Cattle and environmental sample-level factors associated with the presence of Salmonella in a multi-state study of conventional and organic dairy farms

The objective of this study was to evaluate associations between cattle-level factors and environmental samples with the isolation of Salmonella from dairy farms in Minnesota, Wisconsin, Michigan, and New York. The study farms included 129 conventional and organic farms enrolled without regard to previous history of Salmonella infection. Herds were sampled at two-month intervals over a one-year period. Cattle groups more likely to be associated with Salmonella shedding (compared to preweaned calves) were cows designated as sick by farm personnel (OR = 2.5, 95% CI: 1.7, 3.7), cows within 14 days of calving (OR = 1.8, 95% CI: 1.1, 2.8), and cows due for culling within 14 days (OR = 1.9, 95% CI: 1.0, 3.4). State of origin was also associated with the presence of Salmonella in samples from cattle and the farm environment; Midwestern states were more likely to have Salmonella-positive samples compared to New York. Cattle treated with antimicrobials within 14 days of sampling were more likely to be Salmonella-negative compared with nontreated cattle (OR = 2.0, 95% CI: 1.1, 3.4). Farms with at least 100 cows were more likely to have Salmonella-positive cattle compared with smaller farms (OR = 2.6, 95% CI: 1.4, 4.6). Season was associated with Salmonella shedding in cattle, and compared to the winter period, summer had the highest odds for shedding (OR = 2.4, 95% CI: 1.5, 3.7), followed by fall (OR = 1.9, 95% CI: 1.2, 3.1) and spring (OR = 1.8, 95% CI: 1.2, 2.6). Environmental samples significantly more likely to be Salmonella-positive (compared to bulk tank milk) included, in descending order, samples from sick pens (OR = 7.4, 95% CI: 3.4, 15.8), manure storage areas (OR = 6.4, 95% CI: 3.5, 11.7), maternity pens (OR = 4.2, 95% CI: 2.2, 8.1), haircoats of cows due to be culled (OR = 3.9, 95% CI: 2.2, 7.7), milk filters (OR = 3.3, 95% CI: 1.8, 6.0), cow waterers (OR = 2.8, 95% CI: 1.4, 5.7), calf pens (OR = 2.7, 95% CI: 1.3, 5.3), and bird droppings from cow housing (OR = 2.4, 95% CI: 1.3, 4.4). Parity, stage of lactation, and calf age were not associated with Salmonella shedding.