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Posted on November 11, 2012 by Administrator

Hindsight, they say, is 20-20. Reviewing the EPM literature results in a paradigm shift that changes ones way of thinking about the disease. A shift in our opinions about EPM and its treatment resulted in realizing it is a treatable disease. Here we introduce some commonly held beliefs and illustrate an alternative interpretation. A cursory understanding of some molecular biology techniques and a willingness to review published data is all it takes to form your own opinions. Read on and unlock the mysteries of EPM!

There are five concepts that are readily accepted about EPM—so accepted that they are urban myths. The five myths that deserve scrutiny are: 1) The prevalence of S. neurona is high but disease is low; 2) Many strains of S. neurona cause disease; 3) Sarcocystis neurona is neurotropic; 4) Horses have a genetic predisposition for EPM and 5) Disease is due to parasites in the central nervous system.

Changing the perspective on these five issues makes EPM not only comprehensible, but highly treatable.

For example, most horses (when tested by Western blot tests) have antibodies detected in the serum (true) – remember, Western blots are interspecies tests. Sarcocystis neurona can be isolated from horses with EPM, with difficulty—post mortem—thus it is one etiologic agent of EPM. The majority of horses that have signs consistent with EPM have antibodies (when tested by Western blot tests) to S. neurona.  Lesions most often associated with EPM are inflammatory–serum antibody tests don’t detect inflammation. We use C-reactive protein to detect treatable inflammation. Lots of horses have antibodies against S. neurona. Very few horses have parasites in the CNS so disease is low.  However, many horses have signs of EPM.  And more horses with neuromuscular disease have antibodies against S. neurona. It makes sense that the parasites are common,  they stimulate an antibody reaction, and then they are removed.  During infection inflammation is stimulated.  Sometimes the inflammation isn't turned off.

The subjective Western blot test detects both non-specific and specific antigens to S. neurona.  Most horses test positive for antibodies to S. neurona. Why do only a few of these horses succumb to EPM? Why don’t all horses with antibodies and clinical signs have lesions with parasites in the CNS? Why do some horses without antibodies respond to treatment?

The logical conclusion is while the set horses that have S. neurona antibodies detected by Western blot are numerous, this test doesn’t define the set of horses with EPM. The point isn’t that the Western blot is deficient as a diagnostic test for EPM but rather, why isn’t it a good test? A most obvious answer is that EPM is a syndrome that includes inflammation.  That is why we spent years studying inflammation.

The name Sarcocystis neurona implies that it is neurotropic–meaning that the organism readily enters the CNS where it causes damage to these sensitive tissues. Does the rare occurrence of disease indicate that there are a few, particularly rare strains that are responsible for disease? Or is it the horse.  Are some horses genetically predisposed to get EPM due to some idiosyncrasy of the immune system? And, once in the CNS, do the parasites cause career and sometimes life-threatening lesions that renders them useless? Is it possible that there are two diseases? Or, a syndrome, two aspects of one disease? If EPM is really inflammation initiated with parasites that induce an antibody reaction that in turn, removes them but leaves inflammation that is really the disease... Treatment is possible!

Sarcocystis neurona myeloencephalitis is rare. Protozoal myeloencephalitis by its name indicates inflammation is significant to the pathogenesis of disease. Why is the inflammatory component of the disease ignored in the treatment plan? Inflammation (a histological diagnosis) is always mentioned in papers that examine tissues from horses with EPM. In fact, EPM is so linked to inflammation, that a diagnosis “consistent with EPM” is based on histological evidence of inflammation in the absence of parasites. If one were to make a distinction between cases with parasites detected in the CNS (the definition of EPM) versus inflammatory encephalitis, then we propose subsets of horses would be identified that require different treatments. Two subsets of EPM suspect animals may be those with protozoal infections and those with inflammatory disease. (Update: this was posted in 2013, read our 2017 paper on the three diseases associated with EPM found on our home page!) Three diseases are sarcocystosis, S. fayeri, and polyneuritis equi.

An effort to identify S. neurona-induced inflammatory encephalitis versus other causes of inflammatory encephalitis requires that all disease causing S. neurona strains are identifiable and that strains or species of Sarcocystis that don’t cause disease are excluded from the analysis. This distinction can’t be made using inter-species antigens (antigens that are shared among Sarcocystis species). That is why we like specific tests.

The idea that S. neurona is neurotropic, that means it has a predilection to enter the CNS, has its roots in the name of the strain associated with EPM. And with that mind set, it makes sense to treat protozoa with drugs that enter the CNS. Sarcocystis reportedly can enter the CNS of their intermediate hosts. If there is nothing special about S. neurona’s proclivities, then what is the role of inflammation in EPM? Inflammatory lesions are far more common when compared to protozoa-associated lesions in the EPM literature. What if inflammation is the primary mediator of the signs of sarcocystosis?

The inflammatory response to pathogens elicit tissue injury. There are several interconnecting mechanisms of inflammation.  The most familiar may be the potent mediators of inflammation that are derivatives of arachidonic acid. One familiar principal pathway of arachidonic acid metabolism is the cyclooxygenase (COX) pathway that responds well to steroids and NSAID’s. A lack of response by an EPM horse to steroids and NSAID’s indicates that an alternative inflammatory path may be involved in the pathogenesis of disease.

Acute inflammation and chronic inflammation may respond differently to treatment. Chronic inflammation can indicate the inflammatory response is out of regulatory control. The result can be more damage to the body than the agent itself would have produced. The clear research focus on the pathogen rather than treating acute and chronic inflammation present in cases of EPM has denied many horses useful life.

Treating inflammation without a clear understanding of the inflammatory mechanisms involved in the underlying pathology of disease is as unproductive as treating EPM with anti-protozoal drugs has been. Adding NSAID’s to FDA approved anti-protozoal drugs is not effective. Understanding the role of inflammation in EPM can not be under-emphasized.



Previously posted on June 7, 2011

EPM has three definitions depending on who you are talking to.  “Gold Standard” disease is defined  as Sarcocystis neurona residing in the central nervous system of a horse displaying neurological signs attributed to lesions in one or more regions of the central nervous system. For a definitive diagnosis, the organism must be isolated from the CNS. Isolation of the pathogen is post mortem evidence that the horse had EPM. Less stringency is used by clinicians at university equine hospitals. In these clinics EPM is recognized by the organism in the tissues or by PCR (polymerase chain reaction).

Also included are histopathological lesions consistent with EPM.  The tissues don’t show any S. neurona, but specific inflammatory lesions that are similar to those found in classical disease are used for the diagnosis. This inflammatory criteria was used in all the Ohio model infections and many experiments that examine antibodies in “diseased” animals.

To the rest of us, EPM for practicing veterinarians or “clinical EPM”, suspect EPM horses are surviving animals that have neurologic signs consistent with asymmetric or multifocal central nervous system lesions or both for which other likely differential diagnoses were excluded. Ancillary diagnostics used by practicing veterinarians can include serum testing, CSF testing, and response to treatment.

We all know that the etiologic agent of EPM is Sarcocystis neurona. Only antigen type I or II are involved in EPM, antigen types are defined by 32 markers! The recognition that antigen type I or II are involved in EPM is the major evidence that gives the SAG 1, 5, 6 ELISA credibility to identify infections, determine drug resistance, and response to treatment.

A diagnostic test for EPM isn’t available. Licensing a diagnostic is a long and complicated procedure that can not be achieved for EPM, mainly because signs result from inflammation and not the actual parasite. A diagnostic can be licensed to identify antibody in a sample if it is prepared as a kit. There are high dollar machines that read the tests, ambulatory veterinarians are unlikely to make this investment.  Antibody tests (that detect antibody to S. neurona) do not have to be licensed. These tests are “validated”. Validation is a strict set repeat tests between and in laboratories.

An antibody test detects immune responses to infections.  Infections with S. neurona may lead to EPM–the presence of clinical signs are the key factor. It is an important distinction between an antibody test to detect antibodies against S. neurona and a test for EPM. Antibody tests are used to rule in or rule out the possibility or the probability that a horse has an active infection or even parasites in the CNS, however these tests can have false positives and false negatives.  No antibody test can definitively diagnose EPM.

Sarcocystosis is a visceral infection that causes a measurable immune response in horses. Equine sarcocystosis is caused by S. fayeri, S. bertrami, and S. neurona. Equine sarcocystosis is pretty common. Equine sarcocystosis produces antibodies that are measured by tests such as IFAT, SAG 2, 3-4 ELISA, and Western blot. Tests using SAG 1, 5, and 6 are specific for S. neurona, they don't detect S. fayeri, we have a specific test for measuring a disease-associated protein of S. fayeri.

Apicomplexan parasites, like Sarcocystis, have common, or shared, antigens. These shared SAG’s, 2 and 3 and 4 are surface antigens that are common to all organisms in the genus Sarcocystis.

Sarcocystis falcatula have SAG’s as well. Some are unique. It is the common, or shared, SAG’s that get confused between tests that use common antigens. The shared SAG’s are interspecies markers because they have highly similar nucleic acid sequences and are antigenically identical, close enough so a horse can’t tell them apart. That makes them cross-reactive.

Sarcocystis neurona has distinguishing traits, the ones we use are molecular and can be identified by antibody reaction, these are neurona serotypes. Serotype is defined as antigen type and is represented by one of the mutually exclusive SAG’s that classify S. neurona strains into one of three groups: SnSAG1, SnSAG5, or SnSAG6. Phenotypes are antigenically unique and indicate some virulence and drug resistance factors that occur between strains.

SAG 1, 5, 6 ELISA testing uses recombinant proteins representing the phenotypes of S. neurona (SAG’s 1, 5, and 6) and determines the serotype of the infecting organism in horses, dogs and cats. Mixed infections are common and are detected by our test. Opossums have mixed infections. Opossums shed a mixture of oocysts so it’s no surprise that horses get several infections at the same time. The limits of the SAG ELISA are to serotype. A 2-4 fold increase in titer 3-4 weeks apart indicates active infection.

Other IgG tests include SAG 2-3-4 ELISA, IFAT, Western Blot. These commercial tests detect sarcocystis infections in horses. The limits of these tests are to the genus Sarcocystis. No serotype determination or ability to determine the identification of mixed infections are possible using these tests. Active infection is not distinguished from cyst degradation, possible in S. fayeri infections. Drug resistance can’t be determined using these tests.

Anti-protozoal drug susceptibility is measured by in vitro and in vivo assays. There are differences in the efficacy of anti-protozoal drugs that are demonstrated by dose and phenotype of the challenge infection. This is an important area for phenotype determination.


EPM 2011

Many groups of scientists have researched Sarcocystis neurona and EPM. Each group has added new knowledge to form a big picture of infections and the relationship to disease. Until recently, the pieces didn’t seem to fit into one picture and that leaves different groups of scientists with polarizing ideas.

We are exploring the antigenicity, pathogenicity, and molecular traits of Sarcocystis neurona in horses. We also explore the horse’s reactions to infections beyond antigens. Our results give us new insights into past research. Hindsight, as you know, is 20-20.

About Hosts

Realizing and proving that the opossum was a definitive host for S. neurona was a big step forward in EPM research, attributed to Clara Fenger, University of Kentucky. Researchers could focus on sporocysts, which are infective to horses. Sporocysts used in challenge experiments were important in order to determine the intermediate host(s) and complete Koch’s postulates for EPM. For example, nude or immunodeficient mice (ID mice) are susceptible to S. neurona  sporocysts.

The lab strain, isolated from horse with clinical EPM, and the sporocysts from feral opossums, when compared, were 99.8% the same genetically. One important gene of S. neurona was merely 0.2% different than the same gene in S. falcatula. Once the genetic similarity was determined, intermediate host susceptibility took center stage. Surprisingly, the intermediate host specificity for S. falcatula includes budgies, not immunodeficient mice. The biological difference, the ability to infect one host and not another, is a founding principle in pathogenic protozoal identification. Intermediate host specificity highlighted important biological differences between the two closely related parasites. The idea that S. neurona was identical to S. falcatula was wrong. Twenty years later, the molecular differences between S. neurona and S. falcatula remain complicated, but distinct. Antigenic differences are complicated and confound our understanding of EPM.



Equine protozoal myeloencephalitis was recognized in horses in the 50’s and S. neurona was isolated in 1991. The cultured S. neurona enabled Clara Fenger, then a student at the University of Kentucky, and her associates to compare the in vitro material to sporocysts obtained from wildlife. Fenger’s group used molecular markers of Sarcocystidae to suggest a definitive host would be in the dog family. (It was later discovered that S. neurona does infect dogs–there are ongoing investigations to determine if S. canis could be S. neurona). These markers were used to identify S. neurona in feces and intestinal digest of wildlife specimens. These authors concluded that the opossum was involved in EPM. Aflutter with excitement over her new and important data, Dr. Fenger visited another research group and discussed her ideas. The secret was out and history reveals that it was highly suspicious  that S. neurona cycled between opossums and birds.

The race was on to experimentally induce EPM from opossum oocysts. Fenger and her co-workers used wild caught opossum oocysts to infect horses and induce clinical signs of EPM. The infections didn’t result in organisms in the CNS. They failed to complete Koch’s postulates because they failed to re-isolate S. neurona from the CNS of an experimentally infected horse. They described ataxia in the challenged horses and inflammatory lesions that did not include the organism. Likewise, S. falcatula failed to induce EPM. There were no antibodies detected (using what were the antigens du jour) in the S. falcatula experiments when the samples were tested by the Western Blot (EBI, KY). The field was open to those that could identify sporocysts as S. neurona, produce sufficient numbers for challenge experiments and isolate the organism from the CNS of a horse. This task remains unfulfilled to this day. What followed was gathering data about the intermediate host range of S. neurona and the realization that the opossum harbors more than S. rileyi, S. neurona, and S. falcatula as was thought in 1997.

Dr. Fenger and her group identified an outbreak of EPM in 12 of 21 horses on a farm. She found “EPM may develop as an epizootic. Fenger reported subtle clinical signs that were originally considered unimportant that ultimately progressed to obvious neurologic signs.” She co-patented the use of pyrimethamine and trimethoprim-sulfamethoxazole for the treatment of EPM. Also, she reported “adverse effects associated with EPM treatment (pyrimethamine and trimethoprim-sulfamethoxazole) included worsening of neurologic signs, anemia, abortion, and leukopenic and febrile episodes.”

From these papers we found the roots of some long held ideas. The body of work contributed by CK Fenger and her associates (1994-1997) identified the opossum as the definitive host of S. neurona, a cornerstone in this field. They were able to recognize that subtle signs are important in horses with EPM. They recognized that disease is not isolated to an individual horse when sufficient infectious material is present. It is expected that farms with one case of EPM will have others. Most important, in hind sight, is realizing there are CNS lesions associated with clinical signs but no parasites.

Despite recognizing and reporting the initial subtlety and ubiquity of EPM, there is a long standing belief by some that only a few horses are susceptible to infections, presumably due to some defect of their immune systems that allow them to succumb to disease. Sharon Witonsky, in conjunction with Pathogenes used our research model to show that the parasite itself can manipulate the equine immune system. The parasite uses strain specific down regulation measured by proliferation responses. Our research showed that any horse is susceptible to infection using pathogenic strains. Most horses have mild infections that can resolve. Our work also shows that the level of challenge for a horse is low, in the thousands, not millions of organisms, as used in most studies. We also observed that infected horses have a statistically significant rise in titer the longer the infection continues. The higher the titer, the longer the infection. It is beneficial for the diagnosis of EPM to show that a horse has a two to four fold rise in titer in conjunction with signs consistent with EPM.

Fenger’s group presented the proof that opossums are definitive hosts for S. neurona and they clearly believed that S. neurona and S. falcatula were synonymous. They conducted an experimental challenge in horses using oocysts derived from opossums fed sparrows, hosts of S. falcatula. We now know S. neurona and S. falcatula are not the same genetically or biologically. Foals in their study immuno-converted on immunoblots post challenge and demonstrated clinical signs consistent with EPM–this observation was the opposite of the University of Florida S. falcatula (Florida) infection challenge in which horses did not seroconvert, again based on Western blot (EBI, KY). In 2010 we determined that S. falcatula (Florida) displays a surface antigen that is genetically identical to a surface antigen of S. neurona.

Experimental infections of these horses with oocysts were, at best, a mixed oocyst population challenge because wild caught opossum oocysts (these oocysts could have S. neurona or S. falcatula) were added to the challenge dose derived from sparrows. There is no definitive evidence that S. neurona was present in this challenge study, they reported seroconversion on immunoblots, this in contrast to the UF S. falcatula study and indicates S. neurona may have been present . They did prove, by bird challenge (budgie), that S. falcatula comprised some, if not all, of the oocyst population that they used. They demonstrated inflammation and saw clinical signs of ataxia.

In hindsight, we know that some S. falcatula strains could be confused with S. neurona on immunoblots. These confounding S. falcatula strains may infect horses, induce antibodies, and share surface antigens that are almost identical to one phenotype of S. neurona. Fenger’s work illustrates that some S. falcatula strains induce ataxia in horses, but S. falcatula doesn’t cross into the central nervous system. An alternate view is that they introduced S. neurona from the wild caught opossums using a mixed oocyst challenge. A challenge with a small number of S. neurona oocysts could also support their results and based on the work of others is the most likely scenario.

Repeated exposure of horses in an environment contaminated with S. neurona is highly likely. Dr. Fenger’s work supports the idea that it would take very few oocysts to infect horses, ataxia would be seen clinically, and no organisms would be present in the central nervous system, although there would be lesions consistent with inflammation. It is apparent that treatment for organisms that do not enter the CNS would differ from those that do and that the pathology of the neural tissue inflammation is crucial to our understanding of EPM in the horse.



For over 30 years the parasite S. neurona was placed at the center of EPM research. Despite overwhelming evidence, inflammation was ignored as a worthy target for treating clinical signs. As discussion about treating the signs of EPM grows, we propose a mechanism to explain why treatment can resolve clinical signs.  It’s all based on data driven research, our work and work published by others. We offer a slide show that outlines our current thinking. We will give you references to the material in the presentation on request.

Inflammation and c-reactive protein

We learned a lot in 2012—and perhaps you did as well.  A step forward was the addition of the C-reactive protein test in our work.  We compared two published tests and found that the ELISA, although expensive, was superior.  This is a capture ELISA, that means we capture nanomolar amounts of the protein from serum.  We report a micromolar value, normal is 0-10 micrograms/ml in the serum.

The test results indicate the presence of inflammation (the source isn’t unique to S. neurona or EPM) and most important, the values change in response to treatment.  We tested hundreds of horses and compared our data to other published data.  The level of serum CRP measured in µg/ml in normal horses is 7.4 +/- 2, (n=10).

C-reactive protein is elevated in disease–pneumonia 19, +/- 9 (n=10), enteritis 16 +/- 6, (n=10) and arthritis 11 +/- 3, (n=10).  We tested the level of serum CRP in horses. Those with a presumptive diagnosis of EPM (n=183) and found the value ranged between 11µg/ml-35µg/ml. We determined that 63% of horses with a provisional diagnosis of EPM have an elevated CRP. Currently we are evaluating the relationship between a drop in CRP and clinical signs post treatment by pre-and post treatment sampling.

Further analysis reveals a statistically good "normal" cut off is 16 µg/ml  in a population of "EPM" suspect horses.  Other associated disease, polyneuritis equi, is linked to a value of 39 µg/ml.  These last two observations were added in 2017.

Preventing re-infection

We learned that horses can be re-infected, the re-infection is consistent with a rise in antibody titer before the clinical signs appear.  If a horse had, and is treated for the infection, a second exposure induces a quick antibody response.  An inflammatory response can accompany the humoral (B-cell antibody response), and the inflammation causes clinical signs.

In order to prevent EPM there are several requirements.  the horse has to be exposed to oocysts shed by the opossum.  Exposure is detected by a rise in antibody titer, usually in the spring and the fall.  Monitoring the SAG 1, 5, 6 antibody levels, and if indicated, a C-reactive protein, can indicate a horse that is at risk and a candidate for palliative treatment.  While it is possible that horses could harbor encysted S. neurona (this makes them a definitive host) there is little published evidence in support of this theory.  Experiments feeding infected horse muscles to opossums—never published to the best of my knowledge, were unfruitful, or logic says the data would have been published or discussed. Horses can harbor S. fayeri, that is another story revealed in 2016.

Email data

We found video sent via email very helpful to us.  (2017 update: use the GaitScore app available from the Apple Store or Google Play). Video illustrates the degree of the neurological signs.  It is also satisfying to see the progress made in positive outcomes.  We are considering using email video as part of our analysis for our FDA programs, but for now getting a completed submission form is a worthy goal.

S. neurona in dogs and cats,

We published our data on S. neurona antibody found in dogs early in the year.  There is no doubt there is disease due to S. neurona in both dogs and catsWe are testing samples that come in, however there is a lot of research that needs to be completed before much interpretation can be made on a serum sample test result.

Another area of our current investigations are the association between encysted small strongyles and clinical signs of inflammatory encephalitis.  It’s a complicated scenario and we are trying to reduce everything down to a couple of tests—for now we offer C-reactive protein testing to monitor sub-clinical and clinical inflammation.  I anticipate enough meaningful data in the next three months so that we can make interpretations and recommendations that are useful in the field.  For now, it’s on a case-by-case basis.

Continuing Education for Veterinarians

An exciting part of EPM is our CE program.  Pathogenes is a certified State of Florida CE provider, we offer 3 hours of CE’s to veterinarians.  Our goal is to shift the paradigm, we want to change the conversation from “EPM, the disease” to “EPM, the syndrome”.  The EPM syndrome is composed of infection and inflammation, both need to be treated effectively.  We hope for no less than veterinarians discussing inflammatory encephalitis with their clients.  In 2013, we will visit several states to meet with veterinarians and share our knowledge about EPM.  Our hope is to meet some of our horse friends as well.

Hot off the press…

And, after so long a wait–our FDA EPM treatment is bagged and ready to go.  The tablets are certified, pressed, and tested.  This formulation will allow us to make some more discoveries.  This is a product that is made specifically for horses suffering from the effects of EPM. The stability of the drug components is a big advancement in the treatment of EPM.  Obtaining our license for both drugs (INAD 012092 and INAD 012219) will still be a large, time consuming hurdle, the paperwork is mind numbing—but we are committed to the task.  We have some in-house experiments to conduct and field data to collect.  We have to challenge horses and show how each active drug component works in the horse; our understanding of the pathogenesis of disease in the horse allows us to design such a study.  Uniquely, we have the tools to prove our ideas and theories.

And finally, thank you Carine, Carine Barrs sent us a holiday picture of Romeo, wishing us a Merry Christmas, Joyeaux Noel, and Happy Holidays.  We wish the same to all of you.