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It is generally accepted that cattle are a major reservoir of E. coli O157:H7, a zoonotic pathogen that has become a significant cause of human hemorrhagic colitis and hemolytic uremic syndrome (Mead et al., 1999). It is poorly understood if animals become colonized with E. coli O157:H7, or if they transiently shed the organism after ingestion of environmental contamination. Khaitsa et al. (2003) reported that fecal shedding of E. coli O157:H7 from feedlot steers ranged from 1% to 80% over a 136 day sampling period. Based on that finding, it is reasonable to believe that at least some animals transiently shed E. coli O157:H7 at some point in their lives. Besser et al. (2001) showed that when calves are orally infected with E. coli O157:H7, they shed the organism consistently for 70 d and intermittently thereafter. Furthermore, E. coli O157:H7 could be cultured from feces, colonic and cecal contents 118 d after infection. Several other studies have shown evidence of persistence of E. coli O157:H7 infection in individual cattle, providing support that at least some animals can become colonized with E. coli O157:H7 (Besser et al., 2001; Brown et al., 1997 and Grauke et al., 2002). It seems reasonable to assume that infected animals can disseminate the organism into the environment, subsequently contaminating pen mates and facilities.
There has been, to a degree, varying evidence of the primary site of E. coli O157:H7 colonization in infected animals. Grauke et al. (2002) found the colon to show the most persistence and proliferation of E. coli O157:H7. More specifically, Naylor et al. (2003) reported that the recto-anal junction of the gastrointestinal (G.I.) tract was the primary site of E. coli O157:H7 colonization. These findings contradicted those who found the rumen to be the major site of propagation (Brown et al 1997; Laven et al., 2003; Rasmussen et al., 1993 and Tkalcic et al., 2000). Additionally, Besser et al. (2001) reported that cecal contents also were contaminated with E. coli O157:H7. E. coli also has been shown to have the ability to translocate to different segments of the mesenteric lymph node complex in laboratory mouse models (Gautreaux et al., 1994).
This study was designed to identify those cattle that are persistently colonized with E. coli O157:H7 and to determine the site of colonization. In addition, animals found to be persistently colonized with E. coli O157:H7 will be compared, both microbiologically and physiologically to animals that intermittently shed and never shed the organism. Results will be used to design pre-harvest E. coli O157:H7 interventions and application parameters to reduce E. coli O157:H7 numbers and prevalence in slaughter - ready animals.
The stated objectives for this work were:
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