Tying-up in horses is a colloquial term for Exertional Rhabdomyolysis (ER), a condition involving exercise-related muscle cramping and damage.

Some horses experience a single episode of tying up whereas others experience recurrent tying-up. During an episode, the affected horse becomes stiff and reluctant to move. Your horse may only take short, shuffled steps.

In severe cases, a horse displays signs of distress including pawing at the ground, excessive sweating, and quick, shallow breathing due to the pain associated with this condition.

Tying-up episodes should be taken seriously. If your horse is displaying signs, a veterinarian should be contacted immediately. The veterinarian can help identify whether this is a sporadic case or whether your horse is genetically susceptible to recurrent tying-up episodes.

Sporadic cases typically arise due to dietary imbalances, excessive electrolyte loss, or mismanaged exercise routines. Correcting these issues will help to prevent future episodes.

Nutrition and exercise management are important for horses that are genetically predisposed to tying-up. For example, horses with polysaccharide storage myopathy (PSSM) have issues with sugar storage in their muscles. They should be maintained on a low sugar and starch diet and given regular exercise to decrease the risk of tying up.

Horses with poor exercise performance may be experiencing subclinical exertional rhabdomyolysis without overt symptoms.

In this article, we’ll explore the different types of exertional rhabdomyolysis, as well as strategies to prevent and manage this condition.

Types of Tying-Up

There are two main types of tying-up that are seen in horses: sporadic cases and recurrent cases. [1]

1) Acute Exertional Rhabdomyolysis

Acute or sporadic cases are not related to an underlying genetic condition. Instead, these episodes can be linked to external factors, such as management, nutrition, or exercise. It most commonly occurs when a horse is exercised beyond its level of conditioning.

Dietary changes and adjustments to your horse’s exercise routine can help decrease risk of tying-up. In particular, the three following interventions can make a significant impact:

  • Ensuring adequate electrolyte and water intake
  • Ensure your horse is meeting vitamin E requirements
  • Ensure your horse is meeting selenium requirements

Avoiding long rest periods before strenuous exercise can also decrease the risk.

2) Chronic Exertional Rhabdomyolysis

Recurrent cases are often due to an underlying genetic abnormality that affects how the muscle cells function. The most common types are:

  • Recurrent exertional rhabdomyolysis (RER)
  • Polysaccharide storage myopathy (PSSM1 and PSSM2)

Fortunately, these horses also benefit from management practices to decrease the frequency of tying-up episodes. Changes to the diet and exercise program may be recommended by your nutritionist or veterinarian.

If your horse has experienced a tying-up episode, is genetically predisposed to the condition, or you if you suspect subclinical tying-up, submit your horse’s diet for a complementary diet analysis to identify potential risk factors and prevention strategies.

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Prevalence and Risk Factors

Acute Exertional Rhabdomyolysis

Sporadic cases of ER can occur in any horse of any breed, age, or discipline. It is most often seen in horses that are exercising beyond their level of conditioning. Other risk factors that can lead to sporadic ER include:

  • Overexertion
  • Heat exhaustion
  • Hot, humid weather
  • Electrolyte imbalances
  • Vitamin and mineral imbalances

Chronic Exertional Rhabdomyolysis

Recurrent exertional rhabdomyolysis (RER) affects 5-7% of racing Thoroughbreds and Standardbreds. In a typical racing season, up to 17% of horses might not be able to compete again due to RER. [2][3]

Horses are thought to be predisposed to tying-up based on several risk factors: [4]

  • Genetics: Recurrent ER in Thoroughbreds and Standardbreds may have been inadvertently selected for by breeding horses for faster race times [2]
  • Sex: Female horses are more likely to be affected than males
  • Age: Young horses are most susceptible to severe episodes
  • Temperament: Horses with a nervous, excitable temperament are more likely to be affected than calm horses
  • Lameness: Horses with any form of lameness are more susceptible to tying-up