Suspected immune-mediated myositis in horses

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.