For centuries this neurologic disease has left veterinarians and scientists ­scratching their heads.

A jerk, a jump, a hop with the hind legs tucked up high. Similar to the function of strings on a marionette, stringhalt seems to make horses yank their legs up and halt them there momentarily before taking their next step. This uncontrollable exaggerated movement of the digital extensor muscles is the outward sign of neurologic disease, sometimes caused by toxicity, sometimes of unknown origin. If time doesn't cure it, the prognosis is poor. But today researchers are hard at work looking into ways to make horses with this condition safe, happy, and free of clinical signs.

History, Clinical Signs, and Causes

Stringhalt appears to be an age-old disease in horses and other hooved animals. Descriptions of the disease date back to the Renaissance. William Shakespeare has even been credited for the name of the disease (as a variation of the term "springhalt," from Henry VIII).

Stringhalt occurs when the digital extensor muscles contract excessively or when the digital flexor muscles lack sufficient opposition, according to Caroline Hahn, DVM, PhD, MRCVS, Dipl. ECEIM, ECVN, senior lecturer and researcher in the neuromuscular disease laboratory of the University of Edinburgh's Royal (Dick) School of Veterinary Studies, in Scotland.

The clinical signs are usually more extreme when the horse is turning or backing up. Some horses have such severe stringhalt they actually kick themselves in the abdomen when trying to walk.

Both hind legs might be equally affected, or one might show more obvious signs than the other. In some cases stringhalt might be apparent in the forelegs. Clinical signs, such as pastern extension when the hock is flexed, can vary from one horse to another, Hahn says. Shivering is another sign of the disease, she adds.

Although stringhalt might look like strictly a muscular problem, neurologic issues are at the root of it. Nerve damage probably causes abnormal activity of certain sensory receptors called spindles, says Hahn. Located in the body of the muscles, spindles are responsible for detecting changes in muscle length. This faulty wiring of the spindles causes certain muscles to contract either too early or too late, or too much or too little, during certain phases of the gait. "Thus, intermittent abduction (movement of the limb away from the body) and caudal (toward the tail) jerking in the hind limb may result from spasticity of the biceps femoris muscle (located along the back of the hindquarter) during the swing phase of the stride," she says.

Veterinarians divide stringhalt into two major categories. "Australian" stringhalt usually is a temporary disease resulting from toxicity, whereas "classic" (or atypical, depending on the geographic region) stringhalt has no known cause and usually does not resolve on its own.

Australian Stringhalt

Stringhalt in outbreaks caused by the ingestion of toxic plants has acquired the common name of Australian stringhalt. It was originally described by veterinarians in Victoria in the late 19th century. However, despite its name, Australian stringhalt can affect horses worldwide and has commonly been reported in the United States and New Zealand, in addition to Australia. Recently, it also has been diagnosed in some South American countries.

Over the past century, false dandelion (Hypochaeris radicata), also known as flatweed, has been considered a major culprit of Australian stringhalt, according to José Allan Soares de Araujo, DVM, PhD, researcher at the veterinary hospital at the Universidade Federal de Campina Grande in Patos, Brazil, and author of a recent study describing two outbreaks of Australian stringhalt on Brazilian Thoroughbred farms. New cases appear most often in late summer or fall, frequently in horses grazing on poor-quality pastures where weeds are abundant.

Australian stringhalt affects the peripheral nervous system (outside the central nervous system, which includes the brain and spinal cord), almost always affecting both hind legs more or less equivalently and sometimes the forelegs, says Hahn.

Horses should be removed from their pasture as soon as clinical signs appear, says Nicholas Kannegieter, PhD, BVSc, DVCS, FACVs, specialist equine clinician based in the Roseville Equine Clinic in New South Wales, Australia. Away from the toxins, the median recovery period for most horses is six to nine months, although for some it takes a few weeks and for others a few years. Horses with signs of stringhalt in their forelegs take the longest to recover.

The horse's largest nerve fibers are the primary targets of Australian stringhalt; these are in the hind leg and the larynx (voice box), which are both usually affected by the disease, Kannegieter says. The hindquarters show the most obvious clinical signs, but some horses also will emit a hoarse "roar" when they whinny. Even when the gait returns to normal, the laryngeal nerves of some Australian stringhalt-affected horses are damaged. Unfortunately, this can leave them with permanent deficiencies in athletic performance (due to airway restriction), he says.

Other plants, including the common dandelion (Taraxacum officinale) and cheeseweed or little mallow (Malva parviflora), also have been linked to Australian stringhalt, but flatweed remains the common indicator plant.

Araujo found that false dandelion was the certain cause of the two Brazilian outbreaks he described. His research group was able to provoke Australian stringhalt in a young horse by feeding him false dandelions picked from a field where an outbreak had occurred. The horse developed clinical signs within three weeks and showed improvement within days of the researchers removing the plants from the diet, Araujo says. (The horse fully recovered 15 days after the experiment ended.)

However, not all false dandelion or other suspect weeds cause stringhalt, and many horses graze in affected paddocks without issue. Toxicity of the same species of plant can vary from one location to another, according to Araujo. Environment might play a role in providing optimum conditions for producing the toxin, says Kannegieter.

Classic Stringhalt

Unlike Australian stringhalt, classic stringhalt does not typically occur in outbreaks and usually has no obvious cause. Clinical signs are similar, although classic stringhalt frequently has stronger effects in a single leg.

Injury to the back, neck, or leg could be the underlying source of the condition. In these cases the stringhalt might improve once the injury heals, Hahn says.

When no injury is obvious, however, the reason a horse develops stringhalt is difficult to determine and might never be known. Arthrosis (degenerative joint disease) and perhaps osteochondritis dissecans (OCD, a cartilage disorder characterized by large flaps of cartilage or loose cartilaginous bodies within a joint) at the joints, primarily in the neck, could be the cause, according to Inge Wijnberg, PhD, DVM, an assistant professor in equine internal medicine at Utrecht University, in the Netherlands. There she focuses on neuromuscular disease in her research.

"We see a lot of young horses in our referral clinic with issues in the C5 to T1 area (certain neck vertebrae)," she says. "Some horses do have problems in the peripheral nerves, but the vast majority of cases in our clinic have lesions in the cervical or upper thoracic (chest) area as opposed to the lumbar (loin) area. These lead to upper motor neuron (connecting the brain stem to spinal cord) abnormalities which seem to be related to the stringhalt."

Frequently the clinical signs progress in these horses, and more intensive work or training might actually make them worse, Wijnberg says. "A lot of times owners will say they noticed something a little funny around age three, but then it gets much worse by the time the horse is five. That's often the age that dressage horses start a lot of the more complex gymnastics activities. So the stringhalt might advance because of age, or training, or both. We don't know yet."

Genetic links are questionable, she says. "But even if there might be some correlation, OCD has so many causative factors that it's difficult to pinpoint genetics."

Diagnosis and Treatment

Horses with stringhalt symptoms should be evaluated by an equine veterinarian with experience in neuromuscular dysfunction, Wijnberg says. However, horses with Australian stringhalt do not require the same extensive diagnostic procedures as those with classic stringhalt, because the cause is more apparent.

Hahn recommends a thorough orthopedic exam, including radiographs (X rays) and ultrasound of the affected leg, to look for potential problems in the lateral (closer to the outside than the midline) digital extensor muscle, tendon, or hock. "Any abnormalities detected often are assumed to initiate abnormal nervous system reflexes, increasing tone in the digital extensor muscles during stride," she says. Vets should consider differential diagnoses such as muscle diseases, including polysaccharide storage myopathy, she adds.

In Wijnberg's clinic, patients with stringhalt symptoms are hospitalized so they can be examined several days in a row, because gait diagnosis is often fairly subjective, she says. She relies on a combination of extensive clinical examinations, which include an orthopedic workup, diagnostic imaging, and electromyographs (EMGs, tests that measure electric signals in the muscles), for accurate diagnoses. "People can have a lot of differences of opinion about the severity or even existence of the disease in certain horses," says Wijnberg, who recently published a report on diagnosing stringhalt via EMG. "The EMG gives us a clear reading of what's going on in the muscle and really takes out the 'I think' part of the diagnosis."

Clinicians must use specific descriptions of the kinds of clinical signs being displayed to reach clear diagnoses, she says. "We try to stay away from fast slang, like 'stringhalt,' when we initially examine a horse," she says. "Is it hypertonia (increased rigidity, tension, and spasticity of the muscles)? Hypermetria (an ataxic muscle disorder characterized by overreaching)? Lateral or parallel? Neurological problems are complicated, so it's important to describe what you see first and foremost."

Muscle relaxants, anticonvulsants, and drugs that work directly on the central nervous system can be helpful for some horses with long-lasting stringhalt, according to Kannegieter.

Unfortunately, these medications might also act as sedatives, and their effects are often temporary, Wijnberg says. Long-term medical treatment is also expensive and risky in competitive situations, as it could result in positive doping tests.

Surgical resection of a part of the tendon running along the hock is another option, but its success rate varies. Many horses do not recover from stringhalt despite these treatments, she says.

New Research

Like many other neurologic diseases, stringhalt is the subject of active, ongoing research. Scientists are seeking new causes as well as treatments, Wijnberg says. Her research unit is collaborating with the University of Bern to deepen their understanding of the origin of stringhalt based on post-mortem examinations. However, lack of cadavers is hindering this research.

The botulinum toxin Botox, commonly used to smooth facial wrinkles in humans, is under investigation at the moment for its use in calming stringhalt symptoms, Wijnberg says. Her recently published results on the subject were promising, showing reduced clinical signs in the affected test horses, and noting Botox might be even more effective in higher doses.

Thus, Wijnberg is now developing new "retraining" protocols, she says. "If it's an upper motor problem based on a vertebral lesion with lots of arthrosis, I can't fix that," she says. "But I can try to retrain the horse to get around the problem."

The goal, she says, is to use Botox temporarily to "get the horse out of his loop" until he has learned to overcome the neurologic issues. It isn't as far-fetched an idea as it might seem: Similar goals are currently being met for people with upper motor neuron disorders.

With a bit of luck, and a lot of skill and patience, equine neurologists might soon know just what it takes to cut the strings of stringhalt horses forever.