Classic and Australian Stringhalt
- Feb 1, 2003
It's a disturbing and distressing sight: You're backing your horse when one hind leg jerks forward and upward, nearly clipping his abdomen. It's the same every time you back your horse--this strange movement where his leg snaps up toward his belly. There's no mistaking it: Your horse has stringhalt.
A neurologic disorder, stringhalt is an involuntary, exaggerated flexion of the hock that can affect one or both hind limbs. There appears to be three different types:
Classic or true stringhalt is an idiopathic (of unknown cause) disease. "Classic stringhalt occurs as isolated cases," explains Kent Carter, DVM, MS, professor of medicine, section chief of internal medicine, and a lameness specialist at Texas A&M University. "You can have a whole herd of horses with just one horse getting it. There is no known cause; the horse just starts developing this gait."
No environmental or nutritional relationship has been detected with classic stringhalt, and there are no apparent predispositions to breed or activity. However, classic stringhalt tends to affect adult horses and not youngsters. Carter notes that classic stringhalt is fairly uncommon, at least in his area.
Australian stringhalt (ASH) occurs in both hind legs, and it has also been seen in horses in the United States. ASH often affects more than one horse in a herd (average incidence is about 10-15% of a group), primarily occurs in late summer or autumn, and is observed in horses which are turned out in weedy pastures.
Says Peter J. Huntington, BVSc (Hons), MACVSc, MRCVS, a long-time researcher of Australian stringhalt with colleagues at the University of Melbourne, "Flatweed is nearly always seen in association with ASH. Flatweed looks like dandelion (Taraxacum officinale), but it is a different plant. Many affected horses are observed to graze flatweed, and this is often the only green plant available. However, many horses grazing similar paddocks are unaffected, so it has not been proven to be the cause of ASH."
While the exact cause of ASH is unknown, Huntington says toxic factors in plants or fungi or nutrient deficiencies of energy or vitamins that cause nerves to die are possible contributors.
First reported in Australia in the mid-1800s, ASH also occurs in New Zealand and has been seen in the United States in Washington, northern California, and mid-Atlantic states. It is a common seasonal disorder in Australia in late summer and autumn, but no reliable prevalence figures are available. Thoroughbreds are most commonly affected--they have accounted for about 48% of the cases Huntington has studied. He also found that the disorder affected taller horses preferentially, with 84% of affected horses being taller than 15 hands high. It also tends to affect mature horses, and produces more severe clinical signs in horses which are normally nervous or apprehensive.
Trauma-induced or false stringhalt occurs secondarily after a blow or laceration to the cannon bone, but can also result from hoof pain, such as that caused by a foot abscess.
The severity varies from horse to horse. "In mild cases, exaggerated lifting of the leg is only obvious under special circumstances, such as when the horse goes backward, is upset, turns sharply sideways, or in cold weather," says Huntington. "In very severe cases, the horse will go to walk off and gets one leg stuck up under its belly, as though the leg was glued to the belly. These horses can often only move by an exaggerated (three-legged) bunny-hopping motion."
Most cases of ASH have dramatic muscle wastage of the digital extensor group (gaskin); see page 52. "Many horses had considerable wastage of the thighs and between the hind legs," says Huntington, "and a few had more generalized muscle wastage of the hindquarters. Only the few horses affected with atypical ASH had muscle wastage of the forelimb. These were generally draft breeds, and most had severe forelimb gait abnormalities. There was no evidence of muscle wastage in the neck or along the back associated specifically with ASH."
Carter says that he hasn't seen significant atrophy with classic stringhalt, although some atrophy can occur with trauma-induced stringhalt involving the nerves.
The Australian form might also lead to laryngeal dysfunction. "A consequence of stringhalt in athletic horses is that many of them become roarers," Huntington notes. "Some degree of laryngeal paralysis is very common."
Diagnosis and Treatment
Diagnosis of stringhalt is based on clinical signs; neither blood tests nor radiographs are helpful. "You just watch the horse go," explains Carter. "If you see a severe case of stringhalt, you're not going to confuse it with anything else. With mild hyperflexion, we might rule out other things such as upward fixation of the patella or feet and joint problems."
Recommended treatment depends on severity and the type of stringhalt involved.
Classic stringhalt generally doesn't resolve on its own. In some cases, the horse can remain useful and comfortable with the disease, so treatment is not recommended.
"It's one of those conditions that probably bothers the owner as much as it does the horse," Carter says. "With mild cases, it's not a painful situation, it's an abnormal gait. Sometimes it's a lot more apparent at a walk than at a trot or canter. It doesn't look nice, but many times we don't treat them at all and these horses do fine. It depends on what the owner's goal is, what they'll live with, what their needs are, and how severely the horse is affected."
Carter notes that affected horses can still be ridable, although the gait can be distracting. "I was watching a major stakes race on TV. This horse being walked up to the starting gate had pretty significant stringhalt in one leg, but he won the stakes race."
Mildly affected horses often can still participate in physical activities such as jumping or cutting, he says, but show hunters and horses in other show/conformation classes will probably not score very high with severe stringhalt.
When treatment is recommended, surgery is the most common option. "The most conventional treatment is to remove the lateral digital extensor tendon," Carter says. This surgery doesn't affect normal limb use.
Postoperatively, the treated limb is kept bandaged for a few days, then the horse should be hand-walked until the sutures are removed, which is usually after a couple of weeks. After that, the horse can slowly be returned to his normal activities.
Prognosis with surgery varies. "A fair number of horses do respond," reports Carter. "I had a horse here a few years ago that was so bad that when you'd take him out of his stall, he'd almost fall down as he was trying to hyperflex both legs at the same time. It was a terrible sight. Within a week after surgery, he was normal.
"Most horses we see obtain some improvement, but not all horses improve after surgery, and not all horses become normal," says Carter. "Some just get less severe. If surgery is successful, many horses appear normal by the time we take the sutures out, and some will continue to improve for several weeks."
Trauma-induced stringhalt is treated using the same surgical techniques as with classic stringhalt and yields a similar response and prognosis, Carter says.
Australian stringhalt usually becomes a little more severe the first month after the disease is noticed, then gradually resolves on its own with muscle atrophy and gait returning to normal.
"Given enough time, spontaneous recovery is the rule rather than the exception," Huntington states. "Some horses have recovered within two weeks, while others have taken two years; the most common recovery period is six to nine months. Many horses can be used fully some months before they are completely recovered, as at that stage they have only minimal alterations in gait." (Horses which become roarers secondary to stringhalt could require surgery to repair laryngeal function.)
Huntington recommends that affected horses be removed from the pasture, although horses have recovered while remaining in the same pasture since the disease is seasonal with causative factors likely persisting for only a short time in the pasture. Horses removed from the pasture in which the disease occurs can be safely returned to the pasture after the autumn break.
Although horses eventually recover without treatment, some horse owners opt to treat the horses, attempting to hasten recovery. "Removal of a portion of an extensor tendon over the hock has been popular, but has very variable results as some horses improve after surgery but then regress again later," says Huntington. "The success rate of surgery for ASH doesn't really justify its use apart from selective cases where it might be necessary to improve the quality of life for the horse."
Medical therapies have also been tried for ASH. "Two new medical treatments have been studied by researchers in Australia in recent years," Huntington reports. "At the University of Melbourne, we investigated the use of the drug Dilantin (extended phenytoin sodium), which was used in people and other animals for the treatment of epilepsy. This was successful in reducing the severity of the clinical signs; after the treatment ceased, some horses showed a residual improvement in their gait. Baclofen (4-chlorophenyl-butanoic acid) has also been used in a limited number of cases at the University of Sydney, and this drug gave some interesting results. However, Baclofen is very expensive, and at this stage it is hard to recommend its general use. There may be some logic in the use of high doses of vitamin E as this is an antioxidant used as a therapy for some other disorders of long nerves in humans and other animals. Magnesium and high doses of vitamin B1 have also been suggested as a therapy, but there is no evidence to support the claims. If these compounds are effective, it is likely to be through symptomatic means rather than treatment of the primary neuropathy."
Overall, Huntington prefers to allow the horse to recover spontaneously. "Treatment in many forms has been unrewarding," he states. "Most horses cope quite well in the paddock with ASH, and it is more
distressing for the owner than the horse."
With causes of classic stringhalt as yet unknown, preventive techniques are also unknown, says Carter. "As far as the trauma-induced stringhalt, try to keep horses out of wire or from traumatizing their legs by kicking through stalls. For the pasture (Australian) form, management of pastures and eradication of flatweed should prevent this form of stringhalt. But for what we typically see in this country--classic stringhalt--I don't know that there is a way to prevent that (until we better understand the causes)."
FLATWEED AND AUSTRALIAN STRINGHALT
Flatweed (Hypochaeris radicata) is often seen in association with Australian stringhalt (ASH). It is a perennial weed found in the eastern United States as far north as New Jersey and as far west as Mississippi, according to information posted at the Virginia Tech Weed ID Guide web site. It resembles the dandelion--leaves are arranged in a rosette and have toothed or irregularly lobed margins. Says Peter J. Huntington BVSc (Hons), MACVSc, MRCVS, a long-time researcher of ASH at the University of Melbourne, flatweed leaves are hairy, while dandelion leaves are smooth and have sharp-toothed margins. Flatweed flowers are on branched stems with multiple flowers that have bright yellow rays with teeth at the tip, while dandelions each have a single flower.
For more information, see www.ppws.vt.edu/scott/weed_id/hryra.htm.
About the Author
Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.