Immune-Mediated Muscle Diseases of the Horse.
Authors: Durward-Akhurst S A, Valberg S J
Journal: Veterinary pathology
Summary
# Editorial Summary: Immune-Mediated Muscle Diseases of the Horse Immune-mediated muscle disorders in horses manifest through distinctly different pathogenic mechanisms, ranging from secondary responses to infection to primary autoimmune disease. Durward-Akhurst and Valberg's 2018 review synthesises the pathophysiology of three major conditions: infarctive purpura haemorrhagica (IPH) following *Streptococcus equi* ss. equi infection or vaccination, breed-predisposed myositis in Quarter Horse-related breeds involving aberrant MHC upregulation and lymphocytic infiltration, and the rare post-inflammatory myositis complicated by systemic calcinosis. In IPH, IgA-streptococcal M protein immune complexes deposit in vasculature causing ischaemia and skeletal muscle infarction; Quarter Horse myositis presents as progressive lumbar and gluteal atrophy driven by autoimmune attack on myofibre antigens; calcinosis-associated myositis carries a characteristically poor prognosis with pathognomonic hyperphosphatemia. Understanding these distinct immunopathological pathways is essential for farriers, veterinarians and rehabilitation specialists to recognise clinical presentations early, tailor treatment according to underlying mechanism, and provide realistic prognostic guidance—particularly regarding the significantly different outcomes between vaccination-related and inherent autoimmune presentations.
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Practical Takeaways
- •Be alert for post-infection or post-vaccination myositis in horses, particularly Quarter Horse breeds, presenting with muscle atrophy and swelling—early recognition is critical as some forms carry high mortality
- •Understand that immune-mediated muscle disease can arise from either external triggers (streptococcal infection/vaccine) or inherent immune dysregulation, affecting prognosis and management strategies
- •Monitor serum phosphate levels in horses with severe myositis as hyperphosphatemia may indicate progression to systemic calcinosis, warranting urgent veterinary intervention
Key Findings
- •Streptococcus equi infection or vaccination can trigger infarctive purpura hemorrhagica (IPH) through IgA-streptococcal M protein immune complex deposition causing muscle ischemia and infarction
- •Quarter Horse-related breeds develop immune-mediated myositis characterized by MHC class I and II upregulation, lymphocytic infiltration of lumbar and gluteal muscles, and progressive atrophy
- •Rare inflammatory myositis can progress to systemic calcinosis with pathognomonic hyperphosphatemia and high fatality rates