Stringhalt is a myoclonic affliction of either one or both hind legs. It occurs when a horse's hind legs overflex and experience spasmodic movements, which may include raising and lowering the leg to various degrees of hyperflexion. Stringhalt is a condition that can affect any horse of any breed, though it is very rare in horses that are under a year old.
Stringhalt is most easily identified by the degree of hyperflexion that occurs in the affected horse's movements. Mild symptoms include spasmodically lifting and then lowering one or both hind feet. In extreme cases, the horse may repeatedly raise the leg all the way to its barrel and then strike it to the ground with significant force, almost as if kicking out at the ground.
In some of the more extreme stringhalt occurrences, the lateral thigh muscles of the hind legs can atrophy. This is most commonly seen in horses with chronic stringhalt, especially that brought on by the Australian strain of the condition, which is thought to be caused by flat weed intoxication. One method of treatment is to remove part of the leg muscles. In the most severe cases, the atrophy can become severe enough that the horse is unable to move properly or comfortably and must be euthanized.
Not all cases of stringhalt are severe. Sometimes stringhalt is only visible when the horse is performing movements that rely heavily on its hind legs, such as backing and sharp turns on the hindquarter. Horses that are affected with mild stringhalt may only show symptoms on the first few steps after being still for a long period of time or in a stall. Symptoms may also come and go with the weather, as stringhalt symptoms tend to be less visible during warm weather.
Causes and Treatment
In the United States, most cases of stringhalt are considered to be caused by sweet pea poisoning, also known as lathyrism. Other plants, such as dandelion, may also play a role. In mild cases, removing the horse from the plants causing the negative reaction may completely cure the problem. Stringhalt can be diagnosed by your veterinarian using a procedure called electromyography. Surgery to remove part of the muscle is a common treatment, as is treating the horse by administering treatments of thiamine and phenytoin.