Escherichia coli O157:H7 and other serogroups of Shiga toxin-producing E.coli (STEC) have emerged over the last several decades as a significant cause of food-borne illness in the United States. Approximately 5-10% of people clinically infected by these bacteria develop a systemic disease, hemolytic uremic syndrome, which has a fatality rate of approximately 5%. The Centers for Disease Control estimates that STEC cause some 110,000 illnesses and 90 deaths annually in the United States (Mead et al. 1999). In addition, the economic consequences of recalling large lots of food for public health reasons are significant. Cattle are considered to be the primary reservoir for STEC. Depending on the season, the methods used for bacterial culture and the age of the animals, the prevalence of E. coli O157:H7 in U.S. cattle ranges from 2-28% (Hancock et al. 1994; Elder et al. 2000). E.coli O157:H7 has also been recovered from other ruminants such as sheep (Kudva et al. 1996) and deer (Keene et al. 1997; Sargeant et al. 1999).