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EPM stands for Equine
Protozoal Myeloencephalitis. Myeloencephalitis
means inflammation ( it is) of the brain (encephala) and
nerves( myelo), so EPM is inflammation of the brain and
nerves in the horse caused by a protozoan parasite.
The most common of these protozoa is Sarcocyctis neurona,
although several other less well-known parasites also
cause disease, particularly one called Neospora.
parasite responsible for EPM has a specific life cycle
that requires several hosts. The primary host is
generally the American opossum. In the primary
host, the parasite undergoes reproduction in the
intestinal tract and sporocysts(eggs) are excreted in
the animal’s feces. In the normal life cycle, this
feces is then inadvertently ingested by an intermediate
host ( usually a raccoon, cat, skunk, or similar).
In the intermediate host, the eggs develop into juvenile
parasites, who then migrate from the intestinal tract to
the animal’s muscles and form cysts, walled off areas
where the parasites hibernate. When the
intermediate host dies, and an opossum feeds on the
carrion, the cysts are ingested. They hatch in the
opossum’s intestinal tract, and the cycle begins anew.
In normal intermediate hosts the parasites do not cause
disease, but simply stay encysted in the muscle until
the animal dies from some other cause and they are
ingested by a opossum.
horse is NOT a normal intermediate host for the EPM
parasite which is where the trouble begins. When a
parasite enters its normal host, it follows chemical
signals to find the “correct” place to be, in this case
the muscles, where it causes little problem for the
hosts. However, since the horse is NOT a normal
host, when the EPM parasite enters the horse, it becomes
unable to detect where it should go. For reasons
we do not yet understand, instead of going to the
muscles, it generally migrates into the spinal cord
the spinal cord tissue, the parasite develops walled off
cysts, just as it would in the muscle of a normal host.
However, this results in the destruction of nerve
tissue, as well as inflammation due to the presence of a
foreign invader in the horses’s body.
signs resulting from this disease are variable. In
80-90% of infected horses, progressively worsening
neurologic disease occurs. This may initially be
so mild that it may be mistaken for lameness or a
performance problem. Over time, however, the
symptoms become more severe and may include
incoordination, crossing of limbs, inability to walk,
turn, or back up normally, gait abnormalities, and
muscle atrophy. Hind end abnormalities are most
common, and most horses with EPM remain bright and alert
even when neurologic signs are quite severe.. A
smaller number of horses will also develop front-end and
head signs, possibly including laryngeal paralysis (
roaring), difficulty chewing or swallowing, depression,
or a head tilt.
a specific diagnosis of EPM can be a challenge.
There are several blood tests, but they only indicate
whether the horse has been exposed to the EPM organism,
not whether the organism is the cause of the horse’s
current illness. UC Davis has a new blood test called
the IFAT test that is more accurate in determining
whether EPM is involved. However, the gold
standard for diagnosis is a CSF tap( spinal fluid tap,
where a long needle is inserted into the horse’s back
and some fluid removed) and the finding of antibodies
against the EPM organism in the horse’s spinal fluid.
Test results combined with the horse’s physical exam and
history will help in making an accurate diagnosis.
Several treatment options exist. EPM requires long-term
treatment of one month or more with medication.
The most commonly prescribed drug is Marquis, an oral
paste made by Bayer.
There are things you can do to prevent EPM infection in
your horses. First, make sure all your feed is
stored in closed containers where scavenging animals
will not be able to reach it. Also, make sure that
your water tanks are cleaned regularly and if possible
position them to discourage wild animals from utilizing
them. Hay can be a source of EPM transmission as
animals climbing into barn areas may sleep or defecate
on it. When possible, keep hay covered or stored
where wild animals can not access it. Discard any
hay that appears to have animal or bird feces on it.
Clean up any spilled or dropped feed quickly to
discourage other animals from coming to “clean up” after
your horses. When possible, feed heat-treated or
extruded grain as treating seems to destroy the EPM
sporocyst. You can also attempt to live trap and remove
oppossums, raccoons, and other hosts from your property.
If you chose to do so, use caution and remember these
animals also carry the rabies virus, and that they will
often return to an area following being trapped and
relocated if that area ( your farm for example)
continues to provide ready access to food, water and
shelter. Free choice food should also not be left
out for barn cats as this is a big attractant for the
wild animals that carry EPM. Finally,
although there is no vaccine for EPM, keeping your
horses healthy by vaccinating and deworming according to
your veterinarians recommendations, and feeding a
balanced and nutritious diet, will help your horse fight
off the parasite more easily if it is ingested.
is a serious disease, but understanding how it is
transmitted can help you protect your horses.
Denise Bickel DVM
Whole Horse Veterinary Services
Phone # 517-474-4050
Fax # 517-764-7710
3906 Seymour Rd
Jackson, MI 49201