Leading researchers in the quest for the solution to equine protozoal myeloencephalitis (EPM) lectured to a packed room at the North American Veterinary Conference. While the protocol didn't allow for debate among the speakers, debate underscored the sessions anyway. Several "givens" about EPM were questioned. Diagnostics and treatment protocols were bandied back and forth. And the unknowns of the life cycle of the causative protozoal agent, Sarcocystis neurona, were brought to the forefront.

Lectures were honestly revealing of on-going research, and they were followed by a lively question-and-answer time. After the end of the sessions, and a prolonged period of answering questions one-on-one with members of the audience, the speakers agreed to an informal discussion of EPM. Those giving lectures and involved in the round table were Robert MacKay, BVSc, PhD, Diplomate ACVIM, of the University of Florida College of Veterinary Medicine; Frank Andrews, DVM, University of Tennessee, Department of Large Animal Clinical Sciences; Steve Reed, DVM, Diplomate ACVIM, of The Ohio State University Veterinary Teaching Hospital; and Martin Furr, DVM, of the Virginia-Maryland Regional College of Veterinary Medicine, Marion duPont Scott Equine Center.

Is the horse a dead-end host?

The consensus answer from the group was maybe. The horse cannot pass a viable parasite in its feces as can the opossum. One infected horse cannot give the parasite to another horse.

However, it has been said in public forums that if you grind up a horse with the S. neurona parasite in its muscles that a carnivore could not get the parasite in a mature-enough form to continue the life cycle.

"That is incorrect information," stated Reed.

MacKay said that it is "highly premature" to conclude that the horse cannot complete the life cycle of S. neurona. "I don't know why we say that," he stressed. "Who has proved that?"

There is nothing in the literature that says the parasite is not viable in the muscles of the horse, but, he added, it is "very unlikely that the horse advances the life cycle" of S. neurona. "If it does take it any further, it is to the sarcocyst stage in the muscle, which means some other animal would have to eat the horse. It is not the most natural intermediate host."

Andrews noted that while sarcocysts have been found in the muscles of horses, it occurs in a low percentage of cases. But, researcher J. P. Dubey in the 1970s, when he was at Ohio State, stated that he thought that up to 30% of horses had Sarcocystis, called S. fayeri in their muscles. At that point, the equipment and tests were not available to differentiate between neurona and other sarcocystis species.

"You would have to think that if Sarcocystis neurona was in the muscles, someone would have found it by now," stated Reed.

MacKay said researchers have found cross-sections of sarcocystis in the muscles of horses, and they were assumed to be Sarcocystis fayeri, which passes normally from the dog to the horse and does not present a problem to the horse.

"The question is whether neurona can do the same thing and advance to the final stage of differentiation, which is called a sarcocyst," said MacKay. "My point is that it hasn't been proven one way or the other."

"It would be fair to say that we are inching our way closer to a better understanding of the life cycle," said Reed, "and really, the unanswered questions have not changed. We've just now highlighted the unanswered questions that show where we thought we knew something, we don't.

"We aren't going backward on this disease, we are going forward. We're just defining what we need to know," he added.

Is the opossum the definitive host of Sarcocystis neurona (defined as a carnivore that eats the parasite-infested muscles of an intermediate host, allows the parasite to mature in its body, and passes the parasite in its feces for the intermediate host to pick up while feeding)?

"We know the opossum is a definitive host, but we don't know if it is the only one," said MacKay, adding that there might well be other definitive hosts capable of passing along the parasite in their feces. "In the Sarcocystis business, it is well established that a single parasite can use multiple species of animals as definitive hosts. I mentioned in my talk Sarcocystis cruzi, which is a very common parasite of cattle. The definitive host is the dog, and also the fox, the jackal, the coyote, and the raccoon of all things. So, it might be tunnel vision to assume it's just the opossum.

"The other thing is that the American opossum basically is in the U. S., Mexico, and Costa Rica," noted MacKay, "but they have tons of EPM in Brazil, Argentina, and Peru, where the American opossum doesn't live. But they have other opossum species there that probably function as definitive hosts in those countries."

What do we know, and not know, about the life cycle?

Finding the intermediate host in the life cycle of Sarcocystis neurona is the "Holy Grail" of research into EPM, according to MacKay. That sentiment was not shared by all the researchers, although it universally was considered of great importance.

All the researchers stressed the need to know the intermediate host. "It will give us strategies for immediate control of exposure of the horse, and a way to control the life cycle in nature, perhaps, by reducing the number of intermediate hosts," said MacKay.

"The most important thing is that it will allow us to capture the life cycle completely in the lab," he continued. "It doesn't matter how many mouse models you have, you ultimately have to produce the disease in the horse. Right now we have to use sporocysts from dead opossums to infect the horse. If we had an intermediate host that we could keep in the lab, we could cycle the parasite between the intermediate host and the opossum. The opossums are easy to keep in the lab, although smelly and repulsive. If it was a rodent or a bird (intermediate host), it would be easy to keep."

(Reed interjected that keeping opossums in the laboratory is easy and has been done in other research studies for years. At The Ohio State, the human spinal cord trauma injury research laboratory keeps many opossums to study because if the spinal cord of the opossum is severed before five days of age, it regenerates. After five days of age, it won't regenerate.)

Why are so many horses exposed to the parasite, but so few get EPM?

One of the big unknowns is that a lot of horses are affected with the EPM parasite, but very few horses get EPM, acknowledged MacKay. The Ohio State has been looking at risk factors, but what factors allow the progression of a benign infection to EPM?

"Let's say that most horses in some parts of the country are exposed to EPM, but only 1-2% come up with EPM," said MacKay. "What other factors (besides exposure rate) lead to this progression from an inapparent infection to a potentially fatal disease.

We don't know, was the consensus.

Potential risk factors include the dose of parasite the horse receives, whether the horse is immunosuppressed, genetic susceptibility, concurrent disease process, and exogenous corticosteroid administration, which depresses the immune system.

The good news is that about 70% of treated horses improve, and most horses are treated with pyrimethamine and a sulfonamide, usually trimethoprim-sulmethoxazole or sulfadiazine. (More on treatment to follow.)

Antoinette Marsh, PhD, of the University of California, Davis (now at Missouri), developed the first mouse model for use in studying Sarcocystis neurona, published in scientific literature last year. In the mouse, it was discovered that immunocompetence could be an important issue in development of the disease.

It's still unknown exactly how long it takes from exposure to disease for a horse in natural environmental conditions. Experimentally, it can be from 30-40 days from exposure to disease, but some of those experiments were with high doses of parasite.

"You can get EPM horses in England, but only in horses from the United States," pointed out MacKay. "One fellow had a horse there for four years that then came down with EPM." The current thought is that horses can harbor the parasite without clinical signs of disease, then when stressed or faced with a lowered immune system, the parasite can cause EPM.

"It was somewhere in that horse," said Reed of the horse in England.

MacKay said in his lecture that he has developed a theory on the life cycle of the S. neurona. The intermediate host, which is as yet unknown, is eaten by the opossum, which produces in its feces the sporocysts that are ingested by the horse. The parasite undergoes asexual reproduction in the horse that magnifies its numbers and stimulates an immune response by the horse. The horse either recovers, or the parasites travel to the central nervous system. At that point, the horse either recovers, or because of stress and an compromised immune system, progresses to clinical EPM.

Because of the low incidence of EPM (about 1%) and the high exposure rate (in some places more than 50%), MacKay and others commented on the possibility that horses are naturally resistant to developing EPM, and that actual disease is the exception rather than the norm.

In his lecture, Andrews pointed out that EPM in horses under two years of age is rare. He also thinks that in older horses, EPM is overdiagnosed and others, possibly concurrent diseases, are underdiagnosed (such as poor nutrition, neoplasia, and pituitary adenoma).

He pointed out that there is no breed predilection, although certain breeds--Arabians, Quarter Horses, Morgans, drafts, Standardbreds, and Thoroughbreds--are known to be predisposed to neurologic diseases.

Andrews also noted that EPM usually is associated with gradual onset of signs and obscure lameness. Horses with acute and severe neurologic signs should be tested for rabies, trauma, herpes, viral encephalitis, toxicity, botulism, tetanus, and parasite infestation. He also recommended using a written neurological examination report so the horse's condition could be compared over time. Radiographs are essential to determine the occurrence of fractures, malformations, navicular disease, or narrowing of the spinal cord. Scintigraphy, thermography, CAT scans, and MRI examinations are useful adjuncts in developing a differential diagnosis.

As was mentioned in lectures at the AAEP convention in December 1998, cerebrospinal fluid (CSF) samples should be checked for blood contamination. Andrews noted that normal parameters of a CSF sample do not rule out EPM because the sample could reflect that it is early or late in the course of the disease, or that the lesion is too far from the sampling site to give a proper diagnosis. He stressed that albumin quotients (that indicate blood contamination) should be done to rule out false positives in CSF Western blot tests.

MacKay, on the other hand, said he felt the albumin quotient and IgG quotient are not very sensitive ways to detect blood contamination and infection.

What about diagnosis and treatment?

"The central nervous system (CNS) is a very unforgiving system," noted Reed in his lecture. "There is little space for swelling due to inflammation, and often it is a difficult area to treat."

Andrews said if you have a normal horse (one without neurologic signs) that has a positive CNS tap, the probability of that horse's having EPM is about 30%. However, if the horse tests negative, then there is a 98% probability that the horse does not have EPM. Again, stress possibly could push a horse into a position to be at greater risk.

The PCR (polymerase chain reaction) test is frequently negative. If it is positive, then there is a very high probability that the horse has EPM. "A negative test tells you nothing," said Andrews.

Most horses are treated with pyrimethamine and a sulfonamide, usually trimethoprim-sulfamethoxazole or sulfadiazine. At The Ohio State, the treatment protocol calls for oral administration of 1 mg/kg once a day pyrimethamine and 20 mg/kg twice a day of sulfamethoxazole. Folic acid is not supplemented. Vitamin E is administered as an anti-oxidant (6,000-9,000 IU per day), and levamisole is given to boost the immune system.

Toxicity with pyrimethamine can occur. Anemia is the most common sign, but there also can be low white blood cell count, poor appetite, abortion, and bone marrow and other damage in newborn foals.

General immune stimulants such as Eqstim and Equimune IV have been used, although not studied, in conjunction with treatment for EPM. MacKay said he gives Equimune once weekly for three weeks. Eqstim can be given daily for three days, then twice weekly. Levamisole is given orally 1 mg/kg twice a day for the first two weeks, then in weeks five, nine and 13. "Interrupted therapy is important," he stated.

Other treatments being used besides the pyrimethamine and sulfamethoxazole are Diclazuril (Clinacox) usually and Toltrazuril (Baycox), both anti-coccidial drugs. Trials have been done using one of two dosage recommendations: 5 mg/kg one time a day or 10 mg/kg one time a day, each for 28 days. There currently are no studies on-going for Diclazuril. The bulk and volume of drug in its current formulation are limiting, noted Furr. A Toltrazuril derivative (toltrazuril sulfone) is being studied by Bayer and research is expected to be completed this spring.

NTZ, or nitazoxanide, is an anti-protozoal drug developed for human medicine and being investigated as a possible treatment for EPM by Blue Ridge Pharmaceuticals.

Combined therapies are rampant, and there have been no studies on these courses of treatment. Furr said you "take your chances" with combining therapies.

Adjunctive therapies might include herbal medicines, T cell stimulators, and gold salts, said Furr.

He believes that immune stimulants can play a role in the treatment of EPM because he feels from some studies he has done that the immune system has a role in the pathogenesis of EPM. He has shown when fragments of S. neurona are added to lymphocytes, the fragments reduce the proliferative response. That suggests that EPM might be immunosupressive in a local area.

Andrews said in using Baycox (Toltrazuril) and Clinacox (Diclazuril 450 grams twice daily for 30 days), he found through spinal fluid assays that the drugs do penetrate enough to kill the parasite.

Furr suggested staying away from IV Diclazuril because of the possibility of liver toxicity. Reed, however, said more than 800 doses have been given to horses without any major problems associated, but he said further investigation is needed.

The question of treating normal horses prophylactically (to prevent disease) was raised. It was not recommended to treat with pyrimethamine and sulfamethoxazole on a short-term basis as a preventative, but Reed said some of the newer drugs might have prophylactic effects and uses.

MacKay stressed that though horses treated continuously from yearlings to two-year-olds (such as sale yearlings pinhooked for two-year-old sales) might be put at risk for later problems, "I don't see anything wrong with putting them on full treatment."

Andrews pointed out that drug resistance only can occur in individual animals since horses are not thought to pass the mature sarcocysts.

Reed also pointed out that foals will seroconvert to EPM-positive after they nurse and obtain antibodies from their dams.

The researchers advised horse owners not to tap yearlings coming out of sales because of the high incidence of false positives.

"If the veterinarian makes the offer to do a post-sale CSF test on a normal horse and comes up with a postive, he has already crossed the Rubicon and has to treat that horse," said MacKay. "That's why we advise not to tap."

"I go through a lot of effort if I don't see a neurologic problem in a horse not to tap it," said Furr, "because once you've tapped it, and it comes back positive, you have no ethical grounds to say to the client not to treat."

Reed said he suggests that if a positive tap is recovered from a clinically normal horse that nothing be done for 30 days and the horse be re-tapped. If the next time the horse is tapped it is negative, then, he said, you know the first tap was a false positive.

"Some owners and trainers are so nervous when they get that first positive back that even if they agree to watch the clinically normal horse for 30 days, a week later they are calling you telling you the horse is stumbling," said Reed. "Horses stumble all the time, and if they didn't know about the positive, they wouldn't have paid any attention."

Three things that horses should have before they are treated, according to the panelists, are neurologic signs as certified by a veterinarian, verification that they are not wobblers, and a positive CSF tap. Those are good indications for a horse needing treatment.

Blood serum positive, but no signs. Don't treat.

CSF and serum positive, but no signs. Don't treat, but watch closely and re-tap in 30 days (keeping in mind that such a horse should not have been tappped in the first place).

A negative serum test goes a long way in ruling out EPM. Negatives have a high predictive value.

"Traditional therapies still are a reasonable thing to do," said Reed. "There are plenty of new treatments on the horizon, but they will be about six months to a year away. Some of the newer treatments might have some prophylactic properties."

"But all of these treatments require that the disease be reproduced easily in the laboratory, and that requires the question of the intermediate host be solved," said MacKay.

Final comments?

None of the researchers have seen a case of EPM in a mule.

"EPM is worth being worried about, but there is progress being made," said Reed.

"Progress in the area could be accelerated greatly by private donations to research," said MacKay.

"There is a tremendous amount of work to be done in various fields," said Furr. "The life cycle, obviously, the treatment area, the prevention area, diagnostics, and just the understanding of why horses get the disease. We need a better diagnostic test, not that the Western blot is not a good test, but it has its limitations that are making it confusing."

The consensus was that there are plenty of things that need to be done to solve the puzzle of EPM. The research required is sophisticated science, and that costs serious money.

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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