A diagnosis of equine polysaccharide storage myopathy (EPSM) was made
in six non-Quarter Horse related breeds. Clinical signs in affected ho
rses included exertional rhabdomyolysis (four horses), stiff pelvic li
mb gait (three horses), poor performance and lack of energy (three hor
ses), and overt muscle atrophy (one horse). After 2 to 3 months of die
tary therapy all horses showed moderate to marked improvement of perfo
rmance, despite frequent markedly increased post-exercise concentratio
ns of serum muscle enzymes. Blood concentrations of Vitamin E and sele
nium determined before and during the dietary trial did nor. support t
he hypothesis that a deficiency of Vitamin for selenium was responsibl
e for the myopathy in these horses. A carbohydrate metabolic defect in
muscle of affected horses is supported by these results, which indica
te that muscle function in horses with EPSM may be markedly improved b
y feeding a low carbohydrate; high fat, and high protein diet.