Changes in the antimicrobial resistance profiles of Salmonella isolated from the same Michigan dairy farms in 2000 and 2009

Antimicrobial Resistance in Fecal Escherichia coli and Salmonella enterica from Dairy Calves: A Systematic Review

The discovery of antibiotics brought with it many advances in the health and well-being of humans and animals; however, in recent years development of antimicrobial resistance (AMR) has increasingly become a concern. Much of the antibiotic use on dairy farms is for disease management in mature cattle, and AMR in fecal organisms is relatively rare in this group. However, young dairy calves often carry high levels of AMR in their fecal Escherichia coli and Salmonella enterica, which could provide a potential reservoir of AMR genes on dairy farms. To develop practical and effective antibiotic stewardship policies for dairy calf rearing, it is vital to have a solid understanding of the current state of knowledge regarding AMR in these animals. A systematic review process was used to summarize the current scientific literature regarding AMR in fecal S. enterica and E. coli and associations between management practices and AMR prevalence in dairy calves in the United States and Canada. Seven online databases were searched for literature published from 1997 to 2018. Multiple studies indicated an association between preweaned calves and increased risk of fecal shedding of resistant bacteria, compared to other animal groups on dairy farms. There also was evidence, although less consistent, of an impact of antibiotic treatment, antibiotic-containing milk replacer feeding, and feeding nonsalable or waste milk (WM) on the presence of AMR bacteria. Overall, the research summarized in this systematic review highlights the need for continued research on the impact of management practices, including antibiotic use, WM feeding, and disease prevention practices in reducing AMR in E. coli and S. enterica in dairy calves. In addition, few data were available on physiological and microbiological factors that may contribute to the high relative populations of resistant bacteria in young calves, suggesting another valuable area of future research.

Within-Farm Changes in Dairy Farm-Associated Salmonella Subtypes and Comparison to Human Clinical Isolates in Michigan, 2000-2001 and 2009

Temporal changes in the distribution of Salmonella subtypes in livestock populations may have important impacts on human health. The first objective of this research was to determine the within-farm changes in the population of subtypes of Salmonella on Michigan dairy farms that were sampled longitudinally in 2000-2001 and again in 2009. The second objective was to determine the yearly frequency (2001 through 2012) of reported human illnesses in Michigan associated with the same subtypes. Comparable sampling techniques were used to collect fecal and environmental samples from the same 18 Michigan dairy farms in 2000-2001 and 2009. Serotypes, multilocus sequence types (STs), and pulsed-field gel electrophoresis (PFGE) banding patterns were identified for isolates from 6 farms where >1 Salmonella isolate was recovered in both 2000-2001 and 2009. The distribution of STs was significantly different between time frames (P < 0.05); only two of 31 PFGE patterns were identified in both time frames, and each was recovered from the same farm in each time frame. Previously reported within-farm decreases in the frequency of multidrug-resistant (MDR) Salmonella were due to recovery of MDR subtypes of S. enterica serotypes Senftenberg and Typhimurium in 2000-2001 and genetically distinct, pansusceptible subtypes of the same serotypes in 2009. The annual frequency of human illnesses between 2001 and 2012 with a PFGE pattern matching a bovine strain decreased for patterns recovered from dairy farms in 2000-2001 and increased for patterns recovered in 2009. These data suggest important changes in the population of Salmonella on dairy farms and in the frequency of human illnesses associated with cattle-derived subtypes.