PSSM in Horses – What You Need to Know

Understanding PSSM in Horses

When a horse frequently accumulates glycogen in his or her muscle fibers, it’s possible the horse is experiencing a condition commonly referred to as either Equine Polysaccharide Storage Myopathy (EPSM) or (PSSM).

Signs or symptoms of EPSM in horses can include muscle stiffness, muscle cramping, resistance to moving, inability to move, tying up, and abnormal sweating. Horses in light, medium, or athletic levels of exercise can experience this concern.

There are two different types of PSSM in horses. PSSM1 can be identified by a gene mutation in the GYS1 gene. Although both PSSM1 and PSSM2 will reveal abnormal glycogen staining when muscle biopsies are performed, the PSSM2 fails to reveal an identifying mutation. Therefore there is no link to heredity in PSSM2, unlike that of PSSM1.

History and studies have shown that certain breeds such as Warmbloods, Quarter Horses, Paint Horses, Appaloosas, and some Draft horses are more prone to experience PSSM symptoms.

How can there be no heredity link in PSSM2 if there are some genetic tests are available to expose PSSM2 in horses?
The reality is that the current understanding of PSSM2 is more complex. Although there are genetic tests available that may reveal a single genetic mutation that causes PSSM2, there are many cases that may still test negative and still be linked or diagnosed by a medical professional as having PSSM2. Therefore claiming a true link in heredity is irresponsible and may lead to someone assuming their horse has no genetic link to PSSM2, when at this time it is still widely accepted that there are likely several genetic mutations that have not been identified that can link to PSSM2 that are not utilized in the current testing. There are also cases in which no genetic mutation is suspected with the symptoms presented.
Studies and sources are still open in expressing that relying on genetic testing for PSSM2 is unreliable at this time as there is no solid and proven correlation to one specific genetic mutation to a majority of horses that are diagnosed with PSSM2.

"Dr. Valberg is not currently using or recommending the genetic testing for the diagnosis of PSSM2 or MFM as there is currently no scientifically validated evidence that the variants for which genetic testing is available are linked to PSSM2 or MFM. To date (September 2020) we have not found any peer-reviewed studies offered by the company providing the commercial tests.

We have now performed two peer-reviewed studies evaluating the commercial genetic test variants P2, P3 and P4 and the results of those studies have not found the commercial genetic test variants to be consistently present in Warmblood or Arabian horses diagnosed with MFM by desmin staining and consistently absent in healthy horses."

"Similar to Type 1 PSSM, Type 2 PSSM also likely has a genetic basis; however the underlying genetic mutation has yet to be identified."

Because of the lack of significant association between a histopathological diagnosis of PSSM2 or MFM and the commercial genetic test variants P2, P3 and P4 in WB and AR, we cannot recommend the use of these variant genotypes for selection and breeding, prepurchase examination or diagnosis of a myopathy.  

Diet is the Main Focus of Equine PSSM

Feeding a horse as they are designed to be fed is the best practice for horses of all stages of health… Today’s popular trends of highly processed feeds and cereal grains are NOT going to support any horse to health. Therefore a horse with a condition such as PSSM symptoms or an official diagnosis, a more natural forage-based diet is best.

Much of the forage that we feed our horses these days is not as the horse was designed to eat, therefore a horse that is unable to balance and properly utilize glucose needs unique modifications to its diet.

Testing and analyzing the non-structural carbohydrates (NSC) in feeds for PSSM horses is recommended. Forage and selecting supplemental feeds with low NSC concentration serve to empower you as the horse owner to reduce the PSSM symptoms that can debilitate your horse and render them reluctant or unable to move.

Adding fat intake for horses with PSSM can provide them with the energy they need to activate their muscles. Although vegetable-based fats are common, alternative options such as chia seed or stabilized rice bran are preferred by many holistic approach professionals.

What’s important to note and can be under-addressed with this additional fat intake for equine PSSM, is the increased oxidative stress that the horse’s body will experience.


Oxidative Stress on Both Sides of the Equation of Equine PSSM

“Whenever fat is added to an equine ration, Vitamin E should also be used to protect against the oxidative damage that could occur when the fat is metabolized.” ~ Meri Stratton Phelps, DVM, MPVM, DACVIM, DACVN

Increased oxidative stress is already a known concern with PSSM in horses, combine that understanding with the risk of further accumulation due to the raised fat intake… Additionally, if your horse is an athlete or you’re exercising them to maintain overall health, oxidative stress is naturally elevated.

Traditional antioxidants have proven over time to simply not be enough, even for a horse without the additional amplifiers of oxidative stress that horses with PSSM face. The time has come to utilize new, effective, and natural options to reduce oxidative stress.

Stepping into what is commonly referred to as biohacking and the use of nutrigenomics – the study of food and its effects on gene expression. Breakthrough discoveries such as activating the Nrf2 pathway can yield results far more effective at reducing oxidative stress. Improvements as drastic as neutralizing free radicals on a 1:1 ratio with antioxidants vs a 1:1,000,000 ratio with Nrf2 activation.

Just as antioxidants don’t serve to prevent, treat, or mitigate the symptoms of PSSM or any other disease or concern, neither does utilizing NRF2 activation. The goal is to optimize health. Support the body by fostering healthy cell function. A crucial perspective to wellness that has long been ignored with the rising number of diseases reflects it.

If you're ready to explore more support options for your horse's PSSM, click here to connect with your Core Wellness Coach today.

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