EPM Tests and Treatment

At first, veterinarians and researchers were scrambling to discover anything about the "new" neurologic disease that was causing such a stir in the horse industry. The disease, equine protozoal myeloencephalitis (EPM), was discovered to be caused by a one-celled protozoal parasite called Sarcocystis neurona. This parasite does not include the horse in its normal life cycle (which means horses can't pass it to other horses). It is known to migrate into the spinal cord or brain of the horse and cause a multitude of asymmetric symptoms, ranging from hardly noticeable to life-threatening.

Looking for antibodies with a blood test revealed an incredible number of positive horses (up to half the equine population or more in some areas, which is still true today). In the beginning, it was thought that a positive blood test meant a horse had EPM. Then researchers realized that a positive blood test only meant the horse had been exposed to the protozoal parasite and had developed antibodies against it.

A new Western blot assay using cerebrospinal fluid was developed that was specific for S. neurona and did not react for the other sarcocystis organisms. Treatments (pyrimethamine and a sulfonamide, usually trimethoprim-sulfamethoxazole or sulfadiazine) were aimed at stopping replication of the organism in the horse's body until the horse could mount a suitable defense. The use of medication in that timespan is advocated at this time. The life cycle of S. neurona was discovered, and prevention in the form of farm management and animal (opossum and birds) control was added to the arsenal. Folic acid supplementation was recommended, especially for mares, because of the mode of operation of drugs used in treatment. It now is thought that folic acid supplementation is not in the best interest of the health of some horses, especially pregnant mares and unborn foals.

Then, the FDA allowed two drugs (Diclazuril and Toltrazuril, both anti-protozoal medications) to be imported from Canada by veterinarians under a special provision of the U. S. government.

The latest round of information concerning EPM came in the form of four scientific presentations on the disease, and quite a bit of talk during the AAEP convention. More is being learned every day about treating--and not treating--horses for EPM. There are new discoveries about current treatments, warnings about treating mares and stallions, and potential new drugs on the horizon.

Steve Reed, DVM, Diplomate ACVIM, of The Ohio State University, and his colleagues have labeled EPM "the most significant equine disease in the Eastern United Sates because of its potential to cause tremendous economic losses."

Be Wary of Positives

Margaret Miller, DVM, of the University of Pennsylvania's New Bolton Center, presented a lecture based on research into blood contamination of cerebrospinal fluid. Researchers and veterinarians knew if a CSF sample was contaminated with blood, it could give a false positive test if the horse had been exposed to S. neurona and developed antibodies in its blood. The feeling shared among most practitioners is that if a horse shows positive on a CSF test, it means the organism has invaded the spinal cord or brain and caused antibodies to be produced there. However, many horses which tested positive on a CSF Western blot test did not show any observable clinical signs of disease.

What Miller and her colleagues discovered was that even spinal fluid samples that appeared clear (not red or blood-tinged) often had microscopic evidence of hemorrhage (red blood cells). Their research showed that small amounts of blood contamination in the cerebrospinal fluid of a horse which has been exposed to S. neurona (is positive on a Western blot test for blood serum) can result in a positive test result on the cerebrospinal fluid.

"Obtaining a CSF sample free of any blood is difficult; thus, if a CSF sample is determined to be weakly positive for S. neurona antibodies, the results should be interpreted cautiously and only in light of clinical findings," Miller noted in her presentation. That is to say, those horses might not have EPM and careful observation, rather than initiation of therapy, may be satisfactory.

Diagnostic Sense

Bill Bernard, DVM, Dipl. ACVIM, of the Rood and Riddle Equine Hospital in Kentucky, for several years has been encouraging veterinarians and owners to be judicious in interpreting positive test results for EPM in the absence of clinical signs. In his talk "Sarcocystis neurona Antibodies in Equine Cerebrospinal Fluid," Bernard noted the following:

"The immunoblot testing for antibodies to Sarcocystis neurona has improved the antemortem (diagnosis in live horses) evaluation of the neurologic equine. As with those for other infectious disease, antibody tests are an adjunct to diagnosis and rarely can be considered a definitive test. The Western blot testing for cerebrospinal fluid antibodies to S. neurona should be considered a useful diagnostic test in the evaluation of equine neurologic disease. However, the use of the Western blot alone or the use of the Western blot in clinically normal individuals may lead to a false-positive diagnosis and inappropriate therapy." He indicated this should be avoided.

Bernard took records from 943 horses in which cerebrospinal fluid analysis was performed. Neurologic exams also were done on those horses. Each animal was placed into one of two groups depending on whether they had clinical signs of neurologic disease, a gait abnormality, or a client complaint of a performance problem, or if they were just being screened for the disease with no clinical signs observed or reported. (The CSF samples were evaluated for blood and were excluded if considered contaminated.)

The test results showed that 79 of the horses (31%) which were neurologically normal were positive on Western blot testing. He also found that 205 of the horses (29%) with neurologic signs or complaints from owners were positive.

Bernard noted that from these test results, "a large number of clinically normal horses are CSF S. neurona Western blot positive. Possible explanations for the presence of antibodies in the CSF of clinically normal horses include blood contamination of the CSF at the time of collection; blood-brain barrier compromise, with leakage of antibodies across the blood-brain barrier; persistence of antibody response subsequent to removal of the antigen; antibody crossing a normal, intact blood-brain barrier possibly as a result of a very high concentration in the blood; and cross-reactivity of the Western blot test with an antigen other than S. neurona." He added that some horses might not exhibit clinical signs in response to the organism.

He said blood contamination of the CSF was a very plausible explanation for the high rate of positives. Bernard said in his presentation that he feels veterinarians are relying too much on the Western blot test and not enough on neurologic signs. Clinically normal horses can have antibodies to Sarcocystis neurona in a CSF sample. Therefore, a positive CSF does not confirm a diagnosis of EPM, but lab tests support the diagnosis made on history and clinical signs.

Treatment Problems?

J. P. Brendemuehl, DVM, PhD, Dipl. ACT, of Oregon State University, noted in his lecture that there were reports in 1998 about congenital defects in foals from mares treated with pyrimethamine and sulfadiazine during their last trimester. He also noted anecdotal implications of that drug treatment therapy in reducing fertility in mares and stallions, but said another study showed no reduction in stallion fertility with that drug treatment regimen (see next section).

He noted that in his study, mares treated with pyrimethamine and sulfadiazine during breeding and early gestation did not result in detrimental effects or early embryonic loss up to Day 25 of gestation. However, on one farm with 50 broodmares, 10 of which had neurologic signs and were positive for EPM, there was a decrease in mean per cycle pregnancy rate and a high percentage of fetal loss.

His take-home message was that practitioners should be careful putting mares on pyrimethamine and sulfadiazine in early pregnancy, and he questioned the possibility of second trimester abortions and third trimester congenital defects possibly associated with this drug regimen. Brendemuehl stressed that more studies need to be done on this issue, but veterinarians have a responsibility to notify owners of possible side effects.

Treatment And Stallions

Sylvia J. Bedford, LV, MS, and Sue McDonnell, PhD, of the University of Pennsylvania's New Bolton Center, reported on the use of trimethoprim-sulfamethoxazole and pyrimethamine and its effects on stallions.

They reported that the treatment of normal breeding stallions with recommended dosages of trimethoprim-sulfamethoxazole and pyrimethamine did not affect semen quality, testicular function, or sexual behavior. There was evidence, they said, of transient musculoskeletal or neurologic deficits (30 days into treatment through 30 days after treatment) that affected copulatory form and ejaculation pattern.

They recommended not arbitrarily treating stallions, even though their results were "encouraging" that the treatments did not affect reproductive ability. They said the major problem in the four treated stallions they observed involved transient changes in mounting position, "which appeared to reflect musculoskeletal stiffness across the back and possible neurologic changes in the hind limbs and ejaculatory apparatus." They noted that in humans, there is evidence that folic acid deficiency might result in spinal cord-related neurologic problems."

EPM For Lunch

One of the features of the convention is the Table Topics, which are held during the lunch hour. With boxed lunch on lap and pen and paper in hand, veterinarians gathered for "Current Treatment For Equine Protozoal Myeloencephalitis," hosted by Reed of The Ohio State University. In a room filled to overflowing, practitioners learned of the latest information for treating the disease and available products.

There are no controlled studies on any of the treatments for EPM. With this fact in mind, Reed launched into the treatment protocol in place at The Ohio State University.

Current treatment for EPM is a combination of one mg/kg of pyrimethamine one time a day along with sulfadiazine 20 mg/kg two times daily. Since these two drugs are used in combination, every two weeks there is a blood count performed. If signs of anemia are present, then the horse might be treated with folic acid (the sulfadiazine/ pyrimethamine treatment is stopped for a few days and folic acid or folinic acid is administered). Generally, OSU does not recommend using folic acid with pyrimethamine and sulfadiazine as there is some belief that the folic acid might be a cause of toxicity in pregnant mares.

Treatment time is a minimum of 90 days. At the end of that period, the horse is retested. Most of the horses at OSU undergo a five-month treatment period. At the university, a horse is treated until it goes clinically normal, then 30 days beyond. Or, it is treated until it has the same neurological test several times in a row. If the horse is going to relapse, it usually will do so within seven to 10 days.

Vitamin E is part of the treatment for all neurologic conditions, with between 6,000-9,000 IU of Vitamin E given per day.

OSU does not use oxytetracyline on any cases; however, some practitioners have reported using it with success.

There are other treatments available for EPM. Diclazuril and Toltrazuril are two drugs that have shown some promise. There are two dosage recommendations for the drugs: 5 mg/kg one time a day or 10 mg/kg one time a day. These anti-protozoal drugs are being tested on 150 horses at seven different sites. The time period for treatment is 28 days. At this point, no one is prepared to say whether these two drugs are more efficacious than any others. It also is too early to tell if the 28-day treatment period is enough time, or if the 10 mg/kg dose is better than the 5 mg/kg dose.

At Ohio State, every horse receiving pyrimethamine and sulfadiazine undergoes a spinal tap once a month. With the Toltrazuril treatment, the horse is tapped before and at the conclusion of the 28-day treatment , then not tapped again until 90 days after the end of the treatment. At the time of the round table, only two horses had reached that point. Both still tested positive, but they showed clinical signs of improvement. Ancillary treatment with anti-inflammatories such as DMSO is not part of the Diclazuril/toltrazuril regimen, but are often included as part of the treatment for any severely affected horses.

The average success rate of horses responding to treatment is about 70%, and lifelong treatment is not recommended. In addition to these particular drugs, companies also are looking at a preventative drug.

EPM still is considered a sporadic disease, with many more horses testing positive for the S. neurona organism than actually contract the disease. All the drugs mentioned are early in the investigation process. Reed said we must adopt a "wait and see" stance before making any concrete or firm judgments.

Ancillary Treatment Shows Promise

Robert MacKay, BVSc, PhD, Dipl. ACVIM, Chief of the Large Animal Medicine Service of the University of Florida, is working with levamisole as a possible ancillary treatment for EPM. Extrapolated from this drug's effectiveness in other species, including humans, levamisole is being used concurrently with other drugs (sulfadiazine and pyrimethamine) to treat EPM in horses.

Originally used as an anthelmintic (dewormer), levamisole was discovered to have the ability to boost the function of T lymphocytes in the immune system, especially when the immune system had in some way been compromised. Also known as T cells, these T lymphocytes are responsible for stimulating a response to intracellular antigens (foreign substances in the body such as viruses, protozoas, etc.). It has been found that levamisole is quite effective as part of treatment protocol against protozoal diseases. It is thought that the added stimulation to the immune system is what makes the drug effective.

Evidence of levamisole's effectiveness can be found in its use to combat cerebral toxoplasmosis in AIDS patients. Toxoplasma gondii is a protozoan that is found in a wide range of mammals, including humans, and in birds. The cat is the definitive host and passes the protozoa through its feces. Most of the time in humans, the infection is asymptomatic or mild symptoms might occur. However, immunocompromised patients are especially susceptible to this particular parasite, such as the case with the cerebral toxoplasmosis in AIDS patients. Like horses with EPM, the human disease is treated with a combination of sulfadiazine and pyrimethamine. Adding levamisole to the treatment regimen in humans has increased the survival rate for a year by 50%.

This is not the only success story in these kinds of situations. Levamisole has been used with similar results in treatments for coccidiosis of pigs, an intestinal parasite, and leishmaniasis in humans, a highly fatal protozoal disease mainly found in Africa.

Since there is similarity between Toxoplasma gondii and Sarcocystis neurona, levamisole was added to the treatment protocol for EPM horses treated at Florida. According to MacKay, "We add it; we don't substitute. It apparently is important to use the agent according to an interrupted schedule. During a typical 12-week treatment regimen with sulfadiazine/ pyrimethamine, we use the drug at a dose of 1 mg/kg by mouth twice daily during weeks one, two, five, and nine of the treatment. There are many forms of levamisole available, any of which would be fine. I prefer Levasole injectable dewormer solution, 13.65% (Mallinckrodt) because it is cheap and concentrated."

MacKay went on to add that although this particular solution is for subcutaneous use in cattle, it must be given orally to horses.

There have been no studies to document the effectiveness of this approach; there have been, so far, no undesirable side-effects.


On yet another front, nitazoxanide (NTZ), an anti-protozoal drug developed in human medicine, also is under investigation as a possible treatment for EPM. NTZ is used for treating Cryptosporidium parvum (a food and water borne parasite that causes diarrhea) in AIDS patients.

Doug Hepler is the Vice President of Research and Development for Blue Ridge Pharmaceuticals, the firm responsible for conducting the field trial to obtain information needed for Federal Drug Administration approval of NTZ in horses. According to Hepler, preliminary tests using this drug against S. neurona have been conducted in vitro in the laboratory. Also, some preliminary work has been done with horses. Both have been encouraging, he said. Plans now are taking shape for an open field trial with veterinarians so data for the FDA can be gathered in order to have the drug approved by the FDA. The company is testing for both safety and efficacy in the development of the oral paste.

Nick Vatistas, DVM, Dipl. ACVS, a researcher from the University of California, Davis, is working on the formulation. An oral paste administered once daily for 28 days has produced fairly good results. So far, Vatistas personally has looked at 18 horses with EPM. The success rate using this protocol has been around 70%. A few of the horse showed signs of relapse, but they had been put back to work almost immediately, so it was thought that stress caused them to relapse. A change in treatment protocol now allows time for recovery.

So far, there are no major side effects with NTZ. Some horses might be off feed or depressed, the researchers noted. Some horses might even have a mild case of diarrhea. One horse became slightly colicky. Some, however, did show a worsening of clinical signs for one to two weeks before they showed improvement (treatment crises, which also occurs with other therapies for treating EPM). For those horses which have more severe cases and are on the edge of becoming unable to stand, this particular side effect could be hazardous.

Vatistas has one important thing to say about the drug: If the horse has had EPM for a year or so, this drug won't help. Even though it might kill the organism, the damage is still there. The same is true for any other drug regimen. Therefore, an early diagnosis is critical.

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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