KEY TAKEAWAYS
- Although it showed a numerical reduction in prevalence, the whole-dairy farm administration of a Saccharomyces cerevisiae fermentation product at pre-harvest did not significantly reduce Salmonella prevalence in lymph nodes (LN) of cull dairy cattle.
- In both studies, although it differed by region, the overall mean within-supplier Salmonella LN prevalence was approximately 14%, which was lower than anticipated.
- Salmonella LN prevalence significantly varied by region/processing plant, where the Southwest region showed a higher prevalence than its Northeast counterpart. These findings were consistent with the baseline evaluation of Salmonella LN prevalence in cull dairy cattle across different U.S. regions and seasons, where region/processing plant played a significant effect on Salmonella LN prevalence, although it also depended on the season.
- Most of the Salmonella isolates in these studies harbored low bacterial levels and less than 0.1% of the isolates were enumerable. Montevideo, Mbandaka and Muenster were the more frequent serotypes, and their presence varied by season.
BACKGROUND
Cattle lymph nodes (LN) have been identified as a potential source of
Salmonella in beef. Since post-harvest interventions cannot effectively control this route of contamination and the complete removal of peripheral LN from beef carcasses is not practically feasible, there is a need for other means of mitigation. Dietary additives fed to cattle before harvest have shown promising results in the reduction of the prevalence and concentration of
Salmonella in cattle LN. The objectives of these studies were to: 1) determine whether the whole-farm use of a commercially available pre-harvest fermentation product (
Saccharomyces cerevisiae) or postbiotic compared to not feeding a fermentation product (i.e., No-postbiotic) is associated with the reduction in the prevalence and concentration of
Salmonella organisms in the subiliac LN of culled dairy cows harvested in two large U.S. commercial processing plants across seasons (Study 1), 2) to determine associations between demographic and management characteristics, season, and plant/region with the prevalence of
Salmonella in bovine LN (Study 1), and 3) to provide a baseline estimate of the
Salmonella LN prevalence and serovar diversity in cull dairy cattle from dairy farms in different U.S. regions over time (Study 2).
Methodology
In collaboration with two commercial processing plants in the Southwestern and Northeastern regions of the U.S., culled dairy cattle lots processed on the same week from dairy farms that fed postbiotic or did not feed the product (No-postbiotic) were identified and sampled. For the baseline prevalence study, LN samples from dairy farms supplying cattle to five processing plants in four U.S. regions (Upper Midwest, Midwest, Northeast and Southwest) were collected. Up to 20 LN were collected from each dairy farm at least once every season, for both studies, at the corresponding processing plants. Samples were analyzed for
Salmonella presence and concentration by culture-based and quantitative PCR methods. Isolates were subjected to serotype identification via classical serotyping and molecular typing assays, as well as to antimicrobial susceptibility testing.
Although a numerical reduction in prevalence was observed, the whole-dairy farm administration of a
Saccharomyces cerevisiae fermentation product at pre-harvest was not effective at significantly reducing
Salmonella prevalence in LNs of cull dairy cattle.
Salmonella LN prevalence only significantly varied by region/processing plant, where the Southwest region showed a higher prevalence than its Northeast counterpart. These findings were consistent with the baseline evaluation of
Salmonella LN prevalence in cull dairy cattle across different U.S. regions and seasons, where both region and season were the main drivers of prevalence. Dominant serotypes of Study 1 included Montevideo, Mbandaka, Muenster, Cerro, Meleagridis, and Anatum. The probability of isolating a
Salmonella serotype did not significantly vary by feed additive status or region, however, it did vary by season. Between 0 and 34 isolates exhibited resistance to each antimicrobial, with streptomycin and ciprofloxacin showing the highest numbers of resistant isolates. There were seven isolates that were considered multi-drug resistant, which was defined in this study as an isolate that was intermediately or uninhibited by three or more antimicrobials. In Study 2, dominant serotypes included Montevideo, Muenster, Mbandaka, Anatum, and Cerro. As with Study 1, the probability of isolating a
Salmonella serotype did not significantly vary by region, but it did vary by season. For each antimicrobial, there were between 0 and 35 isolates that exhibited a resistant phenotype, with streptomycin showing the highest number of resistant isolates. There were four isolates that were considered multi-drug resistant. The longitudinal nature of the data obtained from several processing plants and seasons enhanced knowledge regarding the environmental and ecological factors that may be contributing to the carriage of
Salmonella in cattle LN.
Implications
The whole-dairy farm administration of a
Saccharomyces cerevisiae fermentation product at pre-harvest was not effective in reducing
Salmonella prevalence in LN of cull dairy cattle.
Salmonella LN prevalence only significantly varied by region/processing plant, where the Southwest region showed a higher prevalence than its Northeast counterpart. These findings were consistent with the baseline evaluation of
Salmonella LN prevalence, where region significantly impacted prevalence. Of note is that most of the
Salmonella isolates in these studies harbored low bacterial levels. In both studies, Montevideo, Mbandaka and Muenster were the most frequent serotypes, and their presence varied by season. Up to 2% of isolates exhibited multi-drug resistance. Plant managers could consider streamlining the application of pathogen control measures factors in specific regions and seasons, as these are consistently found to be important drivers of
Salmonella LN prevalence. Distinguishing risk factors that enhance surveillance efforts can help processing plants manage problems before they become overwhelming, protect public health and the beef industry, and ensure food safety. Beef safety requires a comprehensive farm-to-table approach with a commitment from all parties.