EPM Today: Testing and Treatments

Horses need to be serviceably sound and safe to ride to fulfill their functions. It is no wonder that equine protozoal myeloencephalitis (EPM) has been a dreaded disease ever since its rise to prominence in the 1990s, because horses with neurologic disease often don't recover to their previous athletic capacity.

Neurologic problems, in general, can be some of the most unforgiving types of diseases to treat in horses. Other parts of a horse's physiology affected by infection or injury can have normal function restored, once the problem is diagnosed and appropriate treatment is initiated. In the nervous system, damage to neurons can result in permanent alteration of function, even after the inciting injury, infection, or inflammatory episode has been resolved. This can result in devastating loss of athletic capability in horses. Even when there has been a good response to treatment and a horse with neurologic injury responds with, say, 50% improvement, if the horse does not return to its original level of performance, this can leave the owner in a decision- making quandary.

Equine protozoal myeloencephalitis is one of the most common neurologic diseases affecting horses in North America. The disease was first identified in the 1960s, but it didn't become a byword in the lexicon of horse owners until the 1990s, when the disease appeared to increase in prevalence and became the subject of widespread research.

A great deal of information has become available about the life cycle of the organism that causes EPM. Also, horse owners now know the causative agent is the protozoan parasite Sarcocystis neurona, its natural definitive host (the host in which an organism's life cycle is completed and in whose feces the infective stage of the organism is passed) is the opossum, and that inadvertent consumption of opossum droppings is the route of exposure for most horses.

With the understanding of those aspects of the organism's life cycle have come improvements in diagnostic testing and development of different medications that yield more options for treatment. Still, making a conclusive diagnosis of the disease has always been a challenge for veterinarians.

Diagnostic Tests for EPM

Examinations Even though laboratory tests used to aid diagnosis have evolved and changed, the first test practitioners use in evaluating neurologic horses is still critical--the hands-on examination.

Like making a diagnosis of any other type of disease, the process of arriving at the cause of a horse's neurologic signs should start with a general physical examination and detailed systems examination (in this case, the nervous system). Because the organism can infect any part of the central nervous system--from the brain to the end of the spinal cord--there is overlap between the signs of EPM and other disorders that affect the nervous system.

The purpose of the neurologic examination is to determine what part of the nervous system is affected. For example, do the horse's signs resemble those of brain disease? Or are they more consistent with a problem in the spinal cord, or perhaps one of the peripheral nerves, like the radial nerve? If signs seem to originate in the spinal cord, do the signs fit with a single lesion at one segment of the cord (for instance, in the lumbar portion), or are they more consistent with lesions in several different segments?

Findings like focal areas of muscle atrophy and ataxia (incoordinated gait, leading to weakness and swaying, especially in the hind limbs) are very significant, and the muscles and limbs involved are important indicators of what section of the nervous system is involved. This process of determining what portion of the nervous system is involved is called localization.

Armed with examination findings and the horse's medical history, the veterinarian then might recommend laboratory tests (on blood, cerebrospinal fluid, or both) and imaging tests (often radiographs, including a special type of radiograph called a myelogram--in which a special dye is injected into the spinal canal before a radiograph is taken--if the problem is felt to be originating from the portion of the spinal cord that courses through the neck).

If signs point to the cervical (neck) portion of the spinal cord and the horse in question is young and growing, radiographs might be recommended first. This is because of the suspicion of cervical vertebral myelopathy (Wobbler syndrome), a condition in which the cervical vertebral bones impinge on the spinal cord and cause neurologic signs.

If signs appear to originate from multiple parts of the spinal cord, the veterinarian might first recommend laboratory testing of blood and spinal fluid samples, because nonskeletal problems would be considered more likely. When muscle atrophy is asymmetric (for example, affecting the gluteal muscles on the left side of the rump, but not the right), the suspicion of EPM increases.

Laboratory tests With EPM, S. neurona establishes a presence in the neural tissue of the central nervous system, inciting an inflammatory reaction and damaging the neurons. Because the neural tissue in a living horse cannot be extracted and searched for the organism, veterinarians must look for other evidence of infection. That evidence is found in the form of antibodies against the organism circulating in the blood and in the cerebrospinal fluid that surrounds and bathes the brain and spinal cord. Testing techniques exist for detecting S. neurona itself, but because the organism's presence in the fluid is transient, tests used in commercial laboratories to confirm a diagnosis of EPM are usually based on detection of antibodies against the organism using polymerase chain reaction testing (PCR).

Says Jennifer Morrow, PhD, a scientist and equine consultant at Equine Biodiagnostics/IDEXX, a diagnostic laboratory in Lexington, Ky., "Although PCR is very sensitive at detecting nucleic acids from an organism in a test tube, it is not very sensitive for detecting EPM clinically in the patient, because the organism simply isn't present in the spinal fluid for very long, if at all. Therefore, although a positive PCR result is a confirmatory finding for the disease, a negative result does not rule the diagnosis out."

The tests used commercially to find antibodies against S. neurona are the Western blot or immunoblot test, the immunofluorescent antibody test, and an ELISA (enzyme-linked immunosorbent assay) test.

Because antibodies are the only evidence of the horse having been exposed to the organism at some time and are not necessarily an indication of active infection, detection of antibodies against S. neurona in the blood does not constitute conclusive evidence that a horse with neurologic abnormalities has EPM. However, because antibodies are large molecules and do not typically cross from the bloodstream into the cerebrospinal fluid, when they are identified in the cerebrospinal fluid of a horse with neurologic abnormalities and the sample was not contaminated with blood during the collection process, this is considered to be evidence of S. neurona infection.

The Western blot test "The Western blot test performed on serum or cerebrospinal fluid is the oldest test for EPM and has the longest track record," according to Rob Keene, DVM, a veterinarian for IDEXX laboratories. "Different labs use the Western blot test to detect different surface antigens (foreign proteins or parts of proteins that stimulate antibody response) on the parasite, but the immunoblot technique is common to all of them. The Western blot test has undergone the most extensive validation of all tests used at present."

Different "blot tests" detect antibodies against different proteins on the surface of the organism, each with a distinct molecular weight and configuration.

The Western blot test is used on serum and cerebrospinal fluid. It is a qualitative (results are interpreted as "yes" or "no" with regard to detection of antibodies) rather than a quantitative (results are given as a concentration or titer, indicating quantity of antibodies) test. A positive test result indicates previous exposure to the organism, but not necessarily active infection.

Nevertheless, "positive Western blot results on cerebrospinal fluid are very supportive of a clinical diagnosis of EPM, whereas a negative result is a good rule-out of EPM," explains Morrow.

Many veterinarians use the Western blot test on serum as a screening test to determine whether more invasive tests, such as collection of cerebrospinal fluid, are justified. For example, if a horse with neurologic signs has a negative result on serum (meaning that the horse has not been exposed to S. neurona), other diseases should be suspected. If the test result on serum is positive, it means that EPM is a possible diagnosis and the owner and veterinarian might elect to pursue the more invasive testing to confirm the presence of anti-S. neurona antibodies in cerebrospinal fluid.

The immunofluorescent antibody test The immunofluorescent antibody test (IFAT) is performed commercially on blood samples and identifies antibodies against the entire, intact organism instead of antibodies generated against isolated surface proteins. Second, the results are expressed quantitatively in the form of a titer (an expression of the concentration of the antibodies in the blood). According to Nicola Pusterla, DVM, Dipl. ACVIM, an assistant professor in the Department of Medicine and Epidemiology at the University of California, Davis, School of Veterinary Medicine, one advantage of a quantitative test such as the IFAT is since results are expressed as a titer, they can be used to determine whether infection is active and likely to be causing the horse's neurologic signs. (If titer is rising, the infection is active and ongoing.)

The titer is used in conjunction with a mathematic model, and developers of the test hold that a higher antibody titer in the blood yields a higher probability that the protozoan parasite is causing the horse's neurologic signs.

Pusterla says another test based on the IFAT technique can be used to test horses for Neospora hughesi, a less-common cause of EPM.

Like the Western blot, the IFAT can be performed as a screening test. If the titer falls into the middle range, with values that are too high to yield a definite negative result, but too low to be considered a definite positive result, a practitioner might elect to perform a spinal tap and submit the cerebrospinal fluid for additional testing.

Enzyme-linked immunosorbent assay The ELISA also detects antibodies against a particular S. neurona surface protein. Like the IFAT, results obtained with the ELISA are quantitative and are expressed as an antibody titer. Lucy Edens, DVM, MS, Dipl. ACVIM, an equine internal medicine specialist and consultant for Antech Laboratories, says the ELISA has been reported as having high specificity, a term that means there are fewer false positive results. The test detects antibodies against the S. neurona protein located at what is called the 30-kilodalton (kD) band. Unfortunately, some S. neurona strains do not respond at the 30-kD band, so this test might lead to false negatives.

The test's developers report that the change in titer measured in blood samples collected two to four weeks apart can be used to monitor response to treatment.

Irrespective of the type of test used to aid in diagnosing EPM, vaccination can interfere with blood test interpretation. None of the tests in use can distinguish between vaccine-induced antibodies and those generated in response to natural exposure to the organism.

Vaccination induces the horse's immune cells to synthesize antibodies against the organism or its components, which is the goal of vaccinating a horse, but the positive blood test results will confound attempts to rule EPM in or out in a horse with recently developed neurologic signs.

Part of the Puzzle

Laboratory test results are only a single piece of information in the overall picture created when veterinarians perform or order diagnostic tests to determine whether a horse has EPM. Clinical impression, examination findings, and recognition of the various tests' limitations are also important. Veterinarians must be aware of how results from the various types of tests for EPM should be interpreted, because all the available tests are associated with certain advantages and drawbacks.

Adding to the need for caution here is the fact that the prevalence of EPM in a given geographic area served by a laboratory strongly influences the accuracy of the results. For example, in an area where the prevalence of EPM is low, the strength of even a positive test result is, likewise, low. In areas in which the prevalence of the disease is high, the reliability of a positive result is also high. These factors influence all EPM tests and must be taken into consideration when evaluating results. It is important to discuss the pros and cons of the various tests with your veterinarian when a diagnostic plan is being formulated for a neurologic horse.

Response to Treatment

Some veterinarians have changed the way they approach diagnosis of EPM, relying more heavily on clinical signs than on laboratory testing. Instead of attempting to prove conclusively that a horse with neurologic signs has EPM, another tactic is to rule out other likely conditions and prescribe treatment on a presumptive basis.

"In fact," says Keene, "a horse's response to treatment with EPM medication is sometimes used as a form of diagnostic testing itself."

Prompt improvement in a horse's neurologic deficits after initiation of treatment might be an indication that EPM is indeed the disease affecting the horse. For example, the veterinarian could order a test for anti-EPM antibodies on a blood sample. Positive results would be an indication that the horse had been exposed to S. neurona. If radiographs of the neck revealed no vertebral problems and results of blood tests for other diseases that could cause neurologic signs (such as equine herpesvirus-1 and West Nile virus) were negative, the veterinarian might feel reasonably sure that EPM was the likely diagnosis and would prescribe a course of treatment for the disease, even in the absence of confirming blood tests to prove it. Because of the invasiveness of the procedures for collecting cerebrospinal fluid, an increasing number of veterinarians are diagnosing EPM presumptively in this way.


Three FDA-approved treatments are commercially available at present for treating EPM. Potentiated sulfonamide drugs are the oldest types of medication used to treat horses with EPM. Trimethoprim sulfamethoxazole is a common component of many horse owners' medical kits, and it is frequently prescribed for various types of respiratory tract and cutaneous infections involving bacteria. Sometimes it is used in combination with pyrimethamine, which is used to treat protozoal infections.

The medication is also effective against protozoa because it inhibits folate metabolism (needed for the parasite's survival). The bad news is this inhibiting activity is not restricted to only protozoa. Red blood cell production is a mammalian physiologic process that is also dependent on folate availability, so horses that are on long-term treatment with trimethoprim sulfa should be tested for anemia at least monthly, and special recommendations might be made for pregnant mares that must be treated for EPM, as the medication can affect the growing fetus.

Ponazuril (trade name Marquis) is one of a group of medications that is derived from herbicidal agents. According to Tom Tobin, MRCVS, PhD, Dipl. ABT (toxicology), a professor of veterinary science at the University of Kentucky's Maxwell H. Gluck Equine Research Center, S. neurona is an apicomplexan protozoan parasite, a classification of protozoa that contain cellular components with plant-related genetic material. This has implications for both the effectiveness and safety of treatments.

"It's no accident that the herbicide- derived compounds that are used to treat these infections are nontoxic to horses and that they performed well in safety testing," says Tobin, "because they are designed to damage components of plant cells, not mammalian cells."

Ponazuril is highly specific against the EPM organism and is administered with once-daily dosing. "A positive response to treatment is good evidence that the diagnosis is EPM," says Tobin.

Nitazoxanide (trade name Navigator) is another FDA-approved treatment for horses with EPM. This medication acts by inhibiting enzymes necessary to the survival of not only protozoa, but also certain bacteria and viruses. Because the normal population of microbes living in the gastrointestinal tract can be disrupted, horses being treated with nitazoxanide are weighed for accurate dosing and must be monitored closely by the owner and veterinarian for signs of gastrointestinal upset or other problems. Because the mechanism of activity is different from the other two classes of medication, this treatment might be especially useful in horses that have not responded to treatment with another medication or those that had an initial response, then relapsed.

Take-Home Message

Confirming a diagnosis of EPM typically involves combining physical and neurologic examination findings with results of diagnostic tests. Efforts to develop tests with better sensitivity and specificity are continuing, with the intention of enhancing veterinarians' ability to make a rapid and accurate diagnosis and initiate treatment in horses with evidence of neurologic dysfunction suggestive of EPM.

About the Author

Kim A. Sprayberry, DVM, Dipl. ACVIM

Dr. Kim A. Sprayberry, DVM, Dipl. ACVIM, is an internal medicine specialist at Hagyard Equine Medical Institute in Lexington, Ky. When not working with horses, she enjoys pursuits in medical journalism and editing as well as kayaking and American southwest archaeology.

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