EPM Management

The words equine protozoal myeloencephalitis first appeared in this publication more than five years ago, and while the scientific community has made great advances in unraveling certain aspects of this disease, other areas still beg for answers. In the past, increased awareness--and fear--of EPM led to it being "diagnosed" as the cause of everything from lameness to a change in behavior. Now, horse owners and their veterinarians are accepting that EPM is not the cause of every stumble, limp, or failure to perform up to expectations. They instead are relying on good medical diagnoses to eliminate other causes of problems, and are putting horses on available treatments earlier in the disease process if EPM is determined to be the cause of a physical problem.

"We're seeing it just as much as before," said Doug Byars, DVM, Dipl. ACVIM, ACVECC, head of the medicine facility at Hagyard-Davidson-McGee veterinary practice in Lexington, Ky. "Now, we're taking a more rational approach."

That rational approach, however, is tempered by problems with diagnostic testing and unproven medical treatment. While five years of field use of certain drugs and combinations of drugs has given veterinarians means by which to address the problem of EPM, practitioners don't see one as better than any other.

"There are no studies that show one product to be better than another," said Bill Bernard, DVM, Dipl. ACVIM, of Rood and Riddle Equine Hospital in Lexington, Ky. "All seem to give us the same percentage of improvement and response to therapy. But economics have to be considered."

Each of these experienced veterinarians has seen hundreds of cases of EPM in the past half-decade. Their advice to horse owners is the same: Get an appropriate diagnosis to start with, then treat based on what your veterinarian discovers.

Other neurologic problems or diseases need to be ruled out before assuming symptoms are due to EPM. Wobbler syndrome, especially in Thoroughbreds, Thoroughbred crosses, and other fast-growing breeds, is a common cause of neurologic signs in horses. Wobbler occurs when vertebrae in the neck press on the spinal cord, and can be caused by bony compression in the spinal canal, laxity of the intervertebral ligaments, and abnormal bone development, such as osteochondritis dissecans (OCD).

While there aren't many diagnostic tests available for horses with neurologic problems, radiographs of the neck (including mylograms, which involve an injected dye before radiographs to help veterinarians visualize problems with vertebrae) can be diagnostic. Veterinarians actually can see where the vertebrae are pressing against the spinal cord, and in some cases can surgically correct the abnormality.

There are several diseases or problems that can cause neurologic signs in horses, including encephalitis, West Nile virus, botulism, lower motor neuron disease, and rabies. There are diagnostic and serologic tests that can be done to rule out some of these diseases. There also are tests that can be done on horse blood serum and cerebrospinal fluid to detect antibodies for EPM. While used extensively in the early years of the disease's prevalence, they are used less today, and used more wisely. Veterinarians also stress that EPM can be present at the same time as any of these other diseases.

"Owners must realize that diagnostic tests (for EPM) need to be used in the right way," emphasized Bernard. "The positive CSF tap is not diagnostic. It can be good to do the tap because if it's negative, then it's diagnostic (that the horse doesn't have EPM)."

A positive spinal tap could be the result of blood conatamination.

Byars added that a spinal tap is not the only tool used for diagnosis. A negative serum or blood sample means that the horse is less likely to have the disease. Byars said that this test is statistically the most correct, however, it can also be misleading. "There is no perfect test," he said.

Management Is Key

Owners need to understand that diagnosis is difficult, if not impossible, with EPM. A horse can be exposed to the single-celled protozoal parasite that causes the disease, but not have the disease. Most horses are exposed, develop antibodies, and their immune systems fight off the parasite or prevent the disease from causing symptoms. Therefore, management to prevent exposure to the disease is critical.

Management factors you can modify are:

  • Keep feed sealed to prevent contamination.

  • Remove opossums from the farm and barn or discourage their presence.

  • If feeding horses outside, do not feed on the ground to avoid contamination.

  • Reduce or eliminate birds from barns, especially around feed and water.

  • Reduce contamination of water sources by opossum feces.

  • Acquire hay from areas without opossums to avoid contamination by feces.

In the Oct. 15 issue of the Journal of the American Veterinary Medical Association (JAVMA), William J. Saville, DVM, PhD, Dipl. ACVIM, of The Ohio State University, and colleagues reported an analysis of the risk factors for development of EPM in horses. These were factors that either can't be controlled or are difficult to control, such as age, location, and type of performance undertaken by the horse. (Some of this information was presented by Saville at the American Association of Equine Practitioners meeting in 1999 and reported in the AAEP Wrap-Up in The Horse of February 2000.)

In the article, Saville and his colleagues reported that factors associated with an increased risk of a horse developing EPM included:

  • Age.

  • Season.

  • Prior diagnosis of EPM on the prem-ises.

  • Opossums on the premises.

  • Health events prior to admission.

  • Racing or showing as a primary use.

The higher risk of developing EPM seemed to be in younger horses--from one to five years of age--and older horses--greater than 13 years of age.

There was a higher risk of horses having EPM among those admitted during spring, summer, and fall, compared to horses admitted during winter. It could be that heat acts as a stressor for horses, or that the causative parasite, Sarcocystis neurona, is killed or less able to infect equines in sub-freezing weather. Also, most competitions are during the sum-mer, and stress (competition and travel) couldplay a role in advancement of the disease.

The study showed an increased risk for horses living on premises where EPM had been diagnosed. This could be because whatever factors caused the first horse to develop the disease still are present.

An interesting finding of the study was that farms with a creek or river on the property or in close proximity had a lower risk of EPM. Researchers speculate that the preferred habitat could lure opossums away from barns and feed supplies and reduce contamination in horse areas. However, there was a higher risk for horses developing EPM if opossums commonly were seen on the premises.

Other studies have shown that injured or ill horses are less able to fight off disease. Therefore, it would be plausible to assume that horses with an injury or illness would be more susceptible to developing EPM.

Saville and his colleagues reported that using horses for racing or showing was associated with increased risk of developing EPM. They concluded that exercise intensity might affect immune function.

"In horses and humans, moderate exercise enhances the immune system, but high-intensity, exhaustive exercise suppresses the immune system," they reported.

Another study published in the same JAVMA issue by Saville and colleagues was the first controlled study evaluating risk factors associated with clinical improvement and survival of horses with EPM. In that study they stated: "The likelihood of clinical improvement after diagnosis of EPM was lower in horses used for breeding and pleasure activities. Treatment for EPM increased the probability that a horse would have clinical improvement."

Research has suggested a rate of clinical improvement in treated horses between 60%-70%. Reported relapse rates have ranged between 10%-30% in uncontrolled studies. Neither the prognosis for survival nor return to normal function have been reported in controlled studies.

The study noted that horses used for breeding or pleasure activities were less likely to improve after diagnosis than horses used for racing and showing. They suggested this is true because the breeding/pleasure population is made up of older horses with a less-than-optimal exercise program.

There is some disparity in results of treatment based on severity of clinical signs, according to the study.

"The majority of horses with mild neurologic deficits improved after treatment (74.1%); whereas fewer horses with moderate (57.8%) or severe (50%) neurologic deficits improved after treatment. Horses with EPM that were treated were 10 times more likely to improve than untreated horses."

"This study documents the need for controlled studies of compounds to scientifically investigate future pharmacologic treatment for this disease," the researchers stated.


According to Byars, there is no "gold standard" for treatment of EPM. There are no products licensed and labeled for use against this disease in horses. Some have been used based on assumptions from human or other animal medicine, and FDA approval is pending on at least two products.

"I think when the FDA approves one, we will use it more," said Bernard."

Current therapies being used in horses with EPM include pyrimethamine usually in combination with a potentiated sulfa drug, which was the first treatment used and probably still is the most-used. However, it is not recommended for use in pregnant mares in the last trimester.

Other treatments include Nitazoxanide (NTZ), oxytetracycline (a stop-gap mea-sure), Toltrazuril, and Diclazuril. None of these is labeled for use in EPM horses, and the equine community still is awaiting federal approval of these treatments.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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