Rural Heritage Vet Clinic

EPSM—Muscle Disease in Draft Horses

by Beth A. Valentine, DVM, PhD

The muscle disease Equine Polysaccharide Storage Myopathy (EPSM) has been confirmed or suspected in virtually every draft horse breed, including Belgian, Percheron, Clydesdale, Shire, Haflinger, Norwegian Fjord, Irish Draught, Friesian, Gypsy Vanner, draft cross, and a draft mule. This newly-recognized disease, under research at the College of Veterinary Medicine at Oregon State University, has likely been around for hundreds of years. It has been identified in many breeds of horse, but appears to be particularly common, severe, and difficult to detect in the draft breeds. In fact, approximately two-thirds of all draft related horses show evidence of EPSM, if you look at their muscles microscopically.

In drafts EPSM has been identified as a cause of severe muscle wasting and weakness in both young and older horses, of "tying up" (Monday Morning Disease) in older horses, as a cause of poor performance, "shivers," and other abnormal hind limb gaits. Affected horses may easily be misdiagnosed as having foaling complications, colic, or other diseases. Most devastating of all, this disease has been identified as causing recumbency ("down horses") due to weakness during work, associated with foaling, at rest, or following general anesthesia.

EPSM and Other Conditions
"Monday morning disease" is most often described in hard-working draft horses that are given a day off with full grain feed. When they are asked to work the next day, these horses show severe signs of the condition known as "tying up" in saddle horses—muscles become stiff and begin to degenerate; the horse stops moving and may go down.

Massive muscle injury results in release of the pigment myoglobin from damaged muscle, and the urine becomes a dark red-brown (myoglobinuria). Because of this reddish color, the disease is also called "azoturia" ("azo" from the red clothing dye + "t" from who knows where + "uria," referring to urine). Other names include "set fast," "exertional myopathy," and "exertional rhabdomyolysis."

We believe the massive muscle damage in EPSM drafts is due to lack of muscle energy. The common occurrence of slightly increased levels of muscle enzymes in the blood of apparently normal or only mildly affected horses suggests that these horses have low-level muscle injury during exercise. We don't yet understand what puts them over the edge into massive muscle injury, but studies of muscle from horses with signs of Monday morning disease show that EPSM is a common underlying condition, and we believe EPSM is the cause of the disease. Whether or not all horses with Monday morning disease also have EPSM remains to be absolutely proven, but so far evidence suggests they do.

"Stringhalt" (sometimes called "springhalt") is a condition producing abnormal hind leg action, especially when the horse backs or turns. Some people describe this action as a "hitch" or "cramp" in which the horse pauses with its hind leg in the air before stomping it down. It occurs most often when the horse backs or turns in a tight circle, but I have seen horses do it while standing, or on the first step when they get going, or on the last step before stopping. In many cases, horses thought to have "stringhalt" actually have a form of shivers

"Shivers" is a condition that looks similar to stringhalt. It differs in that shivers horses often progress to muscle wasting and weakness, but these severe signs may not show up for years. A horse with classic shivers trembles or quivers and abnormally elevates its tail. As with Monday morning disease, EPSM has been shown to be an underlying condition in many draft horses (and other breeds) with shivers. The abnormal action is apparently caused by a lack of energy to the powerful hind limb muscles. Only further study will determine if shivers has other causes.

An important part of the studies began at Cornell, and continued at Oregon State University study has been the evaluation of diet change as a treatment. Horses with EPSM seem not to be able to derive adequate muscle energy from carbohydrates, the main source of energy in grains, sweet feeds, and pelleted horse feeds. The diet change involves decreasing the amount of dietary carbohydrates and replacing them with fat as an energy source. For details see Dietary Recommendations for a Horse with EPSM.

If you have a confirmed EPSM horse in the barn, it may be easiest to feed all your horses the same diet. Feeding a "normal" horse the EPSM-type diet is not harmful. Several nutrition researchers, including Dr. Harold (Skip) Hintz at Cornell University and Dr. David Kronfeld at Virginia-Maryland Veterinary College, have for many years tested the effects of this high-fat diet. They have discovered no ill-effects and have found some indications that this diet is better for all horses.

You might think high-fat feed is much more expensive than feeding other grains, but high-fat feeds are so high in calories that the amount necessary to provide the same number of calories is much less. Fat provides more than twice the calories per volume compared to carbohydrates. For example, 2 cups of oil (approximately one pound) provide about 4,000 calories, whereas one pound of corn, oats, sweet feed, or other commercial feed provides only 1,200 to 1,400 calories. Factor in the potential costs of veterinary care for the affected horses, or even the potential loss of a horse, and the cost of the new diet looks even better.

Good quality hay and pasture are still vitally important; only the grain is changed. The simplest diet consists of replacing a portion of the grain with alfalfa and adding vegetable oil, but these days we have many different ways to achieve an EPSM diet. The best diet for your EPSM horse is one you are happy buying that your horse is happy eating, and that keeps your horse's muscles healthy.

With dietary therapy, many cases of EPSM show 100% improvement. Dietary therapy is most effective when started in the disease's earlier stages. Severely affected draft horses may die, despite having been started on dietary therapy. We believe they die because the disease is so advanced at the time of diagnosis. Unfortunately, the naturally stoic nature of many draft horse can allow sever changes to occur within the muscles without signs of problems for the owner to observe. By the time these horses show obvious problems the disease may be quite advanced and severe. Changing the diet of draft horses from a carbohydrate-based concentrate to a high-fat low-carbohydrate feed may decrease, delay, or even prevent the signs of EPSM in affected horses. For some EPSM horses, diet therapy has been life-saving. Horses on this diet often have improved muscling and increased energy. Most important, they are able to perform with minimal to no muscle damage. Only time will tell just how well diet therapy works.

After dietary fat, exercise is the second most important thing needed by horses with EPSM. Standing in a tie stall or going for long trailer rides only makes these horses worse. Give an EPSM horse as much daily turnout and as much regular work as possible, allowing the horse to maximally utilize the dietary fat. Be careful with warm-ups, though, and don't over-exert the horse when returning it to work after a lay-up


Beth A. Valentine, DVM, PhD, is involved with EPSM research and other veterinary matters at the College of Veterinary Medicine, Oregon State University. She is this site's virtual vet and co-author of Draft Horses, an Owner's Manual. This article appeared in The Evener 1998 issue of Rural Heritage.

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26 October 2011 last revision