Trends in Salmonella Dublin over time in Denmark from food and animal related isolates

Salmonella enterica serovar Dublin is highly adapted to cattle-adapted and a relatively rare cause of human infections. In Denmark S. Dublin has been endemic in the cattle population for many years. A national surveillance program in the cattle population was established to reduce the occurrence of S. Dublin. In this study, we analyzed 421 S. Dublin genomes from cattle and food in order to determine trend of S. Dublin's population size over time in Denmark and impact of national intervention to bacterial population. A SNP phylogenetic tree exhibited two major clades and one small cluster. The two major clades consisted primarily of either animal or food related isolates. All isolates were ST10. The temporal phylogenetic tree for S. Dublin isolates showed that the most common ancestor was estimated to be in ~1980 for the two major clades. An effective population size overtime based on a Bayesian skyline plot showed that the population size of S. Dublin decreased significantly between 2014 and 2019 in both major clades. This result was concordant with a decrease of S. Dubin in a surveillance program. The strengthening of a surveillance program in Denmark could be the cause for the reduction of S. Dublin's effective population size. This study showed that whole genome sequencing can be used for revealing a trend of S. Dublin's population over time and an impact of an intervention to bacterial population. An effective population size could potentially contribute to improved control efforts to reduce the occurrence of S. Dublin in cattle.