Page 471 - The Veterinary Care of the Horse

The diagnosis is made on the history, the clinical signs and the results of blood tests. When

the muscle cells are damaged, creatine kinase (CK) and aspartate aminotransferase (AST)
VetBooks.ir leak from the cells into the bloodstream. The timing of the blood sampling is important as the
CK reaches its maximum level approximately 4–6 hours after the episode and usually returns

to normal within 2 days. AST peaks at 24 hours and takes about 2 weeks to return to normal.
The enzyme levels do not always correlate with the clinical signs. Horses with severe

symptoms, for example, can show a relatively small increase in enzymes.
In horses with the chronic form of the disease where the symptoms are not so acute, the

condition may be diagnosed with the aid of:

• a muscle biopsy which can show characteristic degenerative changes in the muscle fibres
• an exercise test: blood samples are taken before exercise and between 4 and 6 hours

afterwards

• scintigraphy, as some horses will show increased uptake of the radiopharmaceutical by
affected muscle (this is rarely necessary).

Causes

Exertional rhabdomyolysis is a complex syndrome and why it occurs is not fully understood.
Some or any of the following factors may play a role.

• Inappropriate exercise for the horse’s level of fitness. Pushing the horse too fast or for too
long can trigger an attack.

• Feeding full rations high in starch (cereals) with only irregular exercise increases the

likelihood of ERS. It commonly follows a day off work with no reduction in rations.
• Electrolyte imbalance is suspected as a cause. Sodium, potassium, magnesium and

calcium are important for the normal function of nerves and muscles. A combination of
regular hard work causing sweating and an inadequate diet may cause depletion of any of

these and lead to poor performance and possibly ERS.
• The composition of the diet can affect the horse’s ability to regulate the electrolytes in the

+
+
2+
body. Electrolytes such as sodium (Na ), potassium (K ), magnesium (Mg ) and
2+
calcium (Ca ) have a positive charge and are known as cations. Electrolytes such as
3-
-
chloride (Cl ) and phosphate (PO ) have a negative charge and are called anions. The
4
balance of the cations and anions in the diet, known as the dietary cation/anion balance
(DCAB) has an effect on the horse’s metabolism. A diet consisting mainly of forage has a
high DCAB and this enables the horse to absorb and use the electrolytes efficiently. Diets
containing a lot of cereals and restricted forage tend to be acidic and have a low DCAB.
Electrolytes such as calcium may be lost from the body in processing the acid diet even