Diagnostic accuracy of rectoanal mucosal swab of feedlot cattle for detection and enumeration of Salmonella enterica

Authors: Agga, Getahun; Arthur, Terrance; Schmidt, John; Wang, Rong; Harhay, Dayna

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2016
Publication Date: 4/1/2016
Publication URL: http://handle.nal.usda.gov/10113/62568
Citation: Agga, G.E., Arthur, T.M., Schmidt, J.W., Wang, R., Harhay, D.M. 2016. Diagnostic accuracy of rectoanal mucosal swab of feedlot cattle for detection and enumeration of Salmonella enterica. Journal of Food Protection. 79(4):531-537. doi:10.4315/0362-028X.JFP-15-409.

Interpretive Summary: Cattle are recognized carriers of the foodborne pathogen Salmonella enterica, and fecal shedding of this organism poses a potential human health risk through possible contamination of beef at harvest, or produce through environmental contamination sources. Historically, fecal grab samples have been the predominant method used to identify cattle colonized or infected with Salmonella, however fecal grab sampling can be impractical in certain situations and rectal swabs are a more convenient sample type to collect. Studies comparing fecal grab and rectal swab for the detection and enumeration of Salmonella fecal shedding are lacking, and yet rectal swab sampling is perceived as less sensitive because a smaller amount of feces is cultured. Accordingly, this study used paired samples to evaluate the sensitivity and specificity of rectal swab compared to fecal grab sampling for the detection of Salmonella shedding in individual feedlot cattle, as well as for identifying cattle shedding high-levels of Salmonella. Both rectal swab and fecal grab were Salmonella positive for 90.8% of samples. The sensitivity and specificity of rectal swab were 91% and 100% respectively for Salmonella detection. We conclude that rectal swab is a reliable alternative to fecal grab for assessing cattle Salmonella fecal shedding status, especially for cattle shedding high-levels of Salmonella.

Technical Abstract: Cattle are noted carriers of the foodborne pathogen Salmonella enterica. The perceived need to decrease the potential human health risk posed by excretion of this pathogen has resulted in numerous studies examining the factors that influence cattle shedding of Salmonella. Fecal grab (FG) samples have been the predominant method used to identify cattle colonized or infected with Salmonella, however FG sampling can be impractical in certain situations and rectoanal mucosal swabs (RAMS) are a more convenient sample type to collect. Despite a lack of studies comparing FG and RAMS for the detection and enumeration of Salmonella fecal shedding, RAMS is perceived as less sensitive because a smaller amount of feces is cultured. In a cross-sectional study to address these concerns, paired RAMS and FG samples were collected from 403 adult feedlot cattle approximately 90 days prior to harvest. Samples were processed for Salmonella enumeration (direct plating) and detection (enrichment and immunomagnetic separation). In all, 89.6% of RAMS and 98.8% of FG samples were positive for Salmonella and concordant prevalence outcomes were observed for 90.8% of samples. Mean enumeration values were 3.01 and 3.12 log10 CFU/ml for RAMS and FG respectively. The sensitivity and specificity of RAMS were 91% (95% confidence interval, CI: 87.5-93%) and 100% (95% CI: 48-100%) respectively for Salmonella detection. Furthermore, RAMS Salmonella enumeration was substantially concordant ('c = 0.89; 95% CI: 0.86-0.91) with FG values. We conclude that RAMS are a reliable alternative to FG for assessing cattle Salmonella fecal shedding status, especially for cattle shedding high-levels of Salmonella.