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    Occupational exposure to pathogenic Leptospira from sheep carcasses in a New Zealand abattoir : a dissertation presented in partial fulfilment of the requirements for the degree of Masters in Veterinary Studies at Massey University, Palmerston North, New Zealand
    (Massey University, 2007) Dorjee, Sithar
    This research was undertaken in response to a rising trend in the incidence of notified human leptospirosis cases, particularly in meat workers, between 2000 and 2005 in New Zealand. Meat workers constituted the highest proportion (65% during 2004) of notified leptospirosis cases. Sheep either alone or in combination with other animals constituted the second most important animal contact source of human infection (after cattle). Further justification of this research was due to the fact that a number of cases were reported from meat workers employed in sheep-only abattoirs and concerns were raised that sheep may be a significant source of human infection. A longitudinal study was undertaken to determine the serological and cultural prevalence of two of the most commonly diagnosed serovars, Leptospira borgpetersenii serovar Hardjobovis and Leptospira interrogans serovar Pomona. Lines of sheep and individual sheep were systematically randomly sampled at a sheep-only abattoir in Feilding from May 2004 to June 2005. In addition, an assessment of the risk of occupational exposure of meat workers to carcasses shedding live leptospires was carried out using a stochastic model. The association between white-spotted kidney lesions and the serological and cultural prevalence of leptospirosis in sheep was determined along with the diagnostic value of these lesions to predict serological and culture status at the line and individual carcass levels. The study showed that the prevalence of lines with sheep seropositive to one or both serovars was 44% (95% CI 35–54), corresponding to 45% (95% CI 35–55) of farms. This indicates that nearly half the sampled farms had been exposed to infection previously. The overall individual serological prevalence in the sample of 15,855 sheep processed was 6% (95% CI 5–7). Lambs born in the 2003–2004 season had a significantly higher serological prevalence to one or both serovars at the line and individual animal levels compared with lambs born in the 2004–2005 season, suggesting a strong seasonal effect. The serological prevalence of Hardjobovis was significantly higher than Pomona at the line and individual animal levels. The overall isolation rates of live leptospires from seropositive kidneys of Hardjobovis and Pomona were 22% and 17% respectively, and 1% from seronegative carcasses. From a purposively selected line (suspected of being from a farm with active leptospirosis) all 13 kidneys of seropositive carcasses were culture positive indicating a high risk of exposure of meat workers to leptospires in such a situation. Kidneys from seropositive carcasses were significantly more likely to return culture positive compared with kidneys from seronegative carcasses. The assessment of daily risk of exposure of meat workers indicated moderate risk for eviscerators and meat inspectors ranging from 3–11 (95% CI 0–22) and 6–18 (95% CI 1–34) carcasses potentially shedding live leptospires respectively, and a high risk for offal-handlers that ranged from 18–54 (95% CI 7–91) shedding carcasses. The results from the third study showed that the prevalence of white spotted kidneys was 16% and 91% at the individual sheep and line levels, respectively. Carcasses with white spotted kidneys were 5.2 times (95% CI 3.9–7.1) more likely to test seropositive to one or both serovars, but lesions were poor predictors of serological status as judged by test sensitivity and positive predictive values. Furthermore, a positive linear association between white-spot kidney lesion scores and seropositivity to either or both serovars was evident. Consideration of lesion status of lines rather than for individual animals resulted in higher test sensitivity but still suffered from a low positive predictive value. Leptospires were isolated from 5% (95% CI 4–8) kidneys that were cultured. There was no statistically significant association between white spotted kidney lesion scores and culture test results in the survey data; however, a significant linear positive association was evident when culture data from a purposively sampled farm was merged with the survey data. We conclude that the processing of sheep in sheep-only abattoirs constitutes a definite exposure risk of meat workers to leptospirosis and that exposure risks ranged from moderate to high degrees depending on type of duties performed on the slaughter room floor. Furthermore, since grossly visible white-spotted kidney lesions were positively associated with serological and cultural prevalence of disease it would be advisable for meat workers to take extra care when processing lines with a high prevalence of carcasses with these lesions to reduce the risk of infection. This recommendation is made despite the poor predictability of serological and cultural status of these lesions.
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    A novel model developed for quantitative microbial risk assessment in the pork food chain : a dissertation presented in partial fullfilment [sic] of the requirements for the degree of Doctor of Philosophy at Massey University, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand
    (Massey University, 2007) Titus, Simone Megan
    Food-borne diseases contribute substantially to morbidity and mortality rates worldwide. The deleterious impact of these diseases on human health, concurrent with the associated socioeconomic cost has led to an increased demand for the production of safe food globally. Consequently, agencies such as the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) have resolved to address this issue. In this vein, scientific, risk-based approaches which facilitate estimation of the probability of disease occurrence, the magnitude of the disease and efficacious control measures have been recommended for use internationally. Many pathogens have been implicated as aetiological agents of food-borne disease. The WHO has identified non-typhoidal Salmonella, Escherichia coli and thermophilic Campylobacter as zoonotic food-borne pathogens of greatest importance. These pathogens can be transmitted to humans through pork consumption. This thesis therefore proposes a suite of novel, mechanistic, semi-stochastic, quantitative, modular process risk models describing the propagation of these three pathogens from the live pig at the abattoir, to pork chops sold at retail. The model is developed for use in risk-based, quantitative microbial exposure assessments in New Zealand and can be employed to explore different intervention strategies targeted at mitigating contamination levels of these pathogens on pork chops. The models comprise multiple, coupled, differential and difference equations. These equations explicitly describe bacterial growth, inactivation, removal, cross-contamination and food partitioning occurring in continuous and discrete time in abattoirs and at retail. Distributions of pathogen numbers on the surface of carcasses, and prevalence levels are output by the models at different stages of abattoir processing and pork chop production. Both dressed pork carcases exiting abattoirs in New Zealand and pork chops at retail are predicted to contain low surface contamination levels of the pathogens under consideration, while a small percentage is estimated to be highly contaminated. Median contamination levels on dressed pork exiting the abattoir are predicted to be less than one cfu/cm2. Generally, there are large reductions in surface bacterial numbers for all three organisms from the time the live pig enters the abattoir, to sale of the pork chop at retail. The introduction of a second singeing procedure immediately postevisceration in the abattoir is predicted by our models, to be an effective mitigation strativ egy, with estimated reductions in median pathogen levels of 100%. This control measure is considered to be more effective than coverage of the anal region of the pig during evisceration. This latter mitigation strategy was predicted to result in 10% – 44% reduction of median pathogen contamination levels. At retail, pork chops are also estimated to contain low numbers of these pathogens. Therefore handling of the raw pork chop soon after purchase from retail outlets may be associated with a low risk of contracting salmonellosis, colibacillosis and campylobacteriosis. This risk can be further reduced by placing pork chops in a blast chiller for 12 hours prior to display. When this mitigation strategy was modelled the outputs indicated a 15% – 61% reduction in the maximum pathogen levels on pork chops, 44 – 100% reduction in the 10th – 90th range and 14% – 50% reduction in pathogen prevalence levels. Detailed investigation revealed the limitations of a specific modelling approach. We determined that the population-based modelling approach is not an appropriate alternative to the individual-based modelling approach when there is a large disparity in contamination levels between processed carcasses. Therefore the former technique should not be used in the presence of large heterogeneity with respect to the number of bacteria on the food unit of interest, or when bacterial populations input into the model are described with large variances. This thesis demonstrates the application of a suite of novel risk models in the pork food chain. We propose use in quantitative microbial exposure assessments. The applicability of these models is not only limited to the pork chain or to the above mentioned pathogens, but by modification of parameters, the entire model, or portions thereof can be extrapolated to other animal species undergoing similar abattoir procedures with pathogens of analogous epidemiological patterns. Finally the information provided by the models can be instrumental in assisting risk managers in their decision-making and policy development undertakings and provide guidance to effectively and strategically funnel limited resources.