Suspected immune-mediated myositis in horses

Susannah S. Lewis, Stephanie J Valberg, Ian L. Nielsen

Research output: Contribution to journalArticlepeer-review

41 Scopus citations


Background: Although immune-mediated myositis (IMM) is commonly reported in other species, this condition is poorly described in horses. Hypothesis: IMM occurs in horses. Animals: Thirty-seven horses with suspected IMM were included in the study. Methods: The database of the Neuromuscular Diagnostic Laboratory was reviewed to identify 37 horses with muscle biopsies characterized by lymphocytic infiltrates. A retrospective standardized questionnaire regarding clinical signs and response to treatment was answered by horse owners. Results: Horses with suspected IMM were predominantly of Quarter Horse bloodlines (33/37 horses) and primarily either ≤8 years or ≥17 years of age. Clinical signs included rapid atrophy, particularly of the epaxial and gluteal muscles, depression, and stiffness. Creatine kinase (CK) activity (mean 9,746; range 260-139,183 U/L: reference range 119-287 U/L) and aspartate transaminase (AST) activity (mean 2880; range 350-9,009 U/L: reference range 138-409 U/L) were high. Exposure to horses with infectious respiratory disease occurred in 39% (9/23) of horses before clinical signs and 47% (9/19) had recurrence of atrophy. Variation in dosage and time elapsed before administration of corticosteroids confounded assessment of treatment efficacy. Macrophages and CD4+ T lymphocytes were the prominent mononuclear cellular infiltrates with lesser numbers of CD8+ cells and small clusters of B lymphocytes in some samples. Myofibers did not stain for equine immunoglobulin G (IgG). Conclusions and Clinical Importance: IMM appears to be a distinct cause of rapid muscle atrophy, particularly in Quarter Horses that may be amenable to treatment with corticosteroids. Diagnosis is best achieved by identifying lymphocytic infiltrates in atrophied muscles.

Original languageEnglish (US)
Pages (from-to)495-503
Number of pages9
JournalJournal of veterinary internal medicine
Issue number3
StatePublished - May 2007


  • Atrophy
  • Lymphocytes
  • Myopathy
  • Rhabdomyolysis


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