Using whole genome sequencing to investigate routes of transmission of Salmonella Dublin to British Columbian dairy farms.

Trade networks and cattle movement are important risk factors for infectious disease spread. As Salmonella Dublin is highly adapted to cattle, transmission between farms is thought to be strongly associated with animal movement compared to non-host-adapted Salmonella strains. This study used whole genome sequencing (WGS) to investigate routes of S. Dublin transmission in British Columbia (BC). In particular, we explore the potential contribution of US dairy cattle importation to the introduction of S. Dublin into BC, assess evidence for re-infection of farms with genetically distinct strains, and examine patterns of relatedness among isolates to better understand possible transmission pathways, including potential links between dairy and human cases. We found that 92% of BC dairy isolates were genetically similar to U.S. S. Dublin isolates, suggesting that cattle imports may play a key role in the pathogen's spread. This was supported by clusters lacking clear regional geographic links, indicating that cattle movement or purchasing may drive transmission. Detection of multiple genetically distinct S. Dublin strains on individual farms over time suggests repeated introductions rather than persistence of a single resident strain. These findings reinforce the hypothesis that animal movement, particularly from the U.S., contributes significantly to S. Dublin infections in BC. Additionally, BC dairy and human isolates often appeared in the same clusters, although human case numbers were low. Further investigation is warranted to assess the role of BC dairy cattle or cattle-derived products in human infections.