Incidence of polysaccharide storage myopathy: necropsy study of 225 horses.
Authors: Valentine B A, Cooper B J
Journal: Veterinary pathology
Summary
Polysaccharide storage myopathy (PSSM) affects a substantial proportion of the equine population, with prevalence varying markedly by breed type according to this necropsy examination of 225 horses and ponies. Muscle tissue samples were histologically evaluated using routine staining and periodic acid-Schiff staining to identify abnormal glycogen accumulation and amylase-resistant polysaccharide deposits, alongside assessment of chronic myopathic changes including fibre size variation and internal nuclei proliferation. The study identified PSSM lesions in 101 animals (45% overall incidence), though this figure ranged dramatically from 27% in Thoroughbreds to 86% in draft-related breeds, with affected horses averaging 14.7 years of age. Horses and ponies with PSSM demonstrated significantly more severe chronic myopathic changes than unaffected animals, suggesting ongoing muscle degeneration associated with polysaccharide accumulation. These findings establish PSSM as a widespread condition across multiple breeds rather than a rare disorder, with important implications for performance horses and breeding programmes, particularly in draft breeds where prevalence approaches that of endemic disease.
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Practical Takeaways
- •PSSM is significantly more prevalent in draft and draft-related horses than light breeds; consider this when evaluating muscle disease and poor performance in these populations
- •The wide age range of affected horses (1-30 years) suggests the condition can manifest at various life stages; clinical suspicion should not be limited by age alone
- •High incidence across breeds means PSSM should be included in differential diagnosis for horses with myopathic signs, lethargy, or exercise intolerance regardless of breed
Key Findings
- •PSSM was identified in 101 of 225 horses (44.9% overall incidence) at necropsy
- •Draft-related breeds showed highest incidence at 86%, while Thoroughbreds showed lowest at 27%
- •Affected horses ranged from 1 to 30 years old (mean 14.7 years) with chronic myopathic changes more severe than unaffected horses
- •PSSM is a common disorder across many breeds, characterized by abnormal glycogen and amylase-resistant polysaccharide accumulation in muscle