Despite its widespread occurrence in the United States, equine protozoal myeloencephalitis remains mysteriousand sometimes misunderstoodSome researchers have estimated that 50% of the horses blood-tested in the United States would test positive for EPMequine protozoal myeloencephalitis, or equine protozoal myelitis.
Understandably, that kind of exposure rate for a disease which can cause debilitating nerve damage and wobbler-type symptoms has put horsemen on guard. It is easy to see why: EPM is most effectively treated when it is recognized early, but it can be overlooked or misdiagnosed because it may look like any of a variety of ailments, from vague hind-end lameness to wobbler syndrome. And once you have determined a horse does have it, treatment can last 412 months or beyond at an estimated monthly cost of $800. Some buyers of Thoroughbreds have considered requiring negative EPM tests before they will bida nightmare for consignors, since mere exposure to the disease can cause a blood positive, even if the horse never comes down with clinical symptoms.
Fear and loathingand better testing procedureshave made EPM one of the most talked-about diseases in the United States. It is hard to know whether the disease is actually occurring more or if we just recognize it more readily, but there is no doubt that horsemen throughout the country are asking more questions. In response, the University of Kentuckys Gluck Equine Research Center in April established an EPM information hotline for phone queries. But even as awareness has risen, EPM remains elusive for researchers and veterinarians, who have tried for years to unlock the basic mystery of its cause and life cycle.
When we know who the culprit is, we can actually do experiments on how to prevent it, said Clara Fenger, a Ph.D. candidate at the Gluck Center who specializes in the study of EPM and is working with EPM expert Dr. Dave Granstrom. There are a number of compelling ideas, like coming up with some kind of a feed supplement, or some kind of safe low-level drug, that you can put directly on the horses feed for life and prevent them from ever becoming infected. And of course a vaccine is something wed like to have eventually.

Search for the culprit
Researchers and horsemen hope a break in the caseidentification of the animal that carries and transmits the disease to horseswill come soon. It is widely believed that possumswhich, like EPM, are native to the Americas and do not naturally occur in Europeare the culprits. Although that commonly held suspicion has not yet been tested and confirmed scientifically, many horsemen are stepping up anti-possum,
-pigeon, -raccoon, and -rat measures in their barns and pastures.
There are some simple things that can be done to minimize the possibility of EPM exposure, said Fenger. Everybody has barn cats, which are necessary ... but if you feed your cats in the barn, cat food attracts a lot of critters. Feed in the day and put the food away at night, when they come out. Cover your trash cans with tight-fitting lids. Dont set your feed at night for the following day, and cover your feed tubs so animals cant get into them.
Anything you can do to get rid of birds, which may help spread the disease, probably also will help, Fenger added.
But until the parasites carrier and life cycle is determined, true understanding of how the disease progresses will be hampered.

A partly known parasite
What is known about EPM is that it is caused by a malaria-related parasite known as sarcocystis neurona, which are passed on in the carriers feces and then ingested by the horse, which promptly mounts an antibody campaign against the organism, resulting in a blood-positive state. The parasite replicates as it travels to the central nervous system, where the horses immune responseincluding dangerous swelling of the spinal cordultimately is triggered as the organism invades and damages nerve tissue.
The horse is a dead-end source, meaning that it cannot transmit the parasite to other animals or people, but the damage sarcocystis neurona can wreak on a single victim can be devastating if the disease is not recognized in time. Veterinarians primarily confirm the disease in one of two waysby Western blot blood test or by analysis of spinal fluid.
The blood test may be simpler from the horsemans standpoint but is limited: A positive blood test indicates only that the horse has been exposed to the parasite, so even an individual who has successfully developed antibodies against it and is free of the organism will still test positive. Analysis of spinal fluidwhich can determine that damage to the central nervous system has occurredis considered far more precise, but the necessary spinal tap requires a certain amount of delicacy to prevent the fluid sample from including false-positive blood.
Recently, researchers have developed a third test which identifies the parasite directly by detecting the presence of specific parasite DNA in spinal fluid. This sensitive procedure is believed to enhance early detection capability.
EPM can cause some dramatic changes, but its multifocal tendencies and often vague early symptoms can throw even an astute observer off track. Even more obvious clinical signs can be mistaken for other problems, like wobbler syndrome.
As a general rule, theyre clinically indistinguishable, Fenger said of EPM horses and wobblers. Most commonly, EPM causes an asymmetric ataxia (lack of coordination) which is worse on the rear end than the front. In general, wobbler disease causes symmetric ataxia, which is worse in the rear end than the front. If the horse is only slightly asymmetric, youre not going to be able to tell the difference. Wobbler disease affects them in the neck, so where the damage occurs is quite predictable. However, EPM can be multifocal, and while it appears to occur most commonly in the neck, it can occur anywhere.

Costly treatment
Standard treatment for the disease is an intensiveand costlycourse of medication with the anti-protozoal medications Daraprim (pyrimethamine) or trimethoprim-sulfa, which is available under several trade names. These often are administered in careful conjunction with anti-
inflammatories such as Bute, Banamine, or DMSO to combat the horses own inflammatory response to nervous system damage.
Daraprim is the only treatment we have available, said Fenger, who added that one problem with Daraprim treatment is that the parasites can become immune to it, particularly if the horses treatment is stopped and restarted according to evidence of clinical signs, or if he is given an amount smaller than 20 tablets (one milligram per kilogram of body weight) per day.
Fenger recommends continuing treatment steadily for a minimum of 412 months but adds that other drawbacksside effects such as anemiacan occur over the long term, which may tempt horsemen to remove the horse from treatment.
Cases vary, with some horses able to continue training during treatment and others requiring nearly five months off while receiving medication. The success rate for returning EPM-affected horses to the racetrack, however, can be quite highprovided the disease is caught and identified earlyand EPM victims have been known to take up rewarding careers after bouts with the disease.
Until the EPM carrier is identified and the parasites complete life cycle is charted and understood, progress toward more effective prevention and treatment will be slow. Until those keys unlock the mystery of EPM, researchers encourage veterinarians and horsemen to be vigilant.
Right now, all we can do is identify it and treat it, Fenger concluded. The prognosis is excellent if the horse is caught early and treated long enough, but very few horses are caught early and treated long enough.