Salmonella in the lymph nodes of cattle presented for harvest
Gragg, Sara E.
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Foodborne illness associated with non-typhoidal Salmonella continues to persist in the United States (US). Cattle are considered an important reservoir for Salmonella and ground beef has been implicated in salmonellosis outbreaks. Bovine lymph nodes, including the subiliac lymph node, can harbor Salmonella and are a source of human exposure when located within adipose tissue that is integrated into ground beef. To improve understanding of the Salmonella burden within bovine lymph nodes, subiliac lymph nodes of feedlot (n=1,501) and cull (n=1,826) cattle were sampled at harvest from commercial abattoirs in three geographically distinct regions of the US throughout a twelve month period. Median prevalence of S. enterica was generally low, with 1.3% of lymph nodes positive. However, in some instances, higher prevalence values were detected, resulting in an arithmetic mean prevalence of 7.5% (95% confidence limits [CL]=3.7, 11.3%). Salmonella prevalence was found to be greater in subiliac lymph nodes of feedlot cattle (14.7% [95% CL=7.4, 21.9%]) compared to those of cull cattle (1.8% [95% CL= -0.01, 3.6%]). Enumeration analysis of a subset of 618 feedlot cattle lymph nodes showed that 66% of those harboring S. enterica (n= 144) did so at concentrations ranging from <1.3 to 2.9 Log10 CFU/lymph node, while 34% carried a higher burden of S. enterica, with levels ranging from 3.0 to >4.8 Log10 CFU/lymph node. Serotyping of S. enterica isolated resulted in the identification of 24 serotypes with the majority being Montevideo (44.0%) and Anatum (24.8%). Antimicrobial susceptibility phenotypes were determined for all isolates and the majority (86.1%) was pansusceptible; however, multi-drug resistant isolates (9.0%) were also occasionally observed. Lymph nodes (mandibular, mesenteric, mediastinal and subiliac), fecal, and hide samples were collected from beef carcasses (n=68) at slaughter in a TIF-Inspected abattoir in Mexico to determine the prevalence and genetic relatedness among Salmonella harbored throughout the lymph system. Prevalence of each sample type was determined with a subset of isolates (n=90) serotyped, characterized using pulsed-field gel electrophoresis (PFGE) and antimicrobial susceptibility phenotypes observed. Salmonella prevalence was 55.9% (mandibular), 91.2% (mesenteric), 7.4% (mediastinal), and 76.5% (subiliac). Fecal and hide samples harbored Salmonella at a prevalence of 94.1% and 100.0%, respectively. Eight serotypes were isolated including: Kentucky (n=28; 31.1%), Anatum (n=26; 28.9%), Reading (n=14; 15.6%), Meleagridis (n=11; 12.2%), Cerro (n=4; 4.4%), Muenster (n=1; 1.1%), Give (n=1; 1.1%), Mbandaka (n=1; 1.1%), and “nontypable” (n=4, 4.4%). Antimicrobial susceptibility was determined for all Salmonella isolates analyzed by PFGE and the majority (n=54; 60.0%) were pansusceptible; however, multidrug resistance was also observed (n=12; 13.3%). Salmonella subtypes obtained from PFGE typing generally clustered based upon serotype; each serotype was associated with multiple sample types, suggesting that Salmonella colonizes the body when given the opportunity via diverse routes of entry and is highly adapted for survival throughout the animal.