Ischaemic myocardial fibrosis and aortic strongylosis in the horse.

The hearts and aortas of 2076 unselected horses of all ages were examined immediately after slaughter. Focal zones of fibrosis, observed in the myocardium of 14.3 per cent of hearts examined, were found in both atria and ventricles and were unrelated to age. Microscopically the majority of lesions involved myocardial fibre lysis and replacement fibrosis, although acute infarction was present in some cases. Intramyocardial arterioles in or adjacent to the lesions exhibited occlusive arteriosclerotic changes whereas those elsewhere remained patent. The evidence strongly suggests that the myocardial lesions were ischaemic in origin and related to the distribution of intramyocardial arteriosclerosis. Nodular fibrous plaques and mural thrombi associated with migrating larvae of Strongylus vulgaris were observed in the thoracic aorta of 9.4 per cent of horses. These intimal lesions were often present in the aortic bulb and proximal 10 cm of the thoracic aorta indicating that larval migration in this zone is common. Statistical analysis revealed a highly significant association between the occurrence of proximal aortic S vulgaris lesions and the presence of focal ischaemic lesions in the myocardium. The association was not the result of direct larval damage but appeared to be caused by microembolisation from parasitic lesions in the proximal aorta, producing obstructive arteriosclerotic lesions in myocardial arterioles.