A cross-sectional study on herd-level risk factors associated with Salmonella Dublin in dairy herds from Québec, Canada

Salmonella Dublin is an endemic zoonotic bacteria affecting dairy cattle in Canada and is also recognized as an emerging concern in dairy industries worldwide. The objective of this study was to investigate the impact of herd-level risk factors on the farm's S. Dublin status. We used an observational cross-sectional study design, where the S. Dublin status of 302 dairy herds from the three main dairy regions in Québec, Canada, was determined using repeated bulk tank milk and individual animal serum samples collected over 6 months. Geographic coordinates and demographic exposures were obtained from the Québec dairy producers association, and management practices were assessed using a questionnaire. Spatial clusters of S. Dublin positive farms were evaluated. The impact of risk factors on odds of S. Dublin was estimated using logistic regression models based on a directed acyclic graph. Associations between the farm's predictor profile and odds of S. Dublin were also evaluated using dimension reduction coupled with hierarchical clustering techniques. Data from 273 and 196 herds were available for the spatial and risk factors analyses, respectively, and 10 (3.7 %) of the herds met the S. Dublin case definition. We observed a spatial cluster of S. Dublin cases. Farm- (number of dairy farms/km²) and cow-density (number of adult dairy cows/km²), and distance to the nearest dairy farm (in km) were not associated with S. Dublin status. Herds that used practices associated with a higher rate of animal introduction had significantly higher odds of S. Dublin. When considering the farm's predictors profile, only the dimension representing the herd's buying and young stock raising habits was associated with the odds of S. Dublin.