By Annie Lambert

There is little to celebrate when it comes to advances in diagnosing and treating EPM, but optimistic research continues.

Blame it on the opossum! Without the medium-sized marsupial that lives in North and South America, horses might have one less health problem to worry about. More specifically, Equine Protozoal Myeloencephalitis.

As debilitating as EPM is to equines, it is not physically painful for afflicted horses. In fact, owners of infected horses will probably suffer more with the frustration of dealing with the condition than their animals.

The diagnosis of EPM has become infamous due to imperfect testing and hard-to-define symptoms. The search continues for a gene marker to create an accurate field test for the disease. In the meantime, new and more satisfying testing procedures continue to be researched and new drugs, or combinations of drugs, considered.

Hopeful Improvements

Opossum are responsible for EPM.
The opossum, a relatively harmless marsupial, is responsible for EPM infection in equines. Photo courtesy Cody Pope.

The University of California at Davis has IFAT [Indirect Fluorescent Antibody Test], the only quantitative test for EPM to date, which is only available through UC Davis. According to Nicola Pusterla, Assistant Professor in the Department of Medicine and Epidemiology at Davis, the test has been a huge asset in diagnosing EPM for about four years.

“The IFAT pinpoints titers, which gives you positive results,” Pusterla says. “It works for infected horses, and with the titer, our statistician has calculated probability of disease. The higher the titer, the more likely it is that the neurologic horse has the disease.”

EPM is caused by one of two protozoa—Sarcocystis neurona and, less commonly, Neospora hughesi. In addition to being the only quantitative test, IFAT is also the only one that screens for both of these organisms.

Danny Dutton, DVM, Chief of Staff at Equine Sports Medicine & Surgery’s Houston Field Division, uses the UC Davis EPM panel to help diagnose EPM infection. The Texas A&M graduate says testing the cerebral spinal fluid is his best bet to diagnose EPM, but the added risk and overall expense of a CSF tap often leaves it a secondary option.

“The procedure itself isn’t that expensive, but it is a little bit dangerous,” he says. “You could introduce an infection to the spinal cord and cause a meningitis, and when a horse is several hours from a hospital to get that procedure done, sometimes the owners are not willing to go through the added frustration and expense.

Diagnosing and treating EPM involves looking for severe atrophy of muscles.
Severe atrophy of muscles, as shown here over the croup, is a common symptom of EPM. Photo courtesy UC Davis.

“Some people do the tap on a sedated horse standing and take the sample from the [lumbosacral], and some lay the horse down and collect the sample from the [cisterna magna] space up by the skull. There is a much bigger pool of fluid to get into, but those horses have to be anesthetized, and that also increases your risk and expense.”

“I like to run the UC Davis EPM panel, which is the Western Blot and IFAT first,” Dutton continues. “The Western Blot is only useful if it comes back negative, meaning they have not been exposed and can’t have EPM. If the IFAT comes back with a high titer, a high probability that I’m dealing with EPM, I might not do the spinal tap. I might just treat the horse based on the test results. There is still a little bit of a gap, it is not 100 percent accurate, but it is a good guess.”

Dutton expresses a lot of frustration with often facing a less than clear-cut diagnosis. Once he has ruled out neurological diseases that mime symptoms of EPM, x-rayed the neck for injuries and the tests come back at 50 percent or less that the horse has an active EPM infection, he is left with the sole option of a CSF tap.

“There have been improvements, but it is still very difficult to diagnosis,” Dutton says of EPM. “I think both horsemen and veterinarians are frustrated because of those difficulties. Basically, the only way you can be 100 percent sure a horse has EPM is a post mortem diagnosis. Of course, that is less than desirable.”

Infected or Exposed?

Horses can contract EPM from eating or drinking opossum feces.
Horses can contract EPM by eating hay or drinking water that is tainted with opossum feces. Photo by Annie Lambert.

Ingesting the protozoa in feed or water contaminated with opossum feces causes exposure or infection in the horse. In about 98 percent of afflicted horses, the animal’s immune system will clear the protozoa from the blood before it crosses the blood brain barrier, moving into the central nervous system. Horses that do not develop an active infection are referred to as having “exposure to EPM” and may carry antibodies to the protozoa for life. No neurological symptoms are caused by exposure to the EPM protozoa.

In horses that develop an active infection, the protozoa live inside the cells in the central nervous system and are not detected by the immune system. Stress, anything from hauling to poor nutrition to non-related illnesses, can suppress the immune system, allowing the infection to thrive. The protozoa reproduce, break out of their host cells and spread to alternate areas of the brain and/or spinal column. Pronounced neurological symptoms appear as the active protozoa cause lesions to form in the central nervous system.

According to researchers, the incubation period of EPM, the period between exposure and outward symptoms, can be from weeks up to two years. As with nearly all diseases, the sooner the horse has been diagnosed and treatment begins, the better the prognosis for recovery. Untreated, most symptoms progress until the disease causes the death of the horse.

Infection in Action

Researchers have no answer as to why most horses are able to fight off an active infection of EPM, while approximately two percent become clinically ill. They do know EPM is not contagious between horses, that it affects almost all breeds of horses from six months to over 30-years of age and that all genders can be afflicted.

Once the horse has ingested food or water contaminated with opossum feces containing the protozoa, it travels through the digestive tract and into the bloodstream. Once inside the central nervous system, the protozoa reproduce very slowly. The horse’s immune system is unable to detect them because they are hiding inside other cells. It is believed stress causes the protozoa to reproduce more rapidly, breaking out of the host cells, finding new cells and reproducing again as they travel through and populate the central nervous system.

“The brain is at the mercy of the blood brain barrier for keeping things like viruses and bacteria out,” Dutton explains. “Sometimes the blood brain barrier gets compromised and parasites, viruses and things have learned how to get through it. Much like Rabies, which has a proclivity for nervous tissue, the protozoa travel into the brain and set up shop.”

The protozoa ultimately kill their host cells as they leave and move into new cells. Groups of killed cells are lesions (somewhat like scar tissue) within the central nervous system. Growing lesions jumble the communication, both coming and going, between the brain and muscles, causing behavior changes in the horse. In addition, lesions can cause inflammation of the CNS.

Lesions can occur anywhere in the brain or CNS, meaning symptoms can show up anywhere in the head or body. Lesions can cause permanent nerve damage. Horses may learn to compensate for the loss of feeling or may generate new nerve pathways around the damaged cells.

The horse begins to compensate for the growing parasite population within the CNS. Often the only outward signs of the immune system fighting the disease may be the horse acting tired or a lack of coordination causing an occasional stumble. Once the horse is no longer able to compensate, his neurological symptoms become more apparent to humans.

Why only a small percentage of horses exposed to the protozoa diverge into a full-blown, active case of EPM remains a mystery. With few clues to date, researchers suggest the best preventative strategies should be avoiding opossum, maintaining good general health and controlling stress.

A Difficult Diagnosis

In diagnosing EPM, it is caused by one of two protozoa.
EPM is caused by one of two protozoa—Sarcocystis neurona and, less commonly, Neospora hughesi. Photo courtesy PHIL.

Diagnosing EPM is difficult at best, and veterinarians admit it can easily be misdiagnosed due to the fact that symptoms can occur anywhere in the body and mimic the neurological symptoms of several other diseases. Proper diagnosis is generally made from a pattern of symptoms that cannot be ruled out as other diseases or soundness problems.

When giving a neurological exam the veterinarian searches for physical symptoms and grades the severity of each. Deficits—which include muscle atrophy, abnormal facial movements, sweating, eyesight and other abnormal indicators—are graded on the Mayhew Scale, a standard test.

Once EPM infection has been confirmed, treatment should begin pronto. Any delay allows the disease to progress, and the potential of additional, permanent damage to the central nervous system increases.

The usual treatment includes a course of drugs known to kill or retard the reproduction of the protozoa. The drugs require a minimum number of days to build concentration in the cerebrospinal fluid to a level that will kill or impede the protozoa. Research has shown the best results are obtained when horses receive these oral drugs daily and at the same time each day—and like any prescription drug, under the supervision of your veterinarian.

Based on clinical studies, drugs used to treat EPM have a success rate in the 57 to 61 percent range. Successful treatment is one that has reduced the neurological symptoms by ‘one’ on the Mayhew Scale. However, the actual prognosis for the animal may seem less than successful for the owner.

Dutton says only about 50—60 percent of horses that are treated are going to recover and not all of those horses will recover fully.

“That’s why it is discouraging,” he admits. “It is a really tough disease to diagnose, and even with the best treatment, the outcome is not always as good as you wished it would be.”

Beware of Miracles

Equine veterinarians warn that there is no miracle cure for Equine Protozoal Myeloencephalitis. Products that advertise the ability to prevent or cure EPM should be carefully researched and scrutinized.

Any product manufacturer claims should be backed up with clinical studies involving 50 or more horses, and an independent laboratory or university should have completed those studies. Also, animals used in the study should have a confirmed, active infection rather than just having been exposed to the protozoa—98 percent of horses will clear an exposure to EPM on their own.

Treatments that remedy EPM must be able to cross the blood brain barrier to attack the active infection in the central nervous system. They must also have the ability to enter individual cells within the central nervous system to kill the protozoa living within other cells. Remedies falling short will not kill the infection.


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