Stringhalt is a neuromuscular disease of the back legs that almost every horseperson has heard about but may not have seen. It’s a rare but distinctive condition, and affected animals be described as ”stringy,” ”hikey” or ”stringhaltered.” Although painless, stringhalt is an unsoundness that will eventually affect the horse’s ability to move.
With stringhalt, one or both hind legs are overflexed when the horse moves off or backs, sometimes to the point of the horse hitting his abdomen with the leg. The leg is suspended in this abnormally flexed position for a fraction longer than in normal movements and, when the leg comes back down to the ground, it does so quickly and hits the ground with a ”slapping” movement, landing hard and flat.
Early cases of stringhalt may be confused with locking patella (stifle), EPM or ”shivers.” Since shivers also involves overflexion of the hind leg, it mimics stringhalt the most. However, shivers lacks that hard, slapping motion of the leg landing and involves muscular quivering and an elevation of the tail during the abnormal movements. Otherwise, the exacerbation of the hind-leg movement when backing and just moving off is the same as with stringhalt.
In the early stages of stringhalt, the odd movement is most likely if the animal has been still for a while, such as when coming out of a stall, when backed or when turned in a tight circle. In moderate cases, the symptoms may disappear as the horse is worked. As the disease advances, symptoms become more pronounced and are present virtually all the time. Symptoms worsen in cold weather and decrease with hot weather.
All breeds are susceptible to stringhalt, and it usually appears in horses four to five years old. It may have a genetic predisposition. Whether this hereditary link causes these horses to be more sensitive to a toxic or infectious trigger or whether it involves a defect in the nervous tissue itself is basically unknown.
Stringhalt has also been described as a herd problem in horses grazing poor pastures where over 50% of the plants were dandelions and related weeds (flatweed or cat’s ear) or after ingestion of sweet pea (Caley pea) plants in hay or on pasture.
The exact nature of the toxicity is unknown and, in the case of dandelions, it requires a high intake and possibly specific growing conditions as well, such as drought. Interestingly, a stringhalt gait may develop following a wound to the front of the hind cannon bone. The mechanism may be adhesions or odd sensory feedback leading to a reflexively abnormal gait.
The disease involves a degeneration of large nerve fibers, specifically those with a myelin sheath. The degeneration is worst in the longest nerves and particularly involves the lateral digital extensor muscle. Examination of nerves, brain and spinal cord on a body-wide basis has shown the brain and spinal cord to be free of lesions, but the recurrent laryngeal nerve, which innervates the larynx and vocal cords, is commonly affected as well. Many horses with advanced stringhalt will also be roarers as a result. In advanced stringhalt cases, the long nerves to the front limbs may also be involved.
When herd outbreaks occur in horses on poor pasture, removing them from the pasture and supplying a high-quality diet may lead to reversal in a few weeks to months. If it doesn’t, or if a dietary link can’t be established, surgery to remove a section of the lateral digital extensor muscle and its tendon may be effective. The prognosis with surgery overall is guarded to favorable. When a dietary link is suspected, it’s advisable to wait a year before considering surgery to see if the condition will correct on its own.
Stringhalt has also been treated with phenytoin (Dilantin) at dosages of from 15 to 25 mg/kg. Dilantin is primarily used as an antiseizure drug in people but has peripheral nerve effects as well and decreases excitability of nervous tissue. As with surgery, improvement but not necessarily cure is to be expected.