Synchronization of E. coli O157 shedding in a grass-fed beef herd: a longitudinal study.

This study aims to describe in detail the temporal dynamics of E. coli O157 shedding and risk factors for shedding in a grass-fed beef herd. During a 9-month period, 23 beef cows were sampled twice a week (58 sampling points) and E. coli O157 was enumerated from faecal samples. Isolates were screened by PCR for presence of rfbE, stx 1 and stx 2 . The prevalence per sampling day ranged from 0% to 57%. This study demonstrates that many members of the herd were concurrently shedding E. coli O157. Occurrence of rainfall (P < 0·01), feeding silage (P < 0·01) and lactating (P < 0·01) were found to be predictors of shedding. Moving cattle to a new paddock had a negative effect on shedding. This approach, based on short-interval sampling, confirms the known variability of shedding within a herd and highlights that high shedding events are rare.