Equine shivers

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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.

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Most horses with shivers are tall (average affected horse is 17 hands), and they are more commonly male (3:1 ratio of males to females). Shivers is most often seen in Warmbloods, draft horse breeds, and Thoroughbreds, [1] but has also been reported in light harness horses, hacks, Quarter Horses, and other light horse breeds. [2]

Hand (unit) unit of measurement of length equal to 101.6 millimetres (4 in)

The hand is a non-SI unit of measurement of length standardized to 4 inches (101.6 mm). It is used to measure the height of horses in some English-speaking countries, including Australia, Canada, the Republic of Ireland, the United Kingdom and the United States. It was originally based on the breadth of a human hand. The adoption of the international inch in 1959 allowed for a standardized imperial form and a metric conversion. It may be abbreviated to "h" or "hh". Although measurements between whole hands are usually expressed in what appears to be decimal format, the subdivision of the hand is not decimal but is in base 4, so subdivisions after the radix point are in quarters of a hand, which are inches. Thus, 62 inches is fifteen and a half hands, or 15.2 hh.

Warmblood group of horse types and breeds

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 sport. The term distinguishes these horses from both heavy draft horses and refined light saddle horses such as the Thoroughbred, Arabian, and Akhal-Teke. Although modern warmbloods are descended from heavier agricultural types systematically upgraded by hotblood influence, the term does not imply that warmbloods are direct crosses of "cold" and "hot".

Draft horse A type of horse or a horse breed bred to be a working animal doing heavy labor

A draft horse (US), draught horse or dray horse, less often called a carthorse, work horse or heavy horse, is a large horse bred to be a working animal doing hard tasks such as plowing and other farm labor. There are a number of breeds, with varying characteristics, but all share common traits of strength, patience, and a docile temperament which made them indispensable to generations of pre-industrial farmers.

Pathogenesis

The cause of shivers is currently unknown. Horses with shivers have been shown to have degeneration of the axons of their cerebellar Purkinje cells, although Purkinje cell number remains normal. [3]

Axon The long process of a neuron that conducts nerve impulses, usually away from the cell body to the terminals and varicosities, which are sites of storage and release of neurotransmitter.

An axon, or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons, muscles, and glands. In certain sensory neurons, such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body, and from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders which can affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, and group C nerve fibers. Groups A and B are myelinated, and group C are unmyelinated. These groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, and Type IV.

Cerebellum region of the brain that coordinates motor functions and muscle tone

The cerebellum 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 or even larger. In humans, the cerebellum plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language as well as in regulating fear and pleasure responses, but its movement-related functions are the most solidly established. The human cerebellum does not initiate movement, but contributes to coordination, precision, and accurate timing: it receives input from sensory systems of the spinal cord and from other parts of the brain, and integrates these inputs to fine-tune motor activity. Cerebellar damage produces disorders in fine movement, equilibrium, posture, and motor learning in humans.

Purkinje cell

Purkinje cells, or Purkinje neurons, are a class of GABAergic neurons located in the cerebellum. They are named after their discoverer, Czech anatomist Jan Evangelista Purkyně, who characterized the cells in 1839.

The strong breed predilection suggests a genetic basis to the disease. [3] Despite some similarities in breed prevalence, there is no evidence to suggest a link between equine shivers and equine polysaccharide storage myopathy (PSSM). [4]

Equine polysaccharide storage myopathy is an inheritable glycogen storage disease of horses that causes exertional rhabdomyolysis. It is most commonly associated with heavy horse breeds and the American Quarter Horse. While incurable, PSSM can be managed with appropriate diet and exercise. There are currently 2 subtypes, known as Type 1 PSSM and Type 2 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: [1]

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. [5] 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. [2]

Despite histologic changes in the cerebellum, horses with shivers do not show clinical signs typical of cerebellar disease (ataxia, intention tremors). [3] There are usually no changes on serum biochemistry profile, including measurements of muscle enzymes (CK and AST). [2]

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. [6] 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"). [2] Similar signs may be seen when the horse is asked to simply lift a hind foot, or during farriery 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. [2]

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. [2]

Treatment and Prognosis

There is currently no known effective treatment, although additional dietary vitamin E and selenium is often added following diagnosis. [1]

In the early stages of disease, horses with shivers are commonly used as riding and driving animals. However, the disease is often slowly progressive. [1] Muscle spasms usually increase in frequency and severity, and the animal becomes weak, develops muscle wasting, and may become very uncomfortable. [2] 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 or detomidine, has proven helpful in reducing signs during farriery visits. [1] Eventually, euthanasia is often elected due to poor quality of life for the animal.

See also

Related Research Articles

Ataxia is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in eye movements. Ataxia is a clinical manifestation indicating dysfunction of the parts of the nervous system that coordinate movement, such as the cerebellum. Ataxia can be limited to one side of the body, which is referred to as hemiataxia. Several possible causes exist for these patterns of neurological dysfunction. Dystaxia is a mild degree of ataxia. Friedreich's ataxia has gait abnormality as the most commonly presented symptom. The word is from Greek α- [a negative prefix] + -τάξις [order] = "lack of order".

Tremor involuntary muscle contraction

A tremor is an involuntary, somewhat rhythmic, muscle contraction and relaxation involving oscillations or twitching movements of one or more body parts. It is the most common of all involuntary movements and can affect the hands, arms, eyes, face, head, vocal folds, trunk, and legs. Most tremors occur in the hands. In some people, a tremor is a symptom of another neurological disorder. A very common tremor is the teeth chattering, usually induced by cold temperatures or by fear.

Hoof horn structure which surrounds the distal phalange in odd-toed ungulates (horses, donkeys and zebras)

A hoof, plural hooves or hoofs, is the tip of a toe of an ungulate mammal, strengthened by a thick, horny, keratin covering.

Myopathy is a disease of the muscle in which the muscle fibers do not function properly. This results in muscular weakness. Myopathy means muscle disease. This meaning implies that the primary defect is within the muscle, as opposed to the nerves or elsewhere. Muscle cramps, stiffness, and spasm can also be associated with myopathy.

Laminitis disease that affects the feet of hooved animals

Laminitis is a disease that affects the feet of ungulates and is found mostly in horses and cattle. Clinical signs include foot tenderness progressing to inability to walk, increased digital pulses, and increased temperature in the hooves. Severe cases with outwardly visible clinical signs are known by the colloquial term founder, and progression of the disease may lead to perforation of the coffin bone through the sole of the hoof or being unable to stand up requiring euthanasia.

Glycogen-branching enzyme deficiency (GBED) is an inheritable glycogen storage disease affecting American Quarter Horses and American Paint Horses. It leads to abortion, stillbirths, or early death of affected animals.

Wobbler disease is a catchall term referring to several possible malformations of the cervical vertebrae that cause an unsteady (wobbly) gait and weakness in dogs and horses. A number of different conditions of the cervical (neck) spinal column cause similar clinical signs. These conditions may include malformation of the vertebrae, intervertebral disc protrusion, and disease of the interspinal ligaments, ligamenta flava, and articular facets of the vertebrae. Wobbler disease is also known as cervical vertebral instability, cervical spondylomyelopathy (CSM), and cervical vertebral malformation (CVM). In dogs, the disease is most common in large breeds, especially Great Danes and Doberman Pinschers. In horses, it is not linked to a particular breed, though it is most often seen in tall, race-bred horses of Thoroughbred or Standardbred ancestry. It is most likely inherited to at least some extent in dogs and horses.

Cerebellar abiotrophy

Cerebellar abiotrophy, also called cerebellar cortical abiotrophy, or cerebellar degeneration, is a genetic neurological disease in animals best known to affect certain breeds of horses, dogs and cats. It can also develop in humans. It develops when the neurons known as Purkinje cells, located in the cerebellum of the brain, begin to die off. These cells affect balance and coordination. They have a critical role to play in the brain. The Purkinje layer allows communication between the granular and molecular cortical layers in the cerebellum. Put simply, without Purkinje cells, an animal loses its sense of space and distance, making balance and coordination difficult. People with damage to the cerebellum can experience symptoms like unsteady gait, poor muscle control, and trouble speaking or swallowing.

A flexion test is a preliminary veterinary procedure performed on a horse, generally during a prepurchase or a lameness exam. The purpose is to accentuate any pain that may be associated with a joint or soft-tissue structure, allowing the practitioner to localize a lameness to a specific area, or to alert a practitioner to the presence of sub-clinical disease that may be present during a pre-purchase exam.

Stringhalt

Stringhalt is a sudden flexion of one or both hind legs in the horse, most easily seen while the horse is walking or trotting. It is most evident when the horse is backing up slowly, turning on the affected leg, or suddenly frightened. It can involve one or both hind legs of the horse. It is a spasmodic contraction of the lateral extensor tendons of the hind legs.

Lameness is an abnormal gait or stance of an animal that is the result of dysfunction of the locomotor system. In the horse, it is most commonly caused by pain, but can be due to neurologic or mechanical dysfunction. Lameness is a common veterinary problem in racehorses, sport horses, and pleasure horses. It is one of the most costly health problems for the equine industry, both monetarily for the cost of diagnosis and treatment, and for the cost of time off resulting in loss-of-use.

Canine physical therapy

Physical therapy for canines adapts human physical therapy techniques to increase function and mobility of joints and muscles in animals. Animal rehabilitation can reduce pain and enhance recovery from injury, surgery, degenerative diseases, age-related diseases, and obesity.

Comparative foot morphology

Comparative foot morphology involves comparing the form of distal limb structures of a variety of terrestrial vertebrates. Understanding the role that the foot plays for each type of organism must take account of the differences in body type, foot shape, arrangement of structures, loading conditions and other variables. However, similarities also exist among the feet of many different terrestrial vertebrates. The paw of the dog, the hoof of the horse, the manus (foot) and pes (foot) of the elephant, and the foot of the human all share some common features of structure, organization and function. Their foot structures function as the load-transmission platform which is essential to balance, standing and types of locomotion.

Nutritional muscular dystrophy disease caused by a deficiency of selenium and vitamin E in dietary intake

Nutritional Muscular Dystrophy is a disease caused by a deficiency of selenium and vitamin E in dietary intake. Soils that contains low levels of selenium produce forages and grains that are deficient in selenium. Similarly, if the forage is of low quality or is not stored properly it may be deficient in vitamin E. If an animal consumes this type of diet without additional supplementation they become susceptible to this disease. This condition often affects young ruminants, such as calves and lambs.

Purpura haemorrhagica is a rare complication of equine strangles and is caused by bleeding from capillaries which results in red spots on the skin and mucous membranes together with oedema (swelling) of the limbs and the head. Purpura hemorrhagica is more common in younger animals.

Limbs of the horse

The limbs of the horse are structures made of dozens of bones, joints, muscles, tendons and ligaments that support the weight of the equine body. They include two apparatuses: the suspensory apparatus, which carries much of the weight, prevents overextension of the joint and absorbs shock, and the stay apparatus, which locks major joints in the limbs, allowing horses to remain standing while relaxed or asleep. The limbs play a major part in the movement of the horse, with the legs performing the functions of absorbing impact, bearing weight, and providing thrust. In general, the majority of the weight is borne by the front legs, while the rear legs provide propulsion. The hooves are also important structures, providing support, traction and shock absorption, and containing structures which provide blood flow through the lower leg. As the horse developed as a cursorial animal, with a primary defense mechanism of running over hard ground, its legs evolved to the long, sturdy, light-weight, one-toed form seen today.

The treatment of equine lameness is a complex subject. Lameness in horses has a variety of causes, and treatment must be tailored to the type and degree of injury, as well as the financial capabilities of the owner. Treatment may be applied locally, systemically, or intralesionally, and the strategy for treatment may change as healing progresses. The end goal is to reduce the pain and inflammation associated with injury, to encourage the injured tissue to heal with normal structure and function, and to ultimately return the horse to the highest level of performance possible following recovery.

Stay apparatus

The stay apparatus is a group of ligaments, tendons and muscles which "lock" major joints in the limbs of the horse. It is best known as the mechanism by which horses can enter a light sleep while still standing up. It does, however, exist in other large land mammals, where it plays a role in reducing fatigue while standing. The stay apparatus allows animals to relax their muscles and doze without collapsing.

Hyperkalemic periodic paralysis is a genetic disorder that occurs in horses. It is also known as Impressive syndrome, after an index case in a horse named Impressive. It is an inherited autosomal dominant disorder that affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood. It is characterized by muscle hyperexcitability or weakness which, exacerbated by potassium, heat or cold, can lead to uncontrolled shaking followed by paralysis.

References

  1. 1 2 3 4 5 Draper, A. C. E., Bender, J. B., Firshman, A. M., Baird, J. D., Reed, S., Mayhew, I. G. and Valberg, S. J. (2015), Epidemiology of shivering (shivers) in horses. Equine Veterinary Journal, 47: 182–187. doi: 10.1111/evj.12296
  2. 1 2 3 4 5 6 7 Valberg, Stephanie; Baird, John. "Shivers". University of Minnesota Equine Center. University of Minnesota CVM. Retrieved 13 June 2015.
  3. 1 2 3 Valberd SJ, Lewis SS, Shivers JL, Barnes NE, Konczak J, Draper ACE, Armien AG (February 2015). "The Equine Movement Disorder "Shivers" Is Associated With Selective Cerebellar Purkinje Cell Axonal Degeneration". Veterinary Pathology. doi:10.1177/0300985815571668.
  4. Firshman, A.M., Baird, J.D. and Valberg, S.J. (2005) Prevalence and clinical signs of polysaccharide storage myopathy and shivers in Belgian draft horses. J. Am. Vet. Med. Ass. 227, 1958-1964.
  5. Baird JD, Firshman AM, Valberg SJ. Shivers (shivering) in the horse: a review. Proc Am Assoc Equine Pract. 2006;52:359–354.
  6. Draper ACE, Trumble TN, Firshman AM, et al. Posture and movement characteristics of forward and backward walking in horses with shivering and acquired bilateral stringhalt. Equine Vet J.