Tye-Up Muscle Myopathy

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Dr Peter Gillespie. BVSc MACVS.

Tye-up or Exertional Myopathies are the terms given to a group of muscular conditions that commonly affect performance horses..

The common assumption that all cases of tye-up are feed related has led to confusion with regard to treatment. There are in fact a number of muscular conditions that have a similar clinical appearance but quite different causes.

Tye-up can be acute or chronic. Acute cases occur sporadically and are generally related to dietary management. Other names for the condition include Azoturia or Monday Morning Disease, the later so called because of its common occurrence in working horses after having Sunday off on full rations. Symptoms of acute tye-up range from hindleg stiffness and a shuffling gait through to profuse sweating and recumbency- signs that can be easily mistaken for colic. Horses fed high energy diets are usually affected, and like the working horses of last century, the common history is a preceding period of little or no exercise without a reduction in dietary energy intake.

Chronic Tye-Up occurs more commonly and is more difficult to manage. Poor performance is often the only outward sign; more obvious cases will show hindleg stiffness with a shuffling gait.

A form of chronic tye-up is a disorder of muscle contraction known as Recurrent Exertional Rhabdomyolysis (RER). It occurs in all types of performance horses but is more common in fillies and mares, especially those with a nervous temperament. It was originally thought that RER was caused by a build up of lactic acid within the muscles, however research has shown this is not the case. Several causes have been identified, they include;

  • low dietary sodium or potassium.
  • low dietary calcium: phosphorus ratio.
  • hypothyroidism.
  • abnormal intracellular calcium regulation.

A condition known as Polysaccharide Storage Myopathy (PSSM) has been identified where dietary carbohydrates are absorbed more efficiently than normal. They have a history of tying-up even without excessive grain intakes.

Localised Muscle Acidosis is a type of exertional myopathy that occurs when the intensity of work exceeds the level of fitness. It is caused by a build up of L-lactic acid when energy is produced by anaerobic instead of aerobic metabolism. A horse that ‘blows up’ over the back after fast work has an acidosis of the muscles in the back.

Post Exercise Muscle Fatigue is an exertional myopathy resulting from the depletion of water, electrolyte or energy stores during exercise. It is generally only seen in endurance horses.

Stress Tetany a condition caused by the rapid depletion of calcium from skeletal muscles during long float trips. It requires immediate veterinary intervention.

The successful treatment and prevention of Exertional myopathies depends on correctly identifying the cause. In some cases immediate veterinary intervention involving intravenous fluids, pain killers and anti-inflammatory drugs is necessary. In the majority of cases however, treatment comes down to prevention, which in turn comes down to dietary management.

Preventative measures include;

1) The diet should contain the correct amount of energy for the level of work achieved. Fast release energy feeds which are rapidly digested and absorbed in the small intestine should be avoided where possible and replaced with slow release energy feeds which are digested in the hindgut. Hay, bran, copra meal and to some extent, corn, all contain slow release energy. Corn oil can be used as an alternative energy source.

2) The diet should also include a balanced mineral and electrolyte supplement to ensure the necessary sodium, potassium, calcium and magnesium are provided in the diet.

3) Vitamin E and selenium are known to play an important role preventing tye-up due to their anti-oxidant properties. Equine Vitamin E and Equine Selenium K should be supplemented in the diet.

4) With nervous fillies and mares predisposed to RER, the use of low doses od the sedative acetyl-promazine during training is warranted. Besides having a light sedating effect, it also increases peripheral blood flow. A calming formulation such as Equine Relax or a digestive aid like Digest-Rite may also benefit these horses.

Susceptible fillies and mares are often worse when they are in season, hence treatment to either prevent or delay oestrus maybe indicated.

It is essential that the role over-feeding and under-training plays in the onset of the condition, is recognised. Appreciation of this fact will go along way to avoiding the problem.

Selenium K- A safe way to supplement Selenium
Vitamin E – Antioxidant for horses on low pasture, or breeding stock or nervous animals.
Performance Minerals- Provides a full balanced daily requirement.
Electrolytes- Correctly formulated to replace vital salts and minerals lost through sweat.

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