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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
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