When we think of shivering, we think of a body movement that is performed when we are cold. Certainly many things can cause horses to shiver, including pain or cold. However, the condition of shivers in horses is actually a nerve-related condition that makes it difficult for horses to walk backwards. Dr. Stephanie Valberg is one of the experts in muscle disease in horses and she indicates in the Practitioner that forward walking and trotting are normal in the early stages. Another common finding is it is difficult for the farrier to hold the rear limbs flexed when standing. And many times the horses will hyper-flex the hindlimb when walking backwards and start trembling and then rapidly place the foot on the ground; the term shivers originated with the trembling of the hind limbs. In more severe cases, the horse over-flexes the hind limb and moves the limb out and away from the body in a movement called abduction. Lots of these more severe horses cannot even hold the limb flexed for trimming as they will slam it to the ground.
It was thought originally this syndrome was related to polysaccharide storage myopathy and was a muscle disorder but has now been found to be a disorder of the nerves in the area. Unfortunately, there is no effective treatment for shivers. The symptoms may decrease when the horse is rested but recur when work is started back. Supplementing with vitamin E is recommended, and if the horse has polysaccharide storage myopathy and shivers, feeding a diet low in non-structural carbohydrates and high in fat may be helpful. If you have a horse that gives the farrier a difficult time trimming only the hind legs, shivers could be involved.