Shivers, or equine shivering, is a rare, progressive neuromuscular disorder of horses. It is characterized by muscle tremors, difficulty holding up the hind limbs, and an unusual gait when the horse is asked to move backwards. Shivers is poorly understood and no effective treatment is available at this time.

Presentation

Most horses with shivers are tall (average affected horse is 17

hands A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each "han ...

), and they are more commonly male (3:1 ratio of males to females). Shivers is most often seen in

Warmblood Warmbloods are a group of middle-weight horse types and breeds primarily originating in Europe and registered with organizations that are characterized by open studbook policy, studbook selection, and the aim of breeding for equestrian sp ...

draft horse A draft horse (US) or draught horse (UK), also known as dray horse, carthorse, work horse or heavy horse, is a large horse bred to be a working animal hauling freight and doing heavy agricultural tasks such as plowing. There are a number o ...

breeds, and

Thoroughbred The Thoroughbred is a list of horse breeds, horse breed developed for Thoroughbred racing, horse racing. Although the word ''thoroughbred'' is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thorough ...

s, but has also been reported in light harness horses, hacks,

Quarter Horses The American Quarter Horse, or Quarter Horse, is an American breed of horse that excels at sprinting short distances. Its name is derived from its ability to outrun other horse breeds in races of or less; some have been clocked at speeds up to . ...

, and other light horse breeds.

Pathogenesis

The cause of shivers is currently unknown. Horses with shivers have been shown to have degeneration of the

axon An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...

s of their

cerebellar The cerebellum (: cerebella or cerebellums; Latin for 'little brain') is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as it or e ...

Purkinje cell Purkinje cells or Purkinje neurons, named for Czech physiologist Jan Evangelista Purkyně who identified them in 1837, are a unique type of prominent, large neuron located in the Cerebellum, cerebellar Cortex (anatomy), cortex of the brain. Wi ...

s, although Purkinje cell number remains normal. The strong breed predilection suggests a genetic basis to the disease. Despite some similarities in breed prevalence, there is no evidence to suggest a link between equine shivers and equine polysaccharide storage myopathy (PSSM).

Clinical signs

Clinical signs usually are present before the age of 5 years old, and slowly progress throughout the horse's lifetime. Affected horses display muscle tremors of the hindquarters, hind limbs, and tail, and a spastic gait when they are asked to back up. Typical signs of shivers include: *Farriery problems (96% of cases) *Muscle twitching/trembling (85%) *Elevated tail head (74%) *Muscle atrophy (44%) *Reduced strength (33%) *Exercise intolerance (33%) *Facial twitching (19%) Clinical signs may be unilateral or bilateral, continuous or intermittent. They often worsen with excitement, stress, when the animal is asked to move on slippery surfaces, or during prolonged periods of standing. While shivers is classically associated with the hind end, horses may also occasionally show spasmodic contraction of the muscles of the head and neck, including twitching of the ears and lips, and rapid blinking. Rarely, horses will show a spastic gait in the front limbs in which the leg is held in full extension or with the knee flexed, with trembling occurring in the upper limb. Despite histologic changes in the

cerebellum The cerebellum (: cerebella or cerebellums; Latin for 'little brain') is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as it or eve ...

, horses with shivers do not show clinical signs typical of cerebellar disease (

ataxia Ataxia (from Greek α- negative prefix+ -τάξις rder= "lack of order") is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in e ...

intention tremor Intention tremor is a dyskinesia, dyskinetic disorder characterized by a broad, coarse, and low-frequency (below 5 Hz) tremor evident during deliberate and visually-guided movement (hence the name intention tremor). An intention tremor is usua ...

s). There are usually no changes on serum biochemistry profile, including measurements of muscle enzymes (CK and AST).

Progression of the disease

In mild cases, shivers may present only when the horse is asked to move backwards, usually seen as trembling in the muscles of the hind limbs and sudden, upward jerks of the tail. Affected animals may also snatch up their foot when asked to lift it for cleaning. More severely affected horses will produce a backward gait where the hind legs are lifted abnormally high, held abducted for a period of time at the height of flexion, before the limb is slowly extended and the foot is placed on the ground. This pause may last for seconds to several minutes, during which time there is spasming of the muscles of the hind limb and tail, leading to trembling ("shivers"). Similar signs may be seen when the horse is asked to simply lift a hind foot, or during

farrier A farrier is a specialist in equine hoof care, including the trimming and balancing of horses' hooves and the placing of shoes on their hooves, if necessary. A farrier combines some blacksmith's skills (fabricating, adapting, and adju ...

y work, especially when the foot is hammered during shoeing. Horses may progress to a point where they show hyperflexion and abduction of the hind limbs for a few steps after being asked to move forward or turn. In advanced cases, the horse may be unable to move backwards. Although most horses are usually clinically normal when standing, some may stand rigidly with the hind legs in full extension behind their body, heels lifted, supporting their weight only on their toes. Others may intermittently flex and abduct their hind legs, with associated muscle spasms, when asked to stand still for long periods. Severely affected animals become weak, develop muscle atrophy in the hind limbs, and may fall or become reluctant to lie down when confined.

Treatment and prognosis

There is currently no known effective treatment, although additional dietary vitamin E and selenium is often added following diagnosis. In the early stages of disease, horses with shivers are commonly used as riding and driving animals. However, the disease is often slowly progressive. Muscle spasms usually increase in frequency and severity, and the animal becomes weak, develops muscle wasting, and may become very uncomfortable. The horse may become unable to hold its feet up for trimming by a farrier, although use of alpha-2 agonist drugs, such as

xylazine Xylazine is a structural analog of clonidine and an α2-adrenergic receptor, α2-adrenergic receptor agonist, sold under many trade names worldwide, most notably the Bayer brand name Rompun, as well as Anased, Sedazine and Chanazine. Xylazine ...

detomidine Detomidine is an imidazole derivative and Alpha-2 adrenergic receptor, α2-adrenergic receptor agonist, used as a large animal sedative, primarily used in horses. It is usually available as the salt (chemistry), salt detomidine hydrochloride. It ...

, has proven helpful in reducing signs during farriery visits. Eventually,

euthanasia Euthanasia (from : + ) is the practice of intentionally ending life to eliminate pain and suffering. Different countries have different Legality of euthanasia, euthanasia laws. The British House of Lords Select committee (United Kingdom), se ...

is often elected due to poor quality of life for the animal.

References