Equine Shivers
Shivers is a neuromuscular syndrome in horses that has been recognized for centuries.1 At certain times, this condition has been called “Shivers” or “Shivering,” but both refer to the same syndrome. Interestingly, the literature states that throughout its known history of occurrence, Shivers has been considered to be common, uncommon, rare and very rare.
Shivers tends to affect draft horse breeds more than other horses, and has also 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.1 In ponies, Shivers is considered uncommon to rare. Thus, researchers are inclined to believe a genetic component is responsible for horses developing the disease.
While Shivers has been widely acknowledged in certain breeds of horses over hundreds of years, little was known about the cause of this neuromuscular disease until recently. “The equine community knows more about Shivers than it did even 10 years ago, thanks in large part to research efforts, and can finally begin to make progress in terms of treatment,” according to Kentucky Equine Research nutritionist Kathleen Crandell, Ph.D.2
What is Shivers?
Shivers is a chronic, slowly progressive neuromuscular condition characterized by quivering of the hind limbs and tail during backward movement. Shivers can occur at any age, with reports in horses as young as young as one year old.3 Most horses show no signs of Shivers when walking or trotting. Horses developing Shivers often display hyperflexion or hyperextension of one or both hind limbs when asked to walk backward. In these cases, the stifle and hock joints become hyperextended and are held rigid and spastic.2 Trembling of the muscles of the hindquarter may also occur, especially in advanced cases.2,3
According to much of the literature, Shivers is caused by a defect in the cerebellum, a region of the brain that controls muscular activity. The cerebellum of horses with Shivers is believed to lack the mechanism for dampening neuroelectrical impulses, leading certain muscle groups to become involuntarily stimulated. Diagnosing Shivers can be relatively easy, but in the early stages of disease, the signs can be understated and irregular, making diagnosis difficult until the disease is well-established.3
Some little-known facts about Shivers:
- Most horses do not begin showing signs of Shivers before the age of five years.
- Geldings are more likely to develop Shivers than mares.
- Tall horses (over 16.2 hands) are more susceptible than shorter horses, with ponies and Miniature Horses hardly ever being affected.
- Shivers often appears similar to other musculoskeletal and neuromuscular disorders, especially during the early stages of the disease.
- There is no cure or definitive treatment for Shivers.
- While some horses with Shivers can continue to work or perform athletically, others will require retirement or even euthanasia.4
Signs of Shivers in Horses
Shivers is typically characterized by trembling of the tail while held erect, trembling of the thigh muscles and a flexed and trembling hind limb. 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.3 Since Shivers presents as similar to other musculoskeletal and neuromuscular disorders, a veterinary examination should rule out stringhalt, upward fixation of the patella, equine motor neuron disease, and even equine protozoal myeloencephalitis (EPM).
In mildly affected horses, Shivers will present as 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. As indicated above, upon backing, more severely affected horses will raise the hind limb, 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.”4 The spasms then subside, with the limb being slowly extended and the foot brought to the ground. These signs reappear when the horse is again asked to move backwards.
On occasion, there will be involvement of the muscles of the forelimb, neck, or even trunk and face. On attempts to lift a front foot, the limb may be thrust forward in full extension with 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.4 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.5
As the disease progresses, a gradual atrophy of the muscles of the thigh occurs; this may progress to generalized muscle atrophy. Limbs may also become stiff and rigid. Horses that are affected to this degree will sleep standing with their front fetlocks and knees becoming bruised by frequent falls. The horse may adopt an abnormal, wide stance in the hind limbs. Excessive sweating has been noted in some cases.
According to the literature, the signs of Shivers may not be seen when the horse is standing still even in well-developed cases. Many horse owners who have not taken note of the early symptoms begin to suspect that something is wrong when their horse becomes impossible to shoe. 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.2-4 When walking, slippery surfaces can exaggerate the signs of Shivers, particularly when the horse is in harness.3 In severe cases, one or both hind limbs may be held out behind the animal in rigid extension. In more severe cases, the animal may fall over.4 Severely affected horses may be hesitant to lie down when confined indoors and often lose body condition.
The Pitcher Plant for Equine Shivers
The pitcher plant, Sarracenia purpurea, is widely distributed in North America and has a history of being marketed as both a pain therapy and a traditional medicine in many aboriginal communities.5 In some instances, the plant has been used to treat the symptoms of diabetes; the leaf extract demonstrates multiple anti-diabetic activities including cytoprotection in an in vitro model of diabetic neuropathy. The leaf extract of S. purpurea is thought to prevent glucose-mediated cell loss in a concentration-dependent manner.4
The aqueous extracts of S. purpureaare thought to exert analgesic effects by neurolytical action on peripheral nerves, thereby mitigating the neurologic pain and symptoms associated with Shivers.
1Baird, J. et. al. Shivers (Shivering) in the Horse: A Review. In: AAEP proceedings Vol. 52 2006.
2Frank, E.R. Veterinary Surgery, 7th ed. Minneapolis: Burgess Publishing Co., 1964; 333–334.
3Malikides, N., et. al. Neurology. In: Rose RJ, Hodgson DR, eds. Manual of equine practice, 2nd ed.Philadelphia: W.B. Saunders, 2000;503–575.
4Adams, R.H.; Chalkley, L.W.; Buchanan, T.M. Veterinary Treatments & Medications for Horsemen. Equine Research Publications, 1977.
5Harris, Cory S., et al. Characterizing the cytoprotective activity of Sarracenia purpurea L., a medicinal plant that inhibits glucotoxicity in PC12 cells. BMC complementary and alternative medicine vol. 12 245. 5 Dec. 2012, doi:10.1186/1472-6882-12-245.
5Rooney, J.R. Clinical neurology of the horse. Kennett Square, PA: KNA Press Inc., 1971.
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