Epidemiology and Virulence Assessment of Salmonella Dublin

Six strains of Salmonella dublin with distinct antimicrobial susceptibility patterns and/or plasmid profiles were repeatedly isolated from calves in a calf rearing facility. Three of the six strains were isolated from numerous calves during outbreaks of clinical salmonellosis while the other three were not. These strains were compared for their ability to adhere to and internalize in human intestinal epithelial cells (Caco-2) and in bovine alveolar macrophages (BAM), to survive in BAM, and to cause lethal infection in female BALB/c mice. All six strains of S. dublin demonstrated an ability to adhere to and internalize in both Caco-2 cells and in BAM. However, strain differences in the level of adhesion and/or internalization in Caco-2 cells and BAM were demonstrated. Most strains were able to persist but not proliferate in BAM. One outbreak-associated strain which readily attached and internalized in eukaryotic cells in vitro was avirulent to mice at the dose tested. The remaining five strains were virulent to mice. In vitro measures of virulence attributes were not clearly correlated with virulence among S. dublin strains measured either as prevalence in calves during outbreaks of disease or as mouse lethality. Also, there was no association between prevalence of strains in calves during outbreaks of clinical salmonellosis and lethality in mice.