EPM Prevention Summary

There have been several clinical studies aimed at unlocking the mystery of why 98% of exposed horses can clear an EPM protozoa infection, while less than 2% succumb to an active infection that causes neurological symptoms.  So far there are no answers, only a few clues.  Limiting opossum contamination to feed and water, limiting stress, and limiting other diseases are the best prevention strategies.

The bottom line is that a horse with less stress and better health is less likely to incur an active EPM infection.

Amanda Manfredi Photograph 'Happy Trails'
   © Amanda Manfredi   Happy Trails
Opossum Connection

Researchers have identified the primary host of S. neurona as the opossum.  Limiting your horse’s exposure to this animal and it’s contamination of feed and hay will help prevent the horse from ingesting the protozoa.  Please see the page on the opossum for more discussion.


Stress for the horse has been identified as a factor in the horse getting an  infection.  Stress can come in many different forms including active show schedules, heavy training, long distance trailering, stall confinement, and inadequate nutrition.  All of these can suppress the immune system of the horse, making him more vulnerable to any infection.

Clinical trials by Ohio State University involved loading horses onto trailers, and transporting them for 24 hours, stopping for feed and water breaks.  Blood collected during and after the transportation shows suppression of the immune system.  The 'transport stress model' was born.  While the study gives information about stress, the trial was not able to reliably infect horses with EPM.   The link from stress to lowered immune system is made, but the connection from stress to EPM has not been shown. 

Several micro-nutrients are necessary for proper immune system function.  Both a lack of these nutrients and over supplementing can stress the horse.  Work with your veterinarian or an animal nutritionist to create a balanced diet for your horse.  Testing the forage you feed to your horse is a good first step.  Make sure that the horse is getting correct amounts of the building blocks of a healthy immune system:  vitamins A, C, D, E, and selenium.

Habits or conditions that cause the horse pain can also be stressful.  Tooth pain, hock injuries, or a saddle that pinches can all be stressful episodes for the horse.  Repeated on a daily basis they and other painful situations like them can hamper the immune system.

Health Care

It is not uncommon for EPM to be diagnosed along with other health problems.  A disease or injury can strain the immune system, allowing an active EPM infection to take hold.  Use good preventative health care for you horse, appropriate to your geographic location.  Vaccines for other diseases will not prevent EPM, but may prevent the horse from being stressed by another disease.  Currently there is no vaccine available for EPM, but there is ongoing research to find an effective one.

Regular fecal tests and worming as appropriate will result in better utilization of nutrition, and less stress on the health of the horse.  Don’t guess at a wormer rotation.  Ask your veterinarian for a recommendation for your specific situation.

Miracle Prevention and Cures

If a product claims to prevent or cure EPM, and it is not listed on this web site, you will need to research the product carefully before basing your horse's health on the product.  Look for clinical studies involving 50 or more horses that support claims made by the manufacturer.  The studies should have been done by an independent laboratory or university.  Look for study results on horses that have been proven to have active EPM, with a cerebrospinal fluid (CSF) test and neurological exam.  Did the horses in the study have an active infection, or were they just exposed to the protozoa?  98% of horses will clear an exposure to EPM on their own.  Read the fine print, be skeptical, and advocate for your horse.

Any remedy used to treat EPM must be able to accomplish two things.  First, it must be able to cross the blood-brain barrier to attack the active infection in the CNS.  Second, it must be able to enter individual cells in the CNS to kill the protozoa when they live within other cells (intracellular).  If the remedy you choose does not do these two things, then it won’t kill the infection.

Recent EPM Research

“Horses have been classified as aberrant intermediate hosts because the terminal asexual sarcocyst stage that is required for transmission to the definitive host has not been found in their tissues despite extensive efforts to search for them.”  However, one post-mortem study indicates, “In a 4-month-old filly with neurological disease consistent with EPM, we demonstrate schizonts in the brain and spinal cord and mature sarcocysts in the tongue and skeletal muscle, both with genetic and morphological characteristics of S. neurona,” which raises the question if horses could be a natural intermediate host.  This small detail is important to whether S. neurona could be transferred back to the opossum, completing the life cycle.  This would happen only if the horse carcass is consumed by the opossum.

A 2008 clinical trail, funded by Bayer, was performed to study the effect of giving ponazuril (Marquis) to horses as an EPM preventative.  The once weekly dose would kill the protozoa in the gut; however, studies have shown that Sn moves from the gut to infect the lungs and liver in 24 hours.  The preventative use has not been approved by FDA.  Researchers have acknowledged that prophylactic use could cause formation of a drug resistant strain of S. neurona.  They contend that it would not pose a problem, because the sarcocysts do not show up in flesh, and could not make it full circle to the opossum.  It seems that the Precautionary Principle would serve well in this situation. 

There is ongoing research into the daily use of decoquinate for prevention.  This looks much more promising than the triazines, and should stimulate the immune system to fight the specific protozoa.

December 2011


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Am J Vet Res. 2008 Mar;69(3):396-402.

J Parasitol. 2006 Jun;92(3):637-43.



Photograph GenilleCaveBrown1910LOC Genille Cave Brown 1910