Keeping an Eye on EPM this Fall

By Bryant Craig

Though EPM can be diagnosed anytime of year, it is more likely in the fall months. Before prescribing treatment, make sure a thorough diagnostic workup is completed.
Veterinarian giving horse owner Protazil

The risk of equine protozoal myeloencephalitis (EPM) is six times higher in the fall.1 Opossums and other wildlife that serve as hosts to the EPM-causing parasites, Sarcocystis neurona and Neospora hughesi, are more active when the weather begins to cool. Autumn is also a time when months of heavy exercise, transport and other stressful events for horses can take a toll, which can increase their risk of EPM.

Although the incidence of EPM clinical disease is low, up to 90% of the U.S. horse population has been exposed to S. neurona, depending on geographic location.1 This creates a high seroprevalence in healthy horses, which can make diagnosis tricky, yet all the more important.

Diagnose before treating

While early treatment is critical to stopping the disease from causing further nerve damage, if the horse does not have EPM, an EPM treatment product will not be effective. Many diseases cause symptoms similar to EPM but will not respond to EPM treatment. If that horse has another disease, not only have we wasted money on unnecessary treatment, but also time that could have been better used pursuing the true cause of the horse’s problem.

A thorough diagnostic workup is critical to make sure we are treating the horse appropriately and improving his chance of recovering. The best initial course of action is a combination of reviewing the health history of that horse and conducting a thorough neurologic and physical exam.

The organism will generally cause asymmetric deficits. Experts say it is rare to have equal deficits on both sides of the horse, so asymmetry is a tell-tale marker of a suspect EPM case. EPM is a progressive, multi-focal disease, often with muscle atrophy. While other clinical signs may be present, these are generally the more common signs in EPM cases and point to the need for antibody testing to further differentiate EPM from other neurological diseases.

The recommendation for antibody testing is to collect spinal fluid and blood samples and compare the antibody titers in each to determine if there is evidence of a central nervous system infection.

Horses treated with an anticoccidial drug, like PROTAZIL® (1.56% diclazuril) Antiprotozoal Pellets, are 10 times more likely to improve than untreated horses.

Early disease treatment critical to recovery

For those that do succumb to EPM, irreversible damage to the brain or spinal cord is more likely to occur if the parasite has been present for long periods of time. Without treatment, many horses diagnosed with EPM progressively deteriorate to the point of recumbency. Horses treated with an anticoccidial drug, like PROTAZIL® (1.56% diclazuril) Antiprotozoal Pellets, are 10 times more likely to improve than untreated horses.1

Diclazuril, the active ingredient in PROTAZIL, is proven to cross the blood-brain barrier and enter the cerebrospinal fluid within hours of administration at levels high enough to either limit the reproduction of protozoa or kill them outright – without a loading dose.2 PROTAZIL is a top-dressed feed medication, ideal for horses with a reluctance to accept oral medication.

Few diseases are more frustrating than EPM. The industry continues to grapple with and study this “master of disguise,” including how best to definitively diagnose it in a live horse and whether any preventive strategies might be appropriate (or on the horizon). Of certainty, EPM continues to threaten horses in large numbers and because it has no single defining clinical characteristic, any neurological horse could be a candidate – making your task in managing EPM a difficult one.

1 Reed SM, et al. Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment and Prevention. J Vet Intern Med 2016;30:491–502.
2 Hunyadi L, Papich MG, Pusterla N. Pharmacokinetics of a low-dose and FDA-labeled dose of diclazuril administered orally as a pelleted top dressing in adult horses. J of Vet Pharmacology and Therapeutics (accepted) 2014, doi: 10.111/jvp.12176
Copyright © 2020 Intervet Inc., d/b/a Merck Animal Health, a subsidiary of Merck & Co., Inc.

For additional information on EPM

In February 2016, the American College of Veterinary Internal Medicine published an updated consensus statement on equine protozoal myeloencephalitis (EPM), with a focus on parasite biology, diagnosis, treatment and prevention.

Download the consensus statement ›
Important Safety Information
Use of Protazil® (1.56% diclazuril) is contraindicated in horses with known hypersensitivity to diclazuril. Safe use in horses used for breeding purposes, during pregnancy, or in lactating mares has not been evaluated. The safety of Protazil® (1.56% diclazuril) with concomitant therapies in horses has not been evaluated. For use in horses only. Do not use in horses intended for human consumption. Not for human use. Keep out of reach of children.

About the Author

Bryant Craig

Equine Technical Services Veterinarian
Merck Animal Health
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