Understanding the ever-changing world of equine neurologic disease.

If someone says he or she can think or talk about equine neurology without becoming dizzy, that shows only the person has not understood anything about it. This bastardization of a quote uttered by Murray Gell-Mann, an American Nobel Prize-winning physicist, at one point in his esteemed career in reference to quantum physics, succinctly and effectively describes the complexity of the art and science of equine neurology.

To take the dizzy factor out of understanding neurologic disease in horses, I.G. Joe Mayhew BVSc, FRCVS, PhD, Dipl. ACVIM, ECVN, head of Equine Massey and professor of equine studies at New Zealand's Massey University, has dedicated much of his career to helping practitioners and owners understand equine neurology. He undertook one of his latest educational efforts at the 11th Congress of the World Equine Veterinary Association, held in Brazil in 2009. He made several presentations on topics ranging from simple reviews of common neurologic diseases and updates on specific conditions to descriptions of emerging equine neurologic syndromes.

"Lameness, colic, airway disease, and skin disorders are much more common than neurologic problems," says Mayhew. "Spinal injury and wobblers (horses with wobbler syndrome) probably occur at a rate of 1% of young horses per year on breeding studs, and infectious diseases such as West Nile virus, protozoal myelitis, and Eastern encephalitis and toxicities such as ryegrass staggers, stringhalt, and grass sickness are major welfare and financial problems at certain locations and in particular seasons. Thus, neurologic diseases are of major concern and certainly of frightening occurrence worldwide."

This article summarizes some of the information presented by Mayhew.

Neurologic or Lame?

Picture a group of veterinarians evaluating a horse, dividing into two camps, and vehemently defending their stance on whether the horse is neurologic or lame.

"This is not an unrealistic picture," comments Mayhew. "Distinguishing orthopedic from neurologic gait abnormalities is thwart with errors and misinterpretation. At WEVA, I presented a paper that attempted to more clearly define the characteristics of neurologic gait abnormalities and how to distinguish them from orthopedic disorders."

Mayhew advises using the following steps when evaluating patients suspected of suffering from neurologic disorders:

  • Look for irregular and unpredictable limb movement and foot placement, toe dragging, head nodding, and abnormal postures that indicate paresis (weakness) and ataxia (loss of coordinated muscle movements);
  • If ataxia is present, determine if the horse is hypermetric, hypometric, or dysmetric. Hypermetria is a lack of direction and increased range of motion, hypometria is stiff or spastic movement of the limbs with limited flexion of the joints, and dysmetria refers to a combination of both, in which affected horses have a high-stepping gait but limited movement in the distal joints; and
  • Assess postural deficits. The tail and halter pull tests and thoracic limb hopping are the most useful tests for determining weakness in the limbs of horses suffering spinal cord disease.

A Word about Wobblers

"Contrary to popular belief, a wobbler is simply an ataxic horse," says Mayhew. "A horse showing ataxia and weakness is called a wobbler, and this should be used as a generic term, not one used to describe horses with cervical vertebral malformation and malarticulation (CVM)."

Weakness of the limbs can occur due to lesions in the spinal cord, spinal nerve roots and ganglia, neural plexuses, or nerves of the limbs. If a veterinarian has established that the underlying problem is indeed in the spinal cord, then specific diagnostic tests can be employed to better establish the underlying cause. Such causes include vertebral malformations; occipitoatlantoaxial defects (at the articulation of the cervical vertebrae with the head); toxic myelopathies caused by plants such as stinging nettle, or chemicals such as ivermectin, fluphenazine, or sodium bromide; and inflammatory or vascular processes such as bacterial osteomyelitis, fibrocartilage embolic myelopathy, and granulomatous myelitis.

"It behooves the equine clinician to complete a thorough neurologic work-up as early as possible," stresses Mayhew. "Ancillary diagnostic aids--such as neuroradiology, advanced computerized imaging techniques, electrodiagnostic testing, and spinal fluid analysis--can all be of tremendous help in developing a plan for early treatment before permanent neurologic signs arise and outlook for return to function dramatically diminishes."

Emerging Neurologic Syndromes

Parelaphostrongylus tenius is an important internal parasite of domestic ruminants capable of causing neurologic disease. Recently, it has been demonstrated that horses can also be infected with P. tenius, producing a traumatic and inflammatory myeloencephalomeningitis (damage and inflammation to the spinal cord, brain, and meninges--the thin tissue surrounding the spinal cord). Although this is a very rare disease in horses, it has to be considered when there is acquired sudden onset neck twisting (torticollis).

"This fascinating worm lives in the brain case of white-tailed deer, where it lays eggs that finally pass as larvae in their droppings to be eaten by slugs and snails," says Mayhew. "When these mollusks are ingested by deer, the tricky worm makes its way via the spine to the brain without causing any damage. In domestic herbivores such as goats and sheep, damage is caused to the spinal cord during these wanderings."

In horses, damage caused by the worm's walkabouts results in an acute onset of a lateral (sideways) deviation of the vertebrae in the neck region (scoliosis), decreased sensitivity of the skin in the neck and/or thorax, and a mild, unilateral ataxia and paresis, but no atrophy of the epaxial muscles in the trunk adjacent to the spine.

"This can potentially occur wherever horses graze pastures where white-tailed deer co-habit--particularly in the United States and Canada," says Mayhew.

Treatment with fenbendazole or ivermectin might be successful if affected horses are treated as soon as possible after onset of clinical signs. If left untreated, the twisted neck becomes permanent and the patient is usually euthanized.

Knuckling has been sporadically reported in various populations. One outbreak involved 24 animals of a 75-horse herd in Sweden in the 1990s. All but three of the 24 affected horses succumbed to disease, and only one recovered. A second noteworthy outbreak was reported in Norway soon after. In both cases, affected horses had varying degrees of damage to the sciatic nerve, the large nerve emanating from the spinal cord and innervating the hindlimbs from which the tibial and peroneal nerves arise. Some horses showed more involvement of the peroneal nerve (supplying the lower leg, foot, and hoof) and others, the tibial nerve (innervating the muscles of the tibia, such as the gastrocnemius).

According to Mayhew, the fatty white substances that form a protective sheath around nerve fibers and assist in the transmission of nerve signals from the spinal cord to the target muscle were deteriorating in affected horses. The most likely cause is thought to be a plant or non-biological neurotoxin that preferentially attacks peripheral nerves (i.e., those outside of the central nervous system).

"All of the affected horses had access to low-quality feed in late autumn or winter, but no specific toxin has been identified as yet," explains Mayhew.

A different form of knuckling has also been described in a handful of horses by a Japanese team, but in this case the knuckling occurred in the forelimbs. Affected horses dragged their toes and had evidence of (neurogenic) muscle atrophy.

"This appears to be a syndrome in weanlings and yearlings of a prominent acquired upright posture with tendency to overstraighten the knees and fetlocks. During weight support, these joints may buckle forward. More research into this syndrome is required," advises Mayhew.

"Episodic events" are unusual, unexpected behavioral events that typically last seconds to minutes. The term includes such conditions as sleep disorders, seizures or epilepsy, and fainting.

"Episodic events are concerning because they can occur suddenly either in a confined space--such as a stall--or while under saddle, posing a significant risk to the handler or rider," says Mayhew.

"Fainting" has been described in Suffolk, Shetland, Fell, Warmblood, and Miniature Horse foals characterized by daytime sleepiness and episodes of partial or total cataplexy (i.e., flaccid paralysis and lack of normal limb reflexes). While adult onset inherited narcolepsy has not been reported in horses, sporadic idiopathic hypersomnia (sleep attacks) does occur in many breeds. Affected horses become drowsy, hang their heads, buckle at the knees, and can suddenly collapse to the ground. Full recovery generally occurs within seconds.

In both adults and foals the sleep attacks appear to be lifelong conditions, but can sometimes be treated with the antidepressant drug imipramine, although treatment is usually not recommended if the patients don't injure themselves during episodes.

In adult horses inherited epilepsy manifested by repeated generalized seizures with no underlying disease process has not been reported. Instead, generalized seizures in adults appear to be most commonly caused by: viral encephalitis; hepatoencephalopathy; leukoencephalomalacia or moldy corn poisoning due to Fusarium spp mycotoxicosis; other toxicities; brain abscesses; and equine protozoal myeloencephalitis (EPM).

Anticonvulsant therapy is indicated in affected horses with cluster seizures or status epilepticus (more than one seizure per month), or if the patient injures himself and his owner will not consider euthanasia. Phenobarbital alone or with potassium bromide is the treatment of choice. Guidelines for administration are included in Mayhew's book Large Animal Neurology (Wiley-Blackwell, UK, December 2009).

"These animals are unsafe to ride until they have remained seizure-free while off anticonvulsant medication for six months," advises Mayhew.

These events occur only intermittently and can therefore prove to be a diagnostic challenge. Mayhew suggests 24-hour video recording as a powerful tool to capture episodes of sleep and seizures and to help the veterinarian come to an accurate diagnosis and prognosis.

Take-Home Message

Through anatomical, physiological, and clinicopathological studies, we have made numerous advances over the past five years in learning about existing and new disorders in horses, says Mayhew.

"Accurate observations and documentation enable newly emerging disorders to be better identified and prevented," he adds.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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