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.
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.
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
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.
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.
horse that ties upeven if it occurs while an unconditioned horse is being
conditionedshould 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
Horses, an Owner's Manual.