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What Are the Causes of Tremors in Horses?

| Updated September 26, 2017

If your horse starts experiencing muscle tremors, he most likely is suffering from one of several conditions. To narrow down the cause, your vet can make use of various diagnostic tools. These include:

  • diagnostic ultrasound, which can indicate muscle fiber disruption
  • electromyography, which measures the muscle's electrical signals
  • muscle biopsy
  • or genetic testing.

Polysaccharide Storage Myopathy

Polysaccharide storage myopathy, or PSSM, appears in two forms. Type 1 primarily affects quarter horses, draft horses, warmbloods and Morgans. Type 2 also affects quarter horses, but also affects light horse breeds such as the Arab and thoroughbred. Type 1 is the result of a gene mutation. Type 2 horses do not have the mutation, and the cause of Type 2 PSSM is unknown. Symptoms of both types are similar, and the culprit is the same -- too much sugar accumulating in the muscle.

Besides muscle tremors in the flanks, symptoms include stiffness, lack of movement, sweating and muscle hardening in the hind end. Severely affected horses may not be able to stand and are in obvious pain. They may pass dark urine, a sign that muscle proteins are breaking down and heading into the bloodstream.

Proper management can reduce or eliminate signs of PSSM. This includes:

  • changing to a low carbohydrate, high-fat diet
  • regular exercise
  • and weight loss -- many affected horses are "easy keepers" prone to easy weight gain.

Hyperkalemic Periodic Paralysis

Hyperkalemic periodic paralysis, usually referred to as HYPP, is most often found in quarter horses or those with quarter horse blood, including the paint and Appaloosa. Most horses with HYPP can trace their lineage back to the great quarter horse sire Impressive. If someone tells you a horse has "impressive" bloodlines, it's not necessarily a compliment but a warning.

The genetic condition causes too much potassium in the blood, with resulting muscle spasms all over the body and weakness. An affected horse might keel over. Once an attack is finished, the horse appears fine again. Still, in some cases, a severely affected animal may die, as HYPP can also affect respiration.

Managing HYPP includes dietary changes, such as feeding timothy or grass hays rather than legumes rich in potassium such as alfalfa and any feed concentrates containing molasses. Beet pulp without molasses is acceptable. If possible, keep your horse on pasture rather than stalled, and make sure he has regular exercise. Divide your horse's ration and feed him several times a day rather than the typical morning and evening feeding.


  • Many HYPP horses go years between attacks -- but it's always a possibility. There's no reason not to ride a horse with HYPP, and the exercise is good for him. Still, no matter how gentle an HYPP equine, only experienced riders informed of his condition should get on his back. Children should not ride horses with HYPP.

Shivers in Horses

Shivers refers to involuntary spasms, usually in the tail, pelvic area or rear legs. Occasionally, the condition also affects the front legs and head. Draft horses and some warmblood breeds with heavy draft influence are primarily affected.

It's a relatively rare disorder, but mild cases may go unnoticed, since minor symptoms may consist of periodic unusual tail elevation. More seriously affected equines may constantly raise a hind leg and cannot move backwards. The rear end of the animal may exhibit tremors, with the tail quivering. The exact cause of the malady isn't known, although there is likely a genetic component.

There's no cure for shivers, which can progress to general muscle atrophy in the hind end. Careful management can control the disorder, including plenty of turnout. If you have a horse with shivers, you may also:

  • Feed a diet low in carbohydrates but high in fat.
  • Supplement with vitamin E.
  • Use acupuncture or massage therapy to offer relief.