Say Yes To Drugs

Like any neurological disease, EPM is heartbreaking for horse owners, who watch their horses slowly deteriorate, with stumbling, altered sensations/perceptions and overall weakness and muscle atrophy. Add to that the fact that EPM can be difficult to diagnose in the field and even the best test — the Western Blot spinal fluid test for antibodies — can give you false positives.

With the intense focus on EPM a few years ago came a host of nutritional supplements offering anything from a help to a cure. Since rapid cures with EPM are few and far between, owners of these horses became an easy mark for anything that bills itself as additional, or even alternative, treatment. However, they aren’t cures.

EPM is a disease caused by an infectious organism. That means the focus of treatment must be on eliminating the organism, which comes from modern antimicrobial drugs. There’s just no substitute.


Pau d’arco, AKA Lapacho, is a flowering evergreen tree found in South America. The inner bark is most often used medicinally, with claims ranging from anti-cancer to anti-inflammatory and for activity against a variety of organisms, including malarial protozoa. Reports of these wide benefits have actually stimulated quite a bit of study, but formal results haven’t been exciting.

Proponents claim this is because most of the work centers on trying to isolate active chemicals rather than using the whole bark. However, the whole bark also has known analgesic and anti-inflammatory effects that may be masking symptoms rather than curing infections. Basically, there’s still no proof this herbal treatment can actually cure EPM, or any other protozoal infection. We’d skip Pau d’arco.

Both olive-leaf components and garlic have been shown to be toxic to a wide variety of organisms in culture dishes, but there’s little support for their use in whole, live animals or people as an alternative to antimicrobial drugs. There is a theoretical potential for plant substances such as these to work inside the intestine, but the dosages required inside the enormous intestinal tract of a horse would likely be proportionately large. Suffice it to say that once again there’s no solid proof these supplements help prevent or treat EPM.

Immune Support

Although stress (e.g., shipping) and immunosuppression (corticosteroid drugs) increase susceptibility to experimental EPM infection, most horses exposed to EPM don’t develop the disease. Why isn’t clear.

Several studies have demonstrated that lymphocytes from EPM horses don’t respond normally to the antigens from the parasite that cause a strong immune response in cells from horses that don’t have EPM. Whether this means that the parasite itself can prevent the horse’s immune system from attacking it, or there is some specific defect in the immune system of horses susceptible to the infection, is still not known.

A common EPM-supplement ingredient is Eleutherococcus. This herb, in combination with other adaptogenic herbs, can help protect against immunosuppression caused by cancer chemotherapy drugs. Astragalus extracts have also had cell-mediated immune system and macrophage stimulant effects in some studies. These and other Chinese immune stimulant herbs have been shown to improve the response to vaccines, working in essence like adjuvants. However, their use for EPM horses is not proven.

Whether using an immune-system drug or immune-stimulating herb, the effects are more predictable when given before exposure to the organism occurs. Given after the fact, there’s a potential for actually increasing symptoms and the effectiveness in helping eliminate the infection is far from guaranteed. The safety and effectiveness of immune stimulants in horses with EPM have not been tested. Use with caution.


Vitamin E and folic acid are ever-present in nutritionally based EPM formulas. Vitamin E is a vital antioxidant in the nervous system, and its deficiency is proven to cause nervous-system disease. For us, this makes supplementation with vitamin E a good idea for these horses.

There is no ”EPM specific” dose of vitamin E, and the last edition of the National Research Council’s nutritional recommendations for horses wasn’t very definitive in its advice either. But, based on accumulated data, a dose of 1500 to 2000 IU/day as a minimum seems reasonable. Vitamin E has little, if any, potential for toxicity.

Folic acid is a different story. Pyrimethamine is an antimicrobial drug used in combination with sulfa drugs to treat malaria, and it was also used several years ago to treat EPM. It works by inhibiting an enzyme needed for the organism to manufacture folic acid, which is required for the EPM organism to reproduce.

Unfortunately, the drug also inhibits the horse’s natural folic-acid-generating enzymes and can cause anemia and bone-marrow suppression over time. Supplementing horses on this drug with folic acid to help prevent those side effects may sound like a good idea, but folic acid supplementation is controversial.

For one thing, the folic acid you feed the horse is just as available to the EPM organisms in his body as it is to his own cells, so supplementing can defeat the purpose of the drug.

Whether or not you should supplement with folic acid during EPM treatment is largely a moot point today anyway, though, as the more recently developed drugs don’t work by inhibiting folic-acid production.

Sulfa drugs in combination with trimethoprim are sometimes used to treat some horses and are much less expensive than the newer treatments. While sulfas also slow utilization of folic acid by the protozoa, they don’t have the same high risk for causing folic-acid deficiency in the horse. Note: Your vet will monitor the horse on trimethoprim sulfa-drug combinations for changes in the red blood cell, white blood cell or platelet counts. If serious problems develop, treatment may be stopped and the horse’s folic acid repleted. Basically, skip folic acid supplementation unless directed by your vet.

Choline is another ingredient you may see on EPM supplement labels. We’re not sure where the rationale for supplemental choline (a nutrient with properties that are a cross between an amino acid and a vitamin) arose, except that the nervous system is rich in choline. While choline intake may be an issue for highly processed human diets, the equine diet is rich in choline. There’s no reason to think choline supplementation would be helpful.

Bottom Line

With the exception of vitamin E, which is an important antioxidant in the nervous system, there’s no solid reason to waste money buying a supplement when your specific intention for that product is the treatment of an EPM horse.

Our greatest concern involves advertisements and supplements that bill themselves as actual treatments. Don’t believe them. A delay in treatment increases your horse’s risk of irreversible damage. When it comes to EPM, just say ”yes” to drugs.

What did you think of this article?

Thank you for your feedback!