The Epidemiology of Shiga Toxin-producing Escherichia coli in Australian Dairy Cattle

Abstract

Shiga toxin-producing E. coli (STEC) have important public health and food safety implications. Cattle are the primary reservoir for STEC, which are transmitted to humans via contact with cattle or related food products. Dairy farms in particular have been incriminated as an important source of STEC. The broad aim of this study was to examine in depth the epidemiology of STEC on the dairy farm. The presence of STEC on three Australian dairy farms was surveyed. This aimed to provide data on the prevalence and nature of STEC on Australian dairy farms, as well as to examine in more detail the pre-harvest/slaughter ecology of STEC. STEC, E. coli O157:H7 and E. coli O26:H11 prevalences were similar to those from dairy farms in other countries. Replacement heifers were the most important source of STEC on the farms. Calves excreted STEC from an early age, with faecal prevalence peaking at weaning. Higher STEC prevalence was also associated with group housing of calves during weaning. Calf isolates were potential human pathogens based on serotype and virulence markers. Clonal relationships between isolates were analysed. Calf isolates were diverse and had a high clonal turnover. STEC isolated from within the same farm had a higher genetic similarity than those from different farms. Vertical and horizontal transmission were both identified among cattle. The farm environment was also identified as an important source of STEC. Reasons for increased levels of STEC excretion by calves were investigated. Two broad hypotheses for higher faecal shedding were proposed and examined individually. The first was that an animal is more likely to excrete STEC when its exposure to STEC is greater, thus promoting inoculation of the gastrointestinal tract. Calves were experimentally inoculated with a traceable STEC strain to examine the infection dynamics of STEC within cattle groups, and explore the effect of calf management procedures. Calves which were housed in groups and co-jointly fed and managed had a higher prevalence of the inoculation strain than animals housed individually. The test strain was readily isolated from the hides and saliva of inoculated calves, as well as their immediate environments. Calves become infected with STEC via the faecal-oral route, iv either by direct contact with other calves, or indirectly through contact with faecally contaminated materials. The second hypothesis was that individual animals are variably susceptible to intestinal colonisation by STEC, which leads to differing magnitudes and durations of STEC carriage. Factors influencing colonisation susceptibility to STEC and the mechanisms behind these factors were also examined. In order to compare enteric colonisation under a range of different conditions, a suitable experimental system was developed. In vitro organ culture of explanted ruminant colonic tissues provided a laboratory model that was representative of in vivo bacterial-mucosal attachment. The degree of STEC colonisation was enumerated using an immunofluorescent filtration technique. The quantitative colonisation assay was applied to determine the effects of host-dependant variables on STEC colonisation. Colonic tissues from weaning calves and adult cattle did not differ significantly in their susceptibility to colonisation; nor did tissues from cattle fed either high forage or high grain diets. Colonic explants from sheep, however, demonstrated significantly higher numbers of adherent STEC than bovine explants. It was therefore concluded that while species-specific differences in host tissues may mediate STEC carriage differences, this did not explain in vivo variability in age and diet related excretion. Factors that indirectly affect the susceptibility of host tissues to colonisation were examined. E. coli O157:H7 cultured in media designed to represent the enteric contents of a well-fed ruminant colonised the colonic mucosa in reduced numbers, indicating that age and diet may be correlated with differences in STEC carriage and excretion because of differing physiological augmentation of the intra-enteric environment. In conclusion, while group dynamics and management practices may increase STEC shedding prevalences for cattle via increased STEC exposure, factors that modulate an individual ruminant’s gastrointestinal carriage of STEC have a significant role in mediating STEC excretion. Either directly or indirectly, species, age and diet can affect the numbers of STEC that colonise the bowel wall, thereby influencing the magnitude and duration of STEC excretion. Both of these features of ruminant STEC ecology should be addressed in order to reduce the pre-slaughter/harvest presence of STEC