Shivers or shivering is a chronic debilitating nervous or neuromuscular syndrome that has been described in horses for centuries. The disease is characterized by a inappropriate raising of a hind leg that along with the tail has a tremor associated with it, much like a muscle spasm or nervous tic. Shivers is also reported in Warmbloods and Warmblood crossbreds. Rarely lighter breeds of horses including Quarter Horses and Thoroughbreds are involved. In ponies shivers is rare.

We are just beginning to learn about the causes of the disease. This article discusses the clinical signs, differential disease rule outs, some of the conjectured causes and treatment options.

Shivering has been characterized as a chronic, often gradually progressive movement disorder that usually begins before 7 years of age and has a higher prevalence in tall male horses. Shivers can starts at any age and as a mild condition that worsens with time and can occur at any age of the horse. Some reports document horses as young as 1–2 yr of age with shivers. Though often said to start in the young horse a recent study in Belgian drafts found no significant difference was observed in the age and sex distribution of horses with shivers from horses without shivers. Shivers has a remarkable range of presentations depending on the stage of the disease and the individual and is characterized by a periodic involuntary spasm of the muscles in the pelvic region, pelvic limbs, and tail. It can effect one or both hindlimbs and seen when the horse is first moved off.

Mild signs would be tenseness or trembling of the hindlimbs and sudden jerky elevation of the tail. The amount of tail elevation will vary considerably in different cases. In more severely affected animals, the hindlimb is suddenly raised, semi-flexed, and abducted with the hoof held in the air "shivering" for several seconds to a minute or more with the tail elevated and trembling simultaneously. The superficial muscles of the upper leg and quarter will also quiver. After a few moments, the spasms of the limb and tail gradually subside and the limb is then slowly extended and the foot is brought slowly to the ground. Signs are usually exacerbated when the horse is backed and some horses exhibit the problem when a hindimb is raised like may happen for trimming and shoeing. This can progress to the point of not being able to shoe the horse.

Except is severe cases, signs of shivers are not seen while standing though some horses may attract attention by repeatedly flexing a affected hindlimb while standing. In severe cases the horse may hyperextend one or both hindlimbs so that the horse is standing on his toes. The worst effected may fall over.

Besides backing and lifting a limb other events can bring on symptoms. Excitement, watering, and feeding have all been associated with an attack.

Rarely, muscles of the forelimb, neck, trunk and/or face (ears, eyelids, lips, and cheek) may be involved. In the case of the forelimb, when the limb is raided it is thrust forward in full extension with the foot barely touching the ground or the limb and the knee (carpus) flexed and elevated. The extensor muscles above the elbow then quiver until the spasm ends or the foot returns to the ground. In the case of the head or neck ther is spasms of the muscles with resulting in rapid blinking of the eyelids, quivering of the ears, or twitching with spasmodic retraction of the commissures of the lips.