Surprisingly little is known about the movement disorder shivers, despite the long history of the disease. Richard Parker MRCVS brings us up to date with the latest findings

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Few conditions are as poorly understood as shivers, or shivering. This rare and mysterious disease is characterised by involuntary muscle twitching or trembling (shivering) of the hindquarters, difficulty in picking up the hind legs and abnormalities in posture and tail carriage.

The condition has been known for centuries, with early reports dating back to the 1800s. While shivers was quite common in the 1960s when it was thought more likely to affect draught horses, heavy hunters and warmbloods, the prevalence of the disease today is unknown.

However, as ownership of draught and heavy horses has declined, cases of shivering have become less common.

The cause of shivers remains unexplained. Various theories include nerve and muscle disorders, genetic abnormalities, infections and even physical injury of the pelvis or spinal cord as possibilities.

New research findings

This year, an international study has shed light on the disease. The report, published recently in the Equine Veterinary Journal, indicated that shivers most often occurs in male horses measuring more than 17hh. It is more common in draught breeds, to a lesser extent in thoroughbreds and warmbloods, and is extremely rare in ponies.

The study revealed that horses used for pleasure and competition are equally affected. The disease is more likely to develop in young horses between the ages of two and four years, usually beginning before seven.

The most common clinical sign in shiverers is difficulty when the farrier is trimming or shoeing the hind feet (see box, right); other signs include muscle twitching, a raised tail when backing up and muscle wastage and weakness in the hindquarters.

In approximately 20% of cases, horses show twitching of the lips, face and eyes when walking backwards. In approximately 75% of cases, signs worsen with time.

Unfortunately, there is no way of knowing how an individual case will progress over a lifetime.

Signs may be intermittent and can vary from day to day, making diagnosis difficult.

In addition, signs may be triggered or made worse by periods of stress such as box rest, lameness, transport or even attempts to walk the horse backwards.

Some shiverers may deteriorate to the point where they are unable to pick up their hind feet, leading to foot problems such as thrush and overgrown hooves. In extreme cases, the horse may become incapacitated necessitating euthanasia.

Stringhalt versus shivers

There are no blood tests or medical procedures such as muscle biopsy to confirm shivers. Diagnosis is based purely on veterinary examination.

It is important to examine these horses carefully to exclude the presence of lameness or other conditions such as upward fixation of the patella (stifle lock) and stringhalt, which may be confused with shivers.

In a separate study, video clips of horses with shivers and stringhalt were analysed and compared.

Researchers reported that although the conditions can look similar, shiverers were more likely to display upward jerky leg movements when walking forwards.

They also recommended that potential shiverers undergo a complete lameness and neurological examination. This involves walking the horse forwards and backwards for at least 10 strides and in tight circles, and physically picking up each leg.

Fortunately, this is a routine part of a pre-purchase examination performed by vets.

No proven, effective treatment

Anecdotal reports of feeding affected horses a high-fat, low-carbohydrate diet have been shown to have little impact on the progression of the disease. Vitamin E and selenium supplements have been tried with little or no effect, as have anti-inflammatory medication such as phenylbutazone (bute) or muscle relaxants.

Pasture turnout and regular exercise are generally recommended in milder cases to maintain fitness. While there is little evidence of the effectiveness of alternative therapies such as chiropractic and acupuncture, some owners have found these treatments useful in relieving the effects of the disease.

Weighing up the facts

When considering whether or not to buy a known shiverer it is important to discuss with your vet whether the horse is likely to meet your athletic expectations.

Research the history and previous performance of the horse and request that he is examined in detail to assess the severity of the condition.

Shivers is technically regarded as a gait abnormality, and many vets feel that this is unsoundness and as a result the horse may not be recommended for purchase.

Early or mild cases can be very difficult to detect, even after detailed examination, so emergence of the condition after a vetting can come as a nasty surprise.

Studies have shown that although confirmed shiverers may show signs every day, some mild cases may only show signs once a week or so.

Shivers can also cause complications if you buy a horse overseas. In other parts of Europe, views on the condition may differ among vets; some do not consider shivers a problem if the horse is still capable of performing well and remains sound.

While signs worsen in an estimated three-quarters of cases, making further use and future resale of the horse potentially difficult, a diagnosis need not spell disaster.

There are plenty of anecdotal reports of shiverers enjoying long and successful careers. It’s vital, however, to take the known facts into consideration before you buy.

Ref: H&H Thursday 4 December, 2014