Pre-slaughter control of Escherichia coli O157 in beef cattle: a simulation study.

A stochastic simulation model was used to assess the benefit of measures implemented in the pre-slaughter period that are aimed at reducing the contamination of beef carcasses with Shiga-like-toxin-producing Escherichia coli O157. The scenario studied was based on an abattoir processing approximately 1000 head of lot-fed cattle per day. Input assumptions were described using probability distributions to reflect uncertainty in their true values. Control measures that were assessed were based on either a reduction in herd prevalence of infection, reduction in opportunity for cross-contamination in the processing plant by re-ordering of the slaughter queue, reduction of concentration of E. coli O157 in fresh faeces, or a reduction in the amount of faeces, mud and bedding ('tag') transferred from the hide to the carcass. Some control measures evaluated were hypothetical in nature and were included to assist with the planning of research priorities. Simulations suggested that the greatest potential impact is associated with vaccination and with an agent that reduces shedding E. coli O157 in faeces. Knowledge of herd-test results obtained by testing a sample of animals from the herd provides only a minor advantage in control programmes, although application of a rapid test to all animals in all lots might be of some benefit. Under most scenarios, there is ample opportunity for cross-contamination to occur within the slaughter plant as a result of early entry of cattle contaminated with E. coli O157. An industry-wide reduction in the amount of tag attached to hides and addition of a source of cattle having a prolonged average fasting time were not predicted to have a large impact on mean amount of carcass contamination with E. coli O157.