Equine shivers is a progressive neurological disease that affects how a horse moves, most noticeably when backing up. Horses with shivers involuntarily hold a hind leg up and away from the body in an exaggerated flexion, and the leg often trembles visibly. The condition develops gradually, typically worsens over time, and has no cure, though management strategies can help affected horses stay comfortable and functional.
How Shivers Looks in a Horse
The hallmark sign is an abnormal gait when the horse tries to back up. Instead of stepping smoothly backward, the horse will snap a hind leg upward and outward in an exaggerated motion called hyperflexion. The raised leg often shakes or trembles while held in that position. In a less common form, the horse does the opposite: it extends its hind legs further back than normal (hyperextension) rather than flexing them upward.
Early on, you may only notice these signs when asking the horse to back up. As the disease progresses, the abnormal movement can appear during the first few steps of forward motion, especially after the horse has been standing still for a while, or when making tight turns. Some horses also develop involuntary spasms in the muscles of the face and neck.
One important detail: most horses with shivers move normally at a trot. The signs are intermittent and often triggered by specific situations, particularly transitions from standing to moving, farrier work (since holding up a hind leg is difficult), and any request to step backward.
What Causes It
Researchers at Michigan State University identified the underlying problem as degeneration in the cerebellum, the part of the brain responsible for coordinating motor movement patterns. Specifically, nerve fibers from cells called Purkinje neurons break down over time. These cells are critical for smooth, coordinated muscle control, and as they deteriorate, the brain loses its ability to properly regulate hind limb movement. This explains the jerky, exaggerated motions and trembling that define the disease.
A definitive diagnosis is only possible by examining brain tissue after death. During a horse’s lifetime, veterinarians diagnose shivers based on observing the characteristic limb positions, muscle trembling, and gait abnormalities, then ruling out other conditions that can look similar.
Which Horses Are Most Affected
Shivers occurs across multiple breeds, but draft horses, warmbloods, and Thoroughbreds appear most susceptible. The condition shows up more frequently in taller, heavier breeds. Belgian draft horses have been particularly well-studied because of their higher prevalence. Signs typically emerge in young adulthood, though the exact age of onset varies. Because the disease is progressive, horses diagnosed earlier in life face a longer timeline of potential decline.
How Shivers Differs From Similar Conditions
Several other conditions cause abnormal hind limb movement in horses, and distinguishing between them matters for management.
Shivers vs. Stringhalt
Stringhalt produces a rapid, jerky hyperflexion of the hind leg during forward movement that can look nearly identical to shivers during the swing phase of the stride. The key differences come down to timing and context. In stringhalt, the abrupt upward snap of the leg happens earlier in the stride and tends to persist at a trot. Horses with shivers usually trot normally. Stringhalt also tends to appear suddenly rather than building gradually over months or years, and the affected leg typically stays closer to the body rather than swinging outward (abducting) the way it does in shivers.
Shivers vs. PSSM
Polysaccharide storage myopathy (PSSM) is a muscle disorder, common in draft breeds, where the horse stores glycogen abnormally in muscle fibers. It can cause a stiff gait, muscle wasting, and abnormal hind limb movement that overlaps with shivers symptoms. Researchers initially suspected PSSM might be an underlying cause of shivers in Belgian drafts, since both conditions share some clinical features. The two are now understood as separate diseases, though a horse can have both simultaneously. PSSM is diagnosed through muscle biopsy or genetic testing, which helps clarify whether a horse’s movement problems stem from one condition, the other, or both.
Managing a Horse With Shivers
No treatment reverses the cerebellar degeneration that drives shivers. Management focuses on slowing progression, maintaining muscle condition, and adapting the horse’s routine to minimize situations that trigger symptoms.
Regular, consistent exercise appears to benefit affected horses. Keeping the horse moving helps maintain muscle mass and coordination. Long periods of stall rest tend to make symptoms worse, so turnout and daily activity are important. Farrier visits often require patience and adaptation, since holding up hind legs for extended periods is difficult for these horses. Short, frequent trimming sessions are easier to tolerate than longer ones.
Dietary adjustments may help, particularly for horses that have both shivers and PSSM. Owners have reported improvements when switching to a diet high in fat and low in starch and sugar. The idea is to provide calories through fat rather than carbohydrates, reducing the glycogen load on muscles. This approach seems most beneficial when started early in the course of the disease. A typical strategy involves replacing grain-based feeds with fat-supplemented feeds and ensuring the horse gets adequate forage.
Long-Term Outlook
Shivers is classified as a progressive disease, meaning it generally worsens over time. The rate of progression varies widely between individual horses. Some remain mildly affected for years, with symptoms limited to difficulty backing up and occasional trembling during farrier work. Others decline more quickly, eventually showing abnormal movement during forward gaits, developing significant muscle wasting in the hindquarters, and losing the ability to perform routine tasks safely.
For horses used in competition or work, the practical impact depends on the discipline. Activities that require frequent backing, collected movements, or tight turns become increasingly difficult. Many mildly affected horses continue in lighter work for years. In severe cases, the horse may struggle with basic mobility, including getting up from lying down, which can ultimately affect quality of life.