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Understanding and Managing EPSM in Horses

Last updated: June 30, 2023

By: Miles HenryFact Checked

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I have seen my fair share of horse related health issues over the years. So when a new horse owner explained that their horse had been experiencing muscle stiffness, cramping, and reluctance to move, especially after exercise. I became concerned their horse was suffering from EPSM.

EPSM is a muscle disease that happens when there is too much of a specific type of sugar stored in the horse’s muscle tissue. This can damage the animal’s muscles and make it hard for the horse to move normally.

However, all is not lost when you have a horse with EPSM. This blog post explains EPSM and offers practical advice on managing horses suffering from this condition.

What is EPSM?

EPSM stands for Equine Polysaccharide Storage Myopathy. It is caused by a buildup of abnormal sugars, called polysaccharides, in the muscle tissue. These abnormal sugars can cause damage to the muscle fibers and disrupt normal muscle function.

A horse gets EPSM due to how the horse’s body processes and stores carbohydrates. In a healthy horse, carbohydrates are broken down into glucose, which is then used for energy.

In horses with EPSM, the process is disrupted, causing a buildup of abnormal sugars in the muscle tissue. This condition is common in quarter horses but can occur in other performance and large draft breeds.

Picture of a two year old thoroughbred horse.
Two-Year-old Thoroughbred

What Causes EPSM?

The two primary causes of EPSM are genetic predisposition and dietary factors. A genetic predisposition means that certain horse breeds are more likely to develop a disease based on their genetic makeup.

This results from the specific genetic structure inherited from a parent. However, just because a horse has a genetic predisposition for Equine Polysaccharide Storage Myopathy doesn’t mean it will get the disease. For example, Percheron, Belgian, Quarter Horse, and Paints have a higher incidence of the condition, but not all suffer from it.

High-starch diets and limited turnout are common contributing factors to the development of EPSM. When horses are fed diets high in grains, such as corn and barley, they can consume more starch than they can effectively digest.

This can lead to an excess of glucose in the bloodstream, which can then be stored in the muscles as abnormal sugars. It’s essential for horse owners of these breeds to be aware of the risk and manage their horse’s diet and exercise accordingly, to minimize the chances of developing EPSM and to prevent the condition from progressing if it is already present.

What Are the Signs of Epsm in Horses?

A horse with EPSM may not always show signs of the condition, as the severity and frequency of symptoms can vary depending on the individual horse and the stage of the disease.

Some horses with Equine Polysaccharide Storage Myopathy may display symptoms only occasionally, while others may have more frequent and severe symptoms. Additionally, certain activities can trigger or exacerbate the symptoms of EPSM.

Exercise is one of the most common triggers of EPSM symptoms, especially intense or prolonged exercise. This is because exercise increases the muscle demand for energy, which can cause the abnormal sugars to build up more quickly.

The horse will display the symptoms of muscle stiffness, weakness, and cramping after exercise, known as tying-up. Other triggers of Equine Polysaccharide Storage Myopathy signs include changes in diet, such as the introduction of high-starch feeds and periods of stress or illness.

The horse’s overall health and management practices can also affect the severity and frequency of EPSM symptoms. It’s important to work with a veterinarian to establish a management plan for a horse with EPSM, which would include monitoring the horse’s diet, exercise, and overall health.

This will help to identify any triggers of Equine Polysaccharide Storage Myopathy symptoms and to adjust the management plan as needed to minimize the symptoms and prevent further muscle damage.

A horse with EPSM can exhibit various muscle weakness and dysfunction symptoms. These may include:

  • Muscle stiffness: The horse may have difficulty moving and may appear stiff or sore, especially after periods of inactivity.
  • Muscle cramping: The horse may exhibit signs of muscle cramping or spasms, such as twitching or tightness in the muscles.
  • Muscle weakness: The horse may have difficulty supporting its own weight and may appear weak or uncoordinated.
  • Reluctance to move: The horse may be reluctant to move or may move slowly due to discomfort or pain.
  • Exercise intolerance: The horse may become easily fatigued during exercise and may have difficulty maintaining normal activity levels.
  • Recurrent tying-up or Azoturia: The horse may experience recurrent episodes of muscle stiffness and cramping, especially after exercise, known as tying-up.
  • Collapse: In severe cases, the horse may collapse due to muscle weakness.

It’s important to note that the severity of symptoms can vary depending on the individual horse and the stage of the condition. Additionally, these symptoms may be similar to other muscle disorders, so it’s important to consult with a veterinarian to determine the underlying cause.

Picture of a horse suspected to have EPSM.
Older Thoroughbred Mare.

Importance of Early Detection of Epsm in Horses.

Early detection of EPSM is critical in managing the condition and maintaining the horse’s overall health. The abnormal sugars that build up in the muscle tissue can irreversibly damage muscle fibers, and the horse’s quality of life and athletic performance can be greatly affected.

By detecting EPSM early, the horse can receive treatment to reduce the amount of abnormal sugars in the muscle tissue and prevent further muscle damage. This can improve the horse’s mobility, reduce pain and discomfort, and maintain overall health.

Additionally, early diagnosis can also prevent the condition from worsening, leading to more serious complications and a decrease in the horse’s overall performance.

If you discover that your horse has Equine Polysaccharide Storage Myopathy, the first step in treating the condition is to work with a veterinarian to develop a management plan, which will likely include changes to the horse’s diet and exercise.

Below is a helpful YouTube video about EPSM.

YouTube video

Diagnosing EPSM

EPSM diagnoses are made through clinical signs, blood work, and muscle biopsy. A veterinarian will look at the horse’s symptoms and medical history and may also perform blood work to check for elevated muscle enzymes, which can be indicative of muscle damage.

A muscle biopsy is often necessary for a definitive diagnosis. Early detection and treatment of Equine Polysaccharide Storage Myopathy are important to prevent further muscle damage and deterioration of the horse’s overall health.

Genetic testing is not widely accepted as a diagnostic tool for EPSM. In some cases, genetic testing can detect specific genetic mutations associated with EPSM, but it is not considered a definitive diagnostic test. In fact, a horse can have EPSM and not have the genetic mutation associated with the disease.

Picture of a horse eating hay from a net to slow consumption for horses with EPSM.
Using hay nets

How Do I Keep My Horse From Having Epsm Episodes?

The best way to help horses with EPSM is to give them a diet that is high in fat and fiber and low in sugar and starch. Also, giving them as much exercise as possible has worked well to help control this sickness.

Here are the three keys to focus on to help your horse recover from Equine Polysaccharide Storage Myopathy and return to normal activity: turnout, exercise, and diet.

Note: If your horse is currently experiencing symptoms of EPSM, such as stiffness, weakness, or cramping (also known as “tying up”), it is important to give them rest before starting an exercise plan to give the horse’s muscles time to heal and reduce the risk of further damage.

Turn out

Turning a horse out to pasture is good for horses with EPSM because they can move around and exercise. But it’s important to think about the kind of grass they are eating. If the grass has a high sugar content, it can make the EPSM worse.

To avoid this, you can move your horse to a different field, keep it in a dry lot, or use a grazing muzzle to limit how much grass the horse eats.


Feeding horses with EPSM is important to help them feel better. The goal of the diet is to give the horse less sugar and more fat. This means fewer grains, sweet feed, and molasses and more hay like alfalfa hay or grass-alfalfa hay mix instead.

Feeding hay with a low amount of sugar (NSC) at 1-1.5% of the horse’s weight, giving a special feed with low calories, and starting to exercise the horse little by little. The key to treatment is to reduce the amount of starch and sugar in the diet and provide the horse with the necessary nutrients to support muscle health.

Prevention of EPSM is key in managing the condition. It’s essential to work with a veterinarian and nutritionist to develop a diet plan that is appropriate for the individual horse.

This may involve reducing grain and increasing the amount of hay and other forage. Additionally, regular exercise and monitoring the horse’s condition can help to prevent the onset of EPSM.


Exercise therapy is important to help horses with EPSM feel better. It’s vital to let the horse spend as much time outside as possible and not keep them in a stall for long periods.

When you begin your horse’s exercise program, you should walk it daily for fifteen minutes, and as the horse gets better, the exercise can be increased. When the horse can exercise for 30 minutes without any problems, you can start riding it again.

Horses with EPSM will feel better if they change their diet and exercise. Usually, after 3-6 months of this treatment, the horse’s muscles will be better. But, some horses, especially large draft breeds, may never fully recover.

Picture of a young thoroughbred horse in training.
Exercising a horse

Can My Horse Be Cured of Epsm?

EPSM is a chronic condition that can be managed through dietary and exercise changes but cannot be completely cured. With proper management, many horses with EPSM can lead normal lives and maintain good health and performance.

However, some horses may continue to experience symptoms despite management efforts. Dietary changes, such as reducing the amount of starch and sugar and providing specific supplements to support muscle health, can help to control symptoms and prevent further muscle damage.

Exercise therapy, such as daily turnout and a gradual increase in workload, can also be beneficial. It’s essential to work closely with your veterinarian to develop a management plan tailored to your horse’s needs.

Why Is Epsm More Common Today?

Historically, horses were primarily kept on forage-based diets and had ample exercise. This type of management was less likely to lead to the development of EPSM.

The modern horse industry has shifted towards high-performance and endurance activities, which require more energy from the horse. This has led to an increase in the use of high-sugar feeds and a decrease in the amount of forage in the diet, which can contribute to the development of EPSM.

It’s also important to note that EPSM may have been present in horses in the past, but it might have been misdiagnosed or not recognized as a distinct condition. With a better understanding of the condition and improved diagnostic tools, EPSM is now more easily identified and treated.


EPSM is a sickness that affects horses, and it happens when there is too much of a certain kind of sugar in the horse’s muscles. This can hurt the muscles and make it hard for the horse to move around.

Equine Polysaccharide Storage Myopathy is often connected to eating food with a lot of carbohydrates. It can be managed by changing the diet and exercise of the horse, but it can not be completely cured.

Working closely with a vet to make a plan that suits your horse and its specific needs is important. With proper management, horses with EPSM can lead normal lives.


Is EPSM and PSSM the same?

EPSM and PSSM share similar symptoms in that they relate to muscle problems caused by sugar, but they also have some differences. It’s important to have a correct diagnosis to be able to manage your horse’s condition properly.

What is the difference between HYPP and EPSM?

HYPP is a genetic disorder and affects how electrolytes are regulated in the horse’s body. EPSM, on the other hand, is caused by an abnormal buildup of sugars in the muscle tissue. Both conditions can cause horses to tie up.