An involuntary trembling of the hind legs when lifted, Shivers is a progressive condition most commonly seen in heavier-set breeds. The causes are yet to be fully understood and the symptoms tend to worsen over time.
The potential causes of Shivers
There are several theories as to the underlying cause of Shivers. The condition might be the result of abnormalities in affected animal's neurotransmitters, the chemicals which help transmit nerve signals. Alternatively, it could be the result of toxins in the bodies of horses which have suffered from infectious diseases. It has also been proposed that Shivers is caused by abnormal muscle cells or by osteoarthritis impacting nerves. An extensive study published in 2015 revealed that the brains of horses with Shivers are damaged in a specific area of the cerebellum, the area of the brain which is largely devoted to regulating muscular activity.
The genetic basis for the disorder
Shivers is breed-related and so there may well be a genetic basis or predisposition for the disorder. Historical reports suggest that Shivers was prevented in the past by breeding away from the condition. However, no specific genetic pattern has been identified and there is no genetic test available to identify horses which are prone to the condition.
The symptoms of Shivers
Symptoms of the condition include the snatching up of hind legs in an exaggerated fashion and legs which tremble when in the air. It is usually the hind legs which are affected by Shivers but the condition has been known to impact forelegs and other parts of the body including the eyes and neck muscles. The symptoms may strike a horse at any age, but the majority of cases are diagnosed between the ages of 7 and 11. The condition can progress at different rates, making a horse's future unpredictable. Some horses with Shivers will lead relatively normal lives for several years while others deteriorate rapidly. A case of Shivers could lead to muscle wastage, a severe loss of power in the hind legs and even complete incapacitation.
Shivers can be hard to detect in its early stages and mild symptoms mimic those of other conditions including, fibrotic myopathy and equine motor neurone disease. Vets will pick up all four legs in an attempt to diagnose Shivers and will ask a horse to back up so they can look for the signature snatching up of the legs and the trembling.
No treatment for Shivers
Unfortunately, there is no treatment for Shivers. Sedatives are an option as they might dampen the reactions of the nerves and at least enable horses to be shod. Farriers might otherwise struggle to shoe a horse as affected animals will be reluctant to lift their legs. If the condition remains mild, horses can be ridden, but if this isn't possible, plenty of turn out to provide exercise is important. In severe cases of Shivers, horses will eventually become unmanageable and euthanasia will be the only option. More research into this disorder would certainly be welcomed.