Salmonella Typhimurium transmitted between poultry and dairy farms

Salmonella Typhimurium is a zoonotic pathogen which poses a major threat to public health, particularly due to the development of Multi-Drug Resistant (MDR) strains. This generalist serotype can be found in many hosts and the environment where varying selection pressures may result in accumulation of antimicrobial resistance determinants. However, the transmission of S. Typhimurium between food-producing hosts, specifically between poultry layers flocks and nearby dairy herds was never demonstrated. We investigated an outbreak on a dairy in Israel in August 2022, with the aim to (i) determine the role of nearby poultry houses to be sources of infection and (ii) to identify the antimicrobial resistance patterns present in both the dairy and poultry isolates.We collected fecal and environmental drag swab samples from the dairy as part of the outbreak investigation. Salmonella Typhimurium was isolated in four samples (three fecal and one environmental) and Salmonella Mbandaka was isolated in two environmental samples.Six S. Typhimurium strains, isolated from poultry layer farms located less than three-kilometers from the dairy, and identified between January 2019 and August 2022 as part of the National Surveillance Scheme, were retrieved form -80 storage at the Laboratory of the Egg and Poultry Board. We retrieved a further four isolates from poultry layer farms with a diverse geographic distribution, from the same time period to add context to our analysis.Whole Genome Sequencing and antimicrobial resistance tests were carried out on S. Typhimurium isolates from the dairy (n = 4) and poultry (n = 10), and on the S. Mbandaka from the dairy (n= 2). The latter were considered an incidental finding.We identified acquired antimicrobial resistant genes, including the blaCTX-M-55 gene, conferring resistance to extended spectrum cephalosporins, which was exclusive to dairy isolates. Genetic similarity with less than five single nucleotide polymorphism difference between dairy isolates and isolates from nearby poultry flocks, suggested a transmission link.This investigation highlighted the transmission risk between dairy and poultry farms, when found in close proximity. The accumulation of potentially transferable genes conferring resistance to critically important antimicrobials underscores the increased public health risk associated with S. Typhimurium circulation between animal hosts.