EPSM in Horses – What The Vet Says

EPSM in horses is a puzzling and sometimes frightening disease for horses and their caregivers. Typical signs include a horse that stiffens, sweats, and is reluctant to move. These clinical signs are common in other diseases of the equine neuromuscular system but have different causes.

Stay with us as we will explain this pathology in detail, everything you need to know to identify it, and how to help your horse recover.

Brown horse in pasture

What is EPSM in Horses?

EPSM stands for equine polysaccharide storage myopathy. It is a metabolic disorder that affects glycogen deposition in the striated muscle tissue of horses. Glycogen is not available to muscle cells for use as a source of energy, thus altering the normal functioning of the muscles.

This condition occurs in various horse breeds, but it is most common in American Quarter Horses and their related breeds, Draft horse breeds, and warm-blooded breeds.

PPSM’s Symptoms and Signs

Some horses with equine polysaccharide storage myopathy may never develop symptoms or rarely show clinical signs, while others may have more frequent and severe symptoms.

There are predisposing factors that can trigger EPSM symptoms:

  • Exercise is among the most common triggers, especially intense or prolonged.
  • Diet changes, such as the introduction of starchy feed.
  • Periods of stress or illness.

Signs are often seen in horses when undergoing initial training or after a period of inactivity. PSSMs early symptoms are often associated with tying up. These signs usually include muscle stiffness, sweating, cramps, and reluctance to move.

During an episode, horses appear sluggish, have a shifting lameness, tighten the abdomen, and develop tremors in the flank area. When horses stop moving, they may stretch as if to urinate. They are sore, sweating, and have stiff, rigid muscles, especially in the hindquarters. Some horses attempt to paw and roll immediately after exercise.

Episodes of muscle soreness and stiffness can be quite severe, causing the horse to be unable to stand and uncomfortable even lying down. The urine of these horses is usually brown.

How Vets Diagnose PPSM

For diagnosis, the veterinarian will request:


All the current clinical signs and the horse’s history will be scrutinized.


Large amounts of myoglobin enter the bloodstream due to the rupture of muscle fibers. When filtered in the kidneys, it will pass into the urine, staining it dark red. Myoglobinuria is the appropriate name for myoglobin in the urine.

Blood Tests

If muscle enzymes creatinine kinase (CK), lactate dehydrogenase (LDH), and aspartate transaminase (AST) appear elevated, it may state for muscle damage.

Stress testing is an excellent screening test that measures blood enzyme levels after 15 minutes of trotting. Horses with EPSM show increases in CK greater than 1000 U/L, between four and six hours after exercise.

Muscle Biopsy

Provide a definitive diagnosis. They assess muscle damage and measure the amount of glycogen in the muscle. Biopsies can be:


The preferred sites are the semimembranosus or semitendinosus muscles. These sites provide easy access, a high prevalence of abnormal polysaccharides, and inconspicuous scarring.

Bergstrom Needle

The gluteus medius muscle is the muscle of choice for sampling. This technique is easy and does not leave a scar.

Genetic Testing on Hair or Blood Samples

While, at present, a DNA test diagnoses type 1 PSSM, type 2 PSSM still requires the examination of a muscle biopsy.

There are other causes of tying-up besides PSSM, such as malignant hyperthermia (MH), glycogen branching enzyme deficiency (GBED), and myosin-heavy chain myopathy (MYHM), so it is important to rule these out to ensure that the horse receives appropriate treatment.

How Severe is the Condition?

Early detection of EPSM in horses is critical to treating the disease and maintaining the horse’s health.

So, how serious is PSSM in horses? Abnormal sugars accumulating in muscle tissue can damage muscle fibers, and the horse’s quality of life and athletic performance can become affected. The horse can suffer kidney damage from the high amount of protein released into the bloodstream with muscle destruction. When muscle injuries are extensive, the horse may die.

PPSM 1 vs. PPSM 2 – The Differences and Similarities

There are two types of PSSM: type 1 and type 2.

Type 1

A genetic mutation causes PSSM type 1. In consequence, long, unbranched anomalous glycogen chains develop. This polysaccharide resists amylase, so excessive glycogen storage in skeletal muscle. But this sugar is not available for fibrillar metabolism (the muscle cannot use it as an energy source).

As it is an autosomal dominant trait, the horse becomes affected with only one copy of the mutation. Yet, environmental factors, i.e., diet and exercise, play an important role in the onset of clinical signs. PSSM1 occurs in Quarter Horses, related breeds such as Paints and Appaloosas, and draft breeds.

Type 2

PSSM type 2 does not originate from a genetic mutation. The cause or causes remain unknown. Horses affected by PSSM type 2 have an abnormality in the regulation of glycogen synthesis that leads to excessive muscle glycogen storage, but this glycogen, unlike PSSM1, is well constructed. PSSM2 occurs in Quarter Horses.

How Do Horses Get EPSM?

In normal tissues, insulin drives blood glucose into muscle and liver cells for storage as glycogen and later use as energy.

PSSM is a glycogen storage disorder characterized by the abnormal accumulation of the usual form of glycogen in muscle (PSSM type 2) or an abnormal form of sugar (polysaccharide) storage in muscle tissue (PSSM type 1). Muscle glycogen concentrations in affected horses are up to four times higher than in normal horses.

As a result, EPSM horses can become energy-deficient despite adequate rations, and their muscle cells break down, leading to stiffness, cramps, muscle wasting, and even death.

What are the Causes of EPSM in Horses?

The origin is multifactorial. Some of the causes are:

  • Genetic predisposition.
  • Diets high in soluble carbohydrates.
  • Failure to change the diet during rest periods.
  • Rest for several days in a row.
  • Hormonal alterations
  • Stress
Freiberger horse

The Available Treatment Options

The secret to treating both forms of polysaccharide storage myopathy lies in diet and exercise.


Current recommendations include a diet low in starch (cereals), high in fat and fiber, and high-quality proteins.

Low starch diets produce low blood glucose and insulin levels after eating, which can reduce the amount of glucose absorbed by muscle cells. High-fat diets increase blood concentrations of free fatty acids, which may promote the use of fat for energy over glucose metabolism. Reducing rations when the horse is at rest or convalescing is also very important.

To protect muscle cells against oxidative stress, administer dietary supplements based on vitamin E and selenium.


A horse with acute EPSM requires rest before starting an exercise plan to allow the horse’s muscles to heal and reduce the risk of further damage.

Horses prone to PSSM must get out of the stall as much as possible and exercise every day, even if only for 10 minutes.

Managing Horses With PPSM

During the acute crisis of PPSM, the recommendations are:

  • Do not move the horse at all. Not only because of the intense pain we would produce but also because we aggravate the condition.
  • Call the veterinarian. Do not force the horse to move or walk. If possible, the veterinarian should treat the animal at the site of the attack.
  • Transport the horse in a trailer to a place to rest. Ensure the ramp is not too steep, and the horse travels strapped, as it could fall.
  • Store the horse in a dry, clean, draft-free stall. You must be able to see if the horse has urinated.
  • Provide a small amount of hay.

After the acute episode:

  • If the horse has already started to move around in its stall for several hours, there are no signs of pain, and its urine is normal, you can release the horse in a small enclosure on short grass.
  • Place feed and water in places that force the horse to move.
  • If the horse is nervous, it may need medication to relax. In this case, you should consult your veterinarian.
  • In non-acute cases, you can start riding the horse after two or three days of rest in the paddock. In other, more acute cases, you should wait until the muscle CK levels have returned to normal on analysis.
  • Avoid roping, using the walker, or any other work that forces the horse to work in small circles. Also, avoid hill work.
  • Work should increase in strength and duration little by little. If the horse is strong, work him always to remain calm.
  • It should work or at least be loose in an enclosure daily but not have access to a lot of grass.

Preventing EPSM in Horses

Use genetic testing to avoid breeding horses affected by PSSM1. Since PSSM1 is an autosomal dominant trait, there is at least a 50% chance of passing on the causative mutation to each offspring.

Although the cause of PSSM2 is unknown, the suggestion is not to breed horses diagnosed as affected by muscle biopsy, as it could have a genetic basis.

The Outcome

With proper treatment, many horses with EPSM can lead normal lives and maintain good health and performance. Yet, some horses may continue to experience symptoms despite efforts.

Prevention of EPSM is vital in the management of this disease. Good feed and exercise management can prevent a resurgence of PSSM.

Is There a Cure?

EPSM is a chronic disease controllable through changes in diet and exercise but not completely curable.

Why is PPSM More Prevalent Today?

The modern equine industry demands increasing athletic performance from horses. So, the animal has higher energy requirements. The solution to meet this increased energy demand was to increase the use of carbohydrate-rich feed and decrease the roughage in the diet, contributing to the development of EPSM. No less important is the tendency to put horses to rest in a stall when there is any doubt of injury or illness.

It is also important to note that EPSM may have existed in horses but remained underdiagnosed. With better knowledge of the disease and accessible diagnostic methods, EPSM is now easier to identify and treat.

Brown and white horse

Winding Up

EPSM is a widespread disease among equines. Knowing the causes that produce it, as well as the therapeutic measures, is of great help in managing this pathology. Likewise, having access to better diagnostic tools makes it easier to identify it and take the necessary steps in each case.