Clinical and histopathological features of myofibrillar myopathy in Warmblood horses.
Authors: Valberg S J, Nicholson A M, Lewis S S, Reardon R A, Finno C J
Journal: Equine veterinary journal
Summary
# Myofibrillar Myopathy in Warmblood Horses: A Distinctive Hereditary Muscle Disease Warmblood horses presenting with exercise intolerance, reluctance to engage, stiffness and poorly localised hindlimb lameness may be affected by myofibrillar myopathy (MFM), a novel exertional myopathy characterised by progressive muscle degeneration. Valberg and colleagues examined muscle biopsies from ten affected horses and traced the condition through three generations of a Warmblood family, identifying distinctive histopathological hallmarks: abnormal aggregations of the cytoskeletal protein desmin within type 2a and 2x muscle fibres, focal Z disc degeneration, and myofibrillar disruption—findings not associated with glycogen abnormalities as seen in other equine myopathies. Affected horses showed significantly elevated histological scores for internalised myonuclei, anguloid (angular) fibre atrophy and overall myopathic damage compared to controls, with desmin-positive fibres appearing in the founding dam and descendants across two subsequent generations, suggesting genetic inheritance. The presence of desmin accumulation alongside Z disc pathology distinguishes this condition from other known myopathies, allowing practitioners to recognise the clinical and histological pattern when evaluating horses with exercise-related hindlimb dysfunction. Given the apparent heritability demonstrated within the Warmblood pedigree, breeders and veterinarians should consider MFM in the differential diagnosis of exercise intolerance in this population and evaluate whether breeding recommendations are warranted to limit further spread of the condition.
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Practical Takeaways
- •Exercise-intolerant Warmblood horses presenting with stiffness and poorly localised hindlimb lameness should be considered for muscle biopsy to rule out myofibrillar myopathy, particularly if unresponsive to conventional lameness investigations
- •Myofibrillar myopathy appears to have a heritable component in Warmblood lines; breeding decisions should consider this potential genetic predisposition when cases are confirmed in a family
- •Muscle glycogen depletion is not involved in this condition, so glycogen-loading management strategies typical for other equine myopathies may not be relevant to MFM cases
Key Findings
- •Myofibrillar myopathy in Warmblood horses presents with exercise intolerance, reluctance to go forward, stiffness and poorly localised hindlimb lameness
- •Abnormal desmin aggregates found in up to 120 type 2a myofibres with significantly elevated internalised myonuclei (0.83 vs 0.22 in controls) and anguloid atrophy scores (0.95 vs 0.31)
- •Histopathological findings include focal Z disc degeneration, myofibrillar disruption and accumulation of irregular granular material without abnormal muscle glycogen concentrations
- •Evidence of potential heritability with desmin-positive aggregates identified in founding dam and two subsequent generations of a Warmblood family