Halfway through your arena workout, your horse simply isn't acting normal: He's sweating and his muscles are shaking. He doesn't want to move, and discomfort is evident in his posture and facial expressions. Many horse owners and their veterinarians are all too familiar with the signs of tying-up (also known as exertional rhabdomyolysis) in horses. Repeated episodes of this disorder, which affects horses of all shapes and sizes, can be a frustrating problem for owners and riders. Erica McKenzie, BVMS, PhD, Dipl. ACVIM, associate professor of large animal medicine at Oregon State University, gave an overview of tying-up during the 2011 Western Veterinary Conference, held Feb. 20-24 in Las Vegas, Nev.


Horses exhibit a variety of different forms of exertional rhabdomyolysis, and what signs they show often depend on breed type.

McKenzie explained that polysaccharide storage myopathy (commonly referred to as PSSM) is thought to be the "most common condition associated with chronic external rhabdomyolysis in horses." The disorder occurs in 36-50% of Belgian Draft horses, 30% of halter-bred Quarter Horses, and about 8% of Quarter Horse-related breeds (i.e., Paints and Appaloosas). The disorder also affects Warmbloods and other light-breed horses, McKenzie said.

A genetic disorder, PSSM is a debilitating and occasionally life-threatening disease in horses characterized by abnormally high glycogen (the storage form of glucose) concentrations in skeletal muscles.

Thoroughbreds, Standardbreds, and Arabians are also prone to tying-up, however they generally do not suffer from PSSM. Instead, McKenzie said, these lighter breed horses most likely have a disorder involving disturbed muscle calcium regulation, referred to as 'recurrent exertional rhabdomyolysis' (RER).

Recurrent exertional rhabdomyolysis causes affected horses to suffer recurring episodes of muscle cramping, stiffness, excessive sweating, and a reluctance to move after exercise.

McKenzie explained that researchers have found that both PSSM and RER are hereditary, so breeding affected horses is generally discouraged.

Finally, McKenzie added, healthy horses with no underlying genetic disorders can experience episodes of tying-up "if they are exerted beyond their level of conditioning."

Clinical Signs of Tying-Up

McKenzie said typical clinical signs of exertional rhabdomyolysis occur soon after exercise begins or ends. She reviewed these, including:

  • Excessive sweating;
  • Muscle tremors,
  • Muscle stiffness;
  • Reluctance to move;
  • A stretched-out hind-limb stance, as if trying to urinate;
  • Firm muscles that are painful to the touch; and
  • Dark colored or red urine.


McKenzie suggested that managing diets of horses with PSSM and RER carefully might be helpful in reducing the number of exertional rhabdomyolysis episodes a horse suffers.

"Controlled clinical trials have demonstrated that reducing soluble carbohydrate intake in horses with either PSSM or RER can significantly improve clinical signs, despite the different etiologies for these diseases," she said.

She continued to say that the most effective means of controlling the diseases includes feeding a balanced, low-starch diet, adding necessary calories with fat sources (such as oils) and fiber.

In addition to an appropriate diet, owners should keep horses with PSSM and RER on a regular exercise schedule, as breaks in the exercise routine can be a significant triggering factor in horses that are prone to tying up. McKenzie said that there is evidence that consistent exercise is beneficial to these horses, and such activity "appears to provide additional beneficial effects and is probably very important for horses with repeated episodes."

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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