Epidemiology, risk factors, and molecular characteristics of shiga toxin encoding bacteria on Minnesota organic and conventional dairy farms

This study describes the prevalence of shiga toxin-encoding bacteria (STB) and herd- and animal-level risk factors associated with its presence on organic and conventional dairy farms. This study also identified the distribution of serotypes, antimicrobial resistance profiles, virulence factor genes of shiga toxin-producing Escherichia coli (STEC) isolates from dairy cattle and their environment in Minnesota, and from exhibition cows at county fairs from 2001 to 2002.; A total of 2,540 samples were collected from 28 dairy cattle farms (8 organic and 20 conventional), 17 calf pens (5 organic and 12 conventional), and 12 MN county fairs from 2001 to 2002. PCR using specific primers for stx genes was used to determine the presence of STB.; Diverse serotypes and virulence genes were identified from STEC isolates. Identical PFGE patterns of serotype O157:H7 were found between isolates from cattle and human sources. Nearly half of STEC isolates were resistant to at least one antimicrobial. Factors associated with an increased risk for STB shedding in cattle on dairy herds included use of TMR for lactating dairy cows, pasture access for lactating or dry cows, and no use of rumensin (monensin) for weaned calves. Small organic and conventional herds (< 100 cows) were also associated with STB shedding, compared to large conventional herds (≥ 100 cows). In addition, cattle group (calves vs. adult cows) and calf age (≥ 4 vs. < 4 weeks old) were associated with STB shedding.; This study shows that STB can be commonly found from cattle in dairy herds, especially organic farms with repeated sampling. The presence of STB and O157:H7 in county fairs may pose a risk to the public who may have contact with cattle or their environment. With serotype and virulence genes analysis, calves were identified as a high risk animal, carrying a higher proportion of virulence genes commonly associated with human illness. Modifying herd-level management factors related to cattle feeding practices along with identified individual-level factors may synergistically reduce occurrence of STB on dairy farms, contributing to a decrease in contaminated food in the food chain and ultimately a decrease in human STEC infections.