The health and welfare of cattle and humans are inextricably linked, and the momentum behind the "One Health" paradigm in veterinary and human medicine continues to grow. The linkage of livestock and human health is particularly true for zoonotic Salmonella that contaminate the human food supply. Salmonella causes the largest number of foodborne deaths and hospitalizations in the United States; livestock, including dairy farms, are a primary reservoir for those infections. Increases in the antimicrobial resistance and incidence of salmonellosis in humans have been associated with the emergence of novel Salmonella strains in livestock. Therefore, improved knowledge of the frequency and drivers for changes in the population of Salmonella on livestock farms could lead to positive impacts on public health.This study used a long-term longitudinal approach to assess changes in the prevalence, antimicrobial resistance, and genetic subtypes of Salmonella on Michigan dairy farms. The overall goal was to determine genotypic population changes that were associated with changes in the prevalence and antimicrobial resistance of Salmonella within farms. Specifically, this study addressed the following four objectives: 1) Determine within-farm changes in the antimicrobial susceptibility of Salmonella between 2000-2001 and 2009 2) Determine within-farm changes in the prevalence of Salmonella using seasonally-matched sampling visits in 2000-2001 and 2009 3) Identify the serotypes, sequence types, and PFGE banding patterns for Salmonella recovered in each time period 4) Determine the association between antimicrobial susceptibility changes and changes in the population of molecular subtypes.Results from this study suggest that the overall prevalence of Salmonella on Michigan dairy farms has increased between 2000-2001 and 2009. Increases in Salmonella prevalence were associated with increases in herd size and changes in management practices. The proportion of isolates that were resistant to multiple antimicrobials was less in 2009 relative to 2000-2001. Additionally, there were decreases in the minimum inhibitory concentrations (MICs) for nine of the 15 tested antimicrobials. Results of the sequence typing and PFGE profiles show an overall high relatedness, and long-term persistence of Salmonella within farms. Analysis of the antimicrobial resistance changes and genotypic changes suggest that downward shifts in MICs were associated with a shift in the population favoring serogroup C1. Additionally, recovery of MDR strains in 2000-2001, and susceptible strains of the same serotype in 2009 suggest displacement of MDR subtypes by susceptible subtypes.
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