Immune-mediated myositis

Veterinary advice should be sought before applying any treatment or vaccine.

Immune-mediated Myositis

Immune-mediated myositis (IMM) is a rare muscle disorder where the horse's immune system attacks the skeletal muscles, resulting in rapid atrophy of the topline muscles. The disease is most frequently triggered by infection with Streptococcus spp, however 40% of IMM cases did not involve infectious agents.


Rapid muscle atrophy
Loss of appetite
Difficulty standing


  • History
  • Clinical signs
  • Physical exam
  • Blood Chemistry - CK and AST levels
  • Muscle biopsy



CorticosteroidsCombination of dexamethasone and prednisolone
Antibiotic therapyUsually required with leukocytosis, hyperfibrinogenemia or lymphadenopathy
DietBalanced concentrate feed with high quality protein and quality hay


Horses can rapidly improve with early treatment and sometimes don't requrie treatment. .

Scientific Research

General Overviews

Risk Factors

  • Quarter horse breed : 89
  • Under 8 years of age or 17 years of age or older : 86
  • Recently vaccinated
  • Recurring muscle atrophy : 43
  • History of respiratory infection or exposure to horses with infections : 39

Commonly Affected Breeds

Quarter Horse iconPaint Horse icon