Your horse hesitates and steps awkwardly when he walks downhill. He's dragging his toes, too, and a few times he has even stumbled while trotting in the ring. He doesn't seem sore, and your trainer and farrier don't see anything wrong with his feet—but you know he's not right. Could he have one of several equine neurological disorders, like equine herpesvirus type 1 (EHV-1) equine protozoal myeloencephalitis (EPM) or West Nile virus?
Owners dread equine neurological disorders, such as equine herpesvirus type 1, equine protozoal myeloencephalitis or West Nile virus, and no wonder. Many of these problems are hard to diagnose and hard to treat, and they can damage a horse's nervous system in ways that leave him unsafe to ride. But every neurologic case doesn't end badly, and quick action—recognizing signs, getting a diagnosis and starting appropriate treatment—can give your horse the best chance.
If you think your horse might have a neurologic problem, it's time to call your veterinarian. What exactly will your vet do, and what disorders might she find? In this article, we'll walk you through a standard neurologic exam, tell you what else may be needed to make a definitive diagnosis and give you an overview of the most common problems.
Sorting Out the Signs
Signs of neurologic problems in horses run the gamut—seizures, abnormal behavior, abnormal gait, facial paralysis and more, says Debra Sellon, DVM, professor of equine medicine at the Washington State University College of Veterinary Medicine. "The most common neurologic problem equine veterinarians see in the United States is an abnormal gait," Dr. Sellon says. "Affected horses are usually ataxic and weak, meaning that they walk with a staggering or drunken type of gait. They may drag their toes, stumble frequently or sway back and forth when they walk."
Disruptions in a horse's command and control system create these problems. For normal movement, nerve signals must flow from his brain along his spinal cord to the nerves that govern his muscles—and nerves must signal back to his brain, reporting where his limbs are. If the signals don't get through, your horse may become uncoordinated or develop abnormal gaits.
Many neurologic disorders can disrupt the signals. In most parts of North America, Dr. Sellon says, the most common are equine protozoal myeloencephalitis and cervical vertebral malformation ("wobbler syndrome"). But there are plenty of others, including injuries, several viral diseases and degenerative conditions, such as equine degenerative myeloencephalopathy, which has been linked to vitamin E deficiencies in young horses.
"Veterinarians use a fairly standard approach to determine a diagnosis for horses with neurologic disease," Dr. Sellon says. The process begins with gathering information on your horse's history. When did he begin to stumble or show other signs? Has he fallen? Information on your horse's age, breed and use is important because some neurologic problems are more common in certain groups of horses.
Step two is a thorough physical examination. This exam may reveal soreness or other non-neurologic causes for your horse's signs, or it may turn up signs of an injury or a disease that produces neurologic problems. As she examines your horse, your vet compares the right and left sides of your horse's body, looking for asymmetry and loss of muscle mass (atrophy) that may develop when muscles go unused, as happens in some neurologic conditions. She may check the range of motion in his neck by encouraging him to bend to each side, using a carrot or another treat as a lure. Limited range of motion may mean an injury or even fractured vertebrae in his neck (the cervical spine).
The third step is a detailed neurologic examination. "The goal is to determine, to the best extent possible, the site in the nervous system that is affected," says Dr. Sellon. By finding out which functions are impaired, your veterinarian can figure out which nerves are involved. The process, called lesion localization, typically includes these steps:
General assessment: Your veterinarian observes your horse's mental status and behavior. Is he alert or lethargic? Standing or down? Wandering aimlessly, circling or showing other odd behaviors or postures?
Basic reflexes: This part of the exam starts at the head with tests of the cranial nerves, which are involved in functions like hearing, vision, swallowing and facial sensation and muscle control. To test vision, for example, the vet quickly moves a hand toward your horse's eye to trigger the menace reflex; your horse should blink and perhaps jerk away.
Along your horse's neck and back on each side of his spine, your vet uses a ballpoint pen or similar object to touch your horse's skin. A light but firm touch should trigger the panniculus reflex, the skin twitch you see when your horse is pestered by a fly. Lack of a reaction in any area suggests a problem with the nerves that supply that region. At the hind end, the vet checks muscle tone by lifting the tail; a limp tail may be a sign of a spinal cord problem. When his anus is gently stimulated, it should pucker and your horse should clamp his tail.
Maneuvers in hand: These tests show if your horse has control of his limbs and knows where his feet are. The vet watches as your horse is backed and turned in very tight circles in both directions to see how he places his feet. A normal horse keeps his rhythm and steps under his body, while a horse with a neurologic problem may interfere, take confused steps, swing a hind leg wide or pivot on one leg.
On a slope: Your vet may ask to see your horse led up and down a slope, to see if he stumbles, drags his toes or shows other gait abnormalities. Repeating this test with your horse's head raised sometimes makes the signs more obvious.
Tail pull: This helps your vet assess your horse's balance, strength and reaction time. As a handler leads your horse forward, your vet grasps his tail and pulls it firmly to the side. A normal horse will resist the pull; a horse with a neurologic problem may be tipped off balance. The test is repeated on the other side.
Foot placement: These tests help determine your horse's awareness of his limb position. Your vet takes each foot in turn and places it over its opposite number—left front over right front, right front over left front, and the same behind. A normal horse will immediately put each foot back where it belongs; a horse with a neurologic problem may leave one or more feet out of place for a time.
By the end of the exam your veterinarian should know whether your horse's problem is neurologic and, if so, what areas of his nervous system are involved. She may not have enough information for a clear diagnosis yet because many neurologic disorders have variable signs. "Horses with EPM can show a wide variety of signs, ranging from ataxia and weakness, to individual nerve paralysis, to seizures, to problems with urination or defecation," Dr. Sellon says. "The vast majority of wobblers present for examination with ataxia and weakness of all four limbs. That means a horse with EPM often looks different from a wobbler but sometimes looks just the same."
Still, your veterinarian will have enough information to make a list of the diseases or disorders that are most likely the cause and then choose the most appropriate diagnostic tests to confirm or rule out the items on that list. Here's what you can expect for three common conditions.