Neurologic Diseases in Horses

BY HEATHER SMITH THOMAS

There are a number of diseases that can cause neurologic signs in horses. These include equine protozoal myeloencephalitis (EPM, caused by protozoa), cervical vertebral stenotic myelopathy (CVSM or Wobbler syndrome) and EDM (equine degenerative myeloencephalopathy) which is often due to vitamin E deficiency and characterized by degeneration of the brain stem and spinal cord. Clinical signs in all of these diseases are similar. Affected horses may show ataxia (uncoordinated movement of the limbs), an abnormal stance at rest, exaggerated gait with excessive joint flexion, or other general proprioceptive deficits (lack of awareness of where various parts of the body are in space), as well as behavioral changes. Gait abnormalities are usually seen in all four limbs — though the “lameness” may be more severe in the hind limbs.

Dr. Amy Johnson (Large Animal Internal Medicine, New Bolton Center in Pennsylvania) says sometimes the owner, trainer or rider is suspicious there might be a neurologic problem and that’s what prompts them to have the horse evaluated. “Other times they may not know what’s wrong but realize the horse’s performance is not what it should be. It may be just an unevenness or abnormal way of moving,” says Johnson.

“Sometimes the only clue is a change in behavior or the horse loses eagerness to work. The horse may become increasingly spooky or unpredictably bolts, bucks or resists being ridden.” The horse is uncomfortable doing things it used to do willingly.

“When we check the horse, if there is no systemic health problem or lameness that’s clearly the cause, we do a neurologic evaluation. Usually the signs are consistent with spinal cord disease, specifically in the neck region; all four limbs are not moving as precisely and coordinated as they should be. These signs are consistent with what we call general proprioceptive ataxia,” she says.

With any neurological disease there is a spectrum of severity. Some horses continue to compete successfully in an athletic career, even with mild neurologic disease. “The disease may progress over time and the horse struggles to maintain performance. Or perhaps the horse develops a second problem such as lameness,” says Johnson.

“Some horses can compensate fairly well for a single issue or a single problem, but when you start stacking more problems, some of their compensatory mechanisms begin to fall apart. If the horse is lame, some of the neurologic signs may become more evident,” she says.

“It can sometimes be tricky to know how much the neurologic problem is contributing to the overall picture. In some cases it clearly is the primary reason for the poor performance, and in other cases there may be something else at play as well.” This can make it challenging to figure out.

NEUROLOGIC DISEASES – “Most of the neurologic diseases we see most frequently cause signs consistent with a cervical myelopathy, which means dysfunction of the spinal cord in the neck region. Although many diseases can affect the cervical spinal cord, most affected horses fall into one of three categories,” says Johnson.

One is compressive myelopathy. “The most well-known is cervical vertebral stenotic myelopathy (CVSM), often called wobbler disease, but other things like trauma, tumors or abscesses can put pressure on the spinal cord. Something is physically compressing the spinal cord, and causing the clinical signs. This category of disease is relatively common; wobblers and neck arthritis causing secondary spinal cord compression is moderately common in older horses,” she says.

“The second category of neurologic diseases is infectious diseases. EPM is by far the most common infectious neurologic disease in horses, and can sometimes mimic wobbler disease— though you might see additional clues that the horse is suffering from EPM. Horses with EPM tend to have more asymmetric signs, more muscle atrophy, or maybe some signs that would be noticed when examining the head that would be indicative of disease in the brain stem in addition to the spinal cord,” she explains.

“In theory, neurologic Lyme disease could affect this region of the spinal cord, but this is rare. If it’s summer, if the horse has not been vaccinated against West Nile, that virus could infect the spinal cord.” There are several other viruses that can cause acute neurologic disease in horses, and these include equine encephalitis, equine herpes virus, and rabies.

The third category is degenerative disease, and Johnson says this is the trickiest to diagnose. “This category goes by two different names but they are basically the same disease—equine degenerative myeloencephalopathy (EDM) and neuroaxomal dystrophy (NAD). Vitamin E deficiency likely plays a role but it’s not the only contributing factor. Genetics and possibly some environmental factors may be important,” she says.

In earlier years, in earlier cases, veterinarians generally thought this disease was mainly in young horses that were vitamin E deficient. “These horses started showing clinical signs at an early age—by the time they were 1 or 2 years old. Then the incidence of this problem diminished, perhaps because horsemen were supplementing with vitamin E. We were not seeing this disease as often. Then in my personal case population, over the last few years the number of EDM cases I’ve diagnosed has surpassed the number of EPM cases or even wobblers, and these horses are usually between 5 and 15 years of age,” says Johnson.

“Along with ataxia and incoordination there is often a behavior change--usually bad behavior. The owner first notices the bolting, spooking, rearing, bucking, etc. that is out of character for the horse. Later we start to detect mild to moderate ataxia. I don’t know why there has been a resurgence of this disease—whether these horses had some vitamin E deficiency when they were very young and it is manifesting later in life, or if it’s something else. I don’t know what the cause might be, but I’ve seen a lot of these cases,” she says.

“We have good tests for diagnosing EPM, especially if we can take a sample of spinal fluid. When we compare the antibody titers in the blood and in the spinal fluid we can with fairly high accuracy determine whether or not the horse has EPM,” she says.

“We also have pretty good tests for diagnosing spinal cord compression, with a combination of survey radiographs and then myelography. More clinics and universities now have the ability to do CTs of the neck regions so we can usually get a good idea whether or not there is spinal cord compression.”

For the degenerative conditions, however, the only way to currently diagnose these is to look at the brain and spinal

Continued on next page cord under a microscope, which can only be done at necropsy. “To come to that diagnosis in a live horse, we have to rule out everything else and assume it is EDM or NAD. If we have a horse with neurologic disease that’s negative for EPM and no evidence of spinal cord compression, we are suspicious the horse has a neurodegenerative disorder, even though it will be very hard to prove. This is probably the worst of the three options because there’s really no way to treat it,” she says.

The clinical course with degenerative diseases is variable. “Some horses seem to plateau and stay at the same level of function for a long time, even several years, and don’t deteriorate further. The question then is whether it is safe for them to be working, or at pasture or safe for the people handling the horse. In other horses this disease tends to progress more rapidly,” she says.

“We usually advise supplementing with vitamin E, especially if the horse has borderline or low levels of this vitamin. It’s rare, however, that you see marked improvement after starting the supplements. If anything, it simply stabilizes the disease and keeps it from getting worse, but may not reverse any of the signs,” she explains.

Some of these horses may have multiple problems and you don’t always know when it started or what else is going on. “If a horse has a mild case of EDM and is also lame, you might be able to resolve the lameness part, and then that horse may be able to continue to perform.”

It can be frustrating and heartbreaking to encounter a neurological disease in a horse that has been doing well. Trying to figure out the cause can be tricky, though it might be helpful to try to diagnose it early rather than later.

“Sometimes this is hard to do, however, because signs in the beginning can be so subtle that you don’t recognize them as a medical problem. Many of these horses might go through some frustrating periods in their training, where the owner, rider or trainer thinks it’s a behavioral problem and then later we figure out that it’s actually a medical or neurologic issue that prompted the behavior.” Some of the clues may be misread at first.

If owners/riders/trainers can be aware of possible neurologic conditions they may pick up on this quicker. “When a horse is presented for poor performance it probably benefits everyone to do a neurologic exam early on. Most people immediately assume it’s a lameness issue or gastric ulcers because these are more common issues, but it can be important to do a neurologic exam, even if it’s just to be able to say that the horse is neurologically normal and rule out any neurologic disease,” says Johnson.

DIAGNOSIS – Dr. Barrie Grant dealt with many neurologic problems and has worked with wobbler syndrome horses since 1977, and has a consulting practice in California. “We now have more ability to check these horses; there are more CT units available today. These are able to show more of the changes around the cervical vertebrae (5-6, 6-7and 7-1) to show aberrant first ribs, or an abnormality that we couldn’t see with x-rays. We can now do a CT myelogram on a horse in just 30 minutes,” he says.

“Some horses respond favorably to steroid injections in their hocks or neck, but the underlying problem is still there. It would respond temporarily to steroids any place in the body. Even if we simply inject a joint, steroids can get into the bloodstream and go to whatever place is inflamed, and be helpful,” explains Grant.

“Nice horses that become fussy or difficult to shoe, or any horse that changes behavior, may be telling us he’s in pain. Sometimes when we see the x-rays of a horse’s neck, it’s hard to imagine how that horse can endure such pain, and easy to see why he might be fussy or resist having one leg picked up and have to stand with all the weight on the other—when there’s a big facet shoving on the nerves exiting the spinal canal. This can be very painful,” he says. These horses start resisting things we ask them to do.

“Another clue that a problem may be neurologic is when you can’t quite figure out which leg the horse is lame on. The horse may favor one leg, and then another. Don’t just assume that you are not experienced or smart enough to detect it. It may be neurologic rather than a specific lameness in a certain leg,” says Grant.

“There’s also nothing wrong with getting a second opinion. Nothing beats an early accurate diagnosis, with a chance to help the horse. It also pays to take a blood count when the horse is healthy and doing well, to have something to compare with later if he has a problem. It’s good to have a baseline,” he says. “It’s similar to a person having a physical exam once a year or so.”

“We listen to the history, and do a complete physical and neurological exam. It doesn’t take much time to check how a horse moves when you turn him around in circles, do a tail-sway test, make him back up, or go up and down a hill, or walks when doing a serpentine. This can give some clues,” Grant says. TFH