By Heather Smith Thomas

Equine Protozoal Myoencephalitis in horses is a challenging disease. Dr. Sharon Witonsky (Associate Professor and researcher at Virginia Tech) has been involved in EPM research for many years. EPM was in the news a lot in earlier years but not so much today—partly because there have been some improvements in dealing with it and partly because other disease have taken more of the spotlight.

“There haven’t been transformative increases in our knowledge, however, and by comparison we are hearing more about concerns with Lyme disease, and the neurologic form of equine herpes, and a few other diseases, which diminish attention to EPM,” Witonsky said. “EPM is still very much of a concern and we are currently making incremental progress, but not progressing as quickly as we were in the past.”

EPM RESEARCH

“Current EPM research is looking at improving our ability to accurately diagnose cases, as well as improved therapeutics for enhanced recovery. Additionally, there are ongoing studies to try to determine why horses develop EPM and what can be done to improve treatment outcomes. We have a few studies looking at immune dysfunction to try to figure out what is happening with the horses that develop disease—so we can improve treatment outcomes,” Witonsky said.

“Some horses definitely have altered immune responses,” Witonsky continued. “We are trying to further define these changes and how this affects their ability to respond to S. neurona (the major EPM-causing organism) infection. One of the big challenges when looking at horses and their immune responses is that we don’t know the time period from when they get exposed until when we see the disease. Sometimes it’s really difficult, because by the time we see the disease, probably 99% of those horses are trying to make a protective immune response.”

“We have a couple different approaches to examine the altered immune response. One approach is to look at immune cells and the molecules they produce in the EPM-affected horses. We also have on-going studies to determine if there is a genetic predisposition for the disease.”

Other studies are focused on determining the pathologic, including neuropathologic, changes associated with EPM. If we can figure out the mechanisms for the pathologic changes, we may be able to assess human medications which are being used. Investigators are also looking at the potential for other organisms to cause this disease.”

Researchers and veterinarians still don’t always understand why some horses develop neurological signs after picking up the organism, and others do not. We don’t have new data, although there are some hypotheses. A study published in 2000 showed that stressed horses (from work, and young horses in training) have an increased risk for disease.

“We also know there is a breed predisposition as well,” Witonsky said. “Early studies found more cases in Standardbreds, Thoroughbreds and Quarter Horses, but there is no more recent data.”

Regarding a vaccine, research is still in the very preliminary stage.

DIAGNOSING EPM

“The current standard for diagnosis of EPM is being able to get both blood and spinal fluid and look at the antibodies against the organisms. We feel that a horse has EPM if we get antibody production within the central nervous system,” Witonsky said.

“There are different tests available from a few commercial laboratories that run different diagnostic assays. A recent publication from UC-Davis indicates that, over the next several years, with additional studies, we will see how accurate the tests are for diagnosing EPM cases,” she said.

In years past, some people thought that toe-dragging was considered the “classic” clinical sign of clinical EPM, but toe-dragging can be seen in other neurologic diseases as well as EPM.

“The signs we actually look for that are consistent with EPM are the three A’s which are asymmetry, ataxia [incoordination], and atrophy [of muscles]. We also often see behavioral change in addition to these other changes. The clinical signs will depend upon where the organism actually localizes within the brain, brainstem or spinal cord. This is what dictates the clinical signs, which could also include facial nerve paralysis—which is another classic sign. Toe-dragging can also go along with the weakness or incoordination,” Witonsky said.

“We really don’t know which clinical signs indicate active infection of S. neurona causing EPM. When a horse presents with these signs during our exam, we don’t know whether this horse has active EPM or simply residual neurologic deficits. We need to talk to the client, trainer and rider to get more information about history and progression of signs, etc. to get more clarity. With diagnostic tests, we can only see if the titers are increased, but we don’t really have a way of knowing for sure that it is an active infection,” she says.

TREATMENT & PROGNOSIS

“We don’t know if the severity of signs correlate to long-term prognosis. We don’t know, until we start treating the horse, whether it will respond or not. If the horse already has muscle atrophy, it may be more difficult for that horse to respond and recover, compared to a horse that does not have atrophy, but we really don’t know if a horse is going to respond to treatment until we start the treatment,” Witonsky said.

There are currently three FDA-approved medications for treating EPM in horses. Marquis® is the trade name for ponazuril, which is a paste. Protazil® is a trade name for diclazuril, which is in a pellet formulation and ReBalance™ is the trade name for sulfadimethoxine pyramethamine which is a liquid.

“Currently veterinarians are using some combination of antiprotozoal medications as well as other supportive therapies,” Witnosky said. “I use a lot of decoquinate as well as levamisole. There are also other immune supplements available and some people use vegetable oil to increase absorption of Marquis. Other supplements that might help include vitamin E [for muscle health and function], NSAIDS and some people use DMSO.”

In a few cases some clinicians are also using steroids, depending on the severity of the case.

“Other things people need to consider are alternative approaches like acupuncture and herbs, to see if those help restore balance, coordination and recovery,” Witonsky said. “We all have preferences for the medications and treatments we use. When I start treatment for a horse, I decide what I want to use and then reassess to see if the horse is responding at the rate I would expect. When a horse is not responding adequately, I reassess the horse to be as certain as I can, that the horse doesn’t have another concurrent disease. However, if I think the horse is getting progressively worse from EPM, I may combine certain therapies of antiprotozoal medications, levamisole, immunotherapies and potentially have the horse assessed by a veterinary acupuncturist.

“In an early study, 60% of the horses responded to treatment, but only improved approximately one grade. Now we have some improved diagnostic testing and realize we may need to treat some horses longer—especially the ones that we suspect might relapse.

“The combination of improved diagnostics and longer-term treatment, awareness of the owners, etc. may result in a greater success rate. However, no studies have been published to assess this. One paper suggests that about 25% of horses tend to relapse.

“Regarding resolved and new infections, if we treated a horse and it still has neurologic deficits, we don’t have a way to diagnose whether it has active infection or residual deficits. Some of us tend to treat these horses longer because we want to give them the best opportunity to recover. However, we can’t always distinguish between resolved and new infections at this time.

“With any infection, if we treat them and they show recurrent signs after they have recovered, we need to reassess them to see if they have EPM, some other disease, or both EPM and another concurrent disease,” Witonsky said.

“Whether relapses tend to be milder just depends on the horse. From my perspective, relapses can show up with the same clinical presentation the horse had initially. If the horse was initially showing clinical signs/deficits in the left front leg and right hind leg, and the horse presents again with signs in the left front leg and right hind leg again, I’m more likely to think the horse has relapsed. However, we don’t have a way to definitively prove this. We also can’t say that relapses tend to be milder,” she said.

“If we have a horse we are treating for EPM and the horse is not responding the way we’d expect, we reassess that horse to try to figure out if we need to do something different in treatment for EPM or if there is a concurrent disease affecting the horse. Some of these horses seem to have more than one disease, and it’s a good idea to have your veterinarian check and assess them,” Witonsky said.