title: Suspected immune-mediated myositis in horses.
abstract: BACKGROUND
  Although immune-mediated myositis (IMM) is commonly reported in other species, this condition is poorly described in horses.
  
  
  OBJECTIVE
  IMM occurs in horses.
  
  
  METHODS
  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 < or =8 years or > 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 9746; range 260-139,183 U/L: reference range 119-287 U/L) and aspartate transaminase (AST) activity (mean 2880; range 350-9009 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
  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.
authors.affiliation: Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota, St. Paul, MN, USA.
authors.forename: Susannah S
authors.initials: SS
authors.lastname: Lewis
authors.forename: Stephanie J
authors.initials: SJ
authors.lastname: Valberg
authors.forename: Ian L
authors.initials: IL
authors.lastname: Nielsen
mesh_terms: D000305:Adrenal Cortex Hormones / Q000627:therapeutic use*; D000818:Animals; D001706:Biopsy / Q000662:veterinary; D005260:Female; D006734:Horse Diseases / Q000188:drug therapy / Q000276:immunology* / Q000473:pathology; D006736:Horses; D007150:Immunohistochemistry / Q000662:veterinary; D008214:Lymphocytes / Q000276:immunology*; D008297:Male; D018482:Muscle, Skeletal / Q000276:immunology / Q000473:pathology; D009220:Myositis / Q000188:drug therapy / Q000276:immunology / Q000473:pathology / Q000662:veterinary*; D011379:Prognosis; D011795:Surveys and Questionnaires; D016896:Treatment Outcome
chemical_list: D000305:Adrenal Cortex Hormones