Investigations of faecal shedding and super-shedding of Escherichia coli 0157 by dairy heifers

Abstract

Several studies were undertaken to investigate the shedding and super-shedding (>104 cfu/g faeces) of Escherichia coli O157 by dairy heifers. E. coli O157 was detected from replacement heifers being raised and these heifers were monitored in longitudinal studies. These studies identified a vast range of the prevalence of shedding within the cohort over time (9.6 ¬– 94.3% during the longitudinal study) and a vast range in the levels of E. coli O157 shed by heifers. No patterns or trends could be identified in shedding at the herd level with no discernable associations between shedding levels beyond one week. The absence of patterns and associations in herd level shedding was further demonstrated by intensive, daily sampling periods, which also demonstrated the short duration of shedding and the variability within individuals. The level of shedding by individual heifers, specifically super-shedding, showed little association to herd level shedding. Super-shedding from individual heifers was sporadic with a large number of heifers (nearly half in the longitudinal study) identified as super-shedding at some point. This suggests that the identification of these ‘high risk’ animals and removal may not provide a practical solution to control E. coli O157 from cattle. Further analysis of these studies was undertaken to identify factors which may affect shedding at the herd level. Analysis showed a link between animal factors and shedding or super shedding; body score was associated with shedding and faecal consistency and hide cleanliness was associated with super-shedding. Warm and moist climate conditions were shown to be associated with higher levels of shedding and super-shedding which is expected to provide optimal conditions for bacterial growth and survival. These associations were also shown to be short term. The largest effects were associated with rainfall and these effects also lasted over the longest duration, which is likely associated with increased moisture in the environment following rainfall events and especially high rainfall events. Pasture growth was also associated with shedding, but the confounding effects of warm and wet conditions on pasture growth must be considered. The associations between many of these factors are difficult to disentangle: there are likely to be links between climate and pasture growth, pasture growth and faecal consistency, and faecal consistency and hide cleanliness. Which of these factors, or the compounded effects, actually affect E. coli O157 shedding is difficult to identify in field, observational studies. Pulsed field gel electrophoresis of isolates recovered from these heifers identified two very similar Xba1 pulsotypes with a change in the predominant type over time. This may indicate a single genetic event propagating in the population. However, the use of a second enzyme (Bln1) identified some differences between pulsotypes which were indistinguishable by Xba1 indicating the limitations of the method. Isolates recovered from cattle were also shown to be similar to isolates recovered from human clinical cases, but this was the minority of cattle isolates and the limitations of the method should be considered. Taken together, these studies demonstrate that E.coli O157 shedding and super-shedding in replacement dairy heifers is dynamic. Persistently-infected supershedding animals which act as superspreaders within a herd was not supported by the empirical data. Based on these studies, identifying and removing E.coli O157 shedding animals is unlikely to impact on E.coli O157 prevalence. However, understanding risk factors for shedding could allow herd prevalence to be predicted.