Review of epidemiological surveys on the prevalence of contamination of healthy cattle with Escherichia coli serogroup O157:H7.

This paper gathers and critically analyses the results of 26 published epidemiological surveys on the prevalence of contamination of cattle with verocytotoxin-producing E. coli (VTEC) serogroup O157:H7. These surveys have been conducted since 1986 on farms in North America (10 studies), on farms in Europe (6 studies) and at slaughterhouses prior to or just after slaughter (7 studies) or after skinning and evisceration (3 studies). The purpose of this review is to understand the first stages of the epidemiology of the infection in animals and humans (the infection process being obscure in many points) and to prepare herd-based control measures to reduce the risk of O157:H7 human infection. The different statistical methods employed in these surveys, as well as the various laboratory screening methods used for detecting positive animals are presented. The observed frequencies of infected animals (animal prevalence) and herds (herd prevalence) are given as a function of localisation, year, type of industry (beef or dairy) and age. From these measured prevalence values, the risk of contamination of ground beef by E. coli O157:H7 in the first stages of the farm-to-fork continuum is assessed. First, we follow the evolution of contamination frequencies from the living animal on-farm to carcasses before transformation. Then, within each set of measurements (i.e., on farm or at slaughterhouse), we identify the effects of the following factors: target population, sampling strategies and laboratory procedures. We argue that the prevalence values inferred from these measurements are very likely underestimated, due to insufficient sampling and not enough sensitive laboratory procedures (one exception being the immunomagnetic bead separation technique). No firm conclusion can be drawn as to the effects of geographical localisation and season. In those surveys, the effect of hygiene level at slaughterhouse on prevalence values is not quantitatively assessed. In addition, there is growing evidence of other sources of E. coli O157:H7 than live cattle in the farm environment, such as feed, water and water-troughs.