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This information is provided by Provet for educational purposes only.

You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Myositis means inflammation (-itis) of the muscles (myo-) 

Inflammation of the muscles is a painful condition causing spasms and cramps. Muscles may be hard to feel, and appear to twitch or tremor. The gait is very stiff, or unsteady and some horses will refuse to move. A few horses will lie down, roll and sweat mimicking the signs of colic. Myositis is also called azoturia,  "tying-up",  "Monday morning syndrome" , paralytic myoglobinuria and rhabdomyolysis and although the underlying causes may be different the clinical presentation is similar.

Damage in the muscle causes the release of pigment called myoglobin into the bloodstream where it is transported to the kidneys. The myoglobin can damage the kidneys causing kidney failure, and in some cases death.  The myoglobin may be passed out as coffee-coloured urine. 

Early treatment of horses with myositis is important. The disease is most often seen in horses returned to exercise after a period of rest and horses either :

  • Show discomfort after a few minutes exercise (type A myositis)
  • Show discomfort after several hours exercise (type B myositis)

Other possible associations include :

  • Influenza virus
  • Vitamin E/selenium deficiency
  • Overnutrition
  • Stress/excitement
  • Hormonal influences
  • Abnormal energy metabolism in the muscle
  • High lactic acid accumulation in the muscle - seen in horses working at over 20mph
  • Electrolyte losses during endurance work - especially potassium and chloride
  • Dehydration - especially during endurance work 
  • Rest followed by vigorous exercise

At risk horses include :

  • Some breed lines especially heavily muscles and draft breeds (eg Clydesdale, Shires, Belgian, Percheron, Appaloosa and American Quarter Horses). This suggests inherited (genetic) factors and may be a defect in muscle energy metabolism 
  • Females are more likely to develop myositis than males
  • Nervous, highly strung animals are more likely to develop myositis
  • Horses in training fed a rich ration (grain and/or alfalfa) are at greater risk - especially if they are rested for a couple of days then start training again

Various suggestions to try to prevent myositis have been made,  including :

  • Reduce ration intake in horses that are not working
  • Keep horses warm - blankets/rugs in cold, wet weather between exercise periods
  • Don't exercise horses that may have flu virus
  • Warm up horses with gentle exercise before subjecting them to vigorous exercise
  • Proper training programme to build up a horses performance conditioning
  • Do not pour cold water on horses after exercise
  • Worm horses regularly to avoid anaemia


Updated October 2013