"Shivers" or "shivering" have been used to designate a chronic nervous or neuromuscular syndrome in horses that has been recognized for centuries. The syndrome principally affects the draft-horse breeds but has also been reported in Warmbloods, Warmblood crossbreds and occasionally lighter breeds of horses (including light harness horses, hunters, hunter-jumpers, hacks, Quarter Horses, and Thoroughbreds). Shivers is considered a rarity in pony breeds.
Shivers usually has an insidious onset. Gait abnormality can occur at any age (even yearlings), although tends to progress with age. In a recent study examining Belgian draft horses, no significant difference was observed in the age and sex distribution of individuals with shivers from those without.
The clinical spectrum of equine shivers is variable in both degree and visible manifestation. Although diagnosis is relatively straightforward, clinical signs may be intermittent, occasional, and/ or latent and therefore difficult to survey.
The disease is characterized by periodic, involuntary spasms of the muscles in the pelvic region, one or both pelvic limbs and tail. In many instances it appears as though the horse is snatching its leg away when asked to pick up a foot for cleaning. The limb trembles or shivers in suspension, often with concurrent elevation and trembling of the tail.
See what this looks like:
• Shivers less frequently involves the forelimbs, neck, or trunk.
• In horses affected with shivers of the thoracic (fore) limbs, the afflicted
limb is thrust forward in full extension with the foot
barely touching the ground; alternatively, the limb with the carpus
flexed is elevated and abducted. The extensor muscles
above the elbow quiver until the spasm ends or
the foot returns to the ground.
• Occasionally spasmodic contractions are observed in the ears, eyelids, lips, and cheeks. With involvement of the face, there is rapid blinking of the eyelids, quivering of the ears, and twitching (with spasmodic retraction) of the lip commissures.
• Signs of shivers are not typically elicited during forward movement.
• Noticeable atrophy of the thigh (quadriceps and biceps femoris) musculature often becomes apparent with progression of the disease. Recent research suggests that generalized muscle atrophy with obvious pelvic limb muscle weakness occurs in approximately 58% of horses with shivers.
Mildly affected horses may exhibit increased tension or trembling of the hindlimbs as well as sudden extensor movements of the tail that cause it to elevate quickly. The degree of tail elevation varies considerably among cases and doesn't appear to be commensurate with derangement of pelvic gait.
In more severe cases of shivers the hindlimb is suddenly raised, semi-flexed and abducted (held away from the body) with the hoof held in the air and poised in a spastic state from several seconds to as long as several minutes.
The pelvic limbs may become generally stiff or rigid with the horse preferring to stand with the hocks wider apart than normal. Because horses with shivers are often reluctant to lie down, there might be bumps and bruises due to partial falls caused by excessive fatigue. Some owners have noted a disproportionate amount of sweating in horses affected with shivers.
Clinical signs are most evident when affected horses are asked to back-up, move quickly or lift a hind limb for hoof cleaning or trimming/shoeing. The superficial muscles of the thigh and quarter usually 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 horse might experience additional spasms as the foot is replaced onto the ground surface, especially if this occurs during backing.
Stress, anxiety and/or unstable (slippery) footing can exacerbate clinical signs associated with shivers, especially if the horse is fitted in harness (Standardbreds and draft breeds).
In exceptionally severe cases, one or both hindlimbs may be held out behind the animal in rigid spastic extension while standing. The animal may stand on its toes with the heels raised off the ground for an indefinite amount of time.
Although the clinical signs of shivers may remain static (unchanging) in some horses, they are progressive and debilitating for the majority of affected animals.
Many afflicted horses can perform in their intended capacity, primarily due to the absence of clinical signs while moving forward. Horses with a thoracic (forelimb) manifestation of shivers seldom experience a change in type or level of work.
There is no significant difference in baseline serum creatine kinase (CK) and aspartate aminotransferase (AST) activities in horses with shivers compared with horses without shivers. There is also no significant difference in serum selenium and serum vitamin E concentrations in horses with shivers compared with horses without. Blood work, therefore, is often not helpful in confirming a diagnosis of shivers.
Moreover, no specific postmortem lesions along any portion of the central nervous system are evident in horses with shivers.
The etiology of shivers and pathophysiologic alterations associated with the clinical signs have not been determined. Neurological, myopathic, genetic, infectious, and traumatic causes have been postulated.
While no one knows what causes shivers, it most likely has a neurologic pathology (at least in part). Depletion of glycogen leading to muscle cramping has also been suggested, but there has been little correlation between the severity of clinical signs and histopathologic findings to support this view.
Sources that have been implicated include the following:
- Basal nuclei lesions
- Motor and reflex hypertonia of pelvic flexor and/or extensor musculature
- Lesions in the sensory or motor pathways between the brain stem and affected musculature/ associated joint and tendon sensory receptors
- Progressive neuromuscular disease
- Other neurotransmitter defects
- Polysaccaride Storage Myopathy (PSSM)
- Metabolic myopathy
- Systemic Infection
There is currently no known treatment for shivers.
Clinical signs may improve or regress after long periods of rest, but the condition usually recurs when work is resumed.
It has been suggested that a high-fat/ low-starch diet may be beneficial if instituted early in the course of shivers. It should be noted, however, that affected Warmblood or Warmblood-crossbreds fed grass hay and a high-fat supplement instead of dietary grain in a recent study did not exhibit any regression of the disease.
Prognosis for affected individuals is generally unfavorable due to the progressive nature of the disease. The long-term prognosis for athletic function may be even worse, especially in disciplines with a high level of difficulty.
In extremely severe cases, shivers may result in death or euthanasia due to profound muscle wasting, weakness, and/or pain which can render the animal incapacitated.