Stephanie Valberg DVM PhD Dip. ACVIM
College of Veterinary Medicine, University of Minnesota
John Baird BVSc. PhD
Ontario Veterinary College, University of Guelph
- What is Shivers?
- What are the signs of Shivers?
- How do I know if my horse has Shivers?
- What causes Shivers?
- Is Shivers related to Polysaccharide Storage Myopathy (PSSM)?
- Is Shivers inherited?
- How do you treat Shivers?
- How do you feed a horse with Shivers?
- Can Shivers be cured?
- What do I do if I think my horse has Shivers?
Shivers or Shivering has long been recognized as a disease in draft horses and other breeds, but research into the cause and treatment of Shivers has been limited. Researchers at the University of Minnesota and the Neuromuscular Diagnostic Laboratory are performing research into Shivers in conjunction with Dr. John Baird at the Ontario Veterinary College. We are trying to establish the cause of Shivers, if this condition is inherited, and if dietary therapy is effective.
For more information on how you can be a part of our research, visit our Shivers Research website.
A horse with signs of Shivers
1.) What is Shivers?
Shivers or shivering are names that have been applied to a chronic nervous or 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.” Signs of Shivers appear at all ages, with cases reported from one year of age through adulthood. Both genders are affected.
The syndrome principally affects the draft horse breeds. Shivers has been also reported in Warmbloods and Warmblood crossbreds, and occasionally in lighter breeds of horses, including light harness horses, hunters, hunter-jumpers, hacks, Quarter Horses, and Thoroughbreds. In ponies, shivers is considered uncommon to rare. The condition develops slowly and can occur at any age with reports in horses as young as one to two years of age. In a recent study on Belgian draft horses, no significant difference was observed in the age and sex distribution of horses with shivers from horses without shivers.
Many horses that hold a hind limb flexed and tremble are said to have Shivers. However, many disorders can create irritation in the hind limbs of horses and cause this type of sign. The classic disease called Shivers is a neuromuscular disease in horses that is characterized by trembling of the tail while held erect, trembling of the thigh muscles and a flexed and trembling hind limb. It occurs most frequently in the draft horse breeds, but is documented in warmbloods, harness horses and other breeds. 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, although anecdotal evidence suggests it is much less common in non-draft horse breeds
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2.) What are the signs of Shivers?
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 and the tail. Shivers is characterized by periodic, involuntary spasms of the muscles in the pelvic region, pelvic limbs, and tail. Mildly affected horses show tenseness or trembling of the hind limbs and sudden jerky extensor movements of the tail that cause it to elevate. The degree of tail elevation varies considerably in different cases. In more severely affected animals upon backing, the hind limb is suddenly raised, semi-flexed and abducted with the hoof held in the air poised in a spastic state for several seconds to one to several minutes. The limb trembles or “shivers” in suspension, and the tail is usually elevated simultaneously and trembles. The superficial muscles of the horse’s thigh and quarter quiver, while the tail is elevated and tremulous. After a few moments the spasms of the limb and tail gradually subside, the limb is then slowly extended and the foot is brought slowly to the ground. The signs may reappear, however, when the horse is again forced to move backwards.
The most characteristic signs of shivers occur when an attempt is made to move the horse backwards. Occasionally, a horse will exhibit signs if made to move over suddenly in the stall or box. Shivering may also be seen on lifting or attempting to lift a hind foot, or when the horse is being shod, especially when the foot is hammered during shoeing.
In the early stages, the owner notices that the horse snatches up the hind limbs when they are being picked up to clean the feet or to be shod. The condition may progress so that the horse becomes impossible to shoe. Even in well-developed shivering cases, signs may not be seen when the horse is standing still. When the horse is moved forwards there may be no signs, or the signs may be restricted to the first two or three steps. In advanced cases, the affected animal may be unable to move backward more than a few paces, and sometimes this cannot be performed at all. Many shivering horses while standing in harness may attract attention by frequently raising and abducting the shivering hind limb. A slippery surface exaggerates the signs of shivers and more particularly when the horse is in harness. In severe cases one or both hind limbs may be held out behind the animal in rigid spastic extension. The animal may stand on its toes with the heels raised off the ground. In severe cases, the animal may fall over. Severely affected animals may be 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 into a straw bed or wooden floor, may often initiate episodes of shivering. 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 muscular spasms occur in the hindquarters, the fore feet remain implanted on the ground, the body sways backwards, the back is arched, and the tail 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 occasionally 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. The limbs may become more or less stiff or rigid. 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 stance with a base-wide stance in the hind limbs. Excessive sweating has been noted in some cases. Top of Page
3.) How do I know if my horse has Shivers?
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. Stringhalt 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. The severity of the gait abnormality is variable and may be accompanied by hind limb 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. However, in shivers, the limb is flexed, abducted and held in a spastic state for a few moments instead of being immediately returned to the ground as in 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 become momentarily (mild) or permanently (severe) caught above the medial trochlear ridge of the femur. 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 gait in one pelvic limb of a light-breed horse. 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 CNS.
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.
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 CSF.
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4.) What causes Shivers?
The causes of Shivers have not been determined. Neurological, myopathic, genetic, infectious, and traumatic causes have been postulated. There are also reports stating that not infrequently horses will show signs of shivering after a long rail or road journey.
Neurological cause: One possibility is that infections precede Shivers symptoms and cause lesions in the neuromuscular system. These lesions would result in neurological damage and produce the signs of shivers. However, detailed necropsies of two horses with Shivers failed to find neurological lesions. It has been stated, “it will likely take a very thorough, detailed and expensive pathological investigations of freshly harvested central and peripheral nervous system tissues on many cases to unravel the mystery of shivers.” Neurotransmitter defects may be responsible.
Myopathic cause: In one study involving a yearling Clydesdale gelding with a clinical diagnosis of shivers, muscle biopsies showed decreased carbohydrate content. It was thought that horses with shivers might have less stored glycogen and thus deplete their stores more rapidly. This may lead to localized muscle cramping, manifesting itself when the horse was forced to move backwards.
Genetic cause: Many have suggested that shivers is inherited or has a hereditary predisposition, and although this has not been proven, a familial tendency is suspected. In some countries, it is recommended that stallions with this disease not be used for breeding purposes. In the United Kingdom, shivering has been classified as a hereditary disease under the Horse Breeding Act, 1918.
Infectious disease: Some have suggested that the occurrence of shivers may be preceded more or less remotely by an attack of influenza, strangles, or other systemic infections. This led to the suggestion that shivers is connected with neuropathic lesions produced by infection, or toxins derived from an antecedent disease.
Trauma: Cases of shivers have also been attributed to accidental injuries, as might be incurred from a severe fall; however, the connection if any has not been determined. In the 1930s, it was stated, “a horse which is a shiverer will always show some evidence of osteoarthritis, often clinically, but certainly on post-mortem examination.”
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5.) Is Shivers related to Polysaccharide Storage Myopathy (PSSM)?
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 there appearance in the same horse is likely coincidental rather than causal.
For more information of PSSM click here.
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6.) Is Shivers inherited?
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. Currently, there is no specific genetic pattern identified and there is no genetic test.
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7.) How do you treat Shivers in horses?
There is currently no effective treatment for shivers. Occasionally the signs may improve or regress after long periods of rest, but the condition returns when work is resumed. 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 and Selenium are important in the diet. The natural form of Vitamin E, Elevate™, may provide additional benefits over 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 moves around.
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8.) How do you feed a horse with Shivers?
1) Caloric balance: Many horses with Shivers are easy keepers and may be overweight at the time of diagnosis. If necessary, to maintain normal body weight, caloric intake should be reduced by using a grazing muzzle during turn-out, feeding hay with a low nonstructural carbohydrate content (NSC) at 1 to 1.5% of body weight, providing a low calorie ration balancer and gradually introducing daily exercise. Rather than provide dietary fat to an overweight horse, it is best to get weight under control before adding any fat to the diet.
2) Selection of forage: The NSC content of hay should be about 12% NSC or less depending upon the caloric requirements of the horse. Feeding a low NSC hay of 4% provides room to add fat to the diet of easy keepers without exceeding the daily caloric requirement and inducing excessive weight gain. For example, a 500-kg horse on a routine of light exercise generally requires 18 MCal/day of digestible energy (DE). Fed at 2% of body weight, a 12% NSC mixed grass hay almost meets their daily caloric requirement by providing 17.4 MCal/day. This with a 12%NSC hay there is only room for 0.6 MCal of fat per day (72 ml of vegetable oil) in order to achieve 18 MCal of energy. In contrast, a 4% NSC Blue Grama hay would provide 13.5 MCal/day which would allow a reasonable addition of 4.5 MCal of fat per day (538 ml of vegetable oil).
3) Selection of grain: Rather than feeding a high NSC content concentrate such as sweet feed, a medium to low starch concentrate should be selected that provides the right balance of vitamins and minerals to the horse. This may be a low starch high fat feed such as Releve Concentrate or Ultium or a blend of a ration balancer with a fat source (see below). To get the right balance of minerals and vitamins from a fortified feed you need to feed it in the amount recommended by the manufacturer. If you feed a product like ReLeve or Ultium you usually need at least 4 lbs to achieve a balanced diet and this may be too many calories in lightly worked overweight horses. If this amount causes weight gain then a lower calorie feed or ration balancer should be fed.
4) Selection of fat source: It is not completely clear whether fat is necessary in the diet of Shivers horses. Some, but not all horses appear to improve with a diet change. The type and amount of fat to add depends on the individual horse and on the horse's weight and owner's budget. In an easy keeping horse, when you add fat the cheapest way to do so is to add oil or a solid fat supplement onto a pelleted ration balancer that provides enough energy. Check the caloric density of the ration balancer, You may want to use one for overweight horses. Suitable oils include soybean, corn, safflower, canola, flaxseed, linseed, fish, peanut and coconut. The amount of oil can be added gradually monitoring the horses exercise tolerance and weight. The amount added is usually between 1/2 and 2 cups.
5) Supplements: The only supplement we are aware of that may improve signs of Shivers is vitamin E. horse should get about 1000 IU of (alpha tocopherol) vitamin E per Day if they have Shivers. Different forms have different absorption and activity in the body once absorbed. If horses are low in vitamin E the fastest way to increase their body stores is with a natural vitamin E such as NanoE (Ker.com) or Elevate (kppusa.com).
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9.) Can Shivers be cured?
At present, there is no cure and few effective treatments for Shivers. The prognosis for affected individuals is generally unfavorable to poor because the disease is usually slowly progressive. In a horse with shivers, the tendency is for the spasms to increase in both frequency and severity. The long-term prognosis for athletic function is grave. Eventually, shivers may result in death or euthanasia because of profound weakness, muscle wasting, and apparent discomfort, and incapacitation associated with episodic muscle cramping.
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10.) What do I do if I think my horse has Shivers?
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. Your veterinarian will be able to confirm the diagnosis, rule out other possibilities and recommend appropriate treatment and management. In some cases, a muscle biopsy will help to confirm if signs of PSSM are present.
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