By Dr. Stephanie Valberg, University of Minnesota
and Dr. John Baird, University of Guelph
Adapted from Shivers article from University of Minnesota
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 also has been 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.
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.
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.
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.
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.
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.
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 moved around.
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.
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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