EPM Prevalent Among Standardbred Racehorses
April 20, 2004
In the past decade, Standardbred trainers, breeders and owners have been plagued by the disease known as
EPM (Equine Protozoal Myeloencephalitis). Costly to treat and often difficult to diagnose, EPM is a debilitating
disease which effects a horse's coordination and thus, their ability to travel in a consistent manner at high rates
of speed. It is an infection of the brain and spinal cord which also causes lameness, weakness and depression in
the equine athlete.
First identified in 1964 by Dr. Jim Rooney, EPM awareness among veterinarians and horsemen has grown considerably.
In 1974 the first EMP lesions were discovered and identified in horses, and through the 1980s the disease continued
to be sporadic and somewhat rare. However, in the 1990s, the disease began to spread and become more prevalent
throughout North America. Top research laboratories and equine scientists at The Ohio State University, University
of Kentucky and the University of Californian at Davis, to name a few, continue to monitor the disease in hopes of
finding more solid advancements in the diagnosis and treatment of this unusual ailment which plagues Standardbreds
and other breeds of horses throughout much of this country and Central and South America.
For some reason, EPM seems to strike especially hard at young horses, and can be a difficult disease to diagnose
as the symptoms can range from mild to severe, with no consistency whatsoever.
For instance, a trainer may notice that his filly is stiff walking to the track one day and not the next, and that
her symptoms are not symmetrical. In other words, the same inconsistencies in gait may appear first on the right
side of the horse one day, and then on the left side the next morning. Often times, horses seem to nearly trip
over themselves and have a stilted or stiff movement, especially when travelling slowly. In extreme cases, paralysis
of the muscles of the face can develop, as well as seizures and abnormal sweating bouts. Some of the typical symptoms
include a head tilted to one side, a tail that does not clamp down tightly, and frequent stumbling and irregular
crossing of the legs while walking. A severely effected horse may lie down and then will not be able to get back
The parasite which carries EPM is called Sarcocystis Neurona, and is not spread from horse to horse through water
or feed. Rather, this parasite is spread by opossums, who acquire the organism from infected birds and their feces.
The infective stage of the organism, the sporocysts, are passed in the opossum's feces and the horse then comes into
contact with the infective sporocysts while grazing or eating contaminated feed or drinking water. Due to the
transport of harness horses back and forth, from track to track, and from farms to the racetracks, as well as
grains being shipping in from various parts of the country, it puts nearly all horses at risk for the disease.
Once a horse ingests the parasite, the sporocysts migrate from the intestinal track into the bloodstream, and then
make their way into the brain. There, these parasites attack the horse's central nervous system, and the onset of
the disease can either be slow or quite sudden. Left undiagnosed and untreated, EPM can cause devastating and
lasting neurological damage. As of the year 2000, EPM is considered to be the number one cause of neurological
problems in horses through the United States. There is not a trainer on the grounds of any racetrack or training
facility who has not had to deal with this problem. The good news is, however, that this disease is treatable,
and many horses with EPM, once treated, go on to become successful racehorses or continue on with successful
When a trainer suspects that his or her horse has EPM, they will request that their veterinarian take a blood test
and possibly a cerebrospinal tap. This spinal tap (CSF-cerebrospinal fluid), in which a long needle is inserted
through the back and into the spinal canal, is the most efficient means of diagnosing EPM. The blood test, while
useful, is often inconclusive as it may indicate exposure to the organism, but not whether or not the disease has
attacked the central nervous system. A positive blood test only means the horse has been exposed to the parasite,
not that the horse has or will develop EPM.
Once diagnosed, treatment under veterinary supervision begins immediately, and nearly 70 per cent of EPM cases which
are treated aggressively show significant or complete reversal of symptoms, thus allowing horses to reach their full
potential. Treatment, which is expensive, includes feeding the horse either a powder or liquid combination of
pyrimethamine and a sulfonamide (sulfa) antimicrobial with or without trimethoprim, on a daily basis. For optimum
affect, this remedy is typically fed at least one hour prior to feeding the horse hay.
The average length of treatment ranges from 90 to 120 days in typical cases, while in more severe cases, horses
may need to be treated for six months or longer. A small handful of horses may be treated indefinitely to combat
In the last two years researchers at the University Of California at Davis discovered a new cause for EPM in the
form of "Neospora Hughesi," a mutant parasitic organism. Researchers say that it may take at least a year before
an accurate diagnostic test can be developed to isolate this new parasite.
Recently, a new drug called diclazuril was made available to veterinarians. Diclazuril is an experimental drug
that to date seems to be a bit more effective than the pyrimethamine/sulfa combination and have fewer side effects
in clinical studies.
Unfortunately, at the current time, there is no vaccine or definitive program to prevent EMP among horses in North
America. Horse owners can minimize their horse's risk by keeping their barns, stables and facilities clean and
unreceptive to opossums or birds that are possibly effected with EMP by providing screens and fences to keep these
animals away from grain and bedding bins.