Rural Heritage Vet Clinic

EPSM Look-Alike Conditions

by Beth A. Valentine, DVM, PhD

EPSM is a muscle disease of draft horses that can cause symptoms like tying up, stiff gaits, and overall poor performance. The root of the problem is the inability of these horses to properly break down the glycogen (animal starch) in their muscles. The result is an excess of glycogen, leading to muscular cramping and weakness. EPSM horses are physically unable to generate a proper canter and they have abnormal muscle wasting, leaving many horse owners and their veterinarians frustrated as they search for cause and treatment. The following misdiagnoses are typically made in the field:

Equine Protozoal Myeloencephalitis (EPM)
Atrophy of rump muscles and proximal thigh, as well as rear-limb weakness, caused by EPSM may be mistaken for spinal-cord disease due to EPM. Even the western-blot testing on cerebrospinal fluid is not an accurate test when a horse shows equivocal signs of neurologic disease. EPSM horses may be weak and stiff in the rear limbs, but they know where their feet are, and can place them properly when spun in tight circles.

Equine Motor Neuron Disease (EMND)
The equine equivalent of human amyotrophic lateral sclerosis (Lou Gehrig's disease), EMND causes profound muscle atrophy, weakness, trembling, and often slight increases in blood levels of muscle enzymes. The weakness is most profound in the muscles used for maintenance of posture. Horses are most distressed while standing; they shift weight in the hind limbs, tremble, sweat and often lie down. Severe cases of muscle atrophy and weakness due to EPSM look similar. A muscle biopsy test for EMND and/or EPSM will make the distinction. EMND horses have extremely low levels of vitamin E, believed to be the main cause of the disease.

Lyme Disease
The exact nature of Lyme disease in horses is still being researched. Testing, as with EPM, is often inconclusive as horses exposed to the Lyme disease organism may test positively without having clinical Lyme disease. Dr. Rich Jacobsen at the New York State Diagnostic Laboratory has developed a test that accurately makes the determination, although repeat testing may be necessary. As with EPSM, horses with Lyme disease have vague shifting-leg lameness and a stiff gait, and appear to have pain all over. A joint synovial membrane biopsy will confirm if the Lyme organism is present.

Repeated episodes of colic, especially after exercise, may be a sign of EPSM. Careful examination of the horse, including observation of defecation, intestinal sounds, and blood testing for increased muscle enzyme levels help distinguish colic due to gastrointestinal pain and colic due to muscle pain/EPSM.

Poor Mover
The short, stiff, stabbing gait of EPSM horses, often described as "pony-gaited," is sometimes assumed to be the horse's normal way of moving. In poor-moving EPSM horses, however, the gait cannot be attributed to any serious conformation problems.

Hock Problems, Arthritis
EPSM horses often have poor hock flexion at the walk and while backing, and may have difficulty holding their hind feet up for the farrier. Careful examination of the hocks (flexion tests, x-rays, ultrasound, and joint blocks) help differentiate joint disease from muscle disease/EPSM. The stiffness is often mistaken for arthritis due to aging, especially in older draft horses.

Some EPSM horses show low-level anemia, lack of energy, and poor performance that may be blamed on anemia. Although treatments directed at correcting anemia may still be indicated, anemia in EPSM horses often resolves following EPSM-recommended diet change.

Behavioral Problems/Back Soreness
EPSM horses may be grumpy under saddle and may resent being asked to canter, to carry themselves in a forward manner, or to go up or down hills. Some horses with EPSM repeatedly stop and assume the parked-out stance of a horse about to urinate. This may be mistaken for back soreness, poor saddle fitting, or behavior problems. Since EPSM most severely affects the power muscles of the back, rump, and thigh it is not surprising that these horses may be uncomfortable using these muscles. EPSM horses often have difficulty standing on three legs for the farrier, especially when asked to hold up a hind limb.

Tying Up
The exact causes of tying up (known in draft horses as "Monday morning disease") have yet to be determined, and horses do tie up for reasons unrelated to EPSM. To date, however, examination of muscle biopsies from horses that have tied up, and of diet change in affected horses, indicate that EPSM may be a more common cause of tying up than was previously thought. We have not yet seen a muscle biopsy from a horse that was brought to us for difficult-to-treat tying up that did not show evidence of EPSM.

Any horse that ties up—even if it occurs while an unconditioned horse is being conditioned—should be suspected of having an underlying metabolic problem. EPSM horses may also have decreased blood levels of selenium, but these horses still can have problems, even when selenium levels are corrected. Some EPSM horses have had persistently low selenium levels, even when supplemented at high levels or being given injections. This is not surprising, as selenium needs are greatly increased with exercise or any muscular damage. Selenium levels generally return to normal following diet change for EPSM.


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.

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26 October 2011 last revision