Model applications of decision support systems in meat hygiene programs : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Science at Massey University
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Date
1996
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Massey University
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Abstract
Current systems to ensure safety of meat are to a large degree based on a "procedural" approach which specifies what inspection actions will be taken to protect human health. Both knowledge and disease priorities have changed substantially over recent decades, and moreover the scale of the problems created by any breakdowns in protection has escalated greatly, as food trading and consumption patterns have changed. It is now recognized that meat hygiene needs to focus primarily on ways by which the risk that product will represent a hazard to human health can be effectively reduced, rather than merely ensuring compliance with a defined set of procedures. In addition to human food safety, meat inspection has the potential to contribute information to improve animal health on a national and a local scale. This thesis examines example issues in order to identify possible approaches to the development of decision support systems which assist in protecting meat consumers and improving the health of livestock on farms. The main areas which were explored for this purpose were respiratory disease in lambs and chemical residues in slaughter animals. A literature review of pneumonia and pleurisy in lambs showed that numerous factors have been proposed as predisposing causes for these diseases, but there was surprisingly little valid experimental or observational research evidence to support such statements. A hazard analysis was performed for the micro-organisms which have been isolated from pneumonic lungs. The major commonly detected organisms did not appear to cause a risk to healthy people. However there were a number of micro-organisms which are isolated on occasion from pneumonic and sometimes from healthy sheep lungs that might cause human disease. A case-control study was carried out as an exploratory means to identify risk factors and to generate hypotheses about causal processes. A number of risk factors were initially identified at univariate level. At the second stage the importance of some of these risk factors was quantified in a logistic regression model. Finally a third stage analysis showed the interactions between the factors in a logistic path model, which consisted of three clusters. One cluster included characteristics of the farm and paddocks, one cluster included the yards and practices in the yards, and a third cluster included the types and number of animals on the farm. Two intervention studies were subsequently carried out to evaluate the effect of making various management modifications on the prevalence of pneumonia and pleurisy at slaughter. One intervention study evaluated the time lambs spent in the yards after weaning and the use or oral or injectable drenches. The second intervention study evaluated the use of oral versus injectable drenches and the use of a shower dip versus a wand. The intervention studies showed an effect of time in the yards on pneumonia. There was some association between time in the yards and acute localised pleurisy but none between the other measures tested and respiratory disease. The studies showed clear temporal patterns with regard to pleurisy and pneumonia and enabled comparisons to be made between farms. A study of inspection for pleurisy at slaughterhouses was analysed. The analysis identified the temporal patterns of certain types of pleurisy. Comparisons were made between four participating premises. The sensitivity and specificity of meat inspection for the various types of pleurisy was analysed. The pleurisy data over an eleven year period of the entire country were analysed. Differences were shown between islands and regions. The potential for development of components of a decision support system for pneumonia and pleurisy was illustrated with a number of examples. An important component was to determine how farmers could be assisted in improving the health of their lambs with regard to pleurisy. Ideas to improve farmer involvement were developed. The principles of a decision support system which evaluated the issue of cross-contamination due to handling of product by the inspector were developed. Epidemiological principles of chemical residues in slaughter animals were investigated. A number of statistical quality control tests were applied to known data sets to evaluate what sample sizes would be required to detect changing trends or spatial paterns. Temporal simulations were performed to determine how well clusters in time could be detected. The Moving Average approach was used and it appeared that with the given data set sample sizes well beyond those feasible to achieve would be required. Spatial analyses with a number of different statistics were performed. In this case also, large sample sizes were required for reliable results. It was concluded that use of a risk analysis model to define a risk-reduction strategy targeted to avoid any significant risk to the consumer offered a much more effective tool than a fixed sampling system. This model combines a range of possible risk reduction measures in various mixes, and determines whether or not each of the tested strategies achieves the goal of making it very improbable that a consumer would be exposed to sufficient levels of chemical residues in food to even constitute some minimal public health risk.
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Meat inspection, Veterinary epidemiology, Communicable diseases in animals -- Prevention, Zoonoses -- Prevention, Lambs -- Diseases, Agricultural chemicals -- Environmental aspects, Decision-making -- Mathematical models