b'LEARNING/RESEARCHLargely misunderstood metabolic disorderBy Kate Hore RNutr(Animal)A recent increase in calls about polysaccharide storage myopathy (PSSM) has led us to investigate the signs of PSSM and how they can be interpreted and managed successfully. Farriers have a good eye and feel for each horse when they treat them. As regular visitors, farriers have the ability to notice changes in each horse or pony, when the owner can overlook changes due to seeing their four-legged friend each day. Of course farriers have to recommend veterinary attention, but with PSSM in mind, when should we be recommending this and why? Difficulties picking up hindlegsLoss of muscle mass from visit to visitType II PSSM is more often seen in lighter breeds,Lamenessparticularly Warmbloods, but also horoughbreds and Arabs. Gait abnormalities/changes in stride patternAlthough these are the typical breeds affected, PSSM hasMuscle twitching been seen in a wide range of breeds, including Lippizaners, PSSM to give it its full name, is an equine muscle disease,Morgans and Standardbreds. Due to the strong genetic link, associated with disruption to normal glycogen within theparticularly for Type I, it is not recommended to breed from muscles. To understand it, we firstly need to understand theanimals known to have PSSM. normal function of glycogen.Like other equine metabolic conditions, such as insulin Glycogen is the way in which energy is stored withinresistance and equine metabolic syndrome (EMS) it is thought muscle. Molecules of glucose (a polysaccharide) are linked intothat PSSM may have originally been a positive trait, as it long chains by an enzyme known as glycogen synthase, withappears to allow these breeds to work hard on poor quality additional branches of glucose being formed off that chaindiets. However modern equine management, where horses along its length. This glycogen is then stored in the muscles,are in relatively little work but maintained on rich, starchy to be broken down back to glucose by an enzyme known asdiets, turns this positive into a negative. amylase when the muscles need energy. A disruption to thisSignssystem causes a glycogen storage disease, and in horses weHorses appear normal at rest, but experience tying-now recognise two different subsets.up (exertional rhabdomyolysis) with light work, such as Type I PSSM shortened stride, stiffness, cramped muscles, sweating, Type I is a genetic disease caused by a gene mutation thatpain and a reluctance to exercise. Additionally to tying-up, causes abnormally high levels of glycogen within the muscles.PSSM horses may show abnormal gaits, lameness, muscle This glycogen is unusual in that it is made up of very longweakness, stiffness, colic-like pain and muscle fasciculation chains with few branches off. This form of glycogen is resistant(brief muscle twitch). These signs are usually first seen when to the action of amylase, meaning there is a significant slowingstarting work as a youngster, however muscle damage can in the rate at which it can be broken down or used by thebe recognised as early as one month old. PSSM horses often muscles. Two different genes have been recognised, and thosehave elevated levels of creatine kinase (CK), an enzyme that are GSY1 and GBE1.demonstrates muscle damage, even at rest. This is one way to Type II PSSM differentiate the disease from horses with recurrent exertional rhabdomyolysis (RER), as their CK levels will return to normal Type II is a similar condition, causing abnormally high levelsbetween episodes. of glycogen in the muscles, but one not caused by geneSigns vary from subclinical, through to prevention of any mutation. The exact cause of Type II PSSM is, as yet, unknown.exercise and showing signs even at rest. The range of severity Type I PSSM can be diagnosed by a non-invasive bloodmay be impacted by further genetic factors, which as owners or hair sample, while Type II will normally require a musclewe can do nothing about. However, as owners and equine biopsy.managers, we can have a significantly positive effect by Breed predilection implementing the right diet and exercise regimes. Not unusually for a genetic condition, there is a strong breedDietpredilection. Type I PSSM is most common in Quarter HorsesFor most PSSM horses, diet has a significant effect on severity and related American breeds (Paint, Appaloosa); but also inof signs. Severity increases as the level of starchy concentrates European draft horses, especially the Belgian Draft where over(non-structural carbohydrates [NSC]) increases in the diet. a third (36%) have been shown to carry it. UK Draft breeds,Therefore, feeding a high fibre and forage, low concentrate such as the Shire and Clydesdale, are not typically affected.diet is recommended. 22 |FORGE|November 2019 18-30 NOV section 3.indd 22 29/10/2019 10:18'