Transmission and epidemiology of Salmonella enterica in commercial dairy farms in Washington State

An estimated 1.3x106 human salmonellosis cases and over 500 deaths occur annually in the United States, and livestock products are implicated. Salmonella enterica often exhibits emergence and clonal dissemination patterns indicating a major role for inter-herd transmission. This research highlights three features of Salmonella enterica in cattle. First, increased ceftazidime resistance was observed for a Salmonella Typhimurium clade designated WA-TYP035/187, in cattle and humans in the Pacific Northwest. blaCMY-2 plasmid diversity within minimum spanning tree branches of MLVA alleles suggested that resistance was acquired on multiple occasions followed by subclonal dissemination. Because this pattern of increasing resistance was very similar to that for the bovine-adapted serovar S. Dublin reported previously, we concluded that selection pressure in cattle due to ceftiofur use was the most likely explanation. Results presented in second chapter indicates that the rate of new MDR Salmonella strain introduction was 75.5 per 1000 herd-months (95% CI 50.8 119.4), which correlates to 0.91 new strains per farm per year. This indicates that Washington dairy farms acquisition of new MDR Salmonella strains is a common event, though the acquisition rate varies greatly among farms. MDR Salmonella were rarely found in delivered feeds and then only at low concentrations. No transmission from feed was noted. Results presented in third chapter indicate that off-farm heifer raising with co-mingling with cattle from other sources was strongly associated with acquiring any new MDR Salmonella strains. (OR= 8.9, 95% CI: 2.4, 32.80 and the number of new strains acquired per farm OR= 2.3, 95% CI: 1.1, 4.7). Herd size, per 100 animals increment, was significantly associated with farm acquiring new strains (OR= 1.04, 95% CI, 1.01, 1.05 and with the number of new strains acquired (OR=1.02, 95% CI, 1.01, 1.03). Prior diagnosis of clinical salmonellosis was associated with the number of new strains introduced (OR=2.5, 95% CI, 1.3, 5.0). These studies indicate that MDR Salmonella transmission to dairy farms in Washington State is common, that selection pressure for ceftiofur resistance most likely occurs on farms, and that the most important management variable involved in transmission is off-farm heifer raising