EPM Is Becoming A Catch-All Diagnosis

We’ve come a long way since 1964, when Dr. James Rooney first described finding a protozoal organism in the brain of a horse with neurological disease — what we now know as EPM — but we still have a long way to go in preventing it and effectively treating it.

EPM, or equine protozoal encehalomyelitis, is a protozoal infection of the central nervous system. Most cases are caused by the organism Sarcocystis neurona, which the horse picks up from feed, the ground or possibly water contaminated by opossum feces that contain the infective cysts.

The opossum picks up the infection when he eats the muscle of intermediate hosts, which range from birds to skunks, even armadillos. Much confusion has arisen because of the long list of possible intermediate hosts — even to the point of some horse owners considering getting rid of their barn cats! — but the intermediate hosts aren’t the problem. Only the opossum poses a direct threat to horses.

Interestingly, as many as 80 to 90% of horses in high EPM areas test positive for EPM exposure by antibodies in their blood, but a small percentage of these develop EPM. The determining factor is unknown, although it could be due to a defect/weakness in the immune system or high exposures when the horse is under stress.

Note: A few horses have been found with obvious EPM signs and lesions in the brain but still test negative for Sarcocystis organisms. However, EPM can also be caused by another protozoa, Neospora. The likelihood of exposure to Neospora compared to Sarcocystis, and how likely an exposed horse is to develop EPM, are largely unknown.

Diagnostic options for EPM haven’t changed in the last few years, although researchers are at work on better methods. At this time, you need a negative Western blot antibody test, using cerebral spinal fluid (CSF), to rule out EPM.

The downside to the Western blot test is a false positive, due to blood contamination and because a positive CSF titer has been found in experimentally infected horses as a transient phenomenon. However, a positive CSF titer combined with obvious neurological signs is about 95% accurate and remains your strongest indication at this time for EPM.

EPM treatments often refer to the “trads,” which is short for traditional and refers to the most common drug combination, pyrimethamine and sulfadiazine.

The “trads” are “-static” drugs, which means they slow the metabolism and reproduction of the organisms, but they don’t actually kill them. The hope is that by getting the infection under control, the horse’s own immune defenses will then be able to eliminate it.

The antiprotozoal Baycox/toltrazuril is a -cidal drug, meaning it kills. This drug is used to eliminate intestinal infections of protozoa in chickens and pigs but isn’t FDA-approved for use in EPM. However, neither are the “trads.” The Baycox manufacturer suggests a 28-day treatment course, based on field experience and limited trials.

A few other antiprotozoals have been tried, including diclazuril and nitazoxanide, but these are still considered investigational. Even the common antibiotics oxytetracycline and doxycycline have been found to produce some improvement in EPM horses, but the organisms appear to become resistant quickly, so this approach is not recommended.

Hopes of an effective treatment/cure for EPM ran high when Bayer announced FDA approval of Marquis (ponazuril). Ponazuril is also a -cidal drug, and it was believed to be a superior to the trads. However, about 60% of the treated horses showed improvement, which is similar to the rate noted with trads therapy.

But, with Marquis, many horses go through the period of temporary worsening that signals organisms are being cleared earlier in the treatment period than they do when receiving the trads. This is probably related both to the fact that Marquis is -cidal and also to the drug reaching effective concentrations in the horse’s body more quickly.

Many horse owners and some vets mistakenly believe a three-week treatment with Marquis, an expensive drug, will cure EPM. However, while some horses do only require a three-week treatment, many others need longer treatments. Relapses several weeks or months after treatment can occur in Marquis-treated horses just as they can with other therapies.

Supplemental Therapies
A variety of over-the-counter and vet-administered/prescribed supplements and therapies are available for EPM, some as actual antiprotozoal treatments and others designed to complement prescription drugs.

Heavy vitamin E supplementation, 5,000 IU/day or more, is usually recommended. Vitamin E is an antioxidant involved in maintaining the integrity of the nerves and protecting them from inflammatory damage. Supplementation with vitamin E, copper, selenium, zinc, vitamin C, and bioflavinoids is a good idea.

The B vitamins, especially choline, folic acid and pyridoxine, are also often added to counteract any negative effects of EPM medications on the horse’s own B-vitamin status. We suggest using a broad-spectrum, potent B supplement. Stress increases the horse’s need for B vitamins, and EPM certainly qualifies.

High-dose MSM is often used as an anti-inflammatory, which is fine if your horse benefits from it. However, MSM alone may not be enough to handle the inflammation of an active EPM infection or that which occurs when the horse undergoes treatment. At those times, your vet will use either IV DMSO or NSAIDs such as bute or banamine.

Supplements aimed at protecting the integrity of the intestinal tract, such as those from Vita-Royal (www.vitaroyal.com, 810/653-5478, see July 1999), are also a logical support-therapy approach. The horse’s intestinal lining is his first barrier against invasion by these protozoa.

Ulcers, both from stress and periodic use of NSAIDs, can be a problem for EPM horses, and it only makes sense that any breaks in their intestinal lining will increase their risk of reinfection. However, a drug for ulcers may not be the best choice, since the horse’s stomach acid is also an important defense mechanism and these ulcer drugs all work by shutting down acid production.

Bottom Line
If your horse is positively diagnosed with EPM, follow your veterinarian’s advice to the letter and employ good nutrition through the judicious use of antioxidants to boost your horse’s natural immune system. Marquis is not a 28-day miracle cure, but it does lead to rapid improvements. A cure is defined as a clean CSF, and we’d continue some treatment until then.

EPM prevention is nothing more than working to minimize opossums on your farm area, minimizing your horse’s stress loads and mazimizing a healthly immune system through proper nutrition.

Also With This Article
Click here to view ”Most Common EPM Symptoms.”
Click here to view ”What Ab out Alternative Cures’”
Click here to view ”It’s In The Blood.”
Click here to view ”What To Expect From EPM Treatment Programs.”