By: Dr. Lydia Gray, SmartPak Medical Director/Staff VeterinarianUpdated 2/27/2020
The term “shivers” or “shivering” is used to describe a neuromuscular condition in horses that has been recognized for centuries. Thanks to a breakthrough in ongoing research, veterinarians and horse owners now have a better understanding of this syndrome, including the age, gender, and type of horse it is more commonly seen in.
This article will cover the most up-to-date information about Shivers as the condition is currently understood. Included are classic signs and symptoms; an explanation of how the disease process leads to these clinical signs; other conditions of the hind limbs that can be hard to distinguish from Shivers; the diagnosis, treatment, and prognosis of the condition; and what, if anything, can be done in terms of nutrition, environment, and exercise in an attempt to slow its progression.
Based on the latest research, Shivers is now defined as a chronic, often gradually progressive “movement disorder” affecting the ability to walk backwards that usually begins before seven years of age with a greater prevalence in tall, male horses. It is characterized by one of two types of gait abnormality when asked to back up, the classic hyperflexion (overflexion or bent leg) and the less common hyperextension (overextension or straight leg):
To be diagnosed as true Shivers, the horse must show an abnormality in the gait when walking backwards. Simply raising a hind leg and holding it in the air in a bent or flexed position (called “standing hyperflexion”) is no longer sufficient evidence for a diagnosis of Shivers to be made because there are other reasons a horse may display this action. In addition to demonstrating a gait abnormality when asked to back, in advanced cases of Shivers horses may also hyperflex when turned sharply or during the first few forward steps. Signs of Shivers may also occur in the front legs, neck, trunk, or even face, but this is rare.
Findings from an online survey that included video submission by owners of horses with movement disorders was published in 2015 and significantly advanced the understanding of this condition.
Even though height, gender, and even breed seem to be closely associated with Shivers in horses, it is not clear whether the condition is inherited or not. At this time, no specific genetic pattern has been identified and there is no genetic test for Shivers. However, the team of researchers led by Dr. Stephanie Valberg in the Equine Neuromuscular Diagnostic Laboratory at Michigan State University’s College of Veterinary Medicine is testing the theory that Shivers is due to a dysregulation of specific genes in the horse.
Although no underlying cause of Shivers has yet been discovered, experts are now able to point to a specific area in the brain that has become damaged and is therefore not functioning normally. Through a combination of brain, spinal cord, and peripheral nerve tissue analysis; muscle activity measurement; and owner-submitted video and survey answers, scientists have identified that the brains of horses with Shivers are damaged in a specific area of the cerebellum.
As background, the cerebrum is the largest part of the brain and handles higher functions such as thought, emotions, and the five senses: sight, sound, touch, taste, and smell. The cerebellum or “little brain” is responsible for coordination, posture, and balance. Basically, the cerebrum forms the plan and the cerebellum carries the plan out.
The team at Michigan State could find no differences between the muscle tissue, peripheral nerves, or spinal cord of horses that showed signs of Shivers and horses that did not. However, when they looked at brain tissue under the microscope, they found that a significant number of a type of nerve cell within the cerebellum known as a Purkinje cell was damaged in affected horses. In fact, horses with Shivers had 80 times more degeneration in these cells than normal horses of the same age. Since the function of Purkinje cells is to communicate with other nerve cells, telling them what to do, if these cells are damaged then the plan for movement cannot be communicated or carried out well.
Electromyography (EMG) and motion analysis of affected and normal horses supports the theory that cerebellar nerve cell damage leads to the clinical signs of Shivers. EMG electrodes were placed on the hind leg muscles of horses to measure the timing and strength of flexor and extensor muscle activity. When normal horses were asked to back, these two opposing sets of muscle groups – flexors and extensors – worked together in harmony to cleanly and crisply pick up and set down diagonal pairs of legs in order: left, right, left, right. However, when the horses with shivers were asked to back, researchers discovered there was little coordination between the flexors and extensors. Not only did the two opposing muscle groups often fire at the same time, causing muscle spasm, there didn’t seem to be an “off switch” that told muscles when to relax or stop contracting. The result was jerky, uncoordinated muscle contractions that didn’t appear to be under the control of the cerebellum.
This data fits with what was seen in the owner-submitted videos and online survey responses. In these clips, it seemed as if the horses affected by Shivers knew exactly what they wanted to do with their feet and legs – that is, they had a plan for where they wanted to go – they just couldn’t physically do it. The investigators also noted that horses with Shivers appeared confused and frustrated more than painful or weak. Survey responses also confirmed that, despite difficulty backing up or lifting a hind leg for the farrier, horses with Shivers are able to excel athletically, often reaching high levels of competition. Slow, learned, less natural movements such as backing or lifting a hind leg on command are most likely controlled by a different path in the cerebellum than the faster, natural, forward gaits like trotting and cantering.
With a new, clear definition of Shivers (hyperflexion or the less common hyperextension upon backing), diagnosing this condition in horses would seem simple and straightforward. However, in the early stages of the disorder signs may be mild as well as intermittent. Also, an early case of Shivers may be difficult to distinguish from lameness, neurological disease, or early cases of other hindlimb disorders, such as Stringhalt.
Therefore, it is important that the owner carefully document observations while the veterinarian performs a complete physical examination that includes a lameness evaluation and possibly other diagnostic testing like bloodwork, imaging, and biopsy. By noting when episodes occur, what happened just before the horse showed signs, and if anything changed recently in the horse’s routine such as diet, exercise, housing, or stress (e.g. trailering, competition, a new horse, or changing barns), a pattern may emerge that, when combined with veterinary testing, rules in Shivers or rules out other conditions.
Keep in mind that owner-recorded video can be a very helpful diagnostic tool when it comes to movement disorders in horses. This is especially true when the gait abnormality cannot be reproduced reliably or is so subtle that it resembles other conditions affecting the hindlegs. In addition to Stringhalt, some of these conditions include:
Shivers tends to be most often confused with Stringhalt, which is a movement disorder that can affect any breed, gender, or age of horse. The characteristic sign of Stringhalt occurs when the horse moves and involves the hind limb being quickly snapped up towards the belly then just as quickly slapped back down to the ground, in an equine version of “goose stepping” or the formal soldier march. Another term used to describe the Stringhalt horse – especially in the faster gaits – is “bunny hopping.” To the trained eye, what distinguishes it from Shivers is that Stringhalt may occur when backing, turning, walking forward, and even trotting forward, and it does not stop after a few steps but continues with each stride. In addition, horses with Stringhalt flex their hind leg under the belly while horses with Shivers flex their hind leg out to the side.
There doesn’t seem to be difficulty telling Shivers apart from PSSM (Polysaccharide Storage Myopathy), but the question does come up: are they related? The answer is: while there can be an overlap between horses affected by Shivers and horses affected by PSSM, there does not appear to be a direct correlation between the two conditions meaning they are separate issues (with Shivers being associated with the cerebellum and PSSM being associated with abnormal muscle tissue).
Unfortunately, there is no universally effective treatment, medication, or cure for Shivers. In general, things like NSAIDs (non-steroidal anti-inflammatory drugs), muscle relaxants, body work (acupuncture, chiropractic, massage), and supplements containing vitamins, minerals, amino acids, or herbs have not shown consistent, reliable improvement of the condition across horses, although they may help relieve discomfort in the short-term. Instead, veterinarians focus on managing the signs through a combination of nutrition, exercise and turnout, and special considerations designed to make life more comfortable for horses with Shivers and the people that provide care for them, such as farriers.
It was thought at one time that the same high fat/low NSC (non-structural carbohydrate) diet that is recommended for horses with PSSM might also be beneficial for horses with Shivers. However, increasing levels of fat in the diet while reducing sugars and starches only improved those horses that also had PSSM. It made no difference in the signs of Shivers or in horses that only suffered from Shivers.
One ingredient that experts believe could play an important role is Vitamin E, an antioxidant that specifically targets oxidative stress in nerve and muscle tissue. Now that research shows Shivers is associated with degeneration of brain tissue over time, it may be more critical than ever to ensure that horses – especially tall, male horses – have adequate levels of this free radical scavenger.
Feedback from owners of horses with Shivers has indicated that the more they are kept fit and active, the better they do. Giving these horses a break from work such as over the winter or after the competition season, or stalling because of a separate illness or injury, can make the signs of Shivers worse and lead to muscle atrophy. It has also been noted that it can be harder to bring these horses back into shape from time off. Therefore, experts recommend keeping them in a regular fitness program year-round with plenty of turnout and limited stalling so they are always moving around. In fact, a pasture housing situation may be more appropriate for a horse with Shivers than traditional stabling.
Stress and excitement seem to worsen the signs of Shivers temporarily. For example, trailering or cold weather may aggravate the condition, but also just something as simple as leading from one type of footing to another, offering a pail of water from the ground, or stepping over an obstacle may trigger an episode. Experts advise learning what excites or triggers each horse and avoiding or minimizing exposure.
One thing that cannot be avoided is regular hoof care, and it can be challenging to pick up and hold for any length of time a hind limb that is affected by Shivers. By working closely together, teams of owners, farriers, and veterinarians have come up with some creative ways to trim and shoe these horses:
When it comes to Shivers, the majority of horses (60-74%, according to the latest owner survey) do progressively get worse over time, both in terms of the classical signs of Shivers and a decline in performance. The decline in performance is most likely due to weakness caused by a gradual atrophy (wasting) of the thigh muscles, which may eventually become generalized or body-wide muscle wasting.
So far, it has not been possible to predict which horses with Shivers will get worse or how quickly, and euthanasia may be necessary for humane reasons. The good news is that research on this condition has made many advances in the last few years and with an understanding of what causes clinical signs (cerebellar tissue degeneration) may come an understanding of what could possibly prevent or treat those signs.
SmartPak strongly encourages you to consult your veterinarian regarding specific questions about your horse's health. This information is not intended to diagnose or treat any disease, and is purely educational.Article First Published June 2012