EPM: A Debilitating Disease of Horses
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Pet Column for the week of May 29, 2000
Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
By Carrie Gustavson
Information Specialist
University of Illinois
College of Veterinary Medicine
In the past few years, equine protozoal myeloencephalitis, or EPM, has emerged as a major
debilitating disease of horses. Each case of EPM is unique, but common signs include
incoordination, lameness, weakness, muscle atrophy, and paralysis. Though the disease
rarely kills, about 30% of afflicted horses do not improve even after treatment.
The disease is caused by single-celled organism, a protozoan parasite called Sarcocystis
neurona, which is spread by the opossum. "The opossum passes the organism in its feces,
which are then ingested by horses when they eat contaminated feed, hay, or water,"
explains Dr. Doug Hutchens, veterinary parasitologist at the University of Illinois College of
Veterinary Medicine in Urbana. After ingestion, the tiny parasite invades the spinal cord and
central nervous system. When the horse's immune system tries to fight off the invading
parasite, inflammation occurs in the spinal cord or brain. Horses can't transmit the disease
to other horses.
The most common signs of EPM are what veterinarians call ataxia, asymmetric lameness,
and atrophy. That is, a horse may be uncoordinated, may be lame on one side or the other,
and may lose muscle mass, most commonly over the hips or shoulders. "Horses may have
sudden onset of clinical signs or develop the disease slowly over several months," say Dr.
Hutchens. "Typically, owners say that afflicted horses stumble or are 'off' in the hind end,
most commonly the stifle."
The first step to diagnosing EPM must include a thorough physical and neurological
examination by an equine veterinarian. If the neurological exam indicates that further testing
for EPM is warranted, the next step is to test blood serum and cerebral spinal fluid (CSF)
for antibodies to Sarcocytsis neurona.
Many more horses are exposed to the organism and manage to fight it off than actually get
the disease. Whenever a horse is exposed, whether it gets sick or not, it will make
antibodies to the organism and carry these antibodies in its blood. Therefore, a blood test
for serum antibodies can confirm exposure to the EPM organism, but it doesn't necessarily
mean the horse has the disease.
The CSF antibody test presents another diagnostic challenge. Since collecting cerebral
spinal fluid for EPM testing involves poking a needle into the lumbosacral space of the
spinal cord, problems can arise if the needle grazes a vessel and blood happens to enter the
CSF during the procedure. "Any bleeding during the spinal tap can contaminate the CSF
with serum antibodies to the EPM organism," says Dr. Hutchens. "What that means is that a
disease-free animal may test positive."
So don't jump the gun on blood or CSF testing healthy animals for EPM. "The test is
useless in a healthy animal without lameness or neurological signs suggestive of EPM," says
Dr. Hutchens. "But, if a horse has clinical signs, the tests have high reliability." Therefore, a
clinician will usually take into account all the evidence -- a neurological exam and serum and
CSF testing -- to diagnose EPM.
So what can you do to prevent this parasitic infection? "The presence of opossums where
horses are stabled puts horses at risk," says Dr. Hutchens. "Stress is another contributing
factor, and showhorses and racehorses have a higher risk of contracting EPM due to the
increased stress of athletic performance and travel." Dr. Hutchens recommends limiting
access of opossums to stables and keeping all feed storage containers covered at all times.
"Since most horses are exposed to the organism but only a small percentage actually
develop the disease, preventive care to keep a horse healthy and minimize stress appears to
be very important," says Dr. Hutchens. So keep up with all routine vaccinations and
de-worming programs, groom and exercise horses regularly, and give them a balanced diet.
Contact your local equine veterinarian promptly if your horse shows any signs associated
with EPM and for more information on how you can help prevent EPM.