Polysaccharide storage myopathy (PSSM) in horses, is a form of "tying up" or exertional rhabdomyolisis (ER), which means "muscle breakdown with exercise". PSSM is caused by an over-abundance of glycogen in the skeletal muscle, and affects primarily Quarter Horses, Paints, Appaloosas and crossbreds with this stock-type breeding.

Affected horses pull glucose from the bloodstream and deposit it into muscle tissue more quickly than normal horses, thereby creating and storing more glycogen than normal horses. This is mainly as a result of heightened insulin sensitivity. Insulin is a hormone released into the bloodstream by the pancreas, in response to the digestion of carbohydrates. Insulin stimulates the muscle to take up sugar from the bloodstream.

Clinical manifestation includes sweating, stretching as if to urinate, rolling or pawing after exercise, muscle fasciculations and reluctance to move just following the onset of exercise. Confirmation of a PSSM-diagnosis is done through a muscle biopsy. Horses showing these myopathies often have glycogen concentrations of 1,5 to 4 times greater than in normal horses.

Many horses with PSSM can be successfully managed. This involves:

• Limiting sugars and starches
• Providing fat and fermentable fibre for energy instead
• Keeping the horse’s muscles moving so that abnormal polysaccharides do not build up.

It is vitally important to realise that if your horse responds to diet change, that diet should be maintained for the life of the horse.

A balanced diet

Good quality grass (Eragrostis teff or curvula) and/or legume hay (lucern) is acceptable for horses engaged in regular exercise. For those at rest or performing minimal exercise, grass hay is sufficient. Enough forage can be fed to maintain optimal body weight, as the starch content of most hays is low, regardless of whether they are grasses or legumes, or a combination of both.

Horses should receive 1% of their bodyweight in daily forage at the very minimum. Approximately 1,5-2% of body weight is recommended. Note that grain hay such as oat hay is not suitable, as high-starch grain kernels can cling to the stems, increasing starch content in the diet.

As for grains, current recommendations for the maximum amount of sugars and starches range from 10-20% of the total daily energy intake. Horses with PSSM need a complete diet, balanced in vitamins and minerals, especially when they are asked to perform. For horses working hard on a regular basis, a high fat, high fibre diet is recommended, providing energy not from starch, but primarily from fat and fermentable fibre.

A suitable product that I am aware of in South Africa, is the Capstone Balancer Pellet, which requires additional fat to be supplemented, as well as sugar beet shreds to be fed as a source of high fermentable fibre.

Fat replaces the sugars and starch as energy source. Rice bran is a lovely product, if you can get hold of it. I have successfully used vegetable fat in powder or oil form to supply these calories. Vegetable oil, although I have recommended and used up to 1 litre per day, works well at levels of 500 ml per day, sprinkled over lucern pellets/chop and soaked sugar beet shreds and/or balancer pellets.

Note that as horses on high fat diets have increased aerobic metabolism, resulting in oxidative stress and the production of free radicals, additional anti-oxidants such as selenium and especially vitamin E should be fed.

I have found that MSM at 10-20 g per day also helps for this condition. Avoid intramuscular injections of selenium and vitamin E, as this can result in muscle soreness and the risk of introducing infection if sterile injection techniques are not adhered to. Daily oral supplements are more economical and provide higher overall, balanced levels of anti-oxidants.

Remember that diet changes should be accomplished over a period of several weeks, to give the horse’s gastrointestinal tract adequate time to acclimatise to the changes.

Exercise

Finally, it is important to allow PSSM horses lots of exercise. Do not put them in a stable for more than 12 hours per day. In addition to as much turnout as possible, these horses do better if worked every day. Some experts recommend two shorter bouts of exercise per day, with a 20-30 minute walking session in the afternoon.

Take your time warming up and cooling down, and if your horse has had an extended layoff for any reason, start back very slowly, adding on just a few minutes of additional exercise a day, until he is back at the former level of work.

Do not warm down at the trot. Horses with a history of tying up after exercise, seem to stiffen up when being warmed down at the trot after a workout. Trotting requires fast muscular activity leading to mobilisation of muscle glycogen, and horses with chronic tying up are unable to mobilise this muscle glycogen quickly enough.

To avoid possible tying up, finish the fast workout and return to the walk for 10 minutes as a warm-down. Alternatively, warm down initially at a slow canter for 2-3 minutes and then return to walk for 10 minutes to let the horse relax and cool down.

Patience

Give the treatment time to work. After a horse was diagnosed with PSSM and the afore-mentioned changes to diet and exercise have been put into place, the timeline for recovery must be realistic.

Because the horse’s caloric requirements are being met through a different source of energy, it will take the gastrointestinal tract a few months to adapt and for the metabolic changes to occur. Signs of the disease usually fade away as management practices continue over the course of several months.

The prognosis for long-term usefulness of horses with PSSM, depends on the severity of the disease. Almost all horses with mild forms of the disease, and many with more severe forms, will return to a normal life once the correct diet is implemented and free-choice exercise is granted. Serious thought should, however, be given when considering breeding with these animals, due to the genetic nature of the disease.