Rural Heritage Vet Clinic

Signs of EPSM in Draft Horses

by Beth A. Valentine, DVM, PhD

A suspected sign of EPSM is increased temperature (hyperthermia) during recovery form anesthesia. The role EPSM may play in post-anesthetic complications is particularly important, as draft horses have been known for many years to be prone to problems associated with general anesthesia. The results of this study may provide treatments for horses that would otherwise die or be euthanized because of their inability to rise. Other signs include:

HorseShoe Loss of muscle mass or conditioning, especially in the shoulder or hind quarters.
HorseShoe Abnormal gait due to mechanical lameness in one or both hind legs. Abnormal gaits possible in EPSM horses include:

Shivers: Affected hind legs are periodically flexed (bent) and often held up for several seconds. This is most obvious when walking, backing, and turning, but may also be seen when the horse is standing still. This may be mistakenly called stringhalt, or the horse may be described as "stringy."

Fibrotic myopathy-like: Affected legs have a short, "stabby" action with little or no bend in the hock and stifle. The horse often "slaps" the ground. True fibrotic myopathy horses actually pull the hoof back several inches at the end of the forward swing before slapping the hoof on the ground. True fibrotic myopathy horses also often have a thickening due to scar tissue from muscle or nerve damage that can be felt at the back of the stifle, where the ham strings an the back of the thigh attach.

Locking stifle: The lock may be prolonged, in which case, the hock and stifle are periodically fixed in a straight position and the horse walks with the limb dragging behind on the toe. The locking may also be short lived (called intermittent stifle locking), in which case the stifle "catches" during bending and then is flexed upward in an exaggerated bend very much similar to shivers.

HorseShoe Trembling, especially after exercise.
HorseShoe Difficulty rising, backing, or reluctance to back.
HorseShoe Lack of energy.
HorseShoe Poor performance.
HorseShoe Reluctance to pick up feet for shoeing.
HorseShoe Lifting or "stomping" of hind limb or limbs, especially while standing.
HorseShoe Episodes of colic, especially after exercise.
HorseShoe Slightly stiff, awkward, or short strided hind limb gait (often with no hock action).
HorseShoe Tying up.
HorseShoe Lying down suddenly, unable to rise.

Affected draft horses may appear completely normal for many years. Although EPSM may result in slightly abnormal blood levels of muscle enzymes, some EPSM horses have normal muscle enzyme levels. Diagnosis is best made by examining a muscle biopsy for the characteristic changes. The biopsy is taken while the horse is standing and sedated, and the site of the biopsy (the muscle of the caudal rump) is injected with local anesthetic. Taking a biopsy is a simple procedure that results in minimal scarring. Testing your horses for EPSM provides valuable, perhaps life saving, information.


Further information: Data on EPSM Signs in Various Breeds

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