PSSM, or Polysaccharide Storage Myopathy, to give it it’s full name, is an equine muscle disease, associated with disruption to normal glycogen within the muscles. To understand it, we firstly need to understand the normal function of glycogen.
Glycogen is the way in which energy is stored within muscle. Molecules of glucose (a polysaccharide) are linked into long chains by an enzyme known as ‘glycogen synthase’, with additional branches of glucose being formed off that chain along it’s length. This glycogen is then stored in the muscles, to be broken down back to glucose by an enzyme known as amylase when the muscles need energy. A disruption to this system causes a glycogen storage disease, and in horses we now recognise two different subsets.
Type I PSSM
Type I is a genetic disease caused by a gene mutation that causes abnormally high levels of glycogen within the muscles. This glycogen is unusual in that it is made up of very long chains with few branches off. This form of glycogen is resistant to the action of amylase, meaning there is a significant slowing in the rate at which it can be broken down or used by the muscles. Two different genes have been recognised, and those are GSY1 and GBE1.
Type II PSSM
Type II is a similar condition, causing abnormally high levels of glycogen in the muscles, but one not caused by gene mutation. The exact cause of Type II PSSM is, as yet, unknown.
Type I PSSM can be diagnosed by a non-invasive blood or hair sample, while Type II will normally require a muscle biopsy.
Not unusually for a genetic condition, there is a strong breed predilection. Type I PSSM is most common in Quarter Horses and related American breeds (Paint, Appaloosa); but also in European Draft horses, especially the Belgian Draft where over a third (36%) have been shown to carry it. UK Draft breeds, such as Shire and Clydesdale, are not typically affected.
Type II PSSM is more often seen in lighter breeds, particularly Warmbloods, but also Thoroughbreds and Arabs.
Although these are the typical breeds affected, PSSM has been seen in a wide range of breeds, including Lippizaners, Morgans and Standardbreds. Due to the strong genetic link, particularly for Type I, it is not recommended to breed from animals known to have PSSM.
Like other equine metabolic conditions, such as I.R. and E.M.S, it is thought that PSSM may have originally been a positive trait, as it appears to allow these breeds to work hard on poor quality diets. However modern equine management, where horses are in relatively little work but maintained on rich, starchy diets, turns this positive into a negative.
Horses appear normal at rest, but experience ‘tying-up’ (exertional rhabdomyolysis) with light work, such as shortened strides, stiffness, cramped muscles, sweating, pain and a reluctance to exercise. Additionally to tying-up, PSSM horses may show abnormal gaits, lameness, muscle weakness, stiffness, colic-like pain and muscle fasciculation (brief muscle twitch). These signs are usually first seen when starting work as a youngster, however muscle damage can be recognised as early as one month old. PSSM horses often have elevated levels of creatine kinase (CK), an enzyme which shows muscle damage, even when they are at rest. This is one way to differentiate the disease from horses with Recurrent Exertional Rhabdomyolysis (RER), as their CK levels will return to normal between episodes.
Signs vary from subclinical, through to prevention of any exercise and showing signs even at rest. The range of severity may be impacted by further genetic factors, which as owners we can do nothing about. However as owners and equine managers, we can have a significantly positive effect by implementing the right diet and exercise regimes.
For most PSSM horses, diet has a hugely significant effect on severity of signs. Severity increases as the level of starchy concentrates (non-structural carbohydrates, NSC) increases in the diet. Therefore feeding a high fibre and forage, low concentrate diet is recommended. Low starch diets produce low blood glucose and insulin after meals, and may reduce glucose uptake by the muscles. Remember the high fibre, low concentrate diet, is an ideal choice for all horses as ponies, as it’s what they evolved to digest – however, it does need balancing with supplemental vitamins and minerals.
Energy should be provided as fat (oils), which are readily metabolised by horses and encourage the body to use free fatty acids for energy production over glucose. Oils should be introduced gradually to the diet, to allow the body to adjust to fat metabolism. Once adjusted, horses cope very well, and for performance horses we may provide up to 500ml per day of oils such as linseed or soya, as a useful form of energy.
Vitamin E, as the body’s major antioxidant, is well recognised for muscle health, working in synergy with selenium, and adequate levels should be included in the diet of PSSM horses. Further, for fat fed horses, it is important to balance the daily oil with Vitamin E, roughly at a level of 1 I.U. Vit E to 1ml of oil. Fresh, early season pasture is a good source of natural Vitamin E, however levels will drop the season, so it is important adequate Vitamin E is provided year round.
However a complex condition, like PSSM, requires a complex answer, and no single ingredient will be totally effective. Naturally sourced antioxidants, that support the role of Vitamin E are recommended, such as turmeric, rosehip and chlorella; while natural adaptogens, such as ginkgo and omicha allow the body to adapt to stresses such as exercise. As a condition which has a strong metabolic link, it is advised to support a healthy metabolism with supplementation of a daily live yeast probiotic.
Electrolytes are important for all working horses, but particularly those prone to tying up and muscle conditions. Allow free access to a salt lick in the field and stable, and add broad spectrum electrolytes to feed when horses have sweated or exercised.
As horses are also often IR, dietary management may include a restriction of intake to avoid weight gain and to ensure a healthy Body Condition Score is maintained.
As signs are most likely to occur during very light work it is important that a return to work, and build-up of fitness, is done very gradually. Starting with just a few minutes walk on a loose rein, and taking several weeks of careful walk and trot before introducing canter.
However, although exercise must be done very carefully, it is important that it is done. Prolonged periods of rest, particularly on box rest, seem to encourage further episodes. Therefore it’s important that exercise is very well-managed, with a very regular routine and minimum rest days. Limiting the intensity of exercise is more important than limiting the amount of exercise. Daily turnout is advised, though take care on rich, early season grazing. It may be necessary to use grazing muzzles, bare paddocks or schools to allow exercise without free access to lush grass.
PSSM has proven to be a career-limiting condition in the past for those affected horses. However with increased knowledge of the best dietary approaches, and how to manage their exercise regimes, both PSSM I and II can continue to lead active, useful careers so long as careful attention is paid to these two key areas.