A Thesis Presented to The Faculty of Graduate Studies of the University of Guelph by ALAIN A. BOUVET for the degree of Master of Science, October, 1984
Porcine stress syndrome (PSS) imposes a heavy economic burden on the swine industry through death loss prior to slaughter and through deterioration of the meat after slaughter. Pig breeders and scientists recognize that this syndrome is inherited and that attempts to reduce the economic loss from this syndrome depend heavily on the detection of carriers of this trait. Among the methods currently used for the detection of stress-susceptibility in pigs is the halothane test which is efficient and practical in a majority of cases. However, the halothane test is technically involved and invasive and occasionally causes the death of the pigs tested. Since a morphological alteration was noted in platelets of stress-susceptible pigs in a previous study, an investigation was undertaken to test whether or not an ultrastructural alteration exists as a consistent feature of the syndrome and to assess the reliability of this alteration as an indicator of stress-susceptibility in pigs.
Histological and ultrastructural studies were carried out on blood samples collected from 24 normal pigs and 25 pigs identified as stress-susceptible on the basis of their response to halothane challenge.
The most striking alteration detected in the stress-susceptible pigs was the abundance of dilated profiles of the open canalicular system (OCS) in contrast to the relative Paucity of these in normal platelets.
A quantitative rating, based on the extent of OCS dilatation and the proportion of affected platelets, showed that the halothane reactor pigs could be distinguished from normal pigs, on the basis of their OCS score .
The influence of temperature and anticoagulants on platelet morphology was examined in order to determine whether or not the difference observed was the result of external factors. The OCS features of the platelets were noted to be unaffected by the anticoagulants used whe`reas exposure to 20øC for a four hour duration appeared to increase the vacuolation of platelets, irrespective of the halothane status of the pigs
The impact of internal factors including the age and the breed of the pigs appeared to be insignificant in terms of the OCS features whereas the halothane status was highly correlated with the OCS score.
Thus, the three categories of pigs including halothane reactors, non-reactors and siblings of reactors were classifiable in different categories on the basis of their mean OCS score, with the siblings of the reactors which were non-responsive to halothane challenge exhibiting a mean score intermediary to those of reactors and non-reactors. The intermediate value of the sibling group was noted to be due to the difference in their individual OCS scores which were separable in two distinct categories. The discrete nature of the OCS score categories in siblings indicates that platelet alteration may be an inherent component of the porcine stress syndrome and that some of the false negatives in the halothane test may be identified as stress-susceptible on the basis of their platelet features.
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