Shivers has been recognized by horse owners for more than a century and refers to a chronic nervous or neuromuscular condition that in a 1962 text was said to be "as common as dirt." This statement referred to the period when draft horse populations were much larger than today. John Baird, BVSc, PhD, of the University of Guelph's Ontario Veterinary College, presented information on "shivers," at the AAEP Convention 2006.

Besides drafts and draft crosses, shivers is recognized also in Warmbloods and Warmblood crosses, as well as occasional light horse breeds such as Quarter Horses, Standardbreds, and Thoroughbreds. It is rare to find it in pony breeds.

Typically, shivers starts out as difficult to detect, and it progressively worsens with age. The horse experiences involuntary muscle spasms or jerky movements in the pelvis, one or both hind limbs, and tail. Initially, it seems a horse is snatching its leg away when asked to pick up a foot for cleaning.

Signs are most evident when an affected horse is asked to back up, move quickly, or when lifting a hind leg for hoof cleaning or while being shod. The horse abruptly raises the hind leg in a partially flexed position held slightly out from the body, and he is unable to put it down. This posture lasts from seconds to minutes. As the limb is slowly replaced to the ground, the horse might experience spasms again, especially if asked to move backward. The degree of associated tail elevation is variable. Usually, signs are not elicited by asking a shivers horse to move forward.

The problem seems worse on slippery surfaces; stress or excitement can exacerbate clinical signs. This condition might rarely affect the forelimbs, neck, or trunk. Occasionally, the muscles of the ears, eyelids, lips, and cheeks can be affected with spasmodic contractions, rapid blinking of the eyelids, ear quivering, and twitching of the lips.

As the disease advances, initially there is noticeable atrophy of the thigh, and this might progress to generalized muscle atrophy with hind limb muscle weakness in 58% of horses with shivers. The limbs become stiff or rigid, and the horse prefers to stand with his hocks wider apart than normal. Because the horse is reluctant to lie down, there might be bumps and bruises from partial falls due to fatigue, and sometimes the horse will make crouching movements. Some owners have noted a disproportionate amount of sweating in shivers horses.

Stringhalt is often confused with shivers, and although stringhalt is a jerky, rapid flexion of a hind limb, with stringhalt, the limb is violently thrust back to the ground after flexing hard into the abdomen. Stringhalt can occur in all breeds; the abnormal gait typically is elicited when a horse is asked to move, turn, or back up. In contrast, a horse with shivers holds the limb up and away from the body for a period of time.

Another differential diagnosis in a possible shivers case would be upward fixation of the patella that is caused by a transient sticking of the medial patellar ligament above the medial trochlear ridge of the femur. This holds the limb in a hyper- extended position that can release suddenly, making it look like stringhalt.

Fibrotic myopathy is caused by scar tissue restriction along the thigh muscles subsequent to an injury. A horse with this problem slaps the hind limb down abruptly, and he doesn't raise it very high to the abdomen due to mechanical restriction by the scar tissue.

"Stiff-horse syndrome" has been reported in Belgian horses; these horses show intermittent stiffness and spasms in the muscles of the lower back and hind limbs, with contractions triggered by voluntary movement, fright, or noise. These horses experience muscle hypertrophy (enlargement) rather than atrophy.

Another differential diagnosis is equine motor neuron disease (EMND) caused by vitamin E deficiency. It is characterized by progressive weight loss, symmetrical muscle wasting, muscle tremors, excessive sweating, a tucked-up abdomen, abnormal gait, excessive recumbency, a low head carriage, and abnormal tail elevation. On occasion, an EMND horse or a horse with equine protozoal myeloencephalitis (EPM) will demonstrate stringhalt-like movements of front or rear legs if certain areas of the spinal cord are affected.

No specific lesions have been documented in horses with shivers in any portion of the central nervous system. While no one knows what causes shivers, it possibly has a neurologic pathology. Another suggestion has been some depletion of glycogen leading to muscle cramping, but there has been little correlation between the severity of clinical signs and findings on histopathology to support this view.

There is also a question whether shivers is genetic or is genetically predisposed. Although not proven, it is suspected that there is some hereditary component.

There is no effective treatment for shivers. A horse might improve with rest, but shivering will return when exercise resumes. Some owners feed a high-fat, low-carbohydrate diet similar to that used to manage PSSM, and this might help early in the disease. In another study, clinical signs did not resolve when fed a high-fat diet.

Some days are better than others for some shivers-affected horses. In general, this is considered a progressive, debilitating disease with a poor prognosis. Clinical signs eventually increase in frequency and severity, and muscle wasting and weakness worsen. The course of the disease can progress rapidly or take as long as 24 years to render a horse incapacitated.



Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse's AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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