Animal and Equine Medicine (O'Carroll, Davies, Slenning) and Microbiology, Pathology and Parasitology (Correa), Population Medicine Program, College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27606.

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Abstract

Objective

To determine effects of fecal sample storage and delayed secondary enrichment (DSE) on detection of Salmonella spp in swine feces.

Sample Population

Fecal samples obtained from 84 pigs in a commercial herd.

Procedure

Each fecal sample underwent 3 storage treatments: no storage (ie, processed on the day of collection), storage at 4 C for 6 days, and storage at −15 C for 14 days. After assigned storage treatments, all samples were enriched in Rappaport-Vassiladias (RV) broth (single enrichment) and plated on XLT4 agar. Delayed secondary enrichment was performed, using single enrichment broths that were stored for 4 days at room temperature.

Results

Of 504 cultures, 186 (36.9%) were Salmonella positive. A difference in proportions of samples with positive results was not found between same-day processing and storage at 4 C for 6 days. Compared with use of single enrichment for 24 hours (34% positive), use of DSE resulted in a greater proportion (40%; P < 0.001) of samples with positive results. Estimated relative sensitivities for the storage methods were 0.90, 0.85, and 0.71 for same-day processing, storage at 4 C for 6 days, and storage at −15 C for 14 days, respectively.

Conclusions

Where practical, processing of fecal samples on the day of collection is recommended, although storage at 4 C for several days does not result in marked loss of sensitivity. Improved detection associated with DSE warrants further investigation and optimization. (Am J Vet Res 1999;60:359–362)

Abstract

Objective

To determine effects of fecal sample storage and delayed secondary enrichment (DSE) on detection of Salmonella spp in swine feces.

Sample Population

Fecal samples obtained from 84 pigs in a commercial herd.

Procedure

Each fecal sample underwent 3 storage treatments: no storage (ie, processed on the day of collection), storage at 4 C for 6 days, and storage at −15 C for 14 days. After assigned storage treatments, all samples were enriched in Rappaport-Vassiladias (RV) broth (single enrichment) and plated on XLT4 agar. Delayed secondary enrichment was performed, using single enrichment broths that were stored for 4 days at room temperature.

Results

Of 504 cultures, 186 (36.9%) were Salmonella positive. A difference in proportions of samples with positive results was not found between same-day processing and storage at 4 C for 6 days. Compared with use of single enrichment for 24 hours (34% positive), use of DSE resulted in a greater proportion (40%; P < 0.001) of samples with positive results. Estimated relative sensitivities for the storage methods were 0.90, 0.85, and 0.71 for same-day processing, storage at 4 C for 6 days, and storage at −15 C for 14 days, respectively.

Conclusions

Where practical, processing of fecal samples on the day of collection is recommended, although storage at 4 C for several days does not result in marked loss of sensitivity. Improved detection associated with DSE warrants further investigation and optimization. (Am J Vet Res 1999;60:359–362)

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