Understanding Salmonella Dublin in British Columbia through bulk tank milk surveillance

Increased cases of Salmonella enterica ssp. enterica serovar Dublin, a bacterial pathogen that primarily affects dairy cattle, have been noted in British Columbia (BC), Canada, since 2015. The objective of this cross-sectional study was to (1) understand the prevalence and distribution of Salmonella Dublin in BC dairy farms based on bulk tank milk (BTM) serology, (2) to investigate the degree of variability within percent positivity (%PP) in negative and positive farms, and (3) investigate risk factors associated with positivity. All BC dairy herds' BTM was sampled 4 times from September 2021 to April 2023 to determine Salmonella Dublin positivity (at least 1 result ≥35% positivity on BTM serology). Of the 461 herds sampled, 137 (30%) were positive. A multivariable logistic regression model identified 3 risk factors associated with herd level positivity. Specifically, herds with 200 to 500 lactating cattle (odds ratio [OR] = 2.42, 95% CI = 1.14-5.36) and herds with >500 lactating cattle (OR = 21.78, CI = 5.14-152.20) were associated with increased risk of Salmonella Dublin herd level positivity. Farms that had >5 farms within 4 km radius (OR = 1.73, 95% CI = 1.05-2.90) were at greater risk for Salmonella Dublin compared with those with 5 or fewer. Having satisfactory pest control was protective against Salmonella Dublin positivity (OR = 0.28, 95% CI = 0.09-0.80). Percent positivity variability varied greatly among negative, indeterminant (15-34%PP), and positive farms. Negative farms experienced very little variability in %PP (median = 5.79%, interquartile range [IQR] = 3.86%), with variability increasing among the indeterminant farms (median = 16.04%, IQR = 11.94%), and highest among the positive farms (median = 35.84%, IQR = 31.59%). Monitoring variability of BTM %PP could function as an accessible early warning tool for producers and veterinarians to predict if their herds may become positive.