Nickelson, Kathryn Josephine (2018-05). Assessment of Salmonella Prevalence in Lymph Nodes of U.S. and Mexican Cattle Presented for Slaughter in Texas. Master's Thesis.
Thesis
Foodborne salmonellosis has been traced back to undercooked ground beef and other beef products in the past, and due to the widespread presence of lymph nodes (LNs) throughout carcasses, they are a source of Salmonella contamination in ground beef products. The objectives of this study were to determine if Salmonella prevalence differs (1) between cattle of Mexican and U.S. origins when exposed to the same feedlot environment and (2) between warm and cool seasons. To meet these objectives, paired subiliac LNs (n = 800 LNs) were collected from 100 carcasses per origin (Mexico and U.S.), per season (warm and cool). The paired LNs (left and right sides) were pooled yielding one sample per animal (n = 400 samples), aseptically trimmed of fat, and pulverized before microbiological analysis. Overall, Salmonella prevalence in LN samples was 52.0% (208/400). No difference (P = 0.4836) was seen in Salmonella prevalence as a function of country of origin, with 54.0% (108/200) and 50.0% (100/200) of LN samples returning Salmonella-positive results from cattle of Mexican and U.S. origin, respectively. Salmonella prevalence differed (P = 0.0354) between seasons, with 46.5% (93/200) and 57.5% (115/200) Salmonella-positive samples from cool and warm seasons, respectively. Serotyping of PCR-confirmed positive samples resulted in fourteen different serovars with Cerro (21.6 %), Anatum (19.7 %), Muenchen (17.8 %), Montevideo (14.4 %), and Kentucky (12.0 %) comprising the majority. These findings dispel previous concerns that Mexican cattle have a higher prevalence rate of Salmonella than U.S. cattle. These results also suggest that environmental factors may play a large role in the Salmonella prevalence rate in bovine LNs, and that additional research is needed to fully understand factors that influence Salmonella prevalence in bovine LNs.
