What Is PSSM in Horses? Understanding EPSM, PSSM1 & PSSM2, MIM

Equine Polysaccharide Storage Myopathy (PSSM or EPSM) is a muscle disease that affects how horses store, create and use energy — particularly glycogen, the storage form of sugar in muscle cells. It’s a leading cause of tying-up episodes, muscle pain, cramps and deterioration, exercise intolerance, poor performance and a feeling that the horse simply “doesn’t feel right” across a wide range of horse breeds.

• Up to 36% of some Draft breeds and 10% of Quarter Horses are affected. (McCue, et al., 2010)
• Symptoms often look like training problems: laziness, poor topline, “bad attitude,” or deficient back leg movement.
• Without the right management, it can lead to severe tying-up, muscle atrophy, and even not being able to stand up at all out of a sudden.

Many horses with PSSM go undiagnosed or misdiagnosed for years, especially when symptoms are subtle or episodic. Learning to recognize and manage this condition can dramatically improve your horse’s comfort, rideability, and long-term health.

EPSM vs PSSM1 vs PSSM2: What’s the Difference?

The condition we now call PSSM was first described as Equine Polysaccharide Storage Myopathy (EPSM) by veterinary pathologist Dr. Beth Valentine in the early 2000s. Valentine observed muscle abnormalities on biopsy in Draft horses and others with chronic muscle stiffness, weakness, or tying-up.

Since then, researchers have identified two main forms of PSSM:

• PSSM1: Caused by a known genetic mutation in the GYS1 gene (glycogen synthase 1), leading to overproduction and abnormal storage of glycogen in muscle.
• PSSM2: Clinically similar to PSSM1 but without the GYS1 mutation. The genetic cause remains unknown, but it is believed that it isn’t one condition, it’s more like a generalizing term for non-genetic muscle issues, and current commercial genetic tests for PSSM2 are not scientifically validated.

Understanding PSSM1 and PSSM2 in Horses

PSSM1

Type 1 PSSM is caused by a mutation in the GYS1 gene — the enzyme responsible for synthesizing glycogen in muscle cells. This mutation causes the enzyme to be more active than normal in the affected horses, and store abnormally large amounts of glycogen. An abnormal polysaccharide formed of ubiquitin and polyglucosan was also discovered.

What We Know About PSSM1

• It’s an autosomal dominant mutation, so even one copy (n/P1) can cause signs.
• Glycogen levels in muscle can be 1.5–2x higher than normal.
• First documented by Dr. Stephanie Valberg in the early 1990s in Quarter Horses.
• Confirmed via a reliable genetic test (hair or blood) or a muscle biopsy.

DNA Test for PSSM1

• Uses a mane hair root or blood sample
• Detects whether the horse is:
  • n/n – clear (no mutation)
  • n/P1 – heterozygous (one copy)
  • P1/P1 – homozygous (two copies)
• Performed by reliable labs like:
  • UC Davis
  • Animal Genetics
• Highly accurate and not affected by age or symptoms

Breeds Commonly Affected by PSSM1

• Draft horses (Continental breeds like Belgian Trekpaards up to 90% affected.) (United Kingdom derived breeds have a lower prevalence like Shires and Clydesdales)
• Quarter Horses (halter lines are more commonly affected up to 28%), general QH: 6-10% affected.
• Paints & Appaloosas: 6–8%.
• Racing QHs: lowest prevalence.
• Arabians & Thoroughbreds: very rare.

Valberg also notes that some Quarter Horses with PSSM1 carry a second mutation for malignant hyperthermia (RYR1), which greatly worsens symptoms and can lead to sudden death during tying-up episodes.

PSSM2/MIM

Type 2 PSSM is diagnosed when horses show clinical signs of muscle dysfunction and abnormal muscle biopsy results, but do not carry the GYS1 mutation. The exact genetic cause is still unknown — and unlike PSSM1, there is no validated DNA test for PSSM2. It could be that this is a combination of a series of genes that create this disease.

In Germany, PSSM2 has been renamed MIM (Muscle Integrity Myopathy)

Muscle Biopsy

What it detects:

• Excess glycogen accumulation
• Amylase-resistant polysaccharide inclusions
• Muscle fiber abnormalities (atrophy, fiber size variation)
• Signs of Myofibrillar Myopathy (MFM) via desmin staining

Biopsy site:

• Semimembranosus or semitendinosus (hindquarters)
• Procedure: Performed under sedation with local anesthesia
• Samples must be sent to a lab experienced in equine muscle pathology

Horses under ~2 years of age may have false negatives — visible changes in muscle often increase with age.

Desmin staining can help differentiate PSSM2 from MFM, especially in Warmbloods with back pain and gait issues.

Breeds Commonly Affected by PSSM2

• Warmbloods
• Quarter Horses (especially reining, barrel, and cutting lines): ~28%
• Arabians, Morgans, Standardbreds, Icelandics, and Thoroughbreds

Valberg notes: Many Warmbloods with biopsy-confirmed PSSM2 do not experience full tying-up episodes, but do show energy decline and postural changes under saddle — especially in the hind end and lumbar region.

Sometimes the horse (typically warmbloods) can actually have myofibrillar myopathy, which is the structural damage to the muscle fibers allowing glycogen to get stored in between them and therefore thinking that it is PSSM2. This can be differentiated by the accumulation of desmin and Z disc and myofibrillar degeneration in the muscle biopsy.

Clinical Signs of PSSM

If your horse seems “lazy,” refuses to move forward, or has mysterious performance issues that training won’t fix — don’t assume it’s a behavior problem. These may be early warning signs of PSSM, especially in certain breeds.

PSSM1 and PSSM2 share many clinical features, though they often show up in different ways depending on the horse’s breed, workload, and management.

• Low energy, especially after 5–10 minutes of work
• Sore back and hindquarters, reluctance to collect or engage
• Poor rounding over fences (Warmbloods)
• Gait abnormalities or shifting lameness
• Poor back and hindquarter muscles and weakness
• Kidney damage and muscle necrosis
• Recumbency (unable to rise) in severe episodes
• Dark or reddish urine (myoglobinuria)
• Tying-up

These symptoms can be barely noticeable to very severe. In the most severe cases, the horse will be unable to stand up. And horse owners may think of colic or laminitis because of the stance and pain symptoms.

The important part here is to try to catch it as soon as possible when the symptoms don’t evolve into severe problems, or as we will discuss in the following sections, in frequently affected breeds or small even the smallest suspicions, it is better to change the diet accordingly.

Far too many horses with PSSM are labeled as:

• “Cold-backed”
• “Stubborn”
• “Out of shape”
• “Difficult to collect”
• “Resistant to transitions”

In reality, they may be dealing with chronic muscle pain and metabolic dysfunction.

🔬 Diagnostic Overview

Before the discovery of the GYS1 mutation, early cases of this disorder were simply labeled “Polysaccharide Storage Myopathy”, and often referred to as “EPSM” — especially in Draft horses. This term is now outdated and no longer used in peer-reviewed research.

When you see the term EPSM, understand it refers to what we now define as either PSSM1 or PSSM2, depending on the genetic and biopsy findings.

How to Feed a Horse with PSSM

Unfortunately, there is no cure for this disease, so when it comes to managing PSSM, nutrition and exercise are the most powerful tools in your arsenal. The right feeding program can prevent tying-up, improve energy levels, and dramatically improve your horse’s quality of life.

Both PSSM1 and PSSM2 horses benefit from a low non-structural carbohydrate (NSC) diet, combined with targeted fat supplementation, balanced minerals, and consistent exercise.

Step 1: Eliminate High-Starch Feeds

High-starch and high-sugar feeds trigger insulin release, which drives more glucose into the muscle and leads to excessive glycogen storage — the core problem in PSSM horses.

❌ Avoid:

• Sweet feeds
• Oats, corn, barley
• Grain-based pelleted feeds
• Molasses-soaked hay or cubes
• High-NSC hay (>12% ESC + starch)

Valberg demonstrated that diets with <10% digestible energy from starch and 13% of daily DE as dietary fat reduced tying-up episodes in Quarter Horses with PSSM1.

Step 2: Feed Low-NSC Forage First

Forage is still the foundation and preferably free choice (meaning 24/7) — but it needs to be the right kind.

Look for:

• Grass hay with <10% NSC (ESC + starch) ideally <6%
• Soaked hay (30–60 minutes) to reduce sugar in case of higher hydrolyzable carbohydrates, but preferably switch to a different hay
• Use of slow feeders to limit intake without giving less food
• Try to avoid lush pastures with high sugar grasses, think of using grazing muzzles to limit the intake or a track system to promote movement as well which is very helpful for this disease.

Want to be precise? Send your hay for lab testing and use the NSC and ESC + starch values to calculate safe intake.

Never restrict forage below 1.5% of body weight per day — doing so risks ulcers, behavioral issues, and hindgut dysfunction.

Step 3: Add for Calories and Energy – Fat or More Hay?

PSSM horses need calories — but not from sugar. Now here is where it gets interesting. I have been reading a lot and listening to several podcasts with different perspectives and conclusions regarding how we feed our horses. Specially talking about the addition of fats as oils and how this may not be beneficial for horses in contrary of what the feed industry tells us. So I will include both perspectives in this section:

Perspective 1: Fat should be fed for energy

Fat provides clean, insulin-neutral energy, and helps reduce the muscle’s dependence on glucose metabolism. Fat can constitute up to 20% of their caloric needs and if you can choose a source rich in Omega 3 and Omega 6 fatty acids, that would be a great choice. High calorie soluble fibre is recommended as well.

Even horses at a healthy weight may benefit from a smaller amount of added fat (e.g., 1 cup/day) to support muscle metabolism and reduce symptoms. If the horse has a body condition score ≤ 4, increasing calories is strongly recommended.

Best Fat Sources

• Flaxseed oil or ground flax – omega-3s, anti-inflammatory
• Fish oil – high in Omega 3 fatty acids and anti-inflammatory as well
• Rice bran – ~20% fat, but also high in phosphorus (requires Ca balance)
• Stabilized dry fat (prilled fats) – concentrated energy for hard keepers

Horses may take 3–4 months to fully adapt to using fat for fuel.

But… Be Careful With High-Fat Diets

Despite its benefits, too much fat can backfire — especially in insulin-sensitive horses.

Several peer-reviewed studies and case reports have linked excess dietary fat to insulin resistance:

• IV lipid infusions in mares caused insulin resistance lasting a full week
• Mares fed 240 mL of corn or rice bran oil daily had reduced insulin sensitivity
• Shetland ponies fed a diet with 23.8% of calories from fat had a 25-fold increase in insulin levels
• Some owners report cresty necks or laminitic flare-ups in horses on long-term, high-fat diets

Instead of relying solely on fat, consider adding high-calorie, high-soluble-fiber feeds that support hindgut fermentation and don’t spike insulin. These include:

• Beet pulp (unmolassed)
• Linseed meal or pellets
• Alfalfa / Lucerne or now best recommended: Sainfoin

Perspective 2: More Hay is Better than Feeding Fat

While many veterinary sources still recommend high-fat diets for PSSM horses — especially to provide an alternative energy source to starch — not all experts agree.

In a 2021 advanced equine nutrition workshop, Dr. Christina Fritz, a German equine scientist and gut microbiome specialist, strongly cautioned against feeding liquid oils for horses with PSSM or any horses in general.

Here’s a summary of her position, based on translated transcripts and Q&A from the event:

“Yes, it’s quite a weird thinking to give oil to a PSSM horse – the PSSM horse is not energy deficient, just the contrary – they derive a lot of energy from fibre as well as glucose and starch. So you only get PSSM symptoms when there’s too much sugar/starch and not enough work cos the muscle cells are highly sensitive from taking up sugar from the bloodstream, and they’re taking up too much sugar from the bloodstream which is why it accumulates in the muscle cell which then gets into trouble and dies, which we see in tying up or colic symptoms in horses.

I don’t know where it came from to feed high protein/fat to PSSM horses – probably cos over the last 30yrs or so ago the digestion research ended at the small intestine but never looked at the large intestine (hindgut), so when you can’t feed sugar/starch to humans as an energy source you have to switch to feeding protein/fat to bring energy into the body cos we can’t digest fibre to get energy from that, but for horses it’s different.

The most important part of digestion is the hindgut and even PSSM horses are great digesters of cellulose, from which they’ll get propionate, butyrate, acetate (the three energy-forming volatile fatty acids produced by the hindgut microbiome) from their hindgut microbiome which can be used for their muscle work. The thinking used to be that horses have to make glucose from the propionate to use it but latest research shows that they’re able to change propionate to glucose but they can also use propionate directly in the cells as fuel so there’s no need to use high fat/protein diets to horses.

I always recommend low-sugar hay, a good mineral supplement, and daily exercise. When you feed high fat/protein rations you’ll get kidney problems cos excess protein has to be degraded and excreted by the kidneys as urea, so the more protein you feed the more urea you produce. Horses don’t use protein for energy but as building blocks for their body, so with 10yrs of high fat diet you’ll send the kidneys into kidney failure. It should always be good quality hay only.””
— Dr. Christina Fritz
Source: Equinatural

Key Points from Dr. Fritz:

• Fat is a last-resort fuel for horses. They get more efficient, immediate energy from the fermentation of fiber into VFAs (acetate, butyrate, propionate).
• Excess fat and protein are not needed for muscle work and may stress the kidneys or gut microbiome long-term.
• High-fat diets may make sense in humans (who can’t ferment fiber), but horses are hindgut fermenters, and the latest research shows they can even use propionate directly for energy — no conversion to glucose required.
• Her PSSM clients — including competition horses — thrive on:
  • 24/7 low-sugar hay
  • No oils or beet pulp
  • High-quality mineral balancer
  • Occasional sainfoin (not alfalfa) for protein
• She recommends resetting the microbiome before transitioning performance horses to this model, which can take 6–12 months to show full results.

“With this approach, my clients’ horses have fewer tendon issues, fewer muscle cramps, faster post-competition recovery, and longer athletic careers — even racehorses.”
— Dr. Christina Fritz

This is such an interesting topic that I will for sure be making new posts talking about it as soon as I start investigating more about the topic.

However, feeding fat in its natural form — such as ground or whole linseed (flaxseed) — is much better tolerated.
These seeds contain essential fatty acids embedded in a fibrous matrix, allowing for slower digestion that’s compatible with the horse’s physiology. Ground linseed provides both calories and omega-3s in a safer, more bioavailable form that doesn’t coat or disrupt the digestive tract like oils do.

So, if your PSSM horse needs a bit more energy, consider small amounts of freshly ground linseed (up to 1–2 cups daily) rather than reaching for the oil bottle. It’s a smarter way to support calorie intake without compromising gut health.

Step 4: Balance Vitamins and Minerals

Low-NSC diets often eliminate concentrate fortified feeds — unintentionally leaving your horse at risk of nutrient imbalances. Horses almost never get their needs met with only hay. So please use a ration balancer or a vitamin-mineral mix.

Vitamin E and selenium are critical antioxidants that help protect muscle tissue, support recovery, and reduce the risk of cramping and stiffness in PSSM horses.

• Minimums for an average horse (per NRC):
  • Vitamin E: 500–1000 IU/day
  • Selenium: 1 mg/day
• For PSSM horses:
  • Vitamin E: 1500–2500 IU/day (natural form preferred)
  • Selenium: 2–3 mg/day (organic selenium yeast or chelated preferred)

The best approach would be to use a ration balancer or vitamin-mineral mix specially designed for IR/metabolic horses because they will contain no added sugars. Also to avoid having to spend too much time preparing the feed with so many separate ingredients, I would highly recommend trying to find a quality ration balancer that combines all these three (quality amino acids, vitamin E and selenium) + Fat. I will leave some recommended products below.

Product Image	Product Name / Our Rating / Description	Primary Button
	Sport Horse Grass Pellets 4.6 This is a great quality vitamin, mineral, amino acid supplement with beet pulp as a first ingredient. Ideal for horses needing limiting amino acids for muscle support and chelated trace minerals for better absorption and gradual release. This is complementary to grass based diets.	Buy on SmartPak
	Equi-Base Grass Pellets 4.5 This is a vitamin and mineral supplement designed for horses eating and all grass hay/forage or a 50-50 alfalfa grass mix. It has a fortified formula that includes vitamin E and selenium. Great for horses in maintenance or moderate work.	Buy on SmartPak
	Life Data® Forage Balancer 4.8 Is a concentrated nutrient source that contains a minimal amount of calories and starch. Great for Easy Keepers, Insuline Resistance, Cushing's or Metabolic Syndrome. Contains Vitamin E and quality amino acids. If your horse needs additional calories for exercise or weight gain, beet pulp or vegetable/fish oil can be added to the diet.	Buy on SmartPak
	AminoTrace+ Vitamin & Mineral Supplement for Horses | Mad Barn 4.5 This is a top buy for many horse owners, with the enhanced nutrient levels that support hoof growth, metabolic health, muscle function and antioxidant protection. It is specially made for horses with laminitis, EMS, PPID and easy keepers. Includes Vitamin E, selenium, flaxseed, quality amino acids, and more.	Buy on MadBarn

Sport Horse Grass Pellets

4.6

This is a great quality vitamin, mineral, amino acid supplement with beet pulp as a first ingredient. Ideal for horses needing limiting amino acids for muscle support and chelated trace minerals for better absorption and gradual release.

This is complementary to grass based diets.

Buy on SmartPak

Equi-Base Grass Pellets

4.5

This is a vitamin and mineral supplement designed for horses eating and all grass hay/forage or a 50-50 alfalfa grass mix. It has a fortified formula that includes vitamin E and selenium. Great for horses in maintenance or moderate work.

Buy on SmartPak

Life Data® Forage Balancer

4.8

Is a concentrated nutrient source that contains a minimal amount of calories and starch. Great for Easy Keepers, Insuline Resistance, Cushing's or Metabolic Syndrome. Contains Vitamin E and quality amino acids.

If your horse needs additional calories for exercise or weight gain, beet pulp or vegetable/fish oil can be added to the diet.

Buy on SmartPak

AminoTrace+ Vitamin & Mineral Supplement for Horses | Mad Barn

4.5

This is a top buy for many horse owners, with the enhanced nutrient levels that support hoof growth, metabolic health, muscle function and antioxidant protection.

It is specially made for horses with laminitis, EMS, PPID and easy keepers. Includes Vitamin E, selenium, flaxseed, quality amino acids, and more.

Buy on MadBarn

Extra Support

Muscle recovery and mitochondrial support matter.

• Lysine, Methionine, Threonine – help rebuild topline and reduce atrophy.
• L-Glutamine – building and repairing muscle tissue, regulates glucose uptake and supports the immune system.
• Acetyl-L-Carnitine (ALCAR) – this amino acid improves the transportation of fatty acids (omegas 3 and 6) into the mitochondria so that they can produce energy for the cell to do it’s job; shown to reduce stiffness and help with the management of PSSM horses.
  • Dose: ~1 g per 100 lb or 2.2 g per 100kg body weight

PSSM horses often have less functional mitochondria than normal horses, this means that they are not good at turning the fuel into energy (this often makes them unable to keep warm in cold weather), therefore Acetyl-L-Carnitine support should be considered as supplementation in your horse’s diet.

SmartEquine Acetyl L-Carnitine HCL Powder

4.8

$76.79

Buy on Amazon Buy on SmartPak

07/17/2025 10:56 am GMT

Exercise and Lifestyle Management for PSSM Horses

For horses with PSSM1 or PSSM2, daily, consistent movement is as essential as diet. The right exercise program helps normalize muscle metabolism, reduce stiffness, and prevent episodes of tying-up.

But the wrong approach — sporadic riding, forced rest, or sudden exertion — can make symptoms worse.

There are many success stories among horse owners when using a track system, the forced movement helps them thrive, especially because it has always been recommended that horses affected with PSSM spend no more than 12 hours in a row in their box. So if you’re trying to avoid the lush pastures, newly grown grass full of pectins and sugar, and are looking for constant movement and almost no box rest, then it makes sense to incorporate the track system in your property or try it our in a boarding place first.

Why Regular Exercise is Non-Negotiable

According to Dr. Stephanie Valberg and other leading researchers, daily movement reduces abnormal glycogen accumulation in muscle cells — especially in PSSM1 horses. Even 10–15 minutes of light work daily can have a major impact.

Best Practices for Exercising a Horse with PSSM

Some horses do best with light exercise every day, while others can maintain with 4–5 rides per week if they also have daily turnout.

Recovering from a Tying-Up Episode

It may feel intuitive to give your horse complete rest after tying-up — but for PSSM horses, extended stall rest can make things worse.

Once your horse is no longer reluctant to move, reintroduce gentle daily activity immediately — starting with calm turnout or hand-walking.

When reintroducing training after a tying-up episode:

• Start with 3–5 minutes of walk/trot (in-hand, on a lunge, or under saddle)
• Increase by 2 minutes per day
• At 15 minutes, include a 5-minute walk break before continuing
• Wait at least 3 weeks before adding canter or collected work and also introduce gradually
• If exercise is skipped for more than 3–4 days, restart gradually with a small amount of exercise

Webinar with Dr. Stephanie Valberg (Must see)

I thought it would be amazing to include this video here, because of the powerful insight that Dr. Stephanie Valberg has, she must be one of the most knowledgeable people in the world about this disease.

Variants P2, P3, P4, P8, and K1

There is controversy and current research that needs more investigation and papers to fully understand what is happening.

You may come across commercial genetic tests claiming that variants P2, P3, P4, P8, and K1 are responsible for muscle diseases like PSSM2 or myofibrillar myopathy (MFM) in horses. While management advice has emerged for horses testing positive for these variants (especially P3), it’s important to understand what the science currently supports — and what it doesn’t. For example:

What Is P3?

Source: Equinatural by notes from Marion Otto

P3 is a newly recognized genetic muscle condition linked to mutations in the FLNC gene, which affects the stability and repair of muscle fibers. Unlike PSSM, signs often don’t appear until after age 12 and include:

• Progressive muscle weakness (starting near the body, spreading outward)
• Difficulty regulating body temperature (wind, cold, humidity)
• Chronic muscle tightness — especially in the lumbar, girth, and flank areas — often misdiagnosed as muscle disease
• Gut inflammation and poor resilience to stress or weather

P3 does not affect the heart, but muscle repair is compromised due to low BAG3 protein activity.

Nutrition and Feeding for P3 Horses

Testing and Management

Horses have to be blanketed if it is under 20ºC (68ºF), they cannot stand the cold, rain, and humid cold.

• Gastroscopy + ultrasound to check GI health
• Bloodwork for vitamin/mineral status and inflammation
• Fecal microbiome tests to assess gut flora and dysbiosis
• Shelter and blanketing are important in cool, damp climates

A joint peer-reviewed study by researchers at Michigan State University, UC Davis, Oregon State University, and University of Nebraska-Lincoln evaluated whether the P2, P3, and P4 genetic variants (located in genes MYOT, FLNC, and MYOZ3, respectively) are causative of PSSM2 or MFM.

• No statistical association was found between having one or more P variants and being diagnosed with PSSM2 or MFM by muscle biopsy.
• Healthy horses were just as likely to carry one or more P variants as those diagnosed with disease.
• Quarter Horses: 60% of healthy QHs had one or more P variants — the same as affected horses.
• Desmin staining (used to confirm MFM) did not correlate with the presence of P variants in Quarter Horses.

Conclusion: The researchers do not support the use of P2, P3, or P4 tests for diagnosis, treatment, breeding decisions, or pre-purchase evaluations. The presence of these variants appears to be common genetic variation, not disease-causing mutations.

PSSM & MIM Groups or Pages for Support

PSSM Forum

EPSM/PSSM/MIM in Horses

PSSM UK

PSSM & MIM Awareness Facebook Group

PSSM Awareness Youtube Channel by Christine Mahon

PSSM FAQ Equiseq

PSSM isn’t a death sentence — but mismanagement can make it feel like one. Whether your horse tests positive for PSSM1, has clinical signs consistent with PSSM2, or is caught somewhere in the genetic gray zones of P2–P8 and K1, the foundation of care remains the same:

Support the muscles. Protect the gut.

That means:

• Prioritizing low-NSC forage and constant access to fiber
• Eliminating unnecessary starch and sugars
• Meeting their antioxidant, protein, and mineral needs with precision
• Rethinking the “just add fat” advice
• And, above all, moving the horse daily and purposefully

We now know that PSSM is a spectrum, not a one-size-fits-all diagnosis. Check all resources, talk to people that have encountered this problem and talk to your vet as well. If you can find a vet that has experience with managing horses that have PSSM, that would be the best choice.

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