| Escherichia coli (E. coli) O157:H7 is commonly implicated in human cases of hemorrhagic colitis and hemolytic uremic syndrome, in North America. In this study, the role of sheep, another common farm-reared ruminant, as a reservoir for potentially virulent E. coli O157:H7, was investigated. For this purpose, a sensitive selective-enrichment culture protocol that detects 1 CFU E. coli O157:H7/10g ovine feces was standardized. Using this technique, for the first time, E. coli O157:H7 was isolated from healthy sheep. A seasonal variation was observed in the occurrence of E. coli O157:H7 within a single flock of sheep, with animals culture-positive only in the summer months. Of a total 140 ovine E. coli O157:H7 isolates studied, 98% possessed the Shiga toxin genes {dollar}(stxsb1sp+/stxsb2sp+),{dollar} the attaching-effacing gene {dollar}(eaesp+){dollar} and demonstrated Vero cell cytotoxicity, indicating their potential for human pathogenicity. This finding consolidated the role of sheep as reservoirs for E. coli O157:H7. In addition, DNA fingerprinting, via pulse-field gel electrophoresis and toxin gene restriction fragment length polymorphism, indicated that a single animal can simultaneously harbor multiple E. coli O157:H7 strains, and that the strains shed by individuals changed over time. These findings have important epidemiological implications. Sheep were also colonized after oral inoculation with E. coli O157:H7. The animals shed the bacteria over 6 to 60 days. This indicated that sheep could be the alternative to cattle, as economical ruminant models for E. coli O157:H7 colonization studies. In various experimental studies conducted, a diet high in fiber, low in nutrients, diet change, and feed withdrawal, could induce increased shedding of E. coli O157:H7 by sheep. Horizontal transmission and reinfection was also observed. These findings have implications in the pre-harvest management of animals. High solid content, no aeration and low temperatures supported survival of E. coli O157:H7 in farm effluents. These findings indicate the possible role of these effluents in the environmental dissemination of the bacteria and have implications in farm waste management. Preliminary investigations indicate that in sheep, the possible colonization site for E. coli O157:H7 may be the lower gastrointestinal tract (cecum, colon, rectum). |
