Hind-leg shakes in horses can be symptomatic of many things, from the not-so-serious, such as chills, to a neurological or neuromuscular condition The first time you notice any shaking, pay careful attention. If your horse's hind leg is shaking on a consistent basis, contact your veterinarian.
Shivers have long been associated with draft or warm-blood breeds, but can occur in any breed. The condition is usually characterised initially by an abrupt jerking of the horse's hind leg when you pick it up to clean out his foot or when he is being shod. You also may notice shivers when you ask your horse to back up. It is a progressive disease, but many horses live with shivers for years. Increasing turnout and exercise seem to have the most positive impact
Stringhalt is similar to shivers, although unlike the tendency for shivers to be more common in warmbloods, stringhalt does not show any breed preference. A horse with stringhalt will flex his hock toward the abdomen before slamming it quickly to the ground instead of being held, as is usually the case with shivers. Stringhalt can be extremely difficult to diagnose; it is usually observed when the horse is backing up or turning, or otherwise moving slowly.
EPM stands for equine protozoal myeloencephalitis, a parasitic disease that affects the horse's central nervous system. It is transmitted by certain wildlife, such as opossums. Because it affects their nervous systems, many horses develop gait abnormalities and lose control of their limbs. Horses can test positive for EPM without showing signs of the disease; some horses have developed an immunity to it or otherwise have a strong enough immune system to resist it.
Other causes of hind-leg shakes include colic, fibrotic myopathy, equine motor neuron disease, "stiff-horse syndrome" and upward fixation, or locking, of the patella. Some are temporary conditions and others are more serious physical or neurological defects. If you notice anything unusual in your horse's gait or the way he carries his limbs or reacts when you touch them, pay attention. If the condition occurs on a regular basis, consult your veterinarian.