The ageing of bruises in lambs : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University
Bruising in lambs processed for human consumption is a significant economic problem. A reduction in the prevalence of bruises could be achieved relatively efficiently if their important places of occurrence could be identified by ageing these lesions with respect to the known times of occurrence of events of possible aetiological significance. To this end efforts were made to age experimental bruises in lambs by objectively assessing semi-quantitative histopathological data using a mathematical model based on Bayes' theorem of inverse probabilities, by enzyme histochemical and isoelectric focussing studies and by measurement of muscle pH. The Bayesian method for objective histopathological ageing was developed and tested on data representing 178 bruises. It was successful in identifying bruises as either 1-20 hours or more than 24 hours old. The 'accuracy' with which a bruise of known age could be identified as such depended on the nature and number of tissue samples studied. The degree of 'confidence' with which an individual bruise of unknown age could be aged, however, depended both on the 'accuracy' of the method and on the relative number of bruises estimated to belong to each of the two age categories considered. In general a degree of 'confidence' of 80-90% can be expected in practice, and in this respect the performance of the Bayesian method is superior to that achieved by purely subjective means. A pilot survey involving 107 bruises collected from an export meat works established both the practical value of the objective ageing method and its superiority over alternative epidemiological approaches to the problem of utilising data pertaining to trucking times and holding times in meat works yards. Of the bruises studied, 60% were estimated to have been inflicted within the works, and 40% prior to arrival. Enzyme studies on bruises aged 4-144 hours old revealed no detectable relationships between observed changes in either histochemical or isoenzyme activities and bruise ages. In light of contradictory published results pertaining to other types of wounds, this lack of success was thought to reflect the relatively mild nature of the tissue reaction in bruises. Statistically significant relationships could not bo demonstrated between absolute or relative muscle pH and the ages of bruises from 4-48 hours old. A newly recognised condition of 'subcutaneous haemorrhagic speckling' in the carcase adipose tissue of young lambs processed for human consumption was investigated. From histopathological and epidemiological evidence, the primary cause of the lesions was shown to be electrical stunning. However, secondary aetiological factors were proposed as having influenced the prevalence and severity of lesions. Attempts to elucidate the pathogenesis of 'speckling' with the intention of formulating a rational approach to its prevention were unsuccessful.
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