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E. coli O157:H7 has recently emerged as an important food safety pathogen, being responsible for a significant number of cases and deaths every year in the US (CDC, 2000). Healthcare costs associated with treatment of infection are considerable, and add to costs associated with prevention, control and surveillance of E. coli O157-mediated disease (WHO, 1998). Because cattle are considered the primary reservoir for E. coli O157:H7 and consumption of beef is recognized as the principal source of infection in humans, it also represents a serious threat to consumer confidence and trade in beef products. The USDA, through the National Ground Beef Monitoring Program, has declared E. coli O157:H7 an adulterant of raw meat, thus adopting a zero-tolerance regulatory position (Wachsmuth et al., 1997). While aspects of the epidemiology of E. coli O157:H7 in livestock populations have been elucidated, many critical gaps in our knowledge remain. Key among these are reasons for the degree of individual and group-level variation in E. coli O157:H7 excretion. Some trends in excretion have been associated with the age of cattle, diet, season and type of production system, although the underlying factors and mechanisms ultimately regulating excretion are unknown. Factors at the host level are likely to play a large role. One important factor is likely to relate to the potential for cattle to become colonized with E. coli O157:H7.
Until recently, there was no evidence for a true E. coli O157:H7 “carrier” state in cattle, although some form of colonization of the gastrointestinal tract (GIT) must occur in order for cattle to harbor and excrete E. coli O157:H7 for periods >24 hours and at fecal concentrations greater than that exposed to (Freter, 1981; Tarr et al., 2000). Mechanisms and dynamics of GIT colonization by E. coli O157:H7 have been studied and a range of bacterial and host-related colonization factors discovered, although most relate to human pathogenesis. Relatively little is known about how E. coli O157:H7 inhabit the bovine gut, though more recently, some putative adhesins and receptors have been described (Baehler and Moxley, 2000; Stevens et al., 2002). While most studies on bovine E. coli O157:H7 epidemiology have centered on the incidence or prevalence of fecal shedding, it is likely that the concentration of cells excreted by individual cattle is at least as important. Most cattle that excrete E. coli O157:H7 appear to do so at low-moderate levels, i.e. concentrations <103 CFU/g (Omisakin et al., 2003; Ogden et al., 2004). While such cattle may appear to be important contributors to E. coli O157:H7 risk based purely on prevalence determination (especially using highly sensitive detection methods such as immunomagnetic separation, which detect very low concentrations of cells in samples), excretion of low numbers of E. coli O157:H7 is less likely to lead to significant carcass contamination at slaughter, or to transmission to other cattle pre-slaughter.
The stated objectives for this work were:
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