Synchronisation of E. coli O157 in an extensive beef system: a longitudinal study

Purpose: Escherichia coli O157 is a bovine gut-commensal capable of causing serious disease and death in humans. Intermittent patterns of shedding have been shown in calves, but little is known about the shedding pattern of live adult beef cattle. This study aimed to describe in detail the temporal dynamics and risk factors for shedding of E. coli O157 in grass-fed beef cattle. Methods: An observational study was performed in which the presence and concentration of E. coli O157 in faeces was identified on a bi-weekly basis over nine months. Trend in the proportion of animals testing positive to E. coli O157 on each sampling day was estimated by a cubic spline model. Visual assessment of this curve revealed that three discrete peaks in shedding occurred over the study period. The three peaks with an estimated proportion of herd shedding at greater than 20% were described as a “herd shedding event”. Reassessing the data in light of herd shedding events allowed all sampling points to be classified as occurring within six discrete events comprised of three shedding events alternating with three non-shedding events. Generalised linear mixed models were used to assess whether variation in animal, environmental and management variables at each sampling point could be associated with a “herd shedding event”. Results: This study demonstrated that there was synchronisation of shedding amongst cows accompanied by a marked elevation in the concentration of the pathogen in faeces. The occurrence of rainfall (P<0.01) was found to be a predictor of shedding. Conclusion: This approach, based on short-interval sampling, confirms the known variability of shedding within a herd and highlights that high shedding events are rare. The extent of individual animal variability revealed in this work and previous studies casts doubt on findings of risk factor studies where outcomes are based on single assessment of dichotomous shedding status. Relevance: The major findings suggest that animals within a herd can be synchronised in their shedding (and non-shedding) which may have practical implications for the risk of human exposure to E. coli O157 and implementation of control measures at the level of the abattoir.