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A Systems Analysis Methodology to Elucidate and Evaluate the Critical Control Points for E. coli 0157:H7 in Cattle and Sheep from Farm to Abattoir

Institutions
Silsoe Research Institute
Start date
2000
End date
2002
Objective
The overall objective is to develop a systems analysis/network flow QRA methodology for E.coli 0157:H7 in cattle and sheep production, and apply it to a range of systems to identify critical control points and make recommendations for research priorities.
  1. Develop a systems analysis methodology for sheep and cattle production systems and dcouments the methodology for sheep and cattle production the methodology.
  2. Develop a model framework of the life cycle of E.coli 0157:H7 in cattle and sheep and identify and extract the date requered to quantify infection and contamination risks (qualitative and quantitative hazard identification). Document the methodology.
  3. Devlop a general probability network model by quantifying the probabilities associated with those hazards and quantifying them for the general case. Apply the model to identify control points and make recommendations.
More information
Final report summary: A model-based study was conducted to assess means of controlling the risk of verocytotoxin-producing Escherichia coli O157 (VTEC O157) reaching abattoirs from cattle and sheep. An analysis of British cattle and sheep calves was conducted to define the systems to be considered, and a literature review collected information on the epidemiology of VTEC O157, with emphasis on quantative data suitable for model development.

A simulation model was developed with two main aspects

  • a general, deterministic simulation of the interations between animals, feed, enclosures and waste on cattle and sheep farms to study the impact of different factors;
  • a stochastic simulation of the epidemiology of VTEC O157 on such farms to study the likelihood of harmful outcomes.
This was used to predict the effect of interventions on the farm on the prevalence and shedding levels of animals leaving the farm for the abattoir.

The model is capable of simulating almost any type of beef or sheep enterprise. Five representative farms were simulated for this study:

  • Grass based beef breeder-finisher
  • Grass based beef fattener-finisher
  • Intensive beef fattener-finisher
  • Sheep (grass based breeder-fattener)
  • • Mixed beef and sheep
Batches of 100 runs of the model were carried out for each intervention so that statistics could be collected. The outcomes were assessed by considering the mean animal prevalence and mean shedding rate in animals for the abattoir and also the complete distribution of results, through the method of (first-order) stochastic dominance. The interventions for each farm were then ranked by their effectiveness in controlling VTEC O157 in animals being sent to the abattoir.

The main conclusions from the study on the basis of the available data and information (using the shorthand that “risk” means the risk of carrying VTEC O157 to the abattoir) were:

  1. Sheep have a lower risk than cattle.
  2. Mixing cattle and sheep increases the risk in both groups.
  3. Merging groups of animals of the same species into larger groups increases the risk substantially.
  4. Bringing in cattle with a prevalence of 5% increases the risk substantially.
  5. Increasing farm/group size increases the risk, independently of stocking density.
  6. Increasing stocking density increases the risk, independently of group size.
  7. A very high level of barn hygiene reduces the risk; there is little difference between intermediate and poor levels.
  8. Reducing the interval between slurry spreading and grazing from 4 to 2 weeks increases the risk; increasing the interval only produces a small reduction in risk.
  9. Background sources of VTEC O157 in the model have some effect on the prevalence and shedding; these could be influenced in practice by the presence of wild animals, etc., carrying the organism.
  10. There may be a higher risk among intensively housed cattle, but this is related to assumptions made in the parameterisation of the model and should be tested against independent evidence.
  11. The parameters to which the model is most sensitive (and hence those that it is most important to quantify) are the ones related to
    • transmission from grass and enclosures to animals
    • contact between animals
    • pathogen survival on grass, in slurry and in barns
  12. The model could be expanded and refined as new data become available. Indeed it is intended to develop further the waste handling aspects within OZ0709. Other areas where additional data could be used to refine the model are:
    • Infection of animals by farm strains of VTEC O157 under farm conditions (as opposed to high doses of laboratory cultures) to test and calibrate the dose-response model.
    • Quantified monitoring of shedding in cohorts of animals to calibrate the transition probability matrix.
    • Long-term monitoring with quantification of ‘simple’ farming systems to provide validation data sets for this and other models.
Funding Source
Dept. for Environment, Food and Rural Affairs
Project number
OZ0708
Categories
Escherichia coli
Sanitation and Quality Standards
Risk Assessment, Management, and Communication
Bacterial Pathogens
Commodities
Meat, Poultry, Game