1. What is Shivers?
2. What are the signs of Shivers?
3. How do I know if my horse has Shivers?
4. What causes Shivers?
5. Is Shivers related to Polysaccharide Storage Myopathy (PSSM)?
6. Is Shivers inherited?
7. How do you treat Shivers in horses?
8. Can Shivers be cured?
9. What is the prognosis?
10. What do I do if I think my horse has Shivers?
    
11. Watch a seminar by Dr. Valberg on Shivers

Shivers or shivering are names that have been applied to a chronic neuromuscular syndrome in horses that has been recognized for centuries. It has been stated that the condition is reasonably common, uncommon, and rare to very rare. In a comparative neuropathology textbook published in 1962 it was stated that “in the heyday of equine practice” shivering was “as common as dirt.” Most horses with Shivers begin to show signs before 5 years of age, and the majority of cases (74% of horses surveyed in a recent study) are progressive in severity. Usually horses with Shivers will show signs before age 10. Both genders can be affected, but geldings are three times more likely to be diagnosed with the disorder. Horses over 16.3 hands tall are also more susceptible than shorter horses.The syndrome affects several breeds, including draft horses, Warmbloods and Warmblood crosses, and occasionally lighter breeds of horses, including light harness horses, hunters, hunter-jumpers, hacks, Quarter Horses, and Thoroughbreds. In ponies, shivers is considered uncommon to rare.

Many horses that hold a hind limb flexed and tremble are said to have Shivers. However, a number of disorders can create irritation in the hind limbs of horses and cause this type of sign. The classic disease called Shivers is a gradually progressive, chronic neuromuscular disease in horses that is characterized by gait abnormalities when backing up. Other typical signs include trembling of the tail while held erect, trembling of the thigh muscles and a flexed and trembling hind limb. One study of the prevalence of Shivers in Belgian Draft Horses found that 19% of horses examined had signs of Shivers. No investigation into estimates of the prevalence of Shivers in other breeds exists. While it is clear that certain breeds are more frequently affected than others, height is also a strong predictor of Shivers risk.

The clinical spectrum of shivers in horses is very variable in the degree or manifestation of signs. The diagnosis of a characteristic case of Shivers seldom presents a problem, however the signs of Shivers may be intermittent, occasional, or latent and very difficult to confirm. Shivers may be extremely difficult to detect in the early stages, and careful observation may be required before the diagnosis can be made. The disease primarily affects one or both hind limbs with or without tail elevation and trembling.

The most characteristic signs of Shivers occur when an attempt is made to move the horse backwards. Generally, horses with Shivers lack a normal 2-beat contralateral (right front then left hind, left front then right hind) gait when walking backwards. Mildly affected horses show tenseness or trembling of the hind limbs and sudden jerky extensor movements of the tail that cause it to elevate. 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 hindlimbs 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.

In more severe cases, hyperflexion and other signs of Shivers will become apparent during forward walking. However, these signs are not consistently observed and will often only appear during the first several steps or when turning sharply. Signs are not apparent at the trot. Even in well-developed shivering cases, signs may not be seen when the horse is standing still.

Horses with Shivers are usually also resistant to having their hind feet lifted manually, and may hyperflex the opposite hind leg before flexing and abducting the hindlimb 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. When the horse is moved forward there may be no signs, or the signs may be restricted to the first several steps. In advanced cases, the affected animal may be unable to move backward more than a few paces, and sometimes this movement cannot be performed at all. Many shivering horses are hesitant to lie down when confined indoors and as a result may lose body condition, and consequently appear older.

Stress or excitement, such as when the horse is led out of the box into the open, or when the footing changes often initiate episodes of shivers. When turned out onto pasture the “shiverer” horse may lay down and there may be an improvement in the disease. Signs of shivers may also become apparent when affected horses are offered a pail of water on the ground. They become excited, and upon extending the neck muscle spasms occur in the hindquarters, the fore feet remain planted on the ground, the body sways backwards, the back is arched, and the tail is jerked upwards. This behavior, although typical of the disease, is not constant.

Occasionally there is involvement of the muscles of the forelimb, neck, or even trunk and face. Forelimb signs are considered rare. On attempting to lift a front foot the limb is thrust forwards in full extension, the foot barely touching the ground, or the limb with the carpus flexed is elevated and abducted, the extensor muscles above the elbow quivering while the spasm lasts or until the foot returns to the ground. Shivers may also affect the muscles of the ears, eyelids, neck, lips, and cheeks. When the muscles of the head or neck are involved, they contract spasmodically. With involvement of the face, there is rapid blinking of the eyelids, quivering of the ears, and the lips exhibit twitching.

With progression of the disease, a gradual and progressive atrophy of the muscles of the thigh occurs, and this may progress to generalized muscle atrophy. Hindquarter weakness was present in 11 of 19 (58%) horses with shivers. Animals so affected sleep standing, and their front fetlocks and knees are bruised and disfigured by frequent half-falls. Affected horses frequently adopt an abnormal base-wide stance in the hind limbs. Excessive sweating has been noted in some cases.

Shivers is straightforward to diagnose when signs are clear, however, milder cases are more difficult to diagnose. If your horse exhibits muscle quivering, difficulty in backing up, discomfort while being shod on hind hooves, or other signs of Shivers, have your horse evaluated by your veterinarian. A veterinarian must rule out any other possible causes of lameness, as some painful conditions mimic signs of Shivers, especially if only one limb is involved. There are often no abnormalities on serum biochemistry profiles and usually muscle enzymes such as CK and AST are normal.

Stringhalt is the condition most often confused with Shivers. It occurs in all breeds of horses and may occur at any age. Stringhalt is characterized by a spasmodic and excessively rapid flexion of one or both hind limbs, which occurs when the horse is made to move, best seen in the slower paces and particularly in turning or backing. In stringhalt, the hocks are flexed suddenly and violently toward the abdomen and are then brought forcibly and noisily back to the ground in one quick motion as the leg is advanced. The hind limb motion may be mild as a slightly excessive flexion, to violent movements during which the fetlock or toe will contact the abdomen, thorax and occasionally the elbow with attempted steps leading to a peculiar “bunny hopping” and plunging type of gait. Unlike Shivers, signs of stringhalt are often apparent when walking forward, with most or every stride appearing abnormal. Facial twitching and tail head elevation are absent. Signs of stringhalt are also observable at the trot, while Shivers horses trot normally. The severity of the gait abnormality is variable and may be accompanied by hindlimb muscle atrophy and laryngeal paralysis. The diagnosis of stringhalt in the early stages is extremely difficult. In both conditions, the horse may show signs only intermittently and shivers and stringhalt may look identical when viewing backing up from the side. However, in shivers, the limb is flexed outward from the body in slight hip extension, and held in a spastic state for a few moments instead of being snapped up quickly and kept underneath the belly in a forward flexed state as seen with stringhalt.

Upward fixation of the patella may occur in horses as an apparently acquired disorder. Upward fixation of the patella occurs when the medial patellar ligament becomes momentarily (mild) or permanently (severe) caught above the medial trochlear ridge of the femur with the stifle in full extension. When the patella releases, the limb flexes forward rapidly, resembling stringhalt. The acquired condition may mimic stringhalt because the hyperextended limb may release rapidly to hyperflex.

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 hindlimb 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” (SHS) has recently been reported in horses in Belgium with intermittent stiffness and spasms in the axial muscles of the lower back and in the muscles of the pelvic limbs. Voluntary movements, fright, or disquieting sounds typically precipitated the contractures. There was no weakness or muscle atrophy, rather a muscle hypertrophy. After the first steps, the movements became more relaxed and once the horse was walking or trotting no spasms were evident. Because this disease was considered to have many similarities to an entity in human medicine designated “stiff-person syndrome” (SPS) and the condition has been called “stiff-horse syndrome”. “Stiff-person syndrome” is thought to be the result of an immune-mediated deficiency of gamma-aminobutyric acid (GABA), a major inhibitory neurotransmitter in the central nervous system.

Equine motor neuron disease (EMND) is clinically characterized by progressive weight loss despite a good appetite, symmetrical muscle wasting, muscle fasciculations, excessive sweating, tucked up abdomen, abnormal gait, excessive recumbency and an abnormally low head carriage. There are some similarities between the clinical signs of shivering and EMND. Bizarre stringhalt-like movement of a front or rear limb has been observed in some chronically affected EMND horses. Subacute to chronic EMND cases frequently have an abnormal elevation of their tail, lie down more frequently and have more muscle atrophy than a case of shivers.

Equine protozoal myeloencephalitis (EPM). There are rare reports of spinal cord disease because of EPM causing a stringhalt-like gait. This disease may be ruled out on absence of Sarcocystis neurona antibodies in the serum and/or in the cerebrospinal fluid.

The ultimate causes of Shivers remain unknown. However, an extensive study published in February of 2015 revealed that the brains of horses with Shivers are damaged in a specific area of the cerebellum, a part of the brain largely devoted to regulating muscular activity. Horses with Shivers often move forward normally and can even reach high levels of athletic competition, but have extreme difficulty with less natural gaits such as backing up or lifting a hindlimb. Studies suggest that these slow, learned movements are regulated by a cerebellar pathway separate from spinal circuits involved in faster, natural forward gaits, providing a possible explanation for the unique deficits seen in Shivers. Shivers is also characterized by uncoordinated, jerky muscle contraction or myoclonus, an effect linked to cerebellar damage in human studies. Furthermore, the muscles of horses with Shivers show a significant change in fast twitch type 2x muscle fiber type similar to horses in intensive training. While normal horses acquire these fiber changes through exercise, horses with Shivers lack the “off-switch” the cerebellum normally provides for muscle contraction, causing the muscles to constantly be active.

Researchers have noted that the breeds of horses affected by Shivers also have particularly high coincident rates of polysaccharide storage myopathy. However, a study done by researchers at the University of Minnesota showed that the PSSM in Belgian Draft Horses does not have a direct relationship with PSSM and their appearance in the same horse is likely coincidental rather than causal. Some horses with PSSM may have some abnormalities in their gait but the classic signs of Shivers are more likely related to abnormal cerebellar function than a muscle disease.

For more information of PSSM click here.

Because Shivers is breed-related, there may be a genetic basis or predisposition for the disorder. Reports from past centuries suggest that shivers was prevented by breeding away from the condition. In some countries, it is recommended that stallions with this disease not be used for breeding purposes. Currently, there is no specific genetic pattern identified and there is no genetic test.

There is currently no effective treatment for shivers. Occasionally the signs may improve especially with turn out and exercise but they may also regress during painful or stressful situations and when stalled. It has been suggested that dietary treatment of affected draft horses with a high-fat, low carbohydrate feed may be beneficial if instituted early in the course of the disease. However, the clinical signs of shivers in horses did not resolve when affected Warmblood or Warmblood-cross horses were fed grass hay and their dietary grain was replaced with a high fat supplement. These dietary recommendations were combined with a gradually increasing daily exercise program and maximal turnout.

If a horse has both PSSM and Shivers, owners have reported improvements in their horses by feeding a diet high in fat and low in starch. This does not appear to cure horses from shivers but it may reduce the frequency of muscle spasms. Adequate levels of vitamin E are important. Vitamin levels in serum should be checked (blood taken, the blood tube protected from light, kept chilled and spun down as soon as possible). If vitamin E levels are below normal limits then supplementation is recommended. Thhis will not change the current signs of shivers but it may possibly slow progression to ensure the horse is not deficient. The natural liquid form of Vitamin E, Elevate™, NanoE™ provide faster increases in serum vitamin E than powdered acetate or synthetic Vitamin E. Massage and acupuncture may be helpful in keeping a horse comfortable and competitive. It may be harder for Shivers horses to maintain muscle mass if they receive time off from work.

Horses may become tight behind, and may take longer to get the horse back in shape if laid up. As such, keeping horses in work with constant turn out seems to help. Symptoms were relieved the more these horses moved around.

At present, there is no cure and few effective treatments for Shivers.

When discussing prognosis we are referring to a definition of shivers that involves difficulty backing up. Many athletic horses that have shivers can perform at a high level. However, there are other horses that begin to show signs at a young age (2 - 3 yrs-old) that are quite severe and these horses ability to perform is clearly impacted. The impact can be most notable in sports like dressage where fine motor control is important especially at upper levels and in driving where horses must be able to back into a hitch. In the short term (a few years) shivers may not impact performance. In the long term, (many years) half of shivers horses will show gradual progression from difficulty trimming hind hooves to difficulty cleaning the hind feet out to difficulty backing and then potentially hyperflexion intermittently at a walk. As this progression occurs there may be a drop in the level of performance. It is impossible to predict which horses remain static and which horses will progressively worsen and how quickly progression will occur. Shipping, painful events like canker, lameness and stall confinement can all make shivers temporarily worse. In some cases shivers may result in euthanasia because of profound apparent discomfort, and incapacitation associated with episodic muscle cramping.

Horses with possible Shivers need to have a complete veterinary evaluation. A thorough lameness exam should rule out abnormalities in the hooves, bones, joints, and tendons that may cause the horse to show signs similar to Shivers. The exam should involve backing horses up and lifting the hindlimbs in a flexed position. Your veterinarian will be able to confirm the diagnosis, rule out other possibilities and recommend appropriate treatment and management.

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1. Draper ACE, Bender JB, Firshman AM, Baird JD, Reed S, Mayhew IJ and Valberg SJ. Epidemiology of Characterisation of Shivers in Horses Equine Vet J 2014 May 6. doi: 10.1111/evj.12296. [Epub ahead of print]
2. Draper ACE, Trumble, TN, Firshman AM, Baird JD, Reed S, Mayhew IJ and Valberg SJ Posture and movement characteristics of forward and backward walking in horses with Shivers and acquired bilateral Stringhalt. Equine Vet J. 2014 Mar 10. doi: 10.1111/evj.12259. [Epub ahead of print]
3. Valberg SJ, Lewis SS, Shivers JL, Barnes NE, Konczak J, Draper ACE, Armien A. The equine movement disorder “Shivers” is associated with selective cerebellar Purkinje cell axonal degeneration. Vet Path February 24, 2015, doi: 10.1177/0300985815571668
4. Christine Barakat. The mystery of shivers http://equusmagazine.com/article/mystery-shivers-2...