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 from 2000-2001 and again in 2009. The second objective was to determine the yearly frequency (2000 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 Salmonella were due to recovery of MDR subtypes of serotype Senftenberg and Typhimurium in 2000-2001, and genetically distinct, pansusceptible subtypes of the same serotypes in 2009. The annual frequency 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 the frequency of human illnesses associated with cattle-derived subtypes.