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Exercise
Returning
Symptoms of ER
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eerh.com
Equine Exertional Rhabdomyolysis (ER) or Tying Up or Azoturia or Monday
Morning Disease is a syndrome that damages the muscle tissue in horses. It is
usually due to overfeeding a horse carbohydrates.
Causes and Process of ER
There does not seem to be a single cause of ER in horses. Exercise is seen in
every case, but exercise is always accompanied by another factor. It is likely
that several factors must act together in order to cause an ER attack.
Other possible factors include:
The overfeeding of carbohydrates (grain and pellets, for example)
Poor conditioning or fitness, sudden increase of workload
The work of a horse after a period of rest, if the grain diet was not reduced
Electrolyte or mineral imbalances, especially seen with potassium
A deficiency in selenium or vitamin E
Imbalance of hormones, including the reproductive hormones in nervous fillies
and mares and thyroid hormones in horses with hypothyroidism
Heredity
Wet, cold, or windy weather conditions
The more factors that are present, the greater the likelihood that the horse
will develop ER. However, the most common cause of ER is an imbalance between
the animal's diet and his workload, especially when he has a high-grain diet.
ER occurs when there is an inadequate flow of blood to the muscles of an
exercising horse. The muscle cells, lacking in oxygen, begin to function
anaerobically to produce the needed ATP. The anaerobic work creates a buildup of
waste products, acid, and heat. This subsequently alters the cell by preventing
the cell's enzymes from functioning and the myofilaments from efficiently
contracting. The cell membranes may then be damaged if the horse is forced to
continue work, which allows muscle enzymes and myoglobin to leak into the
bloodstream.
The body builds up a store of glycogen from converted carbohydrates in muscle
cells. Glycogen, a fuel used by muscles for energy, is depleted during work and
restocked when a horse rests. Oxygen-carrying blood metabolizes glycogen, but
the blood can not flow fast enough to metabolise the excess stored glycogen. The
glycogen that is not metabolized aerobically (by the oxygenated blood) must then
be metabolized anaerobically, which then creates the cell waste products and
heat, and ER has begun. A horse on a high-grain diet with little work collects
more glycogen in its muscles than it can use efficiently when exercise begins,
which is horses on a high-grain diet are more likely to develop ER.
Proper conditioning can help prevent ER, as if promotes the growth of
capillaries in muscles and the number of enzymes used for energy production in
muscle cells. However, improvement in these areas can take several weeks. Thus,
ER is more common in horses that are only worked sporadically or lightly, and in
horses just beginning an exercise regimen.
A common misconception is that ER is caused by the buildup of lactic acid.
Lactate is not a waste product for a cell, but a fuel, used when the cell's
oxygen supply is insufficient. Lactate does not damage a cell, but is rather a
byproduct of the true cause of cell damage: inadequate blood supply and altered
cell function. Lactate naturally builds up in an exercising horse without
harming the muscle cells, and is metabolized within an hour afterward.
The pain is caused by the inadequate blood flow to the muscle tissue, the
inflammation from the resulting cell damage, and the release of cell contents.
Muscle spasms, caused by the lack of blood to the muscle tissue, are also
painful.
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