Exploring the impact and transmission of Salmonella Dublin in crossbred dairy calves

Salmonella enterica serotype Dublin (S. Dublin) is an increasing concern within the dairy industry, particularly due to its effect on morbidity and mortality in young dairy calves. The objective of this cohort study was to evaluate the associations between S. Dublin infection and health and growth in young crossbred dairy calves, as well as model the transmission of S. Dublin within this cohort. A total of 160 crossbred male and female dairy calves arrived at a commercial calf raising facility. Research staff observed calves twice daily to evaluate fecal consistency and for signs of respiratory disease, with all individual treatments recorded by facility staff. Blood samples were collected upon arrival (0 d) and departure (77 d) from the facility and submitted to the Animal Health Laboratory at the University of Guelph (Guelph, ON, Canada) for S. Dublin testing using an ELISA. Fecal samples were collected at arrival, 7 d, 14 d, and 77 d, with bacterial culture conducted to identify the presence of S. Dublin. Calves that died during the study (n = 23) had a postmortem completed by a veterinarian, with samples submitted for bacterial culture. Poisson regression models, Cox proportional hazard models, and a linear regression model were built using Stata 18 to assess the effect of S. Dublin positivity (defined as positive based on fecal culture, serum ELISA, or both at one of the time points) on respiratory disease, diarrhea, and ADG. The effective reproductive number (R_e) was estimated to quantify the transmission of S. Dublin within the calves. Salmonella Dublin positivity at arrival, 7 d, 14 d, and 77 d were 5.6%, 11.4%, 1.3%, and 24.4%, respectively. Combining serum ELISA, fecal culture, and postmortem testing, 45.6% of calves tested positive for S. Dublin at least once during the study period. Calves that were positive for S. Dublin had a greater number of observations with respiratory scores of ≥5 and fewer observations with a respiratory score of 0, especially in calves weighing <42.6 kg at arrival. There was no association with growth, or the number of observations of diarrhea. The estimated R_e values ranged between 1.32 and 1.79. Salmonella Dublin was associated with increased respiratory disease, though it did not affect growth or incidence of diarrhea. The estimated R_e values underscore the epidemic potential of this bacterium, emphasizing the need for further research on effective measures to limit its spread.