Comparison of Salmonella Presence in Bovine Lymph Nodes across Feeding Stages

Peripheral lymph nodes (LNs) located in the fatty tissues of beef carcasses have been shown to harbor Salmonella, and thus are a potential source of contamination in beef trimmings. Salmonella prevalence within LNs differs among feedlots, although the sources and routes of transmission have yet to be confirmed. The objective of this study was to determine if Salmonella prevalence in bovine LNs varies across feeding stages and feedlot environments in South Texas. Two feedlots with historically different levels of Salmonella prevalence within bovine LNs were selected. Twenty steers were to be harvested at each of 4 feeding stages: (1) post-weaning, (2) stocker, (3) 60 d on feed, and (4) 120 d on feed. Four steers did not complete the study. Left and right subiliac and superficial cervical LNs were collected from each carcass (n = 304), and similar node types were pooled by animal (n = 152). Salmonella, if present, was isolated from the LNs following the USDA-FSIS Microbiological Laboratory Guidebook (MLG) 4.08. Presumptive positive isolates were confirmed using PCR, and isolates were submitted for serotyping. Results showed a difference (P < 0.05) in prevalence of Salmonella in bovine lymph nodes between feedlots and among feeding stages. There was no Salmonella isolated from LN samples taken after feeding stage 1 (weaning) or from Feedlot A. Within feedlot B, there was an increase in Salmonella prevalence as cattle moved into later stages of feeding, at 22.2% (4/18), 77.8% (14/18), and 94.4% (17/18) for feeding stages 2, 3, and 4, respectively. There was a difference (P < 0.05) in LN Salmonella prevalence between stages 2 and 3, and a numerical difference between stages 3 and 4. It appears there is an environmental effect that influences the prevalence of Salmonella in LNs. The cause of these differences is unknown, and provides opportunity for future investigation into pre-harvest environmental conditions relating to Salmonella exposures