Shivers, or shivering, in horses is a progressive neuromuscular condition characterized by an abnormal, jerky gait while backing up. 
Horses affected by Shivers exhibit muscle tremors in the pelvic limbs when forced to back up. Clinical signs typically appear in horses between two and five years old and gradually worsen. 
Shivers is more common in taller breeds, Warmbloods, and draft horses and affects geldings three times more frequently than mares. 
There is no effective long-term treatment for Shivers, but many affected horses can continue participating in certain disciplines for several years following diagnosis. As the condition progresses, horse owners may choose euthanasia to prevent injury or suffering.
Shivers in Horses
Equine Shivers is a chronic neuromuscular syndrome that results in a progressive loss of hindlimb muscle control during backward walking. 
The pelvic limbs may appear to shiver or tremble when attempting to back up. Shivers rarely affects forward movement until later in disease progression. 
The cause of Shivers is still not well understood, but research shows that affected horses have nerve damage in a specific area of the brain known as the cerebellum. Neurons in this part of the brain are involved in modulating learned muscle movements, including backing up and lifting the hindlegs. 
In 74% of cases, symptoms worsen over time. As the disease progresses, increased severity symptoms may lead to the premature retirement of the horse. 
If you think your horse is affected by Shivers or another neuromuscular condition, contact your veterinarian as soon as possible to schedule an examination and receive a diagnosis.
While there is no cure or treatment for this condition, early diagnosis and supportive care can make affected horses more comfortable and may help slow symptom progression.
The prevalence of Shivers is unknown, but it is considered a rare disease. Tall horses (over 16.3 hands) have a higher risk than shorter equines, such as ponies. 
Draft horses and large breeds, including Warmbloods or Thoroughbreds and their crosses, are more commonly affected by Shivers than lighter horses. The condition has also been reported in other breeds, such as the Quarter Horse and Morgans. 
In one breed survey, 19% of Belgian horses were reported to have Shivers. 
Both male and female horses can be affected, but geldings are three times more likely to be diagnosed with Shivers than mares. It is not known why male horses have a higher risk.
There is no genetic testing for Shivers available.
Signs of Shivers in Horses
Shivers tends to develop gradually, but symptom onset can vary between individual horses. The majority of affected horses show signs of the disease by the age of seven, but some are symptomatic as young as one or two. 
Signs often become more apparent after the horse has been rested or stalled for a period of time or when walking in tight circles.  Following periods of stress or excitement, abnormal hindlimb movement may be exaggerated. 
In most cases, Shivers affects both hind limbs. 
Mild Clinical Signs
In mild cases, the hindlimbs of the horse may become tense and tremble when backed up.  Horses may be resistant to having their hindlimbs lifted, making it difficult to provide farrier care. 
Sudden jerking movements of the tail cause it to become erect and elevated. Some affected horses may display quivering ears and eyelids, as well as wobbling of the head when elevated.  Horses may also experience excessive sweating. 
In the early stages, signs of Shivers may be intermittent or occasional, making diagnosis difficult. 
Severe Clinical Signs
In severe cases, individual hind limbs may raise and extend from the body when the horse is backed or moved suddenly to the side. The limb may spasm in the air for a few seconds to several minutes. The thigh and flank muscles quiver while the leg is suspended. 
When backed up, the horse may also extend its hind limbs into a stretched-out or saw-horse stance, placing the feet further back than normal. This hyperextension of the hock and stifle can cause instability if the horse cannot regain control of its movements.
Following a shivering episode, the leg extends outwards away from the body, and the foot drops to the ground. Other signs, such as an erect tail or leg spasms, subside after a few minutes. If the horse is backed up again, clinical signs will likely return.
As the disease progresses, hyperflexion and hyperextension of the hind limbs may become apparent in forward movement. Muscle atrophy may occur in the thigh and surrounding area, leading to progressive weakness in the hind end. 
Affected horses may fall or stumble when sleeping, leading to bruising on the fore fetlocks and knees. Horses may widen their natural stance for stability.
While the exact cause of Shivers is unknown, it is thought to have a genetic or hereditary component due to the strong breed predisposition.
Other suggested causes include: 
- Neurological damage
- Infectious disease (influenza, strangles, etc.)
Horses with Shivers may excel at forward movement but have difficulty lifting the hind limbs or backing up. Although it is not fully understood why Shivers only affects backwards locomotion, different neural pathways regulate these slower learned movements than those involved in the natural fast movement of forward gaits.
This explains why some horses with Shivers can be successful in the show-ring yet struggle to lift their hindlimb for the farrier.
Damage to certain neural pathways may cause muscles to stay active when moving backwards. This over-activation of muscle fibres is believed to result in the jerking, spastic motions associated with Shivers. 
Horses with Shivers are commonly observed to have damage to specific regions of the cerebellum. This is part of the central nervous system (CNS) that regulates coordination, balance and movement.
A recent study discovered a link between Shivers and axonal degeneration in Purkinje cells. These are intricately branched nerve cells in the cerebellum that play a fundamental role in motor coordination. 
Purkinje cells are responsible for transmitting important signals between the horse’s cerebellum and the rest of the body. Electrical impulses are carried down axons of nerve cells (neurons), which are narrow projections that interface with other nerve cells at synapses.
In horses with Shivers, there are signs of degeneration in the distal axons of selective Purkinje cells.  Damaged axons cannot efficiently transmit signals to other nerve cells, potentially leading to impaired or abnormal movement.
Research shows that bubble-like formations called axonal spheroids begin to develop on damaged axons. Horses with Shivers often have significantly higher levels of spheroids compared to healthy horses. 
More research is needed to understand the link between damage to Purkinje cells in the cerebellum and the abnormal hindlimb movement seen in horses with Shivers.
If you suspect your horse has Shivers, a complete veterinary evaluation is recommended, including a neurological examination and lameness assessment. 
In the early stages, Shivers can be difficult to detect and diagnose due to the intermittent nature of symptoms. Repeated observations of the horse may be required to identify the problem. 
Your veterinarian will diagnose Shivers based on your horse’s health history and clinical signs and by ruling out other neurological conditions or causes of lameness (i.e. injury).
Diagnostic signs your veterinarian will look for include: 
- The horse’s inability to walk backwards
- Aversion to a handler lifting and holding the hindlimb in a flexed position (i.e. to pick the hoof)
- Hyperflexion of the leg in locomotion
- Atrophy of thigh muscles in advanced cases
Several other conditions affect hindlimb movement in the horse, which can make it challenging to determine the correct diagnosis for your horse.
In order to make an accurate diagnosis, your veterinarian will conduct a differential diagnosis to rule out other potential causes of muscle tremors or lameness.
Conditions with clinical signs that can mimic Shivers include: 
- Upward fixation of the patella
- Equine protozoal myeloencephalitis (EPM)
- Fibrotic myopathy
- Stiff-horse syndrome (SHS)
- Equine motor neuron disease (EMND)
Refusing to back up and stomping of the hindlimbs are common equine behaviours that may indicate irritation or frustration. Neurological conditions are rarely considered as a cause.
Shivers vs. Stringhalt
Shivers and Stringhalt are two neurological conditions that affect the horse’s movement, although stringhalt affects forward gaits. This condition makes every stride look abnormal, especially at a slower gait. It is usually more obvious and easier to diagnose than Shivers. 
Horses with Stringhalt tend to aggressively flex the hindlimb and kick up at the belly when in forward motion. The motion becomes more exaggerated as the disease progresses.
Shivers and Stringhalt can produce sporadic clinical signs, making diagnosis difficult in the early stages. These conditions may also present identically from certain angles when the horse is backed up.
You can distinguish between Shivers and Stringhalt by observing how the horse flexes the hindlimb in a spasmodic state. Horses with Shivers will usually flex the hindlimb outwards, away from the body, holding it in the air.
In comparison, horses with stringhalt tend to quickly flex the leg upwards and forwards when in motion before stomping it back on the ground.
There is currently no successful treatment or cure for Shivers in horses. The condition is degenerative, meaning it worsens over time regardless of any supportive care or treatment administered to the horse. 
However, in mild cases, affected horses can continue to participate in athletic disciplines or leisure activities for several years before needing to retire. 
Supportive nutrition, care and exercise can help to manage the condition so horses are kept as healthy and comfortable for as long as possible.
All horses need a balanced forage-based diet that provides adequate energy, protein, vitamins and minerals to meet nutritional requirements.
Avoiding nutrient deficiencies is especially important for horses affected by Shivers to support nervous system function and muscle health.
Horses with Shivers are anecdotally believed to do best on a high-fat, low-starch diet.  However, more research is needed to understand the effects of different nutritional interventions.
Horses with neuromuscular conditions often require Vitamin E and selenium supplementation to support muscle function. 
Horses experiencing muscle loss may also benefit from amino acid supplementation to support muscle protein synthesis.
Keeping horses in a consistent exercise routine can help reduce signs of Shivers while maintaining muscle mass.
Long periods of inactivity, such as stall confinement and long-distance trailering, should be avoided to prevent the horse from becoming tight in its hind end. 
Maximizing turnout and safely increasing exercise duration and intensity are top priorities when caring for a horse with Shivers. Sedentary behaviour can increase clinical signs and muscle wasting. 
Horses may also benefit from physiotherapy or acupuncture to support athletic performance and help maintain animal comfort. 
Shivers has a variable prognosis, with some horses developing minor symptoms and the condition remaining stable for years. These horses may be able to participate in steady work or even high-level competition with little impact on their motor control. 
However, other horses worsen quickly and develop progressive mobility issues, including an abnormal gait and difficulty lifting the hindlimbs. 
Horses involved in disciplines that require backward locomotion (i.e. dressage horses and driving horses) may need to retire sooner or pick a different discipline. 
Shivers is usually career-ending at some point in the horse’s life. Owners may choose to humanely euthanize their equine companion to prevent pain, suffering or injury.
It is impossible to tell which horses will be affected by Shivers, which makes the condition concerning for horse owners. Horses with a Shivers diagnosis should not be bred to avoid passing on genes that contribute to the disease.
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- Adair, S. et al. Neurology: shivering. Vetlexicon. Accessed April 25, 2023.
- Shivers. Michigan State University: College of Veterinary Medicine.
- Draper, A. C. E. Epidemiology of shivering (Shivers) in horses. Equine Vet J. 2014.
- Firshman, A. M. Prevalences and clinical signs of polysaccharide storage myopathy and Shivers in Belgian Draft Horses. Am Vet Med Assoc. 2005.
- Shivers, Stringhalt, and Australian Stringhalt. Kentucky Equine Research (KER). 2002.
- Baird, J. D. Shivers (Shivering) in the Horse: A Review. AAEP Proceedings. 2006.
- Aman, J. E. et al. Abnormal locomotor muscle recruitment activity is present in horses with shivering and Purkinje cell distal axonopathy. Equine Vet J. 2022.
- Yong, Y. et al. Axonal spheroids in neurodegeneration. Mol Cell Neurosci. 2021.
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