Dynamics of Escherichia coli O157 in an extensive beef production system

Human infection with Escherichia coli O157 is uncommon but potentially fatal. The organism is regularly present as a commensal in the gastrointestinal tract of cattle. Unravelling the epidemiology of E. coli O157 in cattle has been a complex task. Research over the past quarter century has improved the understanding, although substantial shortfalls remain. The main aim of this thesis was to increase knowledge of the dynamics of shedding of E. coli O157 in grass-fed beef cattle. In order to fulfil this aim, it appeared that new approaches were needed, particularly with respect to study design and analytical method. Initially, the research evaluates the usefulness of factors reported in the literature to be associated with E. coli O157 shedding by cattle, using the extent of agreement amongst experts as an indicator of utility. It was identified that only limited agreement existed between experts in E. coli O157, especially regarding the effectiveness of control measures and the nature of “super shedding”. Moreover, a field study was performed to explore in detail the temporal dynamics of E. coli O157 in individual cattle over a prolonged period of time and to identify animal, management and climate variables associated with the shedding of E. coli O157. Findings indicate a distinctive pattern of shedding over time, particularly linked to rainfall, and synchronisation of shedding amongst cattle. Results of a second field study, in which individual cattle in a small beef herd were tested for E. coli O157 at daily and bi-daily intervals, showed that shedding of the pathogen at both the individual and herd level was unpredictable and chaotic. Furthermore, no cattle were observed to persistently shed a high concentration of the pathogen, further refuting the utility of the term “super shedding”. A simulation
model was devised for investigating the cost and sensitivity of detection based on pooling of samples. Pooling has potential to affordably expand study of the shedding characteristics of beef herds to achieve a level of external validity needed for real progress in control of the pathogen.