Comparison of two variants of Salmonella enterica serovar Heidelberg from the 2015-2017 outbreak in dairy beef calves

Salmonella is one of the top five foodborne pathogens and one of the leading causes of foodborne illness in humans in the United States and worldwide. In the United States, a 2015-2017 multistate Salmonella Heidelberg outbreak was linked to human contact with dairy beef calves. Typically, foodborne isolates of Salmonella cause disease in humans but the carrier food animal does not display any signs of disease. The 2015-2017 Heidelberg outbreak was unique because the isolates associated with the outbreak were recovered from ill humans as well as sick dairy calves. Two main variants of Salmonella Heidelberg were isolated from the intestines of calves during the outbreak and because greater morbidity and mortality in calves was associated with one variant compared to the other, the two isolates were deeply characterized to explore factors likely contributing to their contrasting disease severity. The Salmonella Heidelberg isolate with greater morbidity and mortality in dairy beef calves also had elevated expression of virulence genes and higher invasion of human and bovine epithelial cells, potentially enhancing this isolate's pathogenicity, and contributing to the ecological success in dairy calves.