Q: What is shivers? What is it caused by and how does it affect sporthorses? What are the physical limitations, side effects and the affect on performance or resale value?
Shivers is an ill-defined movement disorder of horses. The classic presentation is a horse who has difficulty coordinating his hind limbs when asked to back up and in doing so will rapidly pick up a hind leg in an exaggerated fashion. Affected horses typically hold the leg in the air for several seconds to a minute before rapidly placing it back on the ground. The limb is usually held in extreme flexion through the hock and stifle, though some horses will instead hyperextend the limb. Many shivers-affected horses will not only overflex the elevated limb but hold their hind limb away from their body (abduct), and their muscles frequently tremble while holding the leg up. Some horses will quiver their tail or hold their tail extended while simultaneously holding their limb in the air. There is also a segment of horses for which the presenting sign is reluctance or inability to have the leg picked up by a handler. While the abnormal hind limb movement is most often demonstrated when the horse is asked to back up, some horses will hyperflex a hind limb in the first few strides of forward walking, especially after standing in place for a period of time, for example in the cross-ties.
Shivers most often affects warmbloods and draft breeds, though it has been reported in Thoroughbreds and other light breeds. It is suspected to have a genetic component.
Frustratingly, the exact cause of this movement disorder remains unknown. A recent investigation was able to identify degeneration of a specialized nerve cell type (Purkinje fibers) in a portion of the brain (the cerebellum) of horses affected with shivers that was not present in normal horses, but there is still no explanation for why this happens. Unfortunately, there is also no specific diagnostic test to confirm a horse has shivers, and so it becomes a diagnosis of exclusion. A veterinarian first has to eliminate lameness or a neurological condition as the cause of the gait abnormality. Only after those possibilities have been eliminated and the movement closely compared to other atypical limb movements (such as stringhalt, fibrotic myopathy or a locking stifle) can the label of shivers be applied. It can be hard to differentiate shivers from other movement disorders that affect horses, especially stringhalt. One distinction is whether the abnormal hind limb movement continues at the trot. Shivers uniquely only affects horses at the walk, while stringhalt will also affect horses at the trot.
The biggest hardship for an owner of a horse with shivers is that their exaggerated hind-limb movement can make it difficult for a farrier to effectively trim the hoof and shoe the horse. Many horses can maintain successful athletic careers as show jumpers and dressage horses, especially in the early stages of the disease when they are only mildly affected. However, the disorder is progressive and owners can expect the horse’s movement to worsen over his lifetime. Some horses show only mild signs for years, yet others experience rapid progression and deteriorate to the point that athletic use is compromised. This timeline is variable and unpredictable, which presents a challenge at purchase or resale. Unfortunately, there is no treatment for shivers. Since many shivers horses, especially draft breeds, are also affected with polysaccharide storage myopathy, a metabolic disease, switching to a high-fat and low-starch ration may delay progression and help reduce clinical signs.
With no diagnostic test or effective therapy, shivers remains a frustrating condition. While many horses can cope with milder versions, it generally progresses over the horse’s lifetime to involve both hind limbs and even the fore limbs and can ultimately compromise his athletic ability.
Christina “Cricket” Russillo, DVM, graduated from the Tufts University School of Veterinary Medicine in 2001. After completing a large animal medicine and surgery internship at Texas A&M, she realized her desire was to work on elite sporthorses. Following 13 years of practice at Fairfield Equine Associates in Newtown, Connecticut, focused on high-level show-jumping and dressage horses, she joined Virginia Equine Imaging in 2015. Russillo relocates to Florida every winter to support her clients and patients. She has competed through Third Level in dressage and in February 2017 she was appointed the U.S. Dressage Team veterinarian. She is also a certified member of the International Society of Equine Locomotor Pathology.
This article originally appeared in the Winter 2020 issue.