Equine Protozoal Myelitis (EPM)

Diagnosing EPM early, and then improving the immune system, is the best way to prevent serious symptoms of this disease in your equine patients.

Equine Protozoal Myelitis (EPM) has been around since the 1960s in the US. It’s caused by two protozoal parasites.

  1. Sarcocystis neurona is the most common, and causes multiple symptoms.
  2. Sarcocystis hughesi affects less than 1% of horses. It invades the central nervous system.

Horses in the US seem to have developed some immunity to EPM, since fewer than 1% of exposed horses develop a clinical disease. The horses I see with severe symptoms are usually very weak, stressed or old.

Common symptoms of EPM

The symptoms I see with my equine patients in Texas include:

  • hind limb weakness
  • problems with proprioception
  • performance problems
  • gait abnormalities
  • un-coordination
  • stiffness
  • head tilt
  • general ataxia
  • extreme irritability for no reason.

My differential diagnosis includes wobbler syndrome, trauma, equine herpesvirus myeloencephalopathy, equine degenerative myelopathy, and West Nile Encephalomyelitis.

Over 85% of the horses test positive for antibodies to EPM, but many do not show any symptoms, or at least none recognized by their owners. Perhaps some were a little “off” for a week or two while their immune systems produced antibodies; or perhaps they were strong enough to mount an immediate response when the protozoa were introduced into their bodies.

How EPM develops

The life cycle of the sporocyst is complicated. Briefly, it involves many intermediate hosts (domestic cat, raccoon, skunk, armadillo, other mammals, and maybe even passerine birds and sea otters) who are then ingested, often as “road kill”, by opossums. The opossum is the definitive host (reservoir) in which the sporocyst develops. This animal excretes both oocysts and sporocysts (environmentally resistant stage) in its feces.

Horses are subsequently infected when they eat feed or water contaminated with sporocysts from opossum feces. The horse itself is a dead-end host, and does not pass sporocysts in his manure. The sporocysts in his body migrate from the GI tract into the bloodstream, where they can cross the blood/brain barrier into the CSF. This means that an infected horse cannot infect another horse. The infected horse can show symptoms immediately after exposure, or can harbor the organism for months and develop signs at a later date, often during periods of stress.

Is environmental prevention possible?

Except for the horse, most of the animals involved in the life cycle of EPM are scavengers, and will eat any dead animal. One way for owners to protect their horses is to keep feed locked up in a manner that prevents feces from getting into it. Cats see stalls as giant litter boxes, so this poses the question about whether or not to keep barn cats to control mouse populations.

There is really no reasonable way to protect pastures from the feces of these animals. I have heard of giant vacuum cleaners being used in pastures, but this is not very practical for most horse owners. One thing horse owners can and should do is pick up any dead animals they find in their pastures.

Conventional treatments

There is no vaccine for EPM. Conventionally, horses are treated with Ponazuril (Marquis), Pyrimethamine-Sulfadiazine products, and Diclazuril. Like me, many of you probably remember treating horses 20 years ago with Diclazuril bagged up to be added to chicken feed. It now comes in many new packages, with many new names, but it is the same ingredient.

I have not had much luck with any of these treatments lasting more than about four months without a relapse. Some farms use these products as a preventative, but what is this long-term treatment doing to the horses? I am certain it creates gastrointestinal imbalances over time, thus weakening the animals’ immune systems.

Integrative diagnosis

Diagnosing EPM early, and then improving the immune system, is the best way to prevent serious symptoms. Treating EPM when it’s first entering the horse, before the protozoa can damage the neurological system, is preferable.

I diagnose EPM using osteopathy. A horse with EPM will first present with performance problems, such as not getting to the bottom of their stops, not sitting at their barrels, being clumsy or irritable, hitting jumps, showing hind limb instability, difficulty with proprioception, or an inability to engage their backs, etc. When I examine the spines of these horses in an osteopathic manner, one side will be very flexible, actually too flexible, and the other side very stiff. If I find this on an osteopathic exam, I put EPM in my differential diagnosis. Horses suffering from an inflammation of the spinal cord associated with EPM will be painful between CO and C1. If I squeeze that area with my hand, the horse will duck his head and twist it to one side. These horses are often positive on a tail pull, giving to one or both sides.

Additionally, I almost always see atrophy of the postural muscles. These horses appear thin at first, but when you evaluate them, you find an atrophy of the postural muscles (often the gluteals and quadriceps). To confirm my preliminary diagnosis, I prefer to use the Indirect Fluorescent Antibody test (IFAT), offered at UC Davis. Some clinics prefer to submit both blood and CSF, measuring the ratio of antibody in the blood to that in the CSF. Using this osteopathic preliminary diagnosis, and a positive IFAT test, I can be relatively certain that the horse has EPM. I have treated over 300 cases of EPM in this manner, mostly diagnosing them with my osteopathic techniques.

Treatment schedule

I begin treating a horse with EPM and L4 larvae at the anterior mesenteric artery (see sidebar on page xx) with a commercial de-wormer. If he does not have a history of de-worming within the past four months, my recommendation is to begin with a dose of Ivermectin. Because Ivermectin causes a slow kill, it will prevent a worm impaction if the horse has a large number of worms. Three weeks later, I move forward with the rest of my L4 larvae protocol:

  1. Because of the EPM, I also immediately start the horse on an immune system booster called Karbo Pellets, and another called Epic Liquid (from Gomer’s Inc). They feature some components that help with the digestive system as well.
  2. I use a Panacur Powerpak with sufficient dosage for the horse’s weight. Ideally, this paralyzes the L4 larvae but does not kill them. This is a daily double dose of fenbendazole for five days in a row.
  3. Next, I wait ten days to allow the horse to recover from the Panacur. Then I administer Quest Plus in an amount equal to the horse’s weight. Quest Plus enters the bloodstream and kills the L4 larvae that are already paralyzed and unable to move away from the blood vessels carrying the chemical.
  4. After waiting ten more days, a regular Quest is administered to ensure as many L4 larvae as possible are eliminated.
  5. After the last dose of de-wormer, I recommend waiting at least five days before starting Sefacon, an equine herbal health supplement that actually kills the protozoa (Endocrine Technologies).

When a horse presents with a mild case of EPM, is not in a stressful situation, and has a history of regular de-worming, I begin with Karbo Pellets and Epic Liquid. I feed two scoops of the pellets and 60cc of the liquid two times a day for three months.

If the horse is in a stressful situation, or the EPM is coming in fast and strong, I will add a product called Sefacon for ten days. The horse is usually very improved by the end of three weeks. It is very important to treat the horse with sufficient Sefacon for his weight. I will use double doses in a large horse (over 1,200lb).

If the horse is already very ataxic and wobbly, I begin with Marquis to kill off some of the protozoa as quickly as possible before I start my regular treatment. I have not seen much true cure with Marquis. Often, the horse will get better for around four months, then relapse, or never return to his previous level of performance. My method of treatment allows for early diagnosis, so the horses return to their previous levels of performance.

Concurrent parasitism

Most horses who come down with EPM have weakened immune systems. This allows the protozoa to enter the CNS and cause inflammation. In Texas, one condition that weakens the immune system involves an abundance of L4 strongyle larvae at the anterior mesenteric artery. Our Texas climate does not desiccate the eggs with heat, nor freeze them with cold, and this results in a huge parasite problem. On osteopathic diagnosis, these horses will be stiffer on the right side of their spines, moreso than a horse with just EPM. They will still have the same reaction to a hand squeeze across the front of the atlas, dropping the head ventrally and rotating to the side.

I always question owners about the de-worming status of their horses. It is my belief that Texas and Florida cannot follow AAEP guidelines for using fecal egg counts to determine which horses need a de-wormer. In Texas, many horses are kept on small pastures where strongyle eggs could be high in number. Since it can take six to seven months for the eggs to develop into egg-laying worms, the L4 larvae have plenty of time to accumulate and do damage to the tissue around the anterior mesenteric artery while the horses maintain a low egg count in their feces.

Prevent EPM with a strong immune system

A strong, healthy immune system will help protect horses from contracting EPM. Keeping my client horses’ on Chlorella (species vulgaris, broken cell wall) helps remove heavy metals from their systems and provides them with a healthy food source. The more stress a horse is under, the more likely he is to have a weakened immune system. I see more cases of EPM in young horses who are just going into training and dealing with the stress of leaving home and being trained; and in performance horses that travel a lot. I like to keep these horses on the immune and digestive health booster, Karbo Combo. It comes as a small pellet that is fed in small amounts every day.

All methods that maximize health will decrease the risk of neurological EPM – a high quality diet, fewer chemicals, stress management with appropriate time outdoors or a decreased show schedule, regular osteopathic treatments, etc.

For those interested in pursuing osteopathy (which I highly recommend as it’s the best tool in my toolbox) visit vluggeninstitute.com. Janek Vluggen teaches equine osteopathy at my clinic in San Marcos, Texas.

Dr. Lu Ann Groves graduated in 1981 from Colorado State University College of Veterinary Medicine. She opened a mobile equine clinic in 1982, then opened The Whole Horse Veterinary Clinic (thewholehorse. com) in San Marcos, Texas in 2004. She offers osteopathy, acupuncture, chiropractic, craniosacral therapy and ozone therapy along with conventional diagnostics and therapies. Dr. Groves teaches basic classes at The Vluggen Institute of Equine Osteopathy, also in San Marcos, Texas.