Regional and Seasonal Trends in Salmonella enterica: A Study of Cull Dairy Cattle Lymph Nodes

The objective of this study was to establish baseline estimates of Salmonella prevalence, serotype diversity, and antimicrobial resistance in subiliac lymph nodes (LNs) of cull dairy cattle procured from farms across different U.S. regions and seasons. LN samples from dairy farms supplying cattle to five commercial processing plants in four regions (Upper Midwest, Midwest, Northeast, and Southwest) were collected. During each season, up to 20 LN samples were collected from cull dairy cattle from each farm. Salmonella prevalence and concentrations were determined in LN samples using culture-based and quantitative polymerase chain reaction techniques. Confirmed isolates were serotyped using classical serotyping and molecular typing methods, and their antimicrobial susceptibilities were tested using the broth microdilution technique. Between May 28, 2021, and July 31, 2023, a total of 2231 LN were collected and tested, which showed an overall Salmonella LN prevalence of 13.8% (307/2231). Based on a multivariable model, the association between region with Salmonella LN prevalence significantly depended on the season when samples were obtained (p = 0.027): Salmonella LN prevalence was significantly higher in the Southwest region during the winter (28.6%) compared to the Upper Midwest region during the spring (9.8%). The dominant Salmonella serotypes isolated were Montevideo, Muenster, Mbandaka, Anatum, and Cerro, but the probability of isolating a dominant serotype did not significantly vary by region; however, it varied by season (p = 0.008). For each antimicrobial tested, between 0 and 35 of the 307 isolates exhibited a resistant phenotype, with four isolates showing multidrug resistance. The longitudinal nature of the data obtained from several processing plants and seasons contributes to our knowledge regarding the regional and seasonal factors that influence the carriage of Salmonella in cattle LN.