Shivers is a chronic movement disorder in horses that primarily affects the hind legs and tail, causing involuntary muscle tremors and abnormal leg movements. It’s most obvious when a horse is asked to back up, and it tends to worsen over time. The condition predominantly affects tall, large-breed horses, with symptoms typically appearing before age seven.
What Shivers Looks Like
The hallmark of shivers is what happens when a horse tries to walk backward. Instead of stepping back smoothly, the horse will pause mid-stride, hold one or both hind legs in an exaggerated flexed or extended position, and the muscles of the hindquarters and tail will visibly quiver or tremor. The tail often elevates and shakes. Many horses become reluctant to back up at all, planting their feet or showing obvious distress when asked.
The abnormal movement isn’t limited to simple trembling. Affected horses swing the hind leg outward (away from the body) during these episodes, combined with either extreme over-flexion or rigid over-extension of the limb. About 19% of horses with shivers also show twitching of the face, lips, or eyes during backward movement. In some cases, the front legs are affected too, though this is less common.
What makes shivers tricky to catch early is that forward gaits usually look normal. A horse can walk, trot, and canter without any visible problem until the condition becomes quite advanced. This means the disorder can go unnoticed for months or years if no one asks the horse to back up or picks up a hind foot for cleaning.
What Causes It
Shivers traces back to damage in the cerebellum, the part of the brain that coordinates movement. A 2015 study in Veterinary Pathology found that horses with shivers had an 80-fold increase in degenerating nerve cell projections within the deep cerebellar nuclei compared to healthy horses. Specifically, the long extensions of cells responsible for fine motor coordination were breaking down at their endpoints, and unusual structural abnormalities appeared in the protective coating around those nerve fibers.
This degeneration explains why shivers produces such specific, context-dependent symptoms. The cerebellum is responsible for calibrating movement, so when its circuitry deteriorates, the result is exaggerated, poorly controlled limb motion (hypermetria) and involuntary muscle jerking (myoclonus), particularly during complex tasks like backing up that require precise coordination.
The underlying trigger for this nerve damage remains unknown. Researchers strongly suspect a genetic component. Studies at the University of Minnesota have investigated several chromosomal regions, including a mutation in a gene involved in DNA repair where all affected horses carried two copies of the variant. However, no single definitive genetic marker has been confirmed yet, and the inheritance pattern is still being worked out.
Breeds and Risk Factors
Draft breeds are the most commonly affected group. Belgians, Shires, Clydesdales, and other heavy breeds carry the highest risk. Warmbloods and Warmblood crosses are also well represented, and shivers occasionally appears in lighter breeds like Quarter Horses, Thoroughbreds, and hunter-jumpers. It’s uncommon to rare in ponies.
Height is a significant risk factor. In epidemiological studies, confirmed shivers cases averaged about 173 cm (roughly 17 hands) compared to 163 cm (about 16 hands) in control horses. Males are also disproportionately affected, with a male-to-female ratio of roughly 3:1 in confirmed cases, compared to 1.7:1 in the general population. Symptoms can appear as early as one to two years of age, though most cases are identified before age seven. Cases reported in horses of all ages suggest that later onset is possible or that early signs were simply missed.
How Shivers Differs From Stringhalt
Shivers and stringhalt can look superficially similar because both involve exaggerated hind leg movement. The key differences come down to when and how the abnormal motion occurs.
- Direction of movement: Shivers is primarily visible during backward walking. Stringhalt shows up during forward gaits, including the walk and trot.
- Leg motion: Shivers produces either extreme flexion or extreme extension of the hind leg, combined with the leg swinging outward. Stringhalt produces consistent hyperflexion (a snapping upward motion) without that outward swing.
- Pausing: Horses with shivers tend to freeze at the peak of the stride, holding the leg suspended and trembling. Stringhalt produces a quicker, more rhythmic snapping motion.
Another condition that can mimic shivers is upward fixation of the patella, where the kneecap locks temporarily and the leg gets stuck in extension. This usually releases with a visible pop or jerk as the horse moves forward, and it doesn’t produce the sustained trembling or tail elevation seen in shivers.
How Veterinarians Diagnose It
There is no blood test or imaging scan that confirms shivers. Diagnosis is based on a physical exam, specifically on provoking the characteristic movement. Your veterinarian will ask the horse to walk backward and will manually lift each hind leg to see if tremors, abnormal flexion, or resistance occurs. The combination of muscle tremors, outward leg swinging, and exaggerated flexion or extension during these maneuvers is diagnostic.
Because forward gaits are often normal in early or moderate cases, a standard lameness exam may not catch the problem. If you suspect shivers, mention it specifically so your vet includes backward walking and hind limb manipulation in the evaluation.
Progression Over Time
Shivers is generally progressive, though the speed varies considerably between individual horses. Some horses remain mildly affected for years and continue to perform under saddle with minimal difficulty. Others deteriorate more quickly, developing exercise intolerance, muscle wasting in the hindquarters, and increasing difficulty with basic tasks like farrier work, trailer loading, or navigating tight spaces that require backing.
In advanced cases, the abnormal movement begins to intrude on forward gaits, and horses may struggle with transitions or collected work. Exercise intolerance is more commonly reported in draft breeds than in lighter horses with the condition.
Management Strategies
There is no cure for shivers, and no treatment reverses the underlying nerve degeneration. Management focuses on maintaining muscle condition, minimizing situations that provoke symptoms, and keeping the horse comfortable.
Regular exercise and consistent turnout are among the most frequently recommended strategies. Horses that stay in regular work and spend ample time moving freely tend to show less severe symptoms than those kept in stalls for long periods. Maintaining hindquarter strength through appropriate exercise may slow muscle loss, though it won’t stop the disease from progressing.
Dietary adjustments are also part of the management conversation. Because shivers shares some features with other equine muscle disorders, many veterinarians recommend reducing starch and sugar in the diet and increasing fat as an alternative energy source. This approach aims to support muscle function and reduce the metabolic stress on already compromised neuromuscular pathways. High-quality forage should form the foundation of the diet, with concentrated feeds chosen for their low glycemic impact.
Practical accommodations matter too. If your horse struggles with backing, avoid situations that require it when possible. Warn your farrier about the condition so they can work with the horse patiently and safely, since lifting hind feet often triggers episodes. Keeping the horse on a consistent routine and avoiding long periods of inactivity can help minimize the severity of symptoms day to day.