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Until recently, the exact cause of
tying up (exertional rhabdomyolysis) in horses was unclear, although
many potential causes were considered including vitamin/mineral
deficiency, electrolyte imbalance, and poor thyroid function. In
humans, more than half of the cases of exertional rhabdomyolysis are
caused by an error in the metabolism of muscle cells. These defects
in metabolism are inherited and specific. It only makes sense that
we would eventually identify a primary defect in muscle metabolism
in horses with this clinical condition.
Polysaccharide storage myopathy (PSSM)
has been identified to be the most common cause of tying up in
Quarter Horses and Draft breeds (some specify the Draft horse
disease EPSM). It is a primary muscle disorder characterized by the
accumulation of unusable carbohydrate the muscle cells. The cause is
a defect in muscle metabolism resulting in inappropriately rapid
processing of carbohydrate material and accumulation of abnormal
polysaccharide in muscle cells. Many breeds have been identified
with PSSM including Quarter Horses, Paints, warmbloods, Appaloosas,
and Draft horses. It is more common in females than males and horses
with PSSM often have a calm, sedate demeanor. A familial linkage has
been identified in Quarter Horse-related breeds and is suspected to
be inherited as a recessive gene.
Poor exercise tolerance is the most
common problem reported by owners of horses with PSSM. Severely
affected horses have numerous tying-up episodes recorded from the
beginning of their training, and mildly affected horses may have 1
episode per year or less. The severity of an episode can range from
mild exercise intolerance to severe, stiffness and pain. During a
mild tying up episode, horses will develop a tucked-up abdomen,
muscle twitching in the flank region, and a camped-out stance after
exercise. Painful back muscles and shortened stride length may be
the only signs of disease.
Diagnosis of PSSM is determined by
evaluation of muscle enzymes (at rest or after exercise) and an
abnormal muscle biopsy. A 15-minute exercise test at a trot on a
lunge line can detect PSSM. Approximately 80% of horses with PSSM
will fail the exercise test, defined by an increase in muscle
enzymes measured from a blood sample obtained 4 hours after
exercise. Draft horses suffering from EPSM commonly have normal
muscle enzyme levels, even after an exercise test.
A change in nutrition to a high-fat,
low-carbohydrate diet is very effective in eliminating clinical
signs of PSSM. Sweet feed (high in carbohydrate) should be
eliminated from the diet. A high-fat diet is achieved by
supplementing fat or providing a specialty ration designed to
provide high-fat, low carbohydrate. Regardless of the source of fat,
the diet should provide 20 to 25% of total daily calories as fat,
which is 2.5 to 3 times the approximately 200 gm of fat provided in
typical equine diets. Rice bran, corn oil, or spray dried fat can be
used to supplement fat in the diet. Corn oil can be fed at 480
ml/day mixed with soaked alfalfa cubes. Introduction of corn oil
should be gradual, beginning with 60 ml/day and addition 60 ml every
2 to 3 days until 480 mls is attained. Sugars and starches
(nonstructural carbohydrates) should constitute less than 15% of the
total daily calories. Corn consists of 71% nonstructural
carbohydrates and should be avoided. Most commercial feed labels do
not provide information regarding nonstructural carbohydrate
content. Concentrate feeds with less than 33% nonstructural
carbohydrates are effective diets when combined with good-quality
grass hay or pasture roughage. Sweet feeds contain approximately 47%
nonstructural carbohydrate, whereas commercial high fat diets
contain 28 to 39%, and alfalfa pellets contain approximately 2%. A
high-protein diet (12 to 17%) is recommended to help build muscle.
Vitamin E and selenium supplementation are recommended. This diet
successfully prevents post-exercise elevation in muscle enzymes and
clinical episodes of tying up within 3 to 6 months. Improvement in
attitude, stride, energy, exercise tolerance, and muscling are also
noted by most owners.
Stall rest for more than 12 hours per
day increases the incidence of tying up. Dietary changes without
alteration in training are insufficient to prevent episodes. Daily
lunging or riding and pasture access are essential to successful
management. A 15 minute exercise test is used to determine the
amount of exercise necessary in the initial training stages. If the
horse does not pass, 2 weeks of pasture turn-out with dietary change
are recommended. If horses successfully pass the exercise test, 15
minutes of lunging per day is recommended. Exercise should be
increased by a few minutes every few days. Once horses are capable
of trotting for 30 minutes on the lunge without difficulty, work
under saddle can begin.
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