Shivers
What is 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.
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 with or without tail elevation and trembling.
The term Shivers has been applied by some veterinarians to horses that solely exhibit hindlimb hyperflexion when manually lifting the limb, however, this clinical sign encompasses many other painful disorders. Because horses with standing hyperflexion do not show hyperflexion with backward gaits, the author suggests that the term standing hyperflexion should be applied to such cases rather than Shivers. Standing hyperflexion is more often unilateral than bilateral and less likely to progress than Shivers. Similar to Shivers, the hindlimb muscles may tremble, and the tail may be elevated, and the horse agitated when lifting the limb. If the limb is picked up without exceeding a certain degree of flexion, hyperflexion may not occur. In its most severe form, the hindlimb is abducted and hyperflexed and horses may lean so far toward the weight-bearing hindlimb that they almost fall over. Stress, pain, and farrier work often exacerbate standing hyperflexion. There appear to be three potential causes of standing hyperflexion. First, any painful condition of the hindlimb can induce a strong withdrawal response when the limb is flexed. Secondly, it may be an early sign of Shivers. If this is the case, the horses are usually < 7 years of age, predominantly geldings and 16.3 hands or taller. Thirdly, horses may have a hyperactive reflex arc, pulling the hoof away when trapped by the handler and then not having the ability to relax the limb consciously.
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. A classification system for Shivers has been developed.
- Shivers-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.
- Shivers- Forward hyperflexion: 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.
- Shivers-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 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.
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.
The diagnosis of Shivers is based on the characteristic gait pattern during backward walking that is absent during trotting and only apparent in advanced cases in <20% of strides. There is no antemortem diagnostic test for Shivers. At one point, a link was made between polysaccharide storage myopathy (PSSM) and Shivers suggesting muscle biopsy or genetic testing might be valuable diagnostic tools. Further research, however, showed that there is a coincidental occurrence of both a high prevalence of Shivers in some draught breeds and a high prevalence of type 1 PSSM with no causal link. Furthermore, Warmbloods and Thoroughbreds diagnosed with Shivers have not had evidence of PSSM in muscle samples.
Potential rule-outs for Shivers:
What causes Shivers?
Muscle recruitment patterns of Shivers horses have been studied using surface electromyography. During backward walking, hindlimb muscle activation patterns in horses with Shivers exhibit sustained elevated simultaneous flexor and extensor muscle activation with loss of the precise agonist and antagonist firing patterns required for a normal gait. Notably, altered and elevated firing patterns are also present during forward walking and trotting even though the gait visually appears to be normal. These results indicate that Shivers arises from a lack of coordinated recruitment of the hindlimb flexor and extensor muscles that is most evident during backward walking but present subclinically during forward walking. As Shivers advances, an interruption in motor recruitment becomes apparent visually at the initiation of forward walking. Selective distal axonal degeneration in cerebellar Purkinje cells was the sole pathologic finding in a thorough neuropathological study of the peripheral and central nervous system of a small number of Warmblood and Thoroughbred horses with Shivers. The cerebellum processes sensory information and shares it with nerve cells in the brainstem and cerebral cortical areas, creating locomotor patterns via an excitatory motor drive. Shivers is hypothesized to originate from reduced or loss of regional cerebellar inhibitory output via the deep cerebellar nuclei leading to enhanced excitation of selected efferent targets that modulate hindlimb muscle tone in response to specific motor actions. Potentially, the inhibitory drive for locomotor modules important for a gait like backward walking are disrupted in horses with Shivers.
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 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.
Click here for more information on PSSM.
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. 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.
How do you treat Shivers in horses?
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 E 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. This 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™, Nano E™ 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.
Can Shivers be cured?
At present, there is no cure and few effective treatments for Shivers.
What is the prognosis?
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.
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. 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.
Additional Resources
Seminar by Dr. Stephanice Valberg
References
- 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.
- 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.
- 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.
- Christine Barakat. The mystery of shivers.