Bayesian estimation of diagnostic accuracy of fecal culture and PCR-based tests for the detection of Salmonella enterica in California cull dairy cattle.

John M Adaska,Pius S Ekong,Sharif S Aly,Kristin A Clothier,Xunde Li,Paul V Rossitto,Deniece R Williams,Terry W Lehenbauer,Edward R Atwill

doi:10.7717/peerj.8310

Abstract

Epidemiological studies of low prevalence disease problems are often hindered by the high cost of diagnostic testing. The objective of this study was to evaluate PCR screening of both individual and pooled fecal samples from culled dairy cows for the invA gene of Salmonella followed by culture to determine if the sensitivity and specificity were comparable to the results from traditional culture methods applied to individual samples. Cows from six different dairies were sampled in all four seasons. A total of 240 individual cow fecal samples, 24 fecal pools and 24 pools of 24-hour tetrathionate enrichment broth were tested. Diagnostic sensitivity of PCR screening followed by culture of PCR positive or indeterminate samples (i.e PCR-CUL method) was lower than that of culture (CUL) when applied to individual fecal samples (94.8%, 99.5%), however the specificity was comparable (99.6% and 97.7% respectively). For pools of five fecal samples and pools of five, 24 h tetrathionate broth samples, the specificity of both tests were comparable (∼98%); however, their sensitivity was only comparable in pooled fecal samples (∼93%) but greater for culture compared to PCR-CUL in pooled broth samples (∼99% versus ∼93%). Compared to culture results from testing of individual fecal samples, testing pooled fecal samples by culture had a relative sensitivity of 74% and relative specificity of 96%, testing pooled fecal samples by PCR-CUL resulted in relative sensitivity of 90% and relative specificity of 96%. Testing of pooled 24-hour enrichment broth by PCR-CUL increased the relative sensitivity and specificity to 100%. PCR testing followed by culture of positive or indeterminate samples is a time saving alternative to traditional methods. In addition, pooling of samples may be a useful method for decreasing cost if study aims can accommodate a moderate loss of relative sensitivity.

Highlights

Nontyphoidal Salmonella enterica infections are estimated to be the leading cause of foodborne illnesses in the United States resulting in over a million cases, about 20,000 hospitalizations, and more than 400 deaths annually (Scallan et al, 2011)
Of the 240 individual fecal samples 60 were positive for Salmonella sp. on culture (CUL)
Two of 48 pools (2/48) had 1 or 3 individual cow fecal samples (IF) samples positive on both culture and PCR-CUL while the fecal samples (FP) sample was negative on both culture and PCR-CUL, but the broth pools (BP) was positive on both culture and PCR-CUL

Summary

Introduction

Nontyphoidal Salmonella enterica infections are estimated to be the leading cause of foodborne illnesses in the United States resulting in over a million cases, about 20,000 hospitalizations, and more than 400 deaths annually (Scallan et al, 2011). Foods of animal origin are important sources of Salmonella infections in humans (Buncic & Sofos, 2012; Pires et al, 2009; Scallan et al, 2011). In a 2002 multi-state study in the US, Varma et al (2006) identified consumption of undercooked ground beef as a risk factor for infection with multi-drug resistant Salmonella Newport infection. About 18% of ground beef produced in the United States are sourced from cull dairy cows (NAHMS, 1996). The prevalence of Salmonella sp. in cull dairy cattle in the US had been reported to range from 0.0% to 93.0% depending on the season and day of the week that the samples were collected (Troutt et al, 2001)

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