in the Fod safety and Inspecton Service (FSIS) draft risk assessment) and feedlot animals defensible?
The arguments against using fecal prevalence alone for risk assessment are related to the wide range of concentrations of E. coli O157:H7 in the feces of colonized cattle and the fact that E. coli O157:H7 occurs in locations other than feces.
On theoretical grounds, animals shedding 105 colony-forming units (CFU) of E. coli O157:H7 per gram of feces would cause much more contamination of meat than animals shedding, say, 102 CFU/g, but they are considered to contribute equally in a model that includes only prevalence. The issue might not be of concern if the distribution of pathogen concentrations were narrow or if one could assume a dependable relationship, at the group level, between prevalence and distribution of concentrations. However, the distribution clearly is not narrow. Experimentally infected animals often reach peak shedding concentrations over 105 CFU/g briefly and typically shed much lower numbers for longer periods (Cray and Moon, 1995; Sanderson et al., 1999). Some cattle that are naturally exposed to an infected animal never shed over 103 CFU/g (Besser et al., 2001). There is a paucity of data on shedding dynamics in field populations, but the seeming consequence of findings from challenge studies is that one would expect only a small fraction of positive animals in a group to be shedding E. coli O157:H7 at over 105 CFU/g on any given day. However, those few animals probably account for the large majority of total E. coli O157:H7 cells produced by the group. The disjunction between prevalence and quantity of E. coli O157:H7 shed has probably been magnified as tests have become more sensitive because, as documented by Sanderson et al. (1995) and Besser et al. (2001), the major impact of increased sensitivity of an assay is its ability to detect lower concentrations. For example, methods based on immunomagnetic separation (IMS) have allowed far better detection of animals shedding 102 CFU/g than older assays (Besser et al., 2001). But one animal shedding 105 CFU/g would yield the same number of E. coli O157:H7 cells as 1,000 animals shedding 102 CFU/g.
The use of fecal prevalence as the sole output of the Production Module requires the assumption that most carcass contamination with E. coli O157:H7 (or fecal bacteria in general) occurs directly from the gastrointestinal tracts of slaughtered animals. The draft defends that assumption by reference to a study showing little or no correlation between visible hide soiling and generic E. coli counts on carcasses (Van Donkersgoed et al.,