pH (Borch et al. 1996).
At present in New Zealand there are 11 abattoirs that slaughter pigs. Six of these are located in the North Island and five in the South Island. In this thesis three abattoirs were visited, two of which were in the North Island. One abattoir in the North Island slaughters pigs on two days in the week, except for the month of December in which increased market demand led to slaughtering of pigs on 3 days in the week. It is estimated that a total of 15,000 pigs are slaughter per week. The number of pigs slaughtered daily in abattoirs in New Zealand can vary from 150 to over 1,000 pigs. The procedures described above are undertaken in all visited plants. There is a procedure called polishing, that results in the removal of any black rind from singeing. This procedure was not modelled as the methods used in New Zealand to execute this procedure are varied, ranging from the manual use of nylon hand brushes to large automated equipment with rubber flails and this procedure is sometimes absent. There appeared to be no standardisation with which the procedure is executed in abattoirs visited throughout New Zealand.
2.12.2 Further processing processes
The production of pork chops is evaluated in this thesis as it is one of the most commonly purchased and consumed pork products in New Zealand. Pork chops can be produced in some abattoirs and/or retail outlets by cutting the longissimus dorsi muscles into desired sizes.
2.13 Conclusion 45 wide by the CAC, it is imperative that risk-based analysis be implemented for pathogens transmitted to humans through pork consumption. To this end, this thesis addresses the challenge of developing an exposure assessment model to be used in QMRAs enabling determination of the risk of exposure to Salmonella spp., thermophilic Campylobacter and E. coli from pork chops locally produced and sold at retail outlets in New Zealand.
C H A P T E R 3
Modelling pathogen dynamics in the abattoir
3.1 Abstract
The consumption of pork products contaminated with food-borne microbial pathogens such as Salmonella, Campylobacter and Escherichia coli may result in illness in suscep- tible individuals. Understanding the mechanism(s) by which these pathogens are prop- agated along the food chain can be critical in formulating and refining efficient control strategies. The purpose of this study is to propose a model simulating the propagation of these three pathogens in New Zealand abattoirs.
To this end, a suite of quantitative, semi-stochastic, modular process risk models (MPRM) sufficiently generic to describe the propagation of Salmonella, Escherichia coli and Campylobacter through the various stages of pork processing in pig abattoir is devel- oped. Dynamics of pathogen inactivation, removal, partitioning and cross-contamination are described and explicitly modelled using a combination of difference and differential equations. Second order modelling is performed to quantify parameter variability and uncertainty. Parameters are estimated from published data and targeted investigations in abattoirs in New Zealand.
Using Monte Carlo simulations, our model predicted that both dehairing and eviscera- tion contributed the most to increased carcass contamination levels. Scalding was demon- strated to be highly effective in reducing pathogen numbers, particularly with respect to Campylobacter. The latter abattoir procedures of evisceration and storage were shown to contribute more to increased pathogen variance than the earlier procedure of scalding.
Distributions of all three pathogens of interest on carcasses (from the time of killing to storage of the completed dressed product) were estimated to be highly right-skewed with the 10th to 90th percentile predicted to be 1 – 2495, 1 – 245 and 1 – 85cfu/half carcass
for Salmonella, Campylobacter and Escherichia coli, respectively. A small percentage of carcasses were highly contaminated, with most carcasses possessing low levels of sur- face contamination. The estimated median contamination levels on the dressed carcasses exiting the abattoir were less than one cfu/cm2for the three pathogens.
The models predicted that at least one cfu was present on most carcasses, therefore resulting in mean prevalence values on the final product of 98% and 94% for E. coli and Campylobacter respectively. If the prevalence level of Salmonella in New Zealand was similar to levels currently reported in Europe, the model predicted prevalence levels on dressed carcasses leaving the abattoir for this pathogen of 100%. However Salmonella in pigs in New Zealand abattoirs is rare. Second order modelling quantified parameter uncer- tainty and variability for all pathogens. Also, model development permitted identification of many data gaps.
We conclude that although the pathogens of interest were predicted to be present on nearly all carcasses, only a small number of these dressed carcasses would be expected to possess large pathogen numbers on exiting the abattoir. Further, we propose that the models developed are sufficiently rigorous yet flexible to be extrapolated to other species with similar abattoir processing. Additionally, since the model outputs distributions of pathogen numbers and prevalence, it can be used in quantitative microbial exposure as- sessments and for investigating the efficacy of intervention strategies to reduce pathogen load and the occurrence of food-borne diseases.