Shivering (shivers)

Shivering or Shivers is a condition most seen in the Draught, Warmblood and Thoroughbred breeds, although other breeds can be affected. It has been reported that geldings 16.3 hands or taller are predisposed to Shivering. Signs are usually present in affected horses by 7 years but can occur as early as 1 year of age in Draught breeds. The condition is best demonstrated when horses are backed or when someone attempts to lift the back legs (usually the opposite hindlimb). In more severely affected animals, it can also be seen when horses begin working forward or when turned. It is rare to see it at a forward walk and not seen at the trot, canter or gallop.

It can start out as an intermittent problem in a single rear limb but will typically progress to become consistent and even in both hindlimbs. It is rare to see Shivering affect the forelimbs.

The horse is reluctant to back up and will suddenly flex and typically abduct (move the limb outwards) the hindlimb, holding the leg in this exaggerated position for several seconds or longer, while having muscle spasms over the rump (gluteals) and other muscle groups. The leg may slam the ground in re-placement. This characteristic gait abnormality can also be induced when trying to lift a rear limb. Elevation of the tail head with tremors can also be observed during backing. There is also a form of Shivering when backing where the there is excessive extension of the stifle and hocks joint, with the limb placed more caudal than normal.

The diagnosis is based solely on signalment and examination of the gait, particularly when backing. A previously postulated link between Shivering and Polysaccharide Storage Myopathy (PsSM) has been disproven.

Shivering is thought to be due to loss of inhibitory activity from the cerebellum. This causes increased excitation of neural inputs to hindlimb muscles in response to specific activities, such as backing up.

There is no proven treatment for Shivering, although it is known that stall rest, reduced paddock turnout, lameness, stress or illness can worsen the condition temporarily. To facilitate farrier work it may be beneficial to use a sedative analgesic drug.

The progression of shivering is difficult to predict, although most horses (perhaps 70%) will worsen over time. Some cases progress quickly while others may decline slowly over many years.