Polysaccharide Storage Myopathy in Horses: PSSM Symptoms, Diet, and Management

Polysaccharide storage myopathy, usually called PSSM, is a muscle disease that affects how a horse stores and uses glycogen, the sugar kept in muscle for energy. In affected horses, abnormal glycogen or related polysaccharide builds up in muscle cells. That can lead to muscle pain, stiffness, weakness, poor performance, and episodes of tying up after work or even with mild exercise.

Vets generally divide the condition into PSSM1 and PSSM2. PSSM1 is caused by a known mutation in the GYS1 gene and is inherited in a dominant pattern, meaning one copy can affect the horse. PSSM2 describes horses with similar muscle biopsy changes but without the GYS1 mutation. PSSM2 is more complex, and current evidence does not support commercially marketed DNA variant panels as a scientifically validated way to diagnose it.

This condition can look different from horse to horse. Some have obvious painful episodes with sweating and firm muscles. Others show more subtle signs, like reluctance to move forward, trouble collecting, shifting lameness, or declining performance. A few horses test positive for PSSM1 and show little to no outward illness.

The encouraging part is that many horses do well with a thoughtful long-term plan. Management usually centers on diet, turnout, and consistent exercise, with the exact plan tailored by your vet and, when needed, an equine nutritionist.

The cause depends on the type of PSSM. PSSM1 is caused by a mutation in the glycogen synthase 1 (GYS1) gene. This mutation causes muscle cells to keep making glycogen, which contributes to abnormal storage in muscle. Because the trait is autosomal dominant, an affected horse can pass the mutation to offspring.

PSSM2 is less straightforward. It describes horses with muscle biopsy findings consistent with abnormal glycogen storage but without the GYS1 mutation. Research continues, and current expert guidance warns that commercial DNA tests marketed for PSSM2-related variants are not scientifically validated as diagnostic tools. That matters because management decisions should be based on a real veterinary workup, not a mail-in panel alone.

Diet and exercise strongly influence whether signs appear. Horses with PSSM often do worse on rations high in starch and sugar, and some appear more sensitive to insulin. Long periods of stall rest or inconsistent work can also trigger flare-ups. In practical terms, a horse may carry the condition genetically but only show obvious signs when feed, turnout, or training routines are not a good match.

Breed patterns also matter. PSSM1 has been identified in more than 20 breeds and is well recognized in Quarter Horse-related breeds, while PSSM2-like presentations are reported more often in Warmbloods. Still, similar signs can also be caused by other muscle disorders, so your vet will want to rule out look-alike conditions before settling on a diagnosis.

Diagnosis starts with the basics: your vet will review your horse’s history, exercise pattern, diet, breed, and exact signs. A physical exam is usually followed by bloodwork to look for muscle enzyme elevations, especially CK and AST. In some horses, your vet may recommend checking those values before and several hours after light exercise to uncover a milder or intermittent problem.

If PSSM1 is suspected, a DNA test on hair roots or blood is often the next step. This is the clearest and least invasive way to confirm the GYS1 mutation. If the horse has muscle disease signs but tests negative for PSSM1, your vet may discuss a muscle biopsy, which can help identify PSSM2 or other muscle disorders such as myofibrillar myopathy.

Diagnosis is also about ruling out other causes of tying up and poor performance. Depending on the case, your vet may consider vitamin E or selenium status, a broader chemistry panel, dietary review, and other inherited muscle diseases. That is especially important because treatment plans can differ.

Typical US cost ranges in 2026 vary by region and whether your horse is seen on-farm or in a hospital. A farm call and exam often run about $75-$200, basic bloodwork may add $50-$150, a PSSM1 DNA test is about $45 through UC Davis for one health test, and muscle biopsy with sampling, handling, and pathology commonly brings the total into the $300-$900+ range.

Prevention means two slightly different things with PSSM: preventing inheritance and preventing flare-ups. For PSSM1, genetic testing can help guide breeding decisions because the GYS1 mutation is inherited in a dominant pattern. If a horse carries the mutation, there is at least a 50% chance of passing it to each foal. For horses diagnosed with biopsy-supported PSSM2-type disease, many vets also recommend caution with breeding because a genetic basis is still suspected.

For day-to-day prevention of symptoms, the biggest tools are diet consistency and regular exercise. Horses with PSSM often do best when starch and sugar are controlled, forage is selected carefully, and long periods of inactivity are avoided. Hay testing can be very helpful, especially if your horse has repeated episodes despite a feed change.

Turnout is part of prevention too. Daily movement helps muscles use stored glycogen more effectively. Many horses do worse after stall rest, missed rides, or abrupt changes in workload. A steady routine usually works better than a cycle of hard work followed by several days off.

Because every horse is different, prevention is not one-size-fits-all. Work with your vet to build a plan around body condition, workload, forage access, and any other health issues. That kind of tailored plan often gives the best long-term control and helps your horse stay comfortable and useful.