A horse with stringhalt can look perfectly normal standing in his stall or pasture. But when he begins to move at a walk or slow trot, his hind leg is jerked up involuntarily towards his belly. In some cases, he actually touches the bottom of his abdomen with the front of his pastern with each step. In others, the movement is less dramatic, but still significantly affects the horse’s gait. When both hind legs are involved, the horse may even end up “bunny hopping” as the two legs kick up at the same time.
The movement is usually worse if the horse has been resting or if the weather is cold, and tends to improve or even temporarily disappear after the horse has been exercising for a period of time or when the horse is working at a canter. Fortunately, the abnormal movements don’t seem to cause the horse any pain.
While, at first glance, the horse owner might imagine that this is a problem with the muscles – since it is a muscle contraction that pulls the leg up in this unnatural way – this is actually a neurological disease. The affected nerves control the muscle and cause the typical stringhalt jerking motion.
Making an Accurate Diagnosis
Stringhalt, also called equine reflex hypertonia, has been seen in horses for hundreds of years (Shakespeare even mentions it in Henry VII, although the name he gives the condition is “springhalt”), and is not particularly rare today, said Dr. Catherine Radtke, a professor at the Atlantic Veterinary College connected to the University of Prince Edward Island. It is seen in all breeds and in horses used in all equestrian disciplines. Dr. Radtke added that she has never seen a foal with stringhalt, but it can happen in adult horses of all ages.
If you suspect your horse has stringhalt, Dr. Radtke advises you to check with your veterinarian as soon as possible. “There are other diseases and conditions which can look like stringhalt, so it’s important to get an accurate diagnosis,” she said. When a horse has fibrotic myopathy, for example, he will jerk the limb backwards just before it hits the ground and this motion can be mistaken for stringhalt. A horse with shivers will also have hyperextension, but a 2015 study by Draper et al found that the movement only looks like stringhalt if the horse is backing up; the horse with stringhalt has the motion of kicking up towards his abdomen when walking forward. Intermittent upward fixation of the patella and nerve degeneration caused by equine protozoal myelitis (EPM) can also cause similar gait abnormalities.
“Stringhalt can vary a lot in how severe it is at first, so the lesson is to have your veterinarian take a look at how the horse is moving if you notice something abnormal,” said Dr. Radtke. When available, electromyography (EMG) examinations can be used to help determine the severity of the condition and distinguish it from other conditions with similar symptoms.
Equally important, your veterinarian will be able to determine which type of stringhalt your horse has.
Two Types to Consider
Not all stringhalt cases are the same. “There are two types, idiopathic and acquired, and the first step is to determine which it is,” Dr. Radtke explained.
According to Dr. Radtke, it is important to know which type of stringhalt the horse has, as the treatment and expected outcomes can be quite different. The acquired form of stringhalt (sometimes called Australian or outbreak stringhalt) frequently causes the unusual movement in both hind legs, not just one. It is caused by the horse eating certain toxic plants while at pasture, and so is often seen in late summer or fall after drier-than-usual conditions, when weeds tend to become more abundant and horses are more likely to eat them because there is little grass.
Although it’s often called the “Australian” type of stringhalt (because it was first recognized there), it’s certainly not limited to that country. Outbreaks have been reported in New Zealand, Canada, USA, Brazil, Italy, Chile and Japan. The plants known to cause the symptoms include Hypochaeris radicata (which is also known as the Australian dandelion, false dandelion, flatweed or catsear), Taraxacum officinael (common dandelion) and Malva parviflora (little mallow or cheeseweed). To confirm the link between the plant and the symptoms, José Allan Soares de Araujo, DVM, Ph.D., researcher at the veterinary hospital at the Universidade Federal de Campina Grande in Patos, Brazil, conducted a study in which he induced stringhalt in a healthy horse by adding false dandelion to the animal’s feed for three weeks. Once they stopped putting the toxic plant in his feed, the horse recovered fully in 15 days.
Often more than one horse in the herd sharing the pasture will show stringhalt symptoms.
Treatments for Acquired Stringhalt
A diagnosis of acquired stringhalt is a hopeful one, said Dr. Radtke, because once the animals have been taken off pasture and are no longer consuming the toxic plants, many horses will recover without any further treatment. In one study, 78 per cent of the horses got better once they were no longer exposed to the toxins, but the length of time required for the horse to be moving normally ranged from just a few days to more than 18 months. In horses where recovery is slow, treatment with phenytoin sodium may be recommended.
Researchers have suggested that toxins in the plants cause neuropathy (damage to the nerves) and this is what precipitates the jerking upwards of the hind legs as the horse walks. In about 60 per cent of the horses with acquired stringhalt, lesions are also found in the left recurrent laryngeal nerve (another effect of the toxins in the plants). This nerve controls the horse’s larynx, and if there is enough damage it can cause the horse’s airways to partially collapse. In those cases, the horse will make a roaring or gasping sound during exercise.
Because most horses will recover without any additional intervention, surgical treatment is not usually recommended. In a 2005 study by Torre, however, 11 horses with acquired stringhalt were treated with the surgery commonly used on horses with idiopathic stringhalt (see page 16 for a more detailed description of the surgery). The results: five were able to walk normally right after the surgery, five were back to normal within the next three months, and the last horse took nine months to make a full recovery. This may be an option if the recovery is slow.
Treatments for Idiopathic Stringhalt
“With idiopathic stringhalt, it’s a bit of a different story,” said Dr. Radtke. “For one thing, it usually only affects one leg, not both. It’s very rare to see this type in both hind legs. Another difference is that we don’t know the cause for the idiopathic type – in fact, there are probably multiple possible causes all leading to an underlying neuropathy.” Various researchers have identified over-extension injuries, hoof pain, spinal cord disease and adhesions of the tendons as factors in the development of the condition, but often the cause is not known. In general, there has been some kind of injury or disease that has damaged some part of the horse’s nervous system.
The other difference: idiopathic stringhalt will not improve on its own, and most cases will get worse.
Muscle relaxants and other anti-spasmodic drugs are sometimes used in mild cases, but are not always successful. A 2009 study by Dr. I. Wijnberg tested the use of Botox injections in the affected area of a small group of horses with stringhalt. The researcher found that the amount of hyperflexion in the horses’ motion was reduced, and the effects lasted up to three months. Further research may help to determine if this could be a long-term solution.
While these treatments may be helpful, the most common way of treating idiopathic stringhalt is surgery, even though the outcome is not always favourable, said Dr. Radtke. “The horse’s owner needs to understand that there may be complications, or that even if the surgery goes well the horse may not make a full recovery. Or, he may recover but it could take more than a year after the operation.” On the other hand, many horses do show improvement and some will have a quick, full recovery.
The surgery is called lateral digital extensor tenectomy and partial myectomy. To translate into less-technical language: the veterinary surgeon will remove between two and 10 centimetres of the lateral digital extensor muscle (which is just above the horse’s hock) and the entire tendon which attaches this muscle to the long digital extensor tendon.
When a smaller amount of muscle is being removed, the surgery can be done with the horse standing and a local anaesthetic given. If the veterinarian feels that a larger section should be removed, it can be done under a general anaesthetic. Dr. Radtke said she has seen both approaches used with success.
Two incisions are made, one above the hock and one below. The tendon is pulled out through the lower incision and cut, then pulled out through the upper incision. The section of muscle at the upper incision (which is larger) with the attached tendon is then also removed, and the incisions are closed up. Pulling out the tendon can be difficult, Dr. Radtke commented, as significant force is required especially if there are any adhesions.
“As with any surgery, there is the possibility of infection and other problems,” said Dr. Radtke. As well, even when the surgery seems to go well, some horses will continue to have abnormal movements.
Recovery from Surgery
After surgery, the entire leg is bandaged and the horse put on stall rest for 10 to 14 days. After this, the stitches are removed and if everything looks good, the horse can be hand-walked daily for the next week. Gradually increasing work would then begin with the expectation that the horse would be back to normal by three or four weeks after surgery.
There is, however, considerable variation in outcomes. Some horses will show only slight improvement after the surgery and never make much progress after that. Others are walking normally as soon as their legs heal. Some horses do make a full recovery but it takes several months after the surgery for the benefits to be seen.
Other horses will continue to have stringhalt symptoms despite the surgery and careful management of the recovery period.
Some alternative practitioners have recommended herbal remedies and homeopathic treatments for stringhalt. Dr. Robert McDowell of Australia recommends (on his website) a mixture of herbs which includes “Comfrey, Dandelion, Magnesium Phosphate, Mugwort, Pine Bark, Sage, Saw Palmetto and Valerian along with the Bach Flowers Heather, Vine, Elm, Vervain and Rescue Remedy. This course is given over a 12-week period and a single dose each day of yogurt (acidophilus) (about a couple of dessert spoons) is given as well.” He states that the treatment is also formulated to help rebuild any muscle wastage resulting from the condition and to reduce sensitivity so that the animal will be much less likely to come down again with stringhalt.
Acquired stringhalt is best prevented by keeping horses from eating the toxic plants known to cause this condition. Keeping pastures in good condition so the horses have plenty of good-quality grass, and knowing how to recognize false dandelion or other plants which may cause these symptoms in your community will help you keep your horse safe, said Dr. Radtke. It is not contagious so there is no need to keep affected horses separated (although if one horse in your herd develops symptoms, moving the entire herd out of that pasture may help prevent the others from eating the toxic weeds), and it is not genetic so cannot be passed down from one generation to the next.
While the causes of idiopathic stringhalt are often unknown, there is some evidence that it can be related to injury, so it may help to avoid overwork and excessive stress. A 1994 study by Crabill found 10 cases of horse developing stringhalt after injury to the dorsal metatarsus (below the hock). For six of the horses, stringhalt symptoms began within three months, while the others took a longer time for the symptoms to appear. Of course, many injuries are simply accidents and can’t be avoided.