Special Report: EPM

The world of equine health will never be the same. That statement seems out of context when talking about one research project about one parasite that causes one disease. But, horse owners know that equine protozoal myeloencephalitis (EPM)--the multi-faceted neurologic disease caused by the single-celled protozoal parasite Sarcocystis neurona--is a tremendous problem in the horse industry. One research group estimated that just the diagnosis and treatment of horses with clinical signs of EPM costs over $1 million each year! And that does not take into account loss of performance time and earnings, nursing care, extra cost of ancillary management and upkeep, and the potential loss of the animal never returning to his original level of competitiveness!

The research project from the United States Department of Agriculture (USDA) and The Ohio State University that carved the path for huge strides to be made against EPM determined that the common domestic cat could be used to complete the life cycle of the parasite in the laboratory (see sidebar below). This knowledge will open doors to create a way to give the disease to horses in order to test vaccines and treatments.

The research project itself was a demonstration of how the world must face disease problems of the future. It was conducted by a group of researchers headed by J.P. Dubey, BVSc, MVSc, PhD (parasitology), a senior scientist with the USDA; and William (Bill) Saville, DVM, PhD, Dipl. ACVIM, of The Ohio State University. Other collaborators were D.S. Lindsay, PhD, of the Virginia-Maryland Regional College of Veterinary Medicine; R.W. (Bill) Stich, BS, MS, PhD (parasitology) of Ohio State; Jon F. Stanek (a student working on his masters who will enter vet school at Ohio State this fall); C.A. Speer, PhD, formerly of Montana State University now at the University of Tennessee; B.M. Rosenthal, PhD, USDA; Chinedu Njoku, DVM, PhD (veterinary pharmacology), a post-doctorate research associate at Ohio State; O.C.H. Kwok, PhD, of USDA; S.K. Shen, PhD, of USDA; and Steve Reed, DVM, Dipl. ACVIM, of Ohio State (clinician). Input also was received from David Granstrom, DVM, PhD, Dipl. ACVIM, a former full-time EPM researcher currently working with the American Veterinary Medical Association (AVMA), and Michael Oglesbee, DVM, PhD, a neuropathologist at Ohio State who assisted the team.

Why did it take so long--and take so many different talents--to solve the life cycle of a single-celled parasite? Because it is a member of a family of parasites that has proven over the decades to be difficult to understand, and control. That family includes malaria, which can be treated in humans, but not prevented or cured. However, that family also includes trichomoniasis in cattle and giardia in dogs and cats, and both of those parasites have a federally licensed vaccine on the market (created by Fort Dodge Animal Health, the makers of the current EPM vaccine released under conditional license; see page 14).

An EPM Society recently was formed to bring together researchers to share information and knowledge in order to encourage even quicker advances. The EPM Society is modeled after one created several years ago to solve the problem of heartworms in dogs. That project involved a wide range of researchers and institutions each contributing to the solution. And now, as pet owners, we know that with proper management and treatment, heartworm is a preventable disease.

Hopefully, horse owners will be able to look back in a few years and say the same thing about EPM.

The Mastermind

There are many areas of study when it comes to EPM. There is the life cycle and what that means to understanding the parasite and how to stop it. There is the way in which it infests and spreads in the horse. There are all the various treatments and preventives that could be used or currently are in use. There are various management techniques and risk factors to be considered and put into use at the farm level.

It should be noted that researchers in the past have fed horses S. neurona sporocysts, had the horses develop neurologic signs and antibodies, but could not recover the parasite in any tissue cultures. This suggests that most horses can eradicate S. neurona from their tissues, and highlights the need for developing a model whereby horses reliably can be given EPM in a laboratory setting.

A group of researchers headed by Dubey recently published a review article in Veterinary Parasitology of all the previously published and known scientific information concerning EPM. Dubey is with the USDA's Agricultural Research Service, Animal and Natural Resources Institute, Parasite Biology, Epidemiology, and Systematics Laboratory at the Beltsville Agricultural Research Center in Maryland. The review paper was co-authored by Lyndsay, Saville, Reed, Granstrom, and Speer. However, it was Dubey who first came up with the idea of using the domestic cat as the laboratory model for the intermediate host in the S. neurona life cycle.

Dubey, a former professor at Ohio State, approached the research group at his former alma mater to assist him in conducting the experiments that completed the life cycle. Using the cat wasn't a new idea to Dubey, who had seen a natural S. neurona infection in a cat in 1994. He had done work with the S. neurona parasite in laboratory mice, which was the only animal model available, but wanted to develop a model in a larger animal.

Besides the cat and the horse, an EPM-like disease also exists in raccoons, minks, skunks, a pony, a zebra, Southern sea otters, and Pacific harbor seals. Completing this first life cycle for use in the horse will also help in solving problems in these other animals.

But while this seems like a breakthrough of only a few short months of work, it actually was typical of research. It took years to come to the point of understanding the disease enough to develop the research, and two years to get to the published, peer-reviewed point we are at today.

"Ohio State played a vital role" in the research, said Dubey. "We've known this (cat model) for about two years, but it took all this time to prove it! We have no idea if the cat is the 'natural' host. We don't know. Theoretically it is possible, but it is not an easy thing to find out."

Ongoing research is looking at the role the cat might play in the farm environment. However, researchers warn that before horse owners start eliminating cats from the barn, they should consider the consequences. What if mice or other small mammals are part of the natural life cycle of S. neurona? If cats are eliminated, then it could make the problem worse! Enough is known at this point to say if you have a dead cat on your property, remove it so opossums can't eat it.

Dubey stressed the most important thing now is understanding the conditions under which the disease is induced in the horse. As was said earlier, many horses are exposed to the parasite, but only a few get the disease. Dubey added the following two points.

"We need to understand the mechanisms of the disease because without that, there is no way to evaluate drugs. We also know that diagnosis is still a problem. We can look at drugs like Diclazuril in cell culture, and that's a stepping stone (in understanding). But we still have to look at things in the whole animal.

"In knockout mice (genetically altered, immunocompromised mice), Diclazuril kills the invading stages, so perhaps it will be useful as a preventive in horses. But will it kill the parasite before it reaches the brain?

"Right now, it's anybody's guess on treatments because we aren't 100% sure if the horse has EPM!" added Dubey. "So if you treated a horse and it got better, you can't actually say the problems were caused by Sarcocystis neurona. Until the model (of creating disease in the horse) can be solved, treatments are guesswork. Only Diclazuril has been scientifically tested in an animal model (mice)."

Dubey reminded horse owners that even though the life cycle has been solved in the laboratory, researchers are far from being able to test drugs. "The disease must be reproduced in the horse reliably," he said.

That means in an experimental setting, researchers can give a horse X amount of the parasite under X conditions and that horse will develop neurologic signs of EPM within X days.

"Horse owners must understand that this is a complicated disease, and there is a lot we don't know," added Dubey. "The new drugs coming out look more promising than the sulfas and pyrimethamines. And the very fact that a company is interested in developing a vaccine is great hope. The government is not interested because the horse is not a food animal. The only two parasitic vaccines marketed (for large animals) are against organisms we discovered (Neospora caninum and Toxoplasma gondii) in cattle."

Dubey also stressed the need and hope for a simpler and more effective test.

EPM In Horses

In 1997, the federal government conducted its first survey of the equine industry to determine what was important, what the problems are, and trends in management practices, among other things. Of the specific infectious diseases listed in the National Animal Health Monitoring System (NAHMS) survey, equine infectious anemia (EIA) and equine protozoal myeloencephalitis (EPM) were the most common top priorities, irrespective of respondents' region, horse use, or number of horses owned.

Horses are an aberrant or dead-end host for the parasite S. neurona. What this means is that the horse can pick up sporocysts (one stage of the parasite life cycle) from the environment, but the life cycle cannot be completed in the horse. However, the sporocysts picked up in the environment can multiply asexually in the horse, so the horse might pick up only a few from the environment, but there might be thousands or millions of merozoites in the infected horse. That initial infection or the multiplication of the parasite triggers the immune system to develop antibodies.

So far, those stages of the parasite have been reported only in the brain and spinal cord of the horse. Researchers have noted that while it is very unlikely that the horse advances the life cycle of S. neurona, sarcocysts (one stage of a parasite life cycle) have been found in the muscles of horses, but so far, not from the species of S. neurona. But, if S. neurona is found in the muscles, that means some other animal could eat that infected muscle and possibly complete the life cycle.

At the present time, it is known that horses do not shed the parasite in their feces, and that horses cannot give the parasite to other horses. It also is known that the parasite S. fayeri, which passes normally from the horse to the dog, does not present a health problem to the horse.

Researchers also note that while the opossum is a definitive host of the S. neurona life cycle, it would be premature to assume that it is the only definitive host. For example, Sarcocystis cruzi is a common parasite of cattle. The definitive hosts are the dog, fox, jackal, coyote, and raccoon.


There are several different medications or combinations of medications currently being used to treat horses with neurologic signs of EPM. As of this writing, however, no medications have been licensed by the FDA for use in treating EPM in horses. Two companies, Bayer Animal Health and Blue Ridge Pharmaceuticals (a subsidiary of Idexx), have submitted paperwork to the FDA on two different treatment medications; Bayer on a Toltrazuril derivative called Ponazuril (toltrazuril sulfone), and Blue Ridge on nitazoxanide, or NTZ). There is no indication when or if either of the treatments will receive approval, or which will be approved first.

Currently, the most common treatment protocol for a horse with EPM is a combination of sulfadiazine and pyrimethamine (which interfere with folic acid necessary for the parasite). These might be followed by or used in conjunction with Toltrazuril. Any treatment involving sulfadiazine/ pyrimethamine will require monitoring of the horse's bloodwork to detect anemia (reduction in red blood cells) or leukopenia (reduction in leukocytes in the blood).

Duration of treatment with sulfadiazine/ pyrimethamine usually is a minimum of five months, or until one month after the neurologic signs have either disappeared or remained the same (stabilized). There is the possibility of relapse after treatment with sulfadiazine/pyrimethamine in about 30% of horses.

(General immune system stimulants also have been tried in conjunction with different treatments, as have herbal supplements and acupuncture.)

A medication called Diclazuril currently is under development by Schering-Plough Animal Health for treating EPM in the horse. Early in the treatment protocols for horses, a related product called Clinicox (for chickens) was imported from Canada under a special license. Now, Clinicox is licensed for chickens in the United States by Schering-Plough Animal Health.

Dubey mentioned earlier that Diclazuril is the only treatment that has been tested in the mouse model. He also said that research has shown that Diclazuril is absorbed quickly after being fed to a horse. Dubey reported that Diclazuril can kill the early stages of S. neurona and "may be useful as a prophylactic against S. neurona infections in horses."

Martin Furr, DVM, Dipl. ACVIM, of the Virginia-Maryland Regional College of Veterinary Medicine's Marion du Pont Scott Equine Medical Center reported last year that Toltrazuril "has potential efficacy for the treatment of EPM."

Ponazuril recently was tested in a treatment study at seven sites involving 100 horses and was reported to have very favorable clinical results.

NTZ also underwent a successful field trial, with previous studies indicating that it had the ability to be absorbed. However, previous studies indicated that NTZ at high doses could cause illness or death.

It has been reported that treating mares with sulfonamides, pyri-methamine, folic acid (as a supplement because of the disruption of folic acid production with sulfonamide and pyrimethamine treatment), and vit-amin E can cause congenital abnormalities in the fetuses of pregnant mares, and possibly affect the performance of breeding stallions.

Risk Factors

Dubey reported only one pony as having clinical signs of EPM. Epidemiologic studies have shown that while ponies, donkeys, and mules do develop antibodies against S. neurona, researchers feel that since there have been no other reported cases, non-horse equids have resistance to the disease. Similarly, seroprevalence for S. neurona was high in draft breeds in one study, but those horses had a lower incidence of disease.

Several case studies have been done on risk factors associated with development of EPM. Risk factors from an Ohio State study under controlled conditions include:

  • Young horses (one to five years) and old horses (greater than 13 years) have a higher risk of developing EPM. The highest risk was in 3-year-olds.
  • EPM risks are higher in spring, summer, and fall as compared to winter. The highest risk reported was in the fall.
  • Disease likely was due to management effects rather than genetics.
  • Stress (such as transportation) was questioned as a factor.
  • Racehorses and show horses had the highest risk for EPM (possibly related to stress). However, moderate exercise is known to strengthen the immune system.
  • Risk of EPM was 2.5 times higher if opossums were seen on the property.
  • Protection of feed from wildlife was associated with a one-third decrease in risk.
  • Presence of water (creek or river) on the property reduced the risk of EPM by 50%.
  • Presence of a wooded area on the property doubled the risk of EPM.
  • Treated horses had the greatest chance of improvement (10 times higher than non-treated horses).

Based on this information, horse owners and managers should reduce stress whenever possible, and should try to manipulate the horse's environment to take advantage of the above known risk factors where possible.

EPM Life Cycle Solved In The Laboratory

The most recent Journal of Parasitology contained an article that opens the door for battling equine protozoal myeloencephalitis (EPM). Researchers J.P. Dubey, BVSc, PhD, senior scientist at the Parasite Biology and Epidemiology Laboratory of the U.S. Department of Agriculture, William J.A. Saville, DVM, Dipl. ACVIM, assistant professor of the Department of Veterinary Preventive Medicine at The Ohio State University, D.S. Lind-say, PhD, of the Virginia-Maryland Regional College of Veterinary Medicine, and others found they could use the common domestic cat as the experimental intermediate host for the life cycle of the causative protozoal parasite Sarcocystis neurona.

It already was known that the opossum is the definitive host, and the horse is an aberrant, intermediate host. The horse is considered to be a dead-end host (the parasite can’t complete its life cycle in the horse, and a positive horse can’t pass the parasite to another horse). This new breakthrough allowed researchers to complete the life cycle in the laboratory, thus allowing them to give EPM to horses in an experimental setting. This means that preventive vaccines and medical treatments for EPM can be challenged and proven effective or ineffective in the laboratory. Research needs to continue to determine if the cat is the natural intermediate host in the life cycle, or if there are others. Saville emphasized that: “At this point in time, cats should not be eliminated until the true natural intermediate host (or hosts) is determined.”

It was stated in the article that, “EPM causes an annual loss of more than $100 million to the equine industry in the United States.” EPM causes mild to severe neurologic symptoms because the parasite migrates to the spinal cord and the brain of horses. Current treatments can halt multiplication of the parasite in the horse, and future treatments (currently under FDA scrutiny) could kill the parasite in the horse. Most horses which test positive for antibodies to EPM do not develop clinical neurologic signs (toe dragging, drooped lip, lameness, incoordination, etc.). However, most horses which develop clinical signs do not recover to their previous level of athletic ability. A small percentage will recover, then relapse when treatments are halted.

Texas A&M Vaccine Research

A new vaccine has been approved for aid in the prevention of equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona. The USDA has granted a conditional license for the vaccine on the basis of evidence of safety and a reasonable expectation of efficacy. The effectiveness of the vaccine has not been evaluated in the field. Noah Cohen, AB, VMD, PhD, MDH, Dipl. ACVIM, of Texas A&M University said investigators from 11 centers (eight veterinary teaching hospitals and three private equine referral hospitals that employ board certified internists) will be conducting an independent study to evaluate the vaccine’s performance under field conditions.

The study will be conducted over a period of three years and will involve comparing the proportion and frequency of vaccination among horses diagnosed with EPM with two groups—a group of horses with neurological diseases other than EPM and horses admitted for non-neurological problems. The three-year time period will be necessary in order to render statistically meaningful data about the vaccine’s performance. The information from this study will help to determine how effective the vaccine will be under field conditions. The study will be coordinated by investigators at the Department of Large Animal Medicine & Surgery, College of Veterinary Medicine, Texas A&M University.

Decoding The Bug

The tenet of sharing information in order to advance solutions to the problem of EPM is nowhere more appropriate than when discussing the research at the Gluck Equine Research Center at the University of Kentucky. Daniel K. Howe, PhD, an Assistant Professor at the Gluck Center, has been able in the last six months to isolate 1,900 partial gene sequences from the single-celled parasite Sarcocystis neurona, which causes EPM. Then, he placed those gene se-quences in a genome database that is sup-ported by the federal government and is available to all researchers.

The federal genome database contains gene sequences from all types of creatures, including man. While a parasite is very different from other organisms, Howe hopes that a gene sequence from another animal can be matched to one (or more) from S. neurona. That would help researchers determine what protein is coded for by that area of genetic material. More importantly, Howe would like to identify the unique sequences in S. neurona (as different from other organisms) since these might provide valuable information for understanding and controlling EPM.

What this means toward solving the problems associated with EPM is simple—genes contain the coding information for creating proteins that make up the parasite; the horse’s immune system recognizes the parasite proteins as foreign substances; the immune system responds differently to different proteins.

Therefore, if particular gene sequences can be identified as important in stimulating a particular immune response in the horse, then researchers could potentially use that genetic material to develop a diagnostic marker, or even create a vaccine to help the horse’s immune system protect itself better.

Howe’s research has been funded in large part by grants from the Amerman Family Foundation.

Diagnosing EPM

While there is no way to positively identify a live horse with or without neurologic signs as having EPM, there are ways to reach a tentative diagnosis. Here are some criteria to be addressed by a veterinarian:

    1. Look for abnormal clinical signs consistent with central nervous system disease.
    2. Determine if there is asymmetrical (not the same on each side, i.e., one eyelid droops) neurologic signs or muscle atrophy (wastage of muscle mass).
    3. Rule out other likely causes of neurologic signs, which could include cervical stenotic myelopathy (wobbler), herpes myeloencephalopathy, trauma, and Eastern or Western equine encephalo-myelitis.
    4. Obtain a positive test on Western blot on a clean cerebrospinal fluid sample (one without blood contamination, which could change a negative spinal fluid test to a positive result).
    5. Evaluate response to treatment, although this will take at least a month.

The last diagnostic is questioned by some researchers because some treatments currently used for EPM also could help other conditions, or the horse could have become better without treatment.

Many veterinarians suggest using the terminology that a diagnosis of EPM is a “reasonable possibility” or a “distinct possibility” based on history, environment, and clinical signs. Veterinarians should also remind horse owners that some subtle neurologic signs might be “normal” in some horses.

Horses can exhibit clinical signs as subtle as dragging one toe, being unwilling to change or take a certain lead, or seeming unbalanced to the rider. If observed carefully, most neurologic signs associated with EPM are asymmetric, or occur on one side and not the other.

Clinical signs can remain subtle, or progress rapidly in some horses to being ataxic (uncoordinated), loss of proprioception (doesn’t know where the limbs are), extreme muscle atrophy, facial paralysis (drooping lip or ear), or getting down and unable to rise.

There can be multifocal signs of neurologic disease if the parasite causes damage in several locations of the spinal cord, brain, or brain stem. Some horses with EPM can have abnormal upper airway function, unusual lamenesses, or even seizures.

Florida’s Multi-Faceted Research

There are multiple projects related to EPM being conducted at the University of Florida College of Veterinary Medicine, said Robert MacKay, BVSc, PhD, Dipl. ACVIM. Florida has been a “hotbed” not only of clinical cases of EPM, but of the research geared toward solving the disease. EPM research has been a high priority at the university for a number of years, and numerous projects have been completed, while others are ongoing.

Florida EPM research ranges from clinical studies to basic laboratory research. Among the current or future projects are: the study of a natural intermediate host (which has yet to be published), a field study of cats as a natural host, a multi-centered study on Diclazuril, evaluation of Ponazuril as a preventive for EPM, refinement of a diagnostic test, the creation of a reliable horse model for demonstrating vaccine and treatment efficacy, and playing a leading role in working with other universities and private clinics on a field study of the EPM vaccine from Fort Dodge Animal Health (see Texas A&M sidebar on page 37).

MacKay emphasized that researchers should share data as soon as possible in order to advance the overall solution to the problem of EPM. He credited the Florida Pari-Mutuel Trust Fund with providing research monies for many of these projects, as well as the USDA and Bayer Animal Health, among others.

Ohio’s Team

Bill Saville, DVM, PhD, Dipl. ACVIM, came to The Ohio State University in 1993 from private practice to get an advanced degree. By 1997, research had been done and papers published on the seroprevalence of antibodies to Sarcocystis neurona in Ohio horses. Among the facts uncovered was that more than 53% of the horses were positive for the antibodies, emphasizing the importance of S. neurona in the state.

Studies proceeded, and in 1998, Bill Stich, MS, PhD (veterinary parasitology) came with a background in cattle parasites, including protozoa (the phylum of S. neurona). Some ongo-ing research has not been published yet, and horse owners and veterinarians can look forward to that upcom-ing information. But two of the big challenges the group set for themselves were developing a model for the disease, and improving diagnostic testing for the disease.

“We now have a basis for the model and are trying to refine it,” said Saville of the breakthrough of completing the life cycle in the lab using the domestic cat. A refereed journal will carry the initial information on the model in the near future, although Saville said the model still needs work. Another project looking at the possibility of the cat as a natural host also is starting, as the cat’s role in nature is still unknown.

Part of the model of giving horses EPM may involve compromising the immune system. That has been sugges-ted by many researchers in the past, and some have tried to accomplish that by giving a horse steroids to depress the immune system prior to inoculating the horse with S. neurona. Those models didn’t work reliably. Saville and his Ohio State team, which includes clinician Steve Reed, DVM, Dipl. ACVIM, and Chinedu Njoku, DVM, PhD (veterinary pharmacology), are working on using transportation (a known stressor) in the model.

“We knew that stress of various kinds was found in many horses that developed clinical signs of EPM,” noted Saville. “That includes medical conditions such as surgery and foaling.”

Saville also said that the work in knockout mice—which are genetically engineered mice with specifically compromised immune sys-tems—also gave them clues on working with horses. But while knockout mice are a good model for the disease, “No matter how many mouse studies are done, we still have to prove our theories in the horse.”

Stich was curious about EPM prior to arriving at Ohio State, and since his arrival has become an important member of the team. He, too, thinks working with the mouse model is important, but stated, “One of my favorite parasitology professors said that there are differences between infection, disease, and immunity. In some cases mice are appropriate as

infection models, and in some cases they aren’t appropriate models of disease or immune protection.” That’s why the group is determined to develop an equine model for EPM.

The initial study using transport stress was successful, but only included three horses. That wasn’t enough to convince this strict group. The transported horses were compared to control, non-transported horses, and horses given dexamethasone to suppress the immune system. The researchers found they got the same neurologic signs in the transported vs. the dexamethasone-treated horses, but the clinical signs were more severe in the transported horses, and there were more lesions.

“This suggests that some part of the immune system is involved in the disease,” said Saville, while cautioning that the disease is far too complicated to stop at that point.

“We’re looking at repeating the transportation stress model until we find something that mimics that stress,” he said.

Another part of the whole complicated aspect of understanding EPM in horses is looking at whe-ther or not the immune system plays a role in development of the disease, and how. Njoku and Stich are looking into the possibility of a problem in the level of nitric oxide in the cerebrospinal fluid in transported horses which get the disease. This is a very complicated process. Suffice it to say that it was noticed that stressed (transported) horses with more neurologic signs had lower levels of nitric oxide oxidation products. The researchers ex-pected higher levels.

So, the new question raised is the possibility of an association between nitric oxide

oxidation product levels and neurologic signs. That could be important, since two experienced practitioners can look at a horse and come up with two different conclusions on the severity--or appearance--of neurologic signs. Therefore, the levels of nitric oxide oxidation products could aid in the diagnosis of EPM.

Reed emphasizes that the basics of the model that Ohio State researchers are using is superior to other models at this time because, “We know what we are feeding to ponies and how much.” The amount of parasite picked up by a horse might not have any correlation to disease, he said, based on the first small study done. “The pony that we gave the lowest dose (of parasite) got the worst neurologic signs,” he said. “We have to know age, stress, time of infection, dose...but our model looks good.”

Reed said part of his duties are to “tell people it’s okay to say a horse has neurologic signs and teach vets in the field to do neurologic exams. I’m not a researcher or an epidemiologist, so I’m focusing on the clinical aspects.”

Reed added Ohio State alone could use $1 million in additional funding to refine the model study. Some of the past research has been paid by the industry (including American Live Stock Insurance) and private individuals, as well as pharmaceutical companies.

While acknowledging there is a competition for research dollars, Reed emphasized the need for cooperation in EPM research. “There already are universities and private clinics working together,” he said. “There are three ongoing or recently completed studies to test treatments where multiple sites were involved. There’s already collaboration with universities. But someone has to take the lead.” Reed and Saville think their group has a slight advantage just because of their diverse team, which includes J.P. Dubey, BVSc, MVSc, PhD, of the USDA.

Saville reminded that the FDA has designated a critical number of horses which must be included in experiments before a treatment will be approved in the future. And, the facilities used for these experiments must be large enough to handle the number of horses needed for the study to be statistically significant, must be designed so stalls can be not just disinfected, but sterilized, and must be “critter-proof” with a properly equipped lab attached.

Until that setting can be built, researchers will have to do the best they can with the limited finances and facilities available.

(Editor’s Note: Individuals wishing to donate to EPM research at The Ohio State can contact Karen Longbrake at 614/688-8433; longbrake.1@osu.edu.)

UC Davis Researches Western Blot

Barbara Daft, DVM, Dipl. ACVP, a veterinary diagnostician at the University of California, Davis, California Animal Food Safety Laboratory System, headed a three-year project of post mortem examinations of brains and spinal cords from 234 horses--65 with neurologic disease and 169 without neurologic disease. (These exams were done on dead or euthanized horses submitted for postmortem examination.)

The results of the examinations were compared to results of the Western blot test for Sarcocystis neurona performed at Equine Biodiagnostics in Kentucky. Following is her report on that research project.

Spinal fluid was taken from these horses either at the time of euthanasia or shortly after death. We examined a minimum of 22 areas in the brain and spinal cord to determine whether inflammation compatible with EPM was present.

There was very good agreement between the horses which had EPM (as determined by brain and spinal cord lesions on necropsy) and positive Western blot results on serum and spinal fluid. This means that the Western blot test usually does very well in detecting horses which have the disease and will not misidentify such animals.

On the other hand, the spinal fluid Western blot identified many horses as positive that did not have EPM. In fact, of the 169 horses without neurologic disease, 71 had a positive spinal fluid Western blot result, whereas only eight of those 71 animals had central nervous system lesions of EPM. It is very important to realize that the Western blot tends to have many false positives.

These results suggest that many horses exposed to S. neurona might have transient or undetectable infection (by postmortem examination) of the central nervous system without ever developing the disease.

The Western blot is a useful test that veterinarians can utilize to help with the diagnosis of equine neurologic disease. A negative Western blot result for either serum or spinal fluid is very good evidence that a horse with neurologic disease probably does not have EPM. However, some veterinarians might want to re-test a negative animal with neurologic signs two to three weeks later, especially if the disease duration has been short and the animal’s symptoms are strongly suggestive of EPM.

Positive results of the spinal fluid Western blot test need to be interpreted with caution and supported by thorough neurologic examination and other tests to rule out other possible causes of the symptoms, such as joint disease, tendon/ligament injury, muscle disease, or even heart disease--all of which can result in symptoms that might appear to be neurologic and therefore could be confused with EPM.

This project was supported by the UC Davis Center for Equine Health and the Southern California Equine Foundation with funds provided by the Dolly Green Research Foundation.


(The following information was taken from previous articles in The Horse and information published by Dubey, et al.)

  • In 1970, James Rooney, DVM, first described a clinical syndrome he called segmental myelitis based on 52 horses.
  • In 1974, the first scientific papers were published that reported protozoa in the lesions from horses with segmental myelitis. However, the parasite was misidentified as Toxoplasma gondii. Illustrations from those early articles later were identified as coming from the Sarcocystis neurona life cycle.
  • In 1974, Dubey reported that the parasite was not Toxoplasma gondii and probably was a member of the Sarcocystis family.
  • In 1976, the current name equine protozoal myeloencephalitis first came into use.
  • In 1991, the name Sarcocystis neurona was given to the single-celled parasite, and it was isolated for the first time from a horse from New York.
  • In 1991, researchers at USDA described the development of the parasite in laboratory cell culture.
  • In 1993, Granstrom developed a Western blot test specific for S. neurona in serum, which became the first diagnostic test available in the living horse.
  • In 1995, it was proposed that the opossum was a definitive host for the parasite’s life cycle.
  • In 1995, researchers suggested the S. neurona and S. falcatula were the same parasite.
  • In 1997 and 1998, researchers proved that S. neurona and S. falcatula were different.
  • From 1997 to date, there have been several serologic surveys indicating that 30-50% of horses in the United States, Argentina, and Brazil have antibodies (have been exposed) to S. neurona.
  • In 1998, Dubey and Lindsay provided conclusive evidence that the opossum was the definitive host for S. neurona.
  • In 1998, researchers developed the first knockout mouse model for use in studying S. neurona.
  • In 2000, S. neurona was isolated from a South American opossum species.
  • In 2000, the life cycle of S. neurona was completed in the laboratory by Dubey, et al., using the domestic cat.

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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