PSSM in Horses: Signs, Testing and Safe Long-Term Management
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By Dr Duncan Houston
A horse that suddenly becomes stiff, sweaty and unwilling to move needs help, even if the exercise seemed easy. Trying to walk off a suspected tying-up episode can make muscle damage worse.
Polysaccharide storage myopathy, or PSSM, is one possible explanation. The useful next step is to identify the muscle problem properly, then build a feeding and exercise routine your horse can sustain.
Quick Answer
PSSM is a muscle disorder associated with abnormal glycogen storage. Type 1 has a validated genetic test; the group historically called type 2 requires more careful specialist investigation. Stop exercise and call your vet for painful stiffness, marked sweating or dark urine. Long-term management can help many affected horses, but it must follow the diagnosis and the horse's energy needs.
What is the difference between PSSM1 and PSSM2?
PSSM1 results from a variant in the GYS1 gene and can affect Quarter Horse-related breeds, draught horses and other breeds. One copy can be enough to cause susceptibility, although clinical severity varies. A positive result does not tell you precisely how much work a horse can perform.
PSSM2 describes biopsy findings in horses without the PSSM1 variant. It is not simply a second version of the same proven genetic disease. Some horses previously placed in this group have other muscle disorders, including myofibrillar myopathy. UC Davis explains the distinction and the limitations of commercial DNA tests for PSSM2.
Which signs matter most?
| What you notice | What to do |
|---|---|
| Repeated poor performance or mild stiffness that resolves | Arrange an examination before increasing work; keep a training and feeding record. |
| Firm painful muscles, sweating or reluctance to move during exercise | Stop immediately and call your vet. Do not force walking. |
| Dark brown urine, severe weakness, collapse or inability to rise | Request emergency veterinary help now. |
Muscle pain can resemble colic. A horse may stretch out, paw or look at its flank. Dark urine can indicate muscle pigment release and raises concern about kidney injury.
How do vets investigate tying-up?
Blood tests, including creatine kinase and AST, help assess muscle injury. Kidney function, hydration, electrolytes, feeding history and recent exercise also matter. Depending on breed and findings, the vet may recommend GYS1 testing, a muscle biopsy or referral.
Other explanations include sporadic exertional rhabdomyolysis after inappropriate exertion, recurrent exertional rhabdomyolysis, other inherited myopathies, nutritional problems, lameness and neurological disease. A negative PSSM1 result does not establish PSSM2. The Merck Veterinary Manual's review of exertional myopathies describes why these distinctions change management.
When is this an emergency?
Severe pain, dark urine, collapse or a horse that cannot move comfortably require immediate veterinary assessment. Stop work, remove tack if safe and leave the horse in a quiet place. Offer water if it can drink normally. Ask the vet whether transport is safe; do not make a severely affected horse walk to the float.
Do not give leftover anti-inflammatory medicine before speaking to the vet. Dehydration and kidney risk can change which drugs are appropriate.
How should diet and exercise be managed?
- Stabilise the episode first. Acute treatment may include pain relief and fluids. The vet decides when exercise can restart.
- Review the whole ration. Include hay analysis, pasture, hard feed, treats and supplements. For PSSM1, reducing starch and sugar is commonly useful.
- Match calories to the horse. Fat may replace some starch when extra energy is needed, but automatically adding large amounts of oil to an overweight horse is unhelpful.
- Build a consistent programme. Once cleared, regular controlled exercise and appropriate turnout usually matter more than occasional hard work.
- Record response. Log stiffness, workload, feed changes and time off. Arrange planned reassessment rather than repeatedly treating flare-ups without reviewing the cause.
The University of Minnesota's PSSM guidance supports coordinated diet and exercise management. Horses with a different myopathy may need a different nutritional approach.
Common mistakes
- Working a horse through painful stiffness.
- Treating an unvalidated genetic panel as a complete diagnosis.
- Adding selenium or multiple muscle supplements without checking the total intake.
- Alternating long rest periods with sudden strenuous exercise.
- Expecting one diet to suit every PSSM-labelled horse.
Can future episodes be prevented?
Keep management predictable, change feed gradually and discuss interruptions to exercise before they become unavoidable. Maintain appropriate body condition and check tack, feet and dental health. For breeding decisions, use validated PSSM1 results with veterinary and genetic advice; a comfortable horse can still pass on the variant.
Frequently asked questions
Can a horse with PSSM still be ridden?
Many can, with suitable management. The safe workload depends on clinical response, fitness and the particular muscle disorder.
Does PSSM spread between horses?
No. PSSM1 is inherited, not contagious.
Will rest cure PSSM?
No. Rest may be needed during an acute episode, but prolonged inactivity can be counterproductive in long-term PSSM1 management.
Does my horse need a high-fat diet forever?
The ration needs ongoing review. Extra fat is a way of supplying calories, not a treatment to add regardless of weight or workload.
The best plan starts with an accurate diagnosis and a routine that keeps your horse comfortable. During an acute episode, protecting damaged muscles takes priority over completing the ride.
For help preparing questions about your horse's results and ongoing care, visit ASK A VET™. Acute muscle pain needs your equine vet directly.
