With some smart bloodlines in his breeding, Rebecca Friend’s young warmblood/Irish Sport Horse gelding has the potential for a successful competition career. Yet Rebecca’s hopes were put on hold last year when six-year-old Sam experienced difficulty when asked to move backwards.
“I first noticed the problem on the yard, when I pushed him back and saw him hold out a hindleg and shake it,” she says. “It quite surprised me. I bought Sam as a four-year-old and backed him myself — nothing untoward has happened to him and I’d never seen him do this before.”
When she realised that Sam might be a shiverer, alarm bells started ringing in Rebecca’s head.“I had hoped that he would one day replace my current competition horse,” she says, adding that her farrier had mentioned that Sam seemed “tight” behind when being shod.
“However, if he couldn’t perform rein-back, then he would surely have little future in dressage or high-level eventing.
“The shivers action was quite extreme and would upset him,” says Rebecca, describing how Sam would slide a hindleg out and hold it as if it was “locked”.
“I wondered if he had any future at all.”
While shivers has been recognised for many years, Alex Draper MRCVS, from The Queen’s Veterinary Hospital for Animals, University of Cambridge, explains that there is still confusion as to exactly what clinical signs are associated with the disease and, importantly, what causes it. The definition was standardised in 2015.
“Primarily, one or both hindlimbs are — often to differing degrees — picked up and held out to one side (termed hyperflexion and abduction), or are excessively rigid (termed hyperextension),” says Alex, adding that there is frequently tail elevation, twitching of facial muscles and ears, and hindlimb muscle trembling.
“Normal horses walk backwards with a two-beat gait: right front leg with left hind, then left front leg with right hind, but this pattern is lost in horses affected with shivers,” she says.
“In the majority of cases, the horse will move forwards with a normal gait in walk, trot, canter and gallop. Picking up his hind feet at standstill can be difficult, however, with either hyperflexion or hyperextension occurring. This often is the first sign the owner notices.
“A mildly affected horse may be reluctant to walk backwards and is typically unable to back up in a straight line, perhaps showing muscle tenseness or trembling over the hindlimbs and infrequent, sudden and excessive flexion or extension,” adds Alex. “A severely affected horse may be impossible to back up. He may struggle to lift his hind feet and may even have difficulty standing up after lying down.
“In rare and extreme cases, horses have shown hyperflexion and abduction of the hindlimbs when first walking forwards.”
The diagnosis of shivers is based entirely upon recognition of classical clinical signs; there is no blood test or imaging available.
“In addition, the exact cause has not been fully elucidated,” says Alex. “Degeneration of the cerebellum (the area of the brain that controls coordination and balance) and abnormal hindlimb muscle group recruitment have both been identified in affected horses, strongly suggesting this is a neurological disorder rather than a primary muscle disease. But further work is necessary.”
Alex explains that most cases are reported to occur with an insidious onset, with no apparent cause, in horses of five years old or more. Three-quarters of cases progress in severity.
“Polysaccharide storage myopathy (PSSM) was once claimed to cause shivers signs, but this has been refuted,” she adds. “Rarely, acute, severe signs are reported to occur in previously normal horses after extreme trauma, such as falling on ice, which abate with appropriate veterinary care and time.
“Male horses, particularly of 16.3hh or more, appear to be more commonly affected, although smaller horses and mares do develop the disease,” says Alex. “With height appearing to be a risk factor, breeds such as draught horses, warmbloods and thoroughbreds are most frequently represented. Due to this breed predilection, shivers has been thought to have a genetic predisposition, but no genetic test has yet been developed.”
Many horses hyperflex a hindlimb, often with excessive muscle twitching and typically during shoeing, so Alex stresses the importance of ruling out disorders that can produce the same clinical signs as shivers.
“Horses with hock pain or suspensory ligament inflammation have shown similar signs,” she says. “Remember, if a horse can walk backwards, it is highly unlikely he has shivers.”
Another neuromuscular condition commonly confused with shivers is stringhalt.
“Stringhalt can be idiopathic, occurring seemingly without any cause, or secondary to toxin ingestion,” explains Alex. “The latter is known as Australian or toxic stringhalt and tends to affect many horses grazing the same contaminated pasture or hay.
“The disease is characterised by excessive, rapid flexion of one or both hindlimbs, while the horse walks forwards and backwards, which is different to most shivers cases,” she adds. “The hindlimb flexion can be mild, and asymmetrical, or violent where the fetlock contacts the horse’s underside and causes a ‘bunny hopping’ gait. But facial twitching and tail head elevation are absent.
“Unlike shivers, signs of stringhalt are observable at trot — and stringhalt sufferers may also have other signs of neurological dysfunction, such as laryngeal paralysis (which is known as roaring).
“Other conditions that cause jerky, excessive flexion of the hindlimbs include upward fixation of the patella (often termed locking of the stifle or knee), fibrotic myopathy or a rare disease called stiff-horse syndrome,” Alex explains. “Additionally, equine motor neurone disease, caused by a lack of vitamin E, has been reported to resemble some of the shivers signs but without the characteristic gait changes.”
What’s the prognosis?
To date, there is no effective treatment for shivers.
“Severe clinical signs may improve, especially with increased turnout and mild exercise, but may recur during painful or stressful situations such as the development of laminitis,” says Alex. “If farriery becomes a problem, ask your vet about sedation with an alpha-2 agonist medication which can temporarily alleviate symptoms. Should blood tests reveal low vitamin E levels, supplementation is advisable as this antioxidant supports muscle health.
“The prognosis appears to be case dependent,” she adds. “In some horses the condition progresses quickly, but the contrary is also true. Many equine athletes who have shivers can perform at a high level.”
‘He is coping a lot better’
With help from chiropractor Olivia (Lil) Jones, Rebecca is now more optimistic about Sam’s prospects.
“Lil has encouraged me to manage the problem by keeping him moving and working in the correct way,” she explains. “He is quite highly strung and can be a bit stressed in the stable, so he now lives in a roomy cowshed and has plenty of turnout. He is also on a high-fibre, low-starch diet, which is thought to help.
“After lots of practice at moving backwards in-hand, Sam is far less anxious,” says Rebecca. “His ridden work is better, too, and we’re starting some gridwork.”
Lil explains: “Horses with shivers can become anxious about picking up their hindlegs and moving sideways or backwards, which can worsen the problem and lead to tension and stabilisation issues through the rest of the body. The aim is to help coordinate movement in the horse’s brain, teaching him to lift his legs calmly in-hand and using flatwork exercises such as trotting poles (pictured, right) to encourage this movement.
“I also use hands-on techniques to restore and help maintain normal movement through Sam’s spine. It’s important that he uses core muscles for support and lifts correctly through his withers, without over- or under-working any areas. He has regular sessions and is coping a lot better; not being too restrained is beneficial for his body and brain, and he seems happier and less stressed.”
Ref Horse & Hound; 23 January 2020