What's New With Equine Protozoal Myeloencephalitis?

It's dangerous. It's debilitating. And sometimes it's deadly. It's equine protozoal myeloencephalitis (EPM), and researchers have been studying this neurologic disease for years to try to understand it better. Scientists have made great strides in their comprehension of how the causative agents work, what they do to the body, and how to prevent them from debilitating a horse to the point of requiring euthanasia.

At the 2011 American College of Veterinary Internal Medicine Forum, held June 15-18 in Denver, Colo., Frank Andrews, DVM, MS, Dipl. ACVIM, professor at Louisiana State University School of Veterinary Medicine, presented the latest information in an overview of EPM.

Andrews began by reviewing the life cycle of EPM's causative protozoa (Sarcocystis) as it passes from the feces of opossum and other less common host species and contaminates feed or forage that is ingested by the horse. Once in the horse's intestinal tract, protozoal sporocysts reproduce, changing form into merozoites, which migrate into the blood where they become engulfed by white blood cells. In this way, protozoa enter the central nervous system (CNS) and do damage within the brain and/or spinal cord.

"The insidious onset of clinical signs can take a while to appear," explained Andrews.

Typical clinical signs include asymmetric muscle wasting, general weakness, ataxia (incoordination), spastic movements, obscure lameness, and/or signs associated with cranial nerve damage. Clinical signs vary because the protozoa's location within the neurologic system determines which signs the horse exhibits.

Current Diagnostic Options for EPM

Diagnostic testing is based on antibodies to Sarcocystis surface antigen (SAG) proteins. There are many tests to evaluate this, each with differing degrees of sensitivity (percentage of true positives) and specificity (percentage of true negatives), but the traditional Western blot test remains the standard for EPM diagnosis. However, Andrews noted that better quantitative tests are being developed and could prove more accurate in diagnosing EPM.

In addition, evaluation of albumin and specific EPM antibodies (immunoglobulin G) in cerebral spinal fluid (CSF) might yield information regarding whether a horse has an EPM infection in the CNS or whether another unrelated neurologic disease is affecting the horse.

A large proportion of horses in the eastern United States (about 63%) show blood titers consistent with exposure to Sarcocystis, whereas less than 1% of the population of neurologic cases tested for EPM exhibit actual disease related to neurologic damage.

Andrews explained, "A negative test is strong evidence that a horse does not have EPM, and a weak positive is interpreted as a negative."

Andrews also noted that there has been no significant change in number of cases of EPM diagnosed over the past 17 years.

Andrews stressed that there is no value in testing asymptomatic horses that have no evidence of neurologic disease--including during prepurchase examinations. Testing is more valuable as an aid in diagnosis to help identify the cause of neurologic signs. A veterinarian should conduct complete physical and neurologic exams before determining whether to proceed with testing and should use information gleaned from these exams to help diagnose a horse with EPM. The best test material for EPM is a non-blood-contaminated sample from a CSF tap.

Current EPM Treatment Options

"The medical goal of EPM treatment is to decrease inflammation," Andrews explained. Anti-inflammatory medications commonly used in EPM treatment include dimethylsulfoxide (commonly known as DMSO), corticosteroids in the initial days of an acute phase, and vitamin E supplementation.

Andrews noted that several FDA-approved medications are designed specifically for EPM treatment. Ponazuril is an effective anticoccidial drug (designed to eliminate coccidial parasites) given for 28 consecutive days as an oral paste. It has been observed to achieve 60% improvement success in horses ("Improvement" is defined as an improvement of at least one grade on the CNS exam at the completion of therapy). Andrews noted that drug concentrations reach effective levels in the CNS after seven to eight days, with neurologic signs sometimes worsening (before improvement begins) around Days 8 to 12 of treatment. Once ponazuril reaches adequate tissue levels in the CNS, its goal is to kill off the causative protozoa. Many veterinarians recommend using NSAIDs as an adjunctive treatment to prevent associated inflammation and worsening of neurologic signs.

Another approved medication, diclazuril, is administered once daily for 28 days (in the form of an alfalfa-based pellet) with an overall improvement rate of 67%.

Andrews stressed, "It is important to understand that although the parasite may be cleared by treatment drugs, the existing CNS damage remains (simply, once the damage has been done, there's no way to reverse it). Therefore, a horse's prognosis depends on severity and duration of the infection prior to treatment."

Andrews also urges owners to include physical therapy (longeing and groundwork) in their horse's regular schedule because in some cases, he said, the horse can learn to compensate for his "disabilities" with continued work. In addition, follow up with a sulfadiazine/pyrimethamine combination for an additional two months to help prevent relapses, which can occur in 20-30% of treated cases.

Andrews advises that prevention is key to success for any disease, but particularly so for EPM. Storing feed and hay properly out of reach of wildlife, especially opossums, reduces or eliminates the risk of exposure in the first place.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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