Polysaccharide storage myopathy in Cob Normand draft horses.
Authors: Larcher T, Herszberg B, Molon-Noblot S, Guigand L, Chaffaux S, Guerin G, Cherel Y
Journal: Veterinary pathology
Summary
Polysaccharide storage myopathy (PSSM) has been well documented in certain equine breeds, but its prevalence and pathological characteristics in Cob Normand draft horses remained unclear until this 2008 investigation. Larcher and colleagues examined gluteus medius biopsies from 53 Cob Normands, identifying amylase-resistant polysaccharide accumulation in 38% of the population and stratifying affected horses into two distinct phenotypes based on the percentage and distribution of storage-laden fibres and associated degenerative changes. The milder form (26% of the cohort) showed minimal fibre involvement (mean 0.98%) with benign clustering patterns and no myopathic lesions, whilst the severe phenotype (11%) demonstrated extensive fibre involvement (mean 18.1%) with chronic inflammatory and degenerative changes scattered throughout the biopsy. Longitudinal sampling revealed that horses in the mild category showed progressive accumulation over time, suggesting the condition may worsen, whereas the severe group's pathology appeared established; additionally, widespread polysaccharide deposition across multiple tissue types, including smooth muscle, was noted in one necropsied individual. For practitioners working with Cob Normands—particularly those presenting with exercise intolerance, stiffness, or poor performance—these findings underscore the importance of screening for PSSM, recognising that early detection of the mild phenotype may offer an intervention window before progression to debilitating myopathic changes.
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Practical Takeaways
- •PSSM appears common in Cob Normand draft horses (38% prevalence); consider breed-specific screening and genetic counseling for breeding programs
- •Two clinical phenotypes exist with different progression rates—early detection through biopsy may help identify horses requiring dietary management and exercise modification before severe myopathic changes develop
- •The presence of smooth muscle involvement suggests systemic metabolic dysfunction beyond skeletal muscle; management should address underlying polysaccharide metabolism rather than treating localized muscle symptoms
Key Findings
- •38% (20/53) of Cob Normand horses exhibited polysaccharide storage myopathy with amylase-resistant material in myocytes
- •Two distinct presentations identified: mild form (26% of horses) with low fiber involvement (0.98%) and no myopathic changes, and severe form (11% of horses) with high fiber involvement (18.1%) and chronic myopathic changes
- •Affected type II muscle fibers showed increased diameter (67.7 vs 57.3 microm in normal fibers)
- •Re-biopsy demonstrated progressive accumulation of aggregates in first group over time, and aggregates were found across multiple muscle types including smooth muscle in the necropsied horse