| Summary: | This thesis is a summation of studies carried out between 1991 and 2004 and attempts to place the work in context with other knowledge to establish the role of animals as a source of human infection with verocytotoxin-producing <i>Escherichia coli </i>(VTEC). VTEC O157 was first isolated from cattle in Scotland in 1992. In a preliminary prevalence study using basic techniques to examine faeces sample routinely submitted to the Scottish Agricultural College (SAC) Veterinary Centres, 0.25% samples from cattle were positive for VTEC O157. The organism was more commonly isolated from calves less than two months of age. A very large prevalence study was commissioned following the Central Scotland outbreak. Using what has now become the nationally adopted technique (immunomagnetic separation following enrichment of 1g faeces in buffered peptone water with no antibiotics), prevalence levels were established with 95% confidence limits as follows. 7.9% (6.5, 9.6) animals sampled (12-30 months of age) were shedding VTEC O157. 22.8% (19.6, 26.3) of farms had at least one animal shedding in the group sampled. There was a significant drop in the proportion of farms where shedding was detected between the three years of the study 1998, 1999 and 2000. When farms were repeatedly visited twelve times, the organism was detected on 87.5% farms. Because of the lack of sensitivity of the test and the uneven distribution of the organism in faeces, these are underestimates of the true prevalence. In a cohort study in beef finishing cattle and a longitudinal study in beef cows risk factors for shedding VTEC O157 were determined from questionnaires followed by univariate and multivariate analysis. Increased levels of shedding were associated with animals being housed rather than grazing. Farms with animals at pasture have lower prevalence if the water is from a natural source. The presence of wild geese was also seen as a risk factor. Farms that spread slurry on grazing land were more likely to have shedding animals. Larger farms were more likely to be positive. There were no significant regional differences in shedding within Scotland. A pilot prevalence study in sheep determined a Group Level Prevalence of 8% with 95% confidence of 2% to 19% and an Animal Level Prevalence of 1%). Enumeration of VTEC O157 organisms gave counts ranging from <5x10<sup>2</sup>g<sup>-1</sup> to >10<sup>4</sup>g<sup>-1</sup>. A similar study in deer in Scotland suggested that the prevalence was low. Finally it is postulated that the regional variation in the rate of infection per unit population in Scotland and the difference between Scotland and England relate to the relative cattle and human populations in the areas being considered.
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