Suspected immune-mediated myositis in horses.
Authors: Lewis Susannah S, Valberg Stephanie J, Nielsen Ian L
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Suspected Immune-Mediated Myositis in Horses Whilst immune-mediated myositis is well-recognised in other species, its characterisation in equines remained poorly defined until this retrospective analysis of 37 horses presenting with muscle biopsy evidence of lymphocytic infiltration. The researchers reviewed diagnostic records and obtained standardised owner questionnaires regarding clinical presentation and treatment response, identifying a striking predisposition toward Quarter Horses (89% of cases) with bimodal age distribution clustering at either ≤8 years or ≥17 years. Clinical presentation involved rapid atrophy of epaxial and gluteal musculature accompanied by depression and stiffness, with dramatically elevated muscle enzymes (mean CK 9746 U/L and AST 2880 U/L—approximately 34-fold and 7-fold above reference ranges respectively); notably, 39–47% of affected horses had preceding exposure to infectious respiratory disease or experienced recurrent atrophy episodes. Immunohistochemical analysis revealed predominantly CD4+ T lymphocytes and macrophages with lesser CD8+ involvement, whilst myofibres tested negative for equine IgG, confirming true immune-mediated pathology rather than primary myonecrosis. For practitioners, this work establishes rapid multifocal muscle atrophy—particularly in Quarter Horse breeds—as a potential indicator warranting muscle biopsy and immunophenotyping, with corticosteroid responsiveness offering a therapeutic avenue, though standardised dosing protocols remain needed to optimise clinical outcomes.
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Practical Takeaways
- •Suspect immune-mediated myositis in Quarter Horses presenting with rapid, focal muscle atrophy (especially epaxial/gluteal areas) accompanied by high blood enzyme levels (CK >1000 U/L, AST >400 U/L) and clinical depression
- •Muscle biopsy showing lymphocytic infiltrates is the definitive diagnostic tool; consider corticosteroid therapy but monitor closely as recurrence occurs in nearly half of cases
- •Recent exposure to horses with respiratory disease may be a risk factor; implement biosecurity measures and be prepared for potential relapse even with treatment
Key Findings
- •37 horses with suspected IMM were predominantly Quarter Horse bloodlines (89%), occurring in horses ≤8 years or ≥17 years of age
- •Clinical signs included rapid muscle atrophy (especially epaxial and gluteal muscles), depression, and stiffness with markedly elevated CK (mean 9746 U/L, range 260-139,183) and AST (mean 2880 U/L, range 350-9009)
- •Muscle biopsies showed lymphocytic infiltrates with macrophages and CD4+ T lymphocytes as prominent cells, with 47% of horses experiencing recurrence of atrophy
- •IMM appears amenable to corticosteroid treatment, though assessment was confounded by variation in dosage and timing; diagnosis is best confirmed by identifying lymphocytic infiltrates in atrophied muscle