Equine polysaccharide storage myopathy (EPSSM or PSSM) is a condition affecting certain fibres of the powerful muscles, especially of the rump, thigh and back of many breeds of horses, but most commonly occurring in draught-related horses and ponies, and Quarter Horse and Warmblood-related breeds.

It has also been found in many other horse breeds including Arabian, Morgan, Standardbred, Welsh pony-related breeds, Tennessee Walkers, Thoroughbreds, Paso Fino, American Saddlebreds, various crossbreds and American miniature horses.

It involves excessive storage of glycogen in the powerful muscles, sometimes in a form insoluble to carbohydrate digestion by enzymes. This feature has been used for diagnosis of the condition using special staining with periodic acid schiff (PAS) stain, which stains up glycogen a dark red. Sections of muscle biopsies taken from the muscles between the base of the tail and the sitbone of affected horses, are used.

The condition is directly related to and precipitated in susceptible horses by a diet high in carbohydrates as a source of energy, which is unnatural in the evolution of ­horses (see other article in this edition). Horses have evolved to be plains grazers, eating predominately stalky grass on a more or less continual basis, interspersed with or assoc­iated with movement. They also have a very good capacity to digest and use fat or oil for energy.

What are the signs?

Suspicion of the condition is based on various muscle-related clinical signs. These may range from severe exercise-induced rhabdomyolysis (muscle breakdown), with associated raised levels of certain serum enzymes (CK and AST) and the transient voiding of muscle pigment (myoglobin) in the urine – turning it a brown colour – to severe muscle weakness resulting in inability to rise, muscle pain, stiffness or spasm, or muscle atrophy (wasting).

Other possibilities include abnormal hind-limb gait, often known as "shivers" and resembling to a degree stringhalt, poor performance and/or exercise intolerance, back soreness, behaviour problems when under saddle or in harness, difficulty working with hind hooves, and even episodes of spasmodic colic.

Males and females are equally likely to be affected and the condition can manifest in horses of less than a year old, to over 20 years. Other factors including temperament, exercise (amount of free movement or turnout allowed daily), as well as environmental temperature (worse in cold weather) also determine if and how severely clinical signs will occur.

Where does it come from?

The latest finding, in the process of being published in the USA, is of two gene mutations which have now been highly associated with PSSM cases in Quarter horse-related breeds. The authors of this work have found one mutation associated with 90% of the PSSM-affected Quarter horses, while the other, which they have called MH, makes signs of PSSM more severe in these breeds.

The PSSM-gene is inherited in a dominant fashion, meaning that one copy of the gene can cause PSSM. Genetic testing is ­available in the USA to determine the ­presence of these genes. Work on the gene­tics involved in Warmblood, Thoroughbred, Arab, Standardbred and other light horse breeds is still in progress, and muscle biopsy in these breeds is still the method of choice to ­confirm a diagnosis.

Treating the condition

The best way to treat and manage cases of PSSM, is to change to an oil-based diet, and cut out carbohydrates as much as possible for the rest of the horse’s life. As much turnout as possible and keeping horses warm during cold weather, are also important.

Supplementation with vitamin E and selenium does, in general, help to minimise muscle damage. Improvement may occur slowly over several months or intermittently, but owners should persist. Some horses may die once they go down with the condition.

(Dr Williams is a senior lecturer in the pathology section of the Department of Paraclinical Sciences at Onderstepoort. References available from the author)