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Shivers in horses is a chronic, progressive neuromuscular disorder that primarily affects the hind limbs and tail. The hallmark sign appears when a horse is asked to back up or to lift a hind foot: the limb hyperflexes upwards and outwards in a jerky, trembling motion, often held aloft for several seconds before returning to the ground. For many owners the first clue is a farrier who struggles to hold up a hind leg, or a horse that seems oddly reluctant to step backwards. Some horses show only mild, stable signs for years, while others deteriorate to the point where foot care becomes dangerous and ridden work is no longer realistic.
The condition is still poorly understood, and there is currently no cure. However, a clear diagnosis and a sensible management routine can keep many affected horses comfortable and useful for a long time. This guide explains what shivers in horses actually is, how to recognise the early signs, what the latest research suggests about its cause, and how vets and owners manage it day to day. It is intended as background information only, and is not a substitute for a hands-on veterinary examination.
What Is Shivers in Horses?
Shivers, sometimes called equine shivering or shivering syndrome, is a movement disorder characterised by involuntary muscle spasms in the hind limbs, pelvis and tail. It has been recognised in working horses for well over a century, yet it remains one of the more enigmatic conditions in equine medicine. The University of California, Davis Center for Equine Health describes it as a neurological disease defined by an abnormal gait when the horse is asked to back up.
Two clinical patterns are recognised. The far more common flexion form causes the hind leg to snap up and away from the body, with the hock and stifle held in exaggerated flexion. A rarer extension form causes the horse to hold a hind leg rigidly out behind it, trembling, with the muscles visibly quivering. In both patterns the tail head is often raised and shaking during an episode. Importantly, most horses look completely normal standing still and when walking forward in a straight line, which is part of why shivers is so easily missed in its early stages.
Early Signs and Symptoms of Shivers
Because signs are intermittent and triggered by specific movements, shivers can go unnoticed until foot care or a routine flexion reveals it. Owners often report a horse that has become "difficult" for the farrier rather than one that looks unwell. Typical signs of shivers in horses include:
- Jerky hyperflexion of one or both hind legs when backing up, turning tightly or being asked to move over
- Difficulty lifting and holding up a hind foot for the farrier or vet
- Trembling of the raised limb and the muscles of the thigh and quarters
- Elevation and quivering of the tail head during an episode
- In the extension form, a hind leg held stiffly out behind the horse
- Muscle wastage over the hindquarters in longstanding or advanced cases
- Reluctance to step backwards, or a horse that appears to "forget" how to reverse
Signs are usually absent at rest and during ordinary forward movement, and tend to be worse when the horse is anxious, cold or has been stabled for a prolonged period. If you notice any of these behaviours, filming a short video of the horse backing up and having its hind feet lifted is one of the most useful things you can do before your vet visits, as episodes can be hard to reproduce on demand.
What Causes Shivers? The Current Research
The underlying cause of shivers has been debated for decades, and the honest answer is that it is not yet fully resolved. Historically the condition was assumed to be a primary muscle problem. More recent work, led largely by researchers at the University of Minnesota, has shifted the focus towards the nervous system. Their studies identified degeneration of the ends of nerve cells, including Purkinje cell axons in the cerebellum and the peripheral nerves supplying the hind limb muscles. This points to shivers being a neurodegenerative disorder that disrupts the fine control of movement, rather than a disease of the muscle itself.
A genetic or heritable component is suspected because certain breeds and family lines are over-represented, but no single causative gene has been confirmed. For a time shivers was linked to polysaccharide storage myopathy (PSSM), yet many affected horses do not carry the PSSM1 (GYS1) mutation, so the two conditions are no longer considered the same thing. Until the mechanism is fully understood, prevention is not possible, and management remains focused on controlling the signs rather than reversing the cause.
Which Horses Are Affected? Breeds and Risk Factors
Shivers shows a clear pattern in the type of horse it affects. It is overwhelmingly a condition of tall, heavy horses, and research has found that almost all affected animals stand at or above 16.3 hands. The breeds most frequently reported include:
- Draught breeds such as the Shire, Clydesdale and Belgian Draught
- Warmblood sport horses, particularly larger dressage and event types
- Some Thoroughbreds and Thoroughbred crosses of substantial build
Sex and age also appear to play a role. Geldings are diagnosed noticeably more often than mares, and signs commonly first appear in young adult horses before deteriorating slowly over subsequent years, although onset later in life is also seen. Height itself is not the cause, but it is a reliable marker: if a small pony is presenting with hindlimb spasms, a condition other than shivers should be high on the list. Owners of large draught and warmblood horses should simply be aware of the pattern so that early signs are taken seriously rather than dismissed as bad manners.
Shivers vs Stringhalt vs PSSM
Several conditions produce abnormal hindlimb movement and are easily confused with shivers. Distinguishing between them matters because the management and prognosis differ. The key differences are summarised below, but a definitive answer needs a vet.
Shivers
Signs are triggered mainly by backing up and lifting the hind feet, and the horse is usually normal walking forwards. The limb hyperflexes and trembles, and the tail lifts. It is slowly progressive and has no specific cure.
Stringhalt
With stringhalt, the exaggerated hyperflexion occurs during ordinary forward walking, with the hind foot snatched sharply up towards the belly at every stride. One form, sometimes called Australian or pasture stringhalt, is linked to eating certain plants and can improve once the horse is removed from the pasture. The timing of the jerk, forward movement rather than backing up, is the clearest way to tell it apart from shivers.
PSSM and Other Myopathies
Polysaccharide storage myopathy and related muscle disorders such as myofibrillar myopathy cause exertional stiffness, cramping and muscle pain, often with raised muscle enzymes on a blood test and signs that appear during or after work. Unlike shivers, these are primary muscle diseases, and PSSM1 can be confirmed with a genetic test. Upward fixation of the patella, or locking stifle, and general lameness should also be ruled out, as should neurological conditions such as ataxia.
How Vets Diagnose Shivers
There is no single laboratory test that confirms shivers in a living horse, so diagnosis is largely clinical and based on excluding other causes. A vet will typically:
- Take a full history, including breed, height, age of onset and how the signs have changed over time
- Watch the horse back up, turn in tight circles and have each hind foot lifted, ideally reviewing owner video of episodes
- Assess for muscle wastage, pain and a normal forward gait to help separate shivers from lameness and myopathies
- Run blood tests to check muscle enzymes and rule out conditions such as PSSM
- Recommend genetic testing for PSSM1 where a muscle disorder is suspected
Because signs can be subtle and intermittent, an accurate diagnosis sometimes takes more than one visit. A definitive diagnosis of the underlying nerve changes can only be made on post-mortem examination, which is why the working diagnosis in the field rests on a characteristic clinical picture with other conditions excluded.
Managing a Horse with Shivers
There is no specific treatment that cures shivers, but consistent management can reduce the frequency of episodes and keep a horse comfortable. The guiding principle from veterinary sources is straightforward: keep the horse moving and minimise time shut in a stable. Any changes to diet, exercise or medication should be made with your own vet, who knows the individual horse.
Turnout and Daily Routine
Maximising turnout is one of the most consistently recommended measures. Horses that live out or spend long periods in the field generally have fewer and less severe episodes than those confined to a stable for much of the day. A settled, predictable routine also helps, as stress and sudden changes can make signs worse.
Exercise
Light, regular work tends to benefit affected horses and helps maintain muscle and condition over the hindquarters. Many horses with mild shivers continue in ridden work for years. Warm the horse up gently, avoid forcing tight turns or repeated backing when it is clearly difficult, and stop or scale back if signs are worsening rather than pushing through them.
Diet
A complete, balanced diet appropriate to the horse's workload is the foundation. Because of the historical link with PSSM, some vets suggest a high-fat, low-starch and low-sugar diet, and this is worth discussing even though it is not a proven cure for shivers specifically. Your vet may also review vitamin E status, as this vitamin supports normal nerve and muscle function. Do not add supplements speculatively; ask your vet or a qualified nutritionist to assess the ration first.
Farriery and Foot Care
Foot care is often the hardest practical problem, because lifting and holding a hind leg is exactly the movement that triggers spasms. Sympathetic, experienced farriery makes a real difference. Useful approaches include keeping sessions short, holding the limb as low as possible rather than high and flexed, allowing the horse frequent breaks, and, where necessary, arranging veterinary sedation for trimming or shoeing. Warn any new farrier in advance so the appointment can be planned safely for both horse and handler.
Prognosis: What to Expect Long Term
The outlook for a horse with shivers is genuinely variable, which makes honest prognosis difficult. A large number of affected horses have mild signs that progress very slowly, and they remain in work and comfortable for many years with good management. In these cases shivers is more of a nuisance, particularly around farriery, than a welfare problem.
In a smaller proportion of horses the condition advances more noticeably, with increasing difficulty backing up, muscle wastage over the hindquarters, loss of condition and, eventually, real problems with routine foot care. In the most severe cases quality of life and safe handling can be compromised. Because progression cannot be reliably predicted at diagnosis, regular reassessment with your vet is the sensible approach, rather than relying on any fixed timeline.
Should You Buy a Horse with Shivers?
Shivers is a common question at pre-purchase examinations, particularly for larger warmblood and draught types. Whether to buy an affected horse depends on the severity of the signs, the job you have in mind and your appetite for a condition that may worsen. A horse with very mild, stable shivers may be perfectly suitable for lower-level riding, whereas the same diagnosis is a bigger concern for a serious competition prospect where resale and long-term soundness matter.
If shivers is identified or suspected during a vetting, discuss it frankly with the examining vet, ask to see the horse backed up and having its feet lifted, and factor in the practical reality of ongoing farriery. Because a hereditary tendency is suspected, breeding from an affected horse is generally best avoided. Going in with clear eyes, rather than discovering the condition after purchase, is the key point.
Frequently Asked Questions
Is shivers in horses painful?
In its milder forms shivers does not appear to cause obvious pain, and many horses show no signs of distress between episodes. In advanced cases, however, the muscle spasms and cramping can become painful and can inhibit movement, so any horse that seems uncomfortable should be assessed by a vet.
Is shivers hereditary?
A heritable tendency is strongly suspected because particular breeds and family lines are over-represented, but no single causative gene has been confirmed. As a precaution, breeding from horses with shivers is usually discouraged.
Can shivers in horses be cured?
No. There is currently no cure and no specific treatment that reverses shivers. Management aims to reduce the frequency of episodes and keep the horse comfortable and in work for as long as possible.
Can a horse with shivers still be ridden?
Many horses with mild shivers continue in light, regular ridden work for years, and gentle exercise is generally considered beneficial. Whether a particular horse should be ridden depends on the severity of its signs and should be decided with your vet.
How quickly does shivers progress?
Progression is unpredictable. Some horses remain mild and stable for many years, while a minority deteriorate more quickly with worsening hindlimb signs and muscle wastage. Regular veterinary reassessment is the best way to track an individual horse.
If you suspect shivers in your horse, the most valuable next step is a veterinary examination, ideally supported by video of the horse backing up and having its hind feet lifted. For further reading, the UC Davis Center for Equine Health and the British Equine Veterinary Association provide reputable, up-to-date information. With an accurate diagnosis and a consistent routine of turnout, light work and sympathetic farriery, many horses with shivers go on to live comfortable, useful lives.
