Factors Associated with Salmonella Presence in Environmental Samples and Bulk Tank Milk from US Dairies: Salmonella Presence on US Dairies

The objective of this study was to evaluate herd characteristics and management practices associated with presence of Salmonella in the farm environment and in bulk tank milk (BTM) in US dairy herds. Herd management data, environmental culture, BTM and in-line milk filter polymerase chain reaction results for Salmonella from 260 US dairy herds surveyed during the National Animal Health Monitoring System Dairy 2007 study were analysed. Herd characteristics and management practices were screened by univariate analysis, and selected variables were used to construct a logistic regression model to identify factors associated with the presence of Salmonella in environmental samples. To identify factors associated with the occurrence of Salmonella in BTM and milk filters, a priori selected variables that were related to milking procedures were analysed univariately and a logistic regression model was constructed. The presence of Salmonella in the farm environment was associated with location of the operation in the East (OR = 4.8; CI: 1.9-11.6), not using a broadcast manure spreader (OR = 3.2; CI: 1.4-7.5), use of bovine somatotropin (BST) (OR = 2.7; CI: 1.5-5.0) and use of anionic salts (OR = 2.2; CI: 1.2-3.9). In the final multivariable model, herds with fewer than 100 cows were at decreased odds (OR = 0.3; CI: 0.1-0.7) of being culture positive for Salmonella as were herds with between 100 and 499 cows (OR = 0.4; CI: 0.2-0.8) compared with herds having 500 or more cows. The presence of culture-positive environmental samples and herd size were significantly associated with Salmonella BTM contamination. The herd-level factors identified in this study were in agreement with prior studies but also identified other potential factors that can be targeted in Salmonella control programmes.