Shivers / Shivering

Shivers is a chronic, progressive neuromuscular condition in horses characterized by involuntary muscle spasms and hyperflexion of the hindlimbs. The classic presentation involves the affected horse suddenly snatching the hindlimb upward and outward when asked to back, pick up the foot, or move in certain ways. The limb may be held in an elevated, hyperflexed position while trembling before slowly returning to the ground, giving the condition its descriptive name. While shivers primarily affects the hindlimbs, the tail and occasionally the forelimbs may also be involved, and the condition typically worsens over time.

Shivers affects a specific population of horses more than others, with tall horses, draft breeds, and warmbloods showing significantly higher prevalence than other populations. Horses over sixteen hands in height have notably increased risk compared to smaller horses. Draft breeds including Belgians, Percherons, Clydesdales, and Shires are particularly susceptible. Warmbloods used for dressage and sport horse competition also show elevated incidence. Males may be affected more frequently than females, though both sexes develop the condition. Onset typically occurs in young adult horses between five and ten years of age, though earlier and later onset can occur.

The impact of shivers on equine health and performance is substantial and typically progressive. Affected horses have difficulty with farrier work, as holding up a hindfoot triggers the characteristic spasms. Backing becomes increasingly difficult or impossible as the condition advances. Athletic performance, particularly in disciplines requiring collection, lateral movements, and precise hindlimb control, deteriorates. Some horses develop compensatory movement patterns that cause secondary musculoskeletal problems. In severe cases, muscle atrophy develops in the hindquarters, and horses may become unable to perform basic functions including rising from recumbency.

No cure exists for shivers, and treatments are limited to management strategies that may slow progression or reduce symptom severity in some horses. The progressive nature of the condition means that horses diagnosed with shivers will likely experience worsening symptoms over time, though the rate of progression varies considerably between individuals. Some horses remain mildly affected for years, while others progress to severe disability relatively quickly. Early recognition allows implementation of management changes that may help maintain function for longer periods.

The exact cause of shivers remains incompletely understood despite extensive research efforts. Current evidence suggests the condition involves dysfunction of the cerebellum, the brain region responsible for coordinating movement, muscle tone, and balance. Pathological studies have identified abnormal accumulations of glycogen and other storage materials in neurons of the cerebellum and brainstem in affected horses. This neuronal storage disorder disrupts normal nerve function and likely explains the characteristic movement abnormalities. The underlying trigger for this abnormal storage is not definitively established but may involve genetic factors affecting cellular metabolism.

A genetic component to shivers is strongly suspected based on breed predisposition and familial clustering of cases. Draft breeds and warmbloods carry significantly higher risk than other breeds, suggesting inherited susceptibility. Some families within affected breeds show multiple affected individuals across generations. However, the specific mode of inheritance has not been determined, and no genetic test currently exists for shivers. The condition likely involves complex inheritance with multiple genetic and environmental factors contributing to disease expression. Not all horses with genetic susceptibility necessarily develop clinical signs.

Environmental and management factors may influence disease expression and progression, though their role is not fully characterized. Some evidence suggests that nutritional factors, particularly high-starch diets, may exacerbate symptoms in some affected horses. Horses maintained on lower starch, higher fat diets may show reduced symptom severity. Stress, whether from training demands, transportation, or environmental changes, appears to worsen signs in some individuals. Cold weather has been reported to increase symptom severity in certain horses. The interplay between inherent susceptibility and environmental triggers remains an area of active investigation.

Risk factors for developing shivers include breed, height, sex, and possibly dietary and management factors. Draft breeds face the highest risk, with estimates suggesting five to ten percent of some draft populations are affected. Warmbloods, particularly those bred for dressage with draft breed influence in their ancestry, show intermediate risk. Horses over sixteen hands have higher prevalence than smaller horses. Males appear to be affected more frequently than females, possibly due to hormonal influences or sex-linked genetic factors. Horses in intensive training programs may show earlier clinical onset or faster progression than those in lighter work.

The pathophysiology of shivers involves dysfunction of cerebellar and brainstem circuits that normally coordinate smooth, controlled movement. Abnormal storage of polysaccharide material within neurons impairs their function and eventually causes cell death. The cerebellum plays a critical role in modulating muscle tone and ensuring movements are smooth and coordinated. When cerebellar function is compromised, the characteristic hyperflexion and tremoring movements result from loss of normal inhibitory control over reflexes. The progressive nature of the condition reflects ongoing neuronal loss and accumulation of storage material. Similar pathology may occur in peripheral nerves, contributing to muscle atrophy seen in advanced cases.

Early warning signs of shivers can be subtle and easily dismissed as behavioral quirks or minor coordination problems. Owners may notice slight hesitation when the horse is asked to back up or a momentary catching of the hindlimb when picking up feet for farrier work. The horse may seem reluctant to hold a hindfoot up for cleaning or shoeing, putting the foot down quickly or shifting weight repeatedly. Mild trembling of the hindlimbs when the horse is stressed, cold, or excited may be an early indicator. These initial signs are often intermittent and may not be present at every observation, leading to delayed recognition of the underlying condition.

The hallmark symptoms of shivers involve characteristic abnormal movements of the hindlimbs that are immediately recognizable once the condition has progressed. When asked to back up, the affected horse involuntarily snatches one or both hindlimbs upward and outward in an exaggerated hyperflexion movement. The limb may be held suspended in this elevated position, often with the hoof held well away from the ground and rotated outward. Visible trembling or shaking of the suspended limb gives the condition its name. The horse cannot voluntarily lower the limb immediately, and it slowly descends to the ground after several seconds. Forward movement typically remains normal or near-normal, especially early in the disease.

Behavioral changes accompany the movement abnormalities as horses learn to anticipate and attempt to avoid triggering activities. Affected horses may become resistant to picking up hindlimbs for cleaning or shoeing, recognizing that this action triggers the involuntary spasms. Reluctance to back up develops, with horses planting their feet or swinging the hindquarters sideways instead of stepping backward. Some horses show anxiety or distress when forced into triggering situations. In advanced cases, horses may become difficult to handle around the hindquarters due to anticipation of the uncomfortable episodes.

Physical signs beyond the characteristic movement abnormality develop as shivers progresses. Muscle atrophy in the hindquarters becomes visible as the gluteal and hamstring muscles lose mass due to disuse and possible neurogenic factors. The topline may appear uneven with the hindquarters showing reduced muscle definition compared to the forehand. The tail may be held elevated or clamped, and some horses show trembling or spasms of tail muscles. Gait abnormalities including a shortened stride behind, difficulty with lateral movements, and abnormal limb placement become more apparent. The hindlimbs may show signs of chronic overextension or hyperflexion at rest.

Symptom progression in shivers is typically gradual but relentless. Early stages may show symptoms only during specific activities like backing or foot handling. Intermediate stages see increased frequency of episodes, involvement of both hindlimbs, and symptoms triggered by a wider range of activities. Advanced stages bring difficulty with forward movement, inability to back at all, severe muscle atrophy, and problems rising from recumbency. Some horses reach a plateau and stabilize for periods before further progression occurs. The rate of progression varies considerably, with some horses showing rapid decline over one to two years while others remain mildly affected for many years.

Emergency symptoms in shivers, while rare, can occur when severely affected horses become unable to rise from recumbency. A horse that goes down and cannot get up requires immediate veterinary attention to prevent complications of prolonged recumbency including pressure sores, myopathy, and respiratory compromise. Additionally, horses with severe shivers may fall if hindlimb dysfunction is severe during movement, potentially causing traumatic injury. Any acute worsening of symptoms beyond the expected gradual progression warrants veterinary evaluation to rule out concurrent conditions.

Physical examination and observation form the cornerstone of shivers diagnosis, as the characteristic clinical signs are highly recognizable to experienced clinicians. The examiner observes the horse at rest, noting hindlimb posture and any spontaneous abnormal movements or trembling. The horse is then walked forward, observing for gait abnormalities that may be subtle or absent in early cases. Crucially, the horse is asked to back up, which typically elicits the characteristic hyperflexion and trembling if shivers is present. Picking up each hindlimb for examination assesses whether this manipulation triggers the involuntary holding and trembling. The tail is observed for abnormal elevation or spasms.

Diagnostic tests for shivers are primarily used to rule out other conditions rather than to confirm the diagnosis definitively. No blood test, imaging study, or other laboratory test can diagnose shivers in a living horse. Complete blood count and serum chemistry may be performed to assess overall health and rule out metabolic conditions. Selenium and vitamin E levels may be evaluated to exclude nutritional myopathy. Electromyography may show abnormalities in hindlimb muscles but findings are not specific for shivers. The diagnosis relies on the characteristic clinical presentation and exclusion of other conditions.

Advanced diagnostics have limited utility for diagnosing shivers but may help exclude other conditions or provide research information. Muscle biopsy may show nonspecific changes including fiber size variation and possible glycogen accumulation but is not routinely performed diagnostically. Spinal radiographs or ultrasound may be considered if vertebral pathology is suspected as a differential. Magnetic resonance imaging of the cerebellum could theoretically identify changes but is not practical for most equine patients. Postmortem examination of the brain and spinal cord provides definitive diagnosis by identifying the characteristic neuronal storage changes, but this obviously precludes treatment.

Differential diagnosis for horses presenting with hindlimb abnormalities similar to shivers includes several conditions requiring exclusion. Stringhalt causes hyperflexion of the hindlimb during forward movement rather than the backing-triggered episodes of shivers, though both conditions can be present simultaneously. Equine polysaccharide storage myopathy causes hindlimb stiffness and abnormal gait that may superficially resemble shivers but has different triggers and can be diagnosed by muscle biopsy and genetic testing. Fibrotic myopathy causes mechanical restriction of hindlimb movement with characteristic slapping gait. Upward fixation of the patella causes the limb to catch in extension. Spinal cord disease from various causes can create hindlimb dysfunction. Careful observation of symptom patterns and response to specific triggers distinguishes these conditions.

No curative treatment exists for shivers, as the underlying neuronal damage cannot be reversed with current medical capabilities. All management strategies focus on slowing disease progression, reducing symptom severity, and maintaining quality of life for as long as possible. Owners must understand from the outset that shivers is a progressive condition and that even with optimal management, most affected horses will experience worsening symptoms over time. This understanding allows appropriate planning for the horse's future care and use.

Medical management of shivers relies primarily on dietary modifications that some practitioners believe reduce symptom severity. Reducing dietary starch and sugar by eliminating grain-based concentrates and limiting access to lush pasture may help some horses. Replacing carbohydrate calories with fat supplementation in the form of vegetable oil, rice bran, or commercial fat supplements provides energy without the potentially exacerbating effects of starch. High-quality forage should form the diet foundation. Some horses are supplemented with vitamin E at levels above normal requirements due to its antioxidant properties and role in neuromuscular function, though evidence for efficacy is limited.

No surgical treatments are available for shivers itself. Surgical interventions may occasionally be considered for secondary problems, such as treatment of injuries sustained due to coordination deficits. In cases where upward fixation of the patella occurs concurrently, medial patellar ligament desmotomy may improve that component of the hindlimb dysfunction, though it does not address the shivers. Neurological surgery targeting the underlying brain pathology is not feasible with current technology.

Supportive care and management modifications help maintain function and reduce triggering events. Keeping the horse in regular, light work may help preserve muscle mass and coordination longer than complete rest. Farrier work should be adapted to minimize stress, with frequent shorter sessions preferred over infrequent lengthy sessions. Some farriers use supports or stocks to reduce the time the horse must hold up each leg. Avoiding cold and stress may reduce symptom expression in some individuals. Regular turnout on even footing allows natural movement patterns.

Rehabilitation approaches for shivers are limited by the progressive neurological nature of the condition. Physical therapy and targeted exercises have not been shown to reverse the underlying damage. However, maintaining overall fitness and muscle mass through appropriate exercise may help the horse compensate for deficits longer. Stretching exercises may help with any secondary muscle tightness. Careful conditioning that avoids overstressing the compromised neuromuscular system balances the benefits of exercise against potential risks.

Treatment decisions for horses with shivers must consider the progressive nature of the condition and the horse's current and future quality of life. Mild cases may continue in work for years with appropriate management. Moderate cases require evaluation of whether continued use is safe for horse and handler. Severe cases where the horse cannot be safely handled for necessary care, cannot rise from recumbency, or shows clear suffering warrant consideration of humane euthanasia. The decision to euthanize is difficult but represents a responsible choice when quality of life cannot be maintained.

Recovery in the conventional sense is not applicable to shivers, as the condition has no cure and is invariably progressive. Unlike injuries or acute illnesses that heal, shivers represents ongoing neurological degeneration that current medicine cannot halt or reverse. Owners hoping for recovery must instead adjust their expectations toward management and optimization of the horse's remaining functional capacity. The goal becomes maximizing quality of life and useful function for as long as possible rather than returning to a pre-disease state.

Post-diagnosis care and monitoring focus on tracking disease progression and adjusting management accordingly. Regular evaluation of the horse's ability to perform required tasks such as backing, hoof handling, rising from recumbency, and general locomotion provides objective assessment of status. Video documentation at regular intervals creates a record of progression that helps identify subtle changes. Veterinary reassessment should occur at least annually or whenever significant changes in symptoms are noted. Body condition and muscle mass should be monitored, with intervention if significant loss occurs.

Prognosis factors for horses with shivers include age at onset, rate of early progression, and severity at diagnosis. Horses with mild symptoms and slow initial progression may remain functional for many years. Those with rapid worsening in the first year after diagnosis often continue to progress quickly. The specific anatomical distribution of neuronal damage likely influences individual prognosis but cannot be assessed in living horses. Larger horses and draft breeds may have somewhat faster progression than smaller affected horses. Response to dietary and management modifications provides some prognostic information, with horses that improve on low-starch diets potentially having better intermediate-term outlook.

Long-term outlook for horses with shivers requires honest assessment and realistic planning. Most affected horses will eventually reach a point where they cannot be safely maintained or their quality of life is unacceptable. This progression may take two to ten or more years depending on the individual. Owners should plan for the eventual likelihood of euthanasia while focusing on maintaining quality of life in the interim. Some horses stabilize for extended periods before further decline. Mildly affected horses may live out normal lifespans, dying of unrelated causes before shivers becomes severely disabling.

Prevention of shivers is currently not possible because the underlying cause is not fully understood and involves suspected genetic factors that cannot be avoided. Unlike conditions caused by infectious agents, toxins, or injuries, shivers appears to develop from inherent susceptibility that expresses clinically in affected individuals. No management practice, vaccination, or intervention has been demonstrated to prevent the condition from developing in susceptible horses. This reality emphasizes the importance of breeding decisions and pre-purchase evaluation.

Nutritional considerations may influence disease expression but have not been proven to prevent shivers in susceptible individuals. Maintaining horses on lower starch diets from an early age might theoretically reduce the likelihood of triggering or accelerating the condition, but this has not been demonstrated in controlled studies. Ensuring adequate vitamin E and selenium throughout life supports neuromuscular health generally. Avoiding nutritional extremes and maintaining consistent, high-quality feeding practices represent reasonable approaches for all horses regardless of shivers risk.

Exercise and conditioning cannot prevent shivers but appropriate management of young horses from high-risk breeds may optimize their neuromuscular development. Avoiding overtraining and ensuring adequate recovery between intense work sessions protects the neuromuscular system from excessive stress. Maintaining horses in regular work rather than prolonged periods of inactivity keeps the neuromuscular system functioning optimally. These practices benefit all horses and may support later function if shivers does develop.

Environmental factors including stress reduction and climate considerations may be relevant though not proven preventive measures. Reducing chronic stress from herd dynamics, frequent transportation, or inconsistent management may theoretically benefit susceptible horses. Providing appropriate shelter from temperature extremes may reduce the cold-related symptom exacerbation some horses experience. These measures represent good management practice regardless of shivers concerns.

Breeding recommendations offer the most meaningful prevention strategy given the suspected hereditary nature of shivers. Horses diagnosed with shivers should not be used for breeding to avoid perpetuating genetic susceptibility in the population. Draft breed and warmblood breeders should be aware of shivers prevalence in their breeding programs and avoid concentrating affected bloodlines. Pre-purchase examination of prospective breeding stock should specifically assess for shivers signs. Buyers of breeds at elevated risk should ensure thorough pre-purchase evaluation including backing assessment and hoof handling observation.

Daily management adjustments for horses with shivers focus on reducing triggering situations while maintaining necessary care. Hoof care represents one of the greatest challenges, as affected horses struggle to hold hindlimbs up for picking and trimming. Developing techniques that minimize the duration of foot elevation helps, such as quickly cleaning each hoof rather than holding it up for extended inspection. Farrier visits should be scheduled when the horse is calm and conditions are optimal. Some horses tolerate front leg work better when hindlimbs are addressed first while the horse is fresh. Daily handling should be calm and consistent to minimize stress-related symptom exacerbation.

Housing and turnout considerations for horses with shivers prioritize safety and ease of movement. Turnout on level, well-maintained footing reduces the risk of falls due to hindlimb dysfunction. Avoiding muddy or uneven ground protects horses with compromised coordination. Adequate shelter protects against cold that may worsen symptoms in some individuals. Stall design should allow the horse to turn around without backing, and doors should be wide enough for entry and exit without requiring the horse to back up. Herd dynamics should be monitored to ensure the affected horse can move away from aggressive herd mates despite coordination challenges.

Exercise modifications balance the benefits of maintained fitness against the risks posed by neuromuscular dysfunction. Light, regular exercise may help maintain muscle mass and coordination longer than complete inactivity. Forward movement typically remains more normal than backing or complex movements, so riding or driving that emphasizes forward work may continue in mildly affected horses. Disciplines requiring collection, lateral work, or backing become progressively more difficult as the condition advances. Trail riding over uneven terrain poses risks if the horse cannot reliably navigate obstacles. Exercise intensity should be reduced if fatigue worsens symptoms.

Monitoring and ongoing care requirements include regular assessment of disease progression and adjustment of management accordingly. Owners should learn to recognize their individual horse's triggering factors and symptom patterns. Weight and muscle mass should be monitored, with dietary adjustments if significant changes occur. Hoof care schedules may need modification if the horse cannot tolerate normal farrier intervals. Secondary musculoskeletal problems from compensatory movement patterns should be identified and addressed. Communication with veterinary and farrier professionals about the horse's condition ensures appropriate care.

Quality of life and use considerations require ongoing reassessment as shivers progresses. Mildly affected horses may continue many activities with minor modifications. Moderately affected horses may need to transition from athletic work to light riding or companion status. Severely affected horses may reach a point where necessary care such as hoof trimming becomes impossible, where the horse cannot safely navigate its environment, or where other indicators suggest poor quality of life. Euthanasia should be considered when the horse's condition precludes humane management rather than waiting until an emergency forces the decision.

Draft horse breeds face the highest documented risk for shivers, with prevalence estimates reaching five to ten percent in some populations. Belgian Draft Horses, Percherons, Clydesdales, and Shire horses all show elevated incidence compared to the general horse population. The massive size and height of these breeds may contribute to their susceptibility, as larger neurological systems may be more vulnerable to the degenerative process. Within draft breeds, certain bloodlines may show higher incidence than others, though comprehensive genetic studies have not been completed. Prospective buyers of draft horses should specifically evaluate for shivers signs during pre-purchase examination.

Warmbloods and sport horses, particularly those with draft breed influence in their ancestry, show intermediate risk for shivers. Breeds developed by crossing draft horses with lighter breeds, such as many European warmblood registries, may have inherited susceptibility from their draft ancestors. Horses bred for dressage, which requires significant hindlimb engagement and collection, may show clinical signs earlier due to the demands of training, or affected horses may be identified more readily due to the precision required of the work. The increasing prevalence of very tall warmbloods may be associated with increased shivers incidence in some populations.

Genetic testing for shivers is not currently available, but breeding recommendations can reduce disease prevalence within affected breeds. Horses confirmed to have shivers should not be bred, as the condition appears to have hereditary components. Breeding stock from breeds at elevated risk should undergo thorough evaluation specifically assessing for shivers signs. Pedigree research identifying affected relatives provides additional risk information. Breed registries and breeding cooperatives should consider tracking shivers incidence and discouraging breeding of affected individuals. Research to identify genetic markers for shivers susceptibility would allow more targeted breeding decisions and eventually enable pre-breeding genetic testing.

Commonly co-occurring conditions with shivers include other movement disorders and musculoskeletal problems. Stringhalt, another condition causing abnormal hindlimb hyperflexion, has been reported to occur concurrently with shivers in some horses, though the conditions have distinct characteristics and triggers. Equine polysaccharide storage myopathy shares some demographic risk factors with shivers, affecting draft breeds and involving glycogen metabolism abnormalities, and both conditions may occur in the same horse. Secondary musculoskeletal problems including hock arthritis and back pain may develop from compensatory movement patterns. Muscle atrophy creates additional challenges for horses already struggling with coordination.

Conditions with similar symptoms to shivers require careful differentiation to ensure appropriate management. Stringhalt causes exaggerated hindlimb hyperflexion during forward movement, typically triggered by walking rather than the backing and hoof-lifting triggers of shivers. Australian stringhalt occurs seasonally related to pasture toxins and may be reversible, unlike shivers. Upward fixation of the patella causes the hindlimb to lock in extension before suddenly releasing with a characteristic snap. Fibrotic myopathy creates a mechanical restriction of hindlimb motion with distinctive slapping gait. Equine protozoal myeloencephalitis can cause hindlimb dysfunction but typically shows asymmetric weakness and ataxia rather than hyperflexion episodes.

Potential complications of shivers extend beyond the primary condition to include consequences of the progressive disability. Injuries from falls occur when hindlimb dysfunction causes the horse to lose balance. Inability to receive proper hoof care leads to overgrown hooves, abnormal wear patterns, and secondary lameness. Muscle atrophy progresses as the horse becomes less able to exercise and as neurogenic factors cause muscle wasting. Pressure sores and myopathy may develop in horses that cannot rise from recumbency. Psychological effects including anxiety and frustration may develop as horses experience loss of control over their movements.