Colonisation of EHEC in ruminants and the development of super-shedding status – knowledge and gaps

Summary

​Since the designation of E. coli O157 as a foodborne pathogen in the 1980's and the identification of cattle as the principal reservoir there has been a concerted effort to understand many aspects of this organism, its hosts and the diseases that it causes.
The colonisation of cattle by E. coli O157 and other EHEC is now an intense international research area.
Understanding the factors that facilitate high prevalence and/or high concentrations of organisms being shed will provide avenues for control of the colonisation, growth and transmission of E. coli O157 which ultimately will reduce carriage of these pathogens in cattle as well as assisting in the prevention of human disease via the food chain or associated environmental contamination.
Greater understanding of the distribution of E.coli O157 in cattle and the variation over time of shedding and 'super-shedding' of this organism from cattle was required within Australian production systems. In order to achieve this, after an initial period of laboratory methods validation and skill enhancement, two systems were investigated in a longitudinal manner – an extensive beef cattle production system (at Charles Sturt University) and a dairy replacement herd (at University of Sydney). Cattle within these herds were followed longintudinally for a number of months with repeated sampling twice a week during this time. In addition, more intensive sampling was also performed daily and twice daily over a period of weeks at both sites. An expert opinion elicitation exercise was also performed, where international experts in the area of E.coli O157 were questioned on multiple areas surrounding shedding of the pathogen, supershedding, risk factors and potential control measures. Finally, a number of simulation models were specified to identify the ability to pool samples in order to reduce the cost of future longitudinal studies, and also to identify key interventions for reduction of prevalence of carcass contamination with E.coli O157, in light of the findings of the current research.
This study found that shedding and super-shedding of E.coli O157 is highly variable both between and within animals. It also identified that synchronisation of shedding may occur within herds, where periods of time associated with high prevalence and concentration of E.coli O157 shed in faeces occurs across the herd. The only consistent risk factor identified for shedding E.coli O157 across the two production systems was rainfall, but a possible relationship between individual animal stress and shedding and concentration of E.coli O157 was also identified and further investigation of the relationships between these variables is required. From the research performed it does not appear that individual supershedding animals are able to be identified for implementation of pre-slaughter control measures. Indeed, all animals appear to have the capacity to supershed at some time. In addition, while the implementation of pre-slaughter control measures might be effective in reducing carcass contamination with E.coli O157, this is only likely in the presence of effective abattoir hygiene that results in a very low probability of transfer of this organism from both gastrointestinal tract and hide to the carcass. In the absence of such control measures at the level of the abattoir there appears to be little benefit to on-farm interventions.
This work provides a marked increase in the knowledge surrounding shedding and supershedding of E.coli O157 in Australian production systems.