Recovery of Salmonella bacterial isolates from pooled fecal samples from horses

Abstract

Background: It is important to determine if a horse is shedding Salmonella spp., but a complete culture series can be cost prohibitive.

Objectives: Determine the optimal pooling technique to maintain high sensitivity of Salmonella spp. culture using spiked samples, and then demonstrate the efficacy of this protocol on clinical submissions.

Hypothesis: Pooled fecal samples are as sensitive as 5 individual cultures for the detection of Salmonella shedding.

Animals: A single Salmonella-negative horse from the university herd, and 19 hospitalized horses.

Methods: Salmonella-free fecal samples were spiked with different amounts of Salmonella spp. (10² , 10³ , 10⁴ , and 10⁵ colony forming units [cfu]) and homogenized to evaluate pooled samples. Five individual fecal samples were collected from 19 hospitalized horses. Ten-gram aliquots of each individual sample were combined to make a pooled sample. Both individual and pooled samples were cultured for Salmonella spp. The identity of bacterial isolates was confirmed by matrix-assisted laser desorption-ionization time of flight mass spectrometry.

Results: A 10² cfu concentration of Salmonella spp. could be recovered from a spiked Salmonella-free fecal sample. Homogenization protocols indicated that the addition of 20 mL of broth to the pooled sample improved recovery, whereas homogenization time did not. Of the 19 horses tested, 5 were positive for Salmonella. In all instances, Salmonella spp. were recovered from the fecal pool as well as individual samples.

Conclusions and clinical importance: Pooling of 5 fecal samples for Salmonella culture is a sensitive and cost-effective diagnostic approach to detect horses that are shedding the organism.

Keywords: culture; horse; pool; series.