09/09/2025
Shivers in Horses
Brian S. Burks, DVM
Diplomate, ABVP
Board Certified in Equine Practice
Shivers is a chronic progressive neuromuscular disease of unknown etiology. It is characterized by involuntary muscle tremors, spasms, and cramping, primarily affecting the hind limbs, tail, and sometimes the head and neck. Occasionally, there is generalized hypertonia and the thoracic limbs are affected. The disease is most common in draft horses, including Belgians, Percherons, Clydesdales, shires, Haflingers, Norwegian Fjord, Suffolk, Irish Draft, Draft crosses, and Draft Mule, but other breeds (Warmbloods, light horses) may be affected. While shivers can affect horses of any s*x, geldings are three times more likely to be diagnosed with the disorder than mares. Horses taller than 16.3 hands are also more susceptible than shorter horses. The majority of horses show signs before the age of five years.
The etiology of shivers is unknown, but there is damage to specialized nerve cells (Purkinje cells) of a specific area of the cerebellum. The cerebellum regulates slow, learned movements, such as backing. Forward, faster gaits are regulated by separate spinal circuits, allowing affected horses to compete, even at high levels. Horses with shivers lack the “off-switch” the cerebellum normally provides for muscle contraction, causing the muscles to be constantly active. This may be a genetic disorder.
Shivers should be distinguished from several diseases:
Stringhalt results in more flexion and occurs when moving forward. Stringhalt has a spasmodic and excessively rapid flexion of one or both hind-limbs. The hocks are flexed violently and suddenly toward the abdomen and are brought forcibly to the ground in one quick movement. Severely, and bilaterally, affected horses have a ‘bunny hopping’ gait. Most strides are abnormal.
Fibrotic myopathy results from scar tissue formation following injury to the semitendinosus and semimembranosus muscles. The gait is usually characterized by an abnormal slapping-type hind-limb gait with a lower arc of the limbs than seen with shivers or stringhalt. Horses with fibrotic myopathy can back normally. Adult Quarter horses are the most frequently affected breed.
Stiff horse syndrome is characterized by intermittent stiffness and spasms in the back and pelvic limb muscles. There is muscular hypertrophy. The movements become more normal when walking more than a few steps, or during trotting. In people, stiff person syndrome is an immune-mediated deficiency of gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter of the central nervous system.
Clinical signs of Shivers can be difficult to detect in mildly affected individuals, and because signs occur at irregular intervals. Affected horses show signs when being asked to back or turn, or when forced to step over an object. The affected limb will be partially flexed and abducted (held away from the body). The muscles of the upper limb and tail may tremor. They may stand base-wide or base-narrow instead of an even stance. Affected horses may exhibit hyperextension (feet are placed further back than normal) of the hind limbs when backing up. In a few cases, muscles of the face and neck may contract spasmodically. In severe cases, clinical signs are observed when the horse moves forward, usually only for a few steps or when turning sharply. Muscle atrophy, especially of the thigh muscles, occurs as the disease progresses. This often leads to hindquarter weakness. Clinical signs usually resolve in a short time, and the horse will appear clinically normal when standing still or trotting, with signs reappearing when the horse is asked to turn or back.
There are two primary categories of Shivers signs observed while backing:
• Hyperflexion: One hindlimb is raised up and away from the body in a spastic state for several seconds to several minutes. The limb trembles or “shivers” in suspension, then the foot is brought rapidly to the ground when the spasms subside. One or both hind-limbs may be affected.
• Hyperextension: The horse places the hind feet further back than normal when moving backwards with the stifle and hock joints hyperextended. Both forelimbs are also extended when backing begins, resulting in a stretched or “sawhorse” stance. In severe cases one or both hind limbs may be held out behind the animal in rigid spastic extension, resulting in instability and even falling if the horse cannot regain control of one leg to catch itself. The horse may stand on its toes with the heels raised off the ground. Most horses are affected in both hind limbs.
Horses with shivers are resistant to hind-limb flexion and may hyperflex the opposite hind leg before flexing and abducting the hind-limb being touched. These signs may occur when the hind feet are picked up to be cleaned or when the horse is being shod, especially when the foot is hammered during shoeing. The condition may progress so that the horse becomes impossible to shoe. Excitement, such as leading the horse from the stall, or when footing changes, may worsen clinical signs. Such horses improve when turned out on pasture. Offering a bucket of water on the ground can also stimulate clinical signs. As the neck is extended, muscle spasms occur in the hindquarters, with the front feet planted firmly. The body sways backward, the back is arched, and the tail is je**ed to an elevated position.
Occasionally the muscles of the forelimbs, neck, trunk and face may occur. The forefoot is thrust forward into extension, with the foot barely touching the ground. The extensor muscles above the elbow quiver. When facial muscles are involved, there is rapid blinking, quivering ears, and lip twitching. These signs are considered rare.
There is no specific diagnostic test for shivers. Diagnosis is based upon clinical signs and ruling out other diseases such as stringhalt, equine protozoal myeloencephalitis (EPM), and others through lameness and neurological examinations along with other diagnostic testing. It is important to recognize that many horses may demonstrate hyperflexion when asked to pick up their hind feet, but they back up normally. These horses do not have shivers but instead likely have an underlying orthopedic issue. A diagnosis of shivers requires the horse to demonstrate clinical signs while backing up.
There is no treatment for the disease other than consistent exercise and a low carbohydrate/high fat diet. Provision of vitamin E is may slow the progression, though it will not improve the clinical signs of shivers. The disease is slowly progressive and the prognosis for affected horses is poor.
Veterinary evaluation of horses with suspected shivers is essential. A thorough lameness exam can rule out other causes of lameness or other abnormalities. The exam should have horses back and the limb should be flexed.
Fox Run Equine Center
www.foxrunequine.com
(724) 727-3481
