Stringhalt in Horses: Causes, Symptoms & Treatment

Stringhalt, or equine reflex hypertonia, is a neuromuscular condition that causes abnormal hindlimb movement in the horse.

Horses with stringhalt have excessive and prolonged flexion of the pelvic limbs while in forward movement, showing signs of the condition at most gaits. ^[15]

One (unilateral) or both (bilateral) legs may be affected. Some horses experience mild cases characterized by involuntary jerking of the hindlimb, while others experience lameness and difficulty standing up.

Horses of all ages and breeds can be affected by stringhalt. In some cases, it is caused by ingesting toxic plants at pasture, but other cases develop quickly without apparent cause.

Diagnosis is complicated due to limited research on equine neurological movement disorders. ^[4] The prognosis and treatment of stringhalt depend on the individual case.

Contact your veterinarian if your horse shows signs of a neuromuscular movement disorder.

Stringhalt in Horses

Equine reflex hypertonia or stringhalt is characterized by involuntary jerking or flexing of the hind limb.

This condition can be acquired and intermittent (temporary), or chronic with progressive symptoms. It is often idiopathic meaning it has no known cause and arises spontaneously.

Mild cases involve sporadically lifting and grounding the foot while walking. More severe cases involve full leg spasms in which the foot is raised to the horse’s stomach and then dropped to the ground.

Other progressive movement disorders with similar presentation include Shivers and Stiff-horse syndrome (SHS).

Clinical Signs of Stringhalt

Horses can develop clinical signs of stringhalt suddenly or gradually over time. These symptoms can range from mild to very severe, where a secondary injury is possible due to involuntary kicking.

Horses that are nervous, excited or agitated may demonstrate more gait abnormality. Some movements may exacerbate signs, such as walking downhill, turning sharply or suddenly stopping. Cold weather, hard exercise and sudden movement after rest can intensify symptoms.

The most common signs of the condition are: ^[5][15]

• Involuntary, exaggerated upward movement of the hindlimb
• Hopping or jerking
• Kicking upwards towards the belly
• Incoordination, dragging hind hooves
• Muscle atrophy of the lower hindlimb
• Inability to stand up without assistance

The clinical presentation of stringhalt varies greatly between individuals, which makes the disorder difficult to diagnose. If your horse is showing signs of stringhalt, contact your veterinarian immediately.

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Idiopathic Stringhalt

Idiopathic stringhalt is sometimes referred to as classic or true stringhalt. The exact cause of this condition is unknown, and it can develop overnight.

The condition tends to affect one hind leg (unilateral), and hyperflexion is particularly evident when the horse is walked, trotted or turned in a tight circle. ^[15]

In horses with idiopathic stringhalt, the nerves that convey impulses and trigger contraction of the lower hindlimb are affected, although the exact cause of neurological dysfunction is unknown.

Damage to the peripheral nerves leads to abnormal smooth muscle contraction. This explains the jerky, random contractions observed in the hindlimb.

Injury to the muscles of the lower hind limbs, back or neck may also lead to stringhalt. ^[15] Unfortunately, the signs of the condition can progress over time, and horses rarely recover from idiopathic stringhalt without surgical intervention.

Pasture-Associated Stringhalt (PSH)

Pasture-Associated Stringhalt (PSH), or sporadic stringhalt, results when horses are exposed to a plant neurotoxin that damages the peripheral nervous system. This is generally defined as neuropathy. ^[5]

This condition usually occurs in an outbreak when horses are kept on poor-quality pasture during late summer and autumn and thus forage on weeds they otherwise wouldn’t be interested in..

The onset of PSH is sudden, and signs can worsen quickly, although the condition is usually temporary. Depending on the severity, recovery can take between 6 – 24 months. Some horses never fully recover.

Horses with PSH usually have bilateral hyperflexion of the hindlimbs. This results in a bunny-hopping gait when in movement.

In extremely rare cases of PSH, the forelimbs can be affected in what is called atypical bilateral stringhalt syndrome. ^[12]

PSH can also affect nerves in the larynx (voice box). Approximately 20-60% of PSH-affected horses demonstrate abnormal vocalization due to laryngeal dysfunction.

This condition limits airflow to the lungs and produces an abnormal, high-pitched noise during inhalation (stridor). ^[7]

Hypochaeris radicata

The pasture weed hypochaeris radicata (flatweed, catsear, false dandelion) is known to cause bilateral stringhalt in horses. This perennial weed is native to Europe, but has been introduced to North and South America, Japan, Australia and New Zealand where it is considered invasive. ^[15][1]

It’s important to be able to identify H. radicata and distinguish it from its harmless lookalikes to keep your pastures healthy. Unlike true dandelions with jagged leaves that grow on a single stem, H. radicata has solid, branching stems and hairy, lobed leaves.

Pasture-Associated Stringhalt (PSH) outbreaks are often associated with dry, overgrazed pastures in which drought-resistant H. radicata can survive. ^[11] Horses will consume this weed if other forages are unavailable.

Other Toxic Plants

Several toxic plants are associated with stringhalt in horses, including Lathyrus species, Taraxacum officiale (common dandelion) and Malva parviflora (marshmallow, mallow weed). ^[2][15]

These plants have been tied to PSH outbreaks in North America, Australia, New Zealand and Brazil.

Most toxic plants have a bitter taste, preventing horses from consuming them. However, horses will eat bitter plants if good quality forage is unavailable.

To prevent poisoning from toxic plants, ensure your horse has access to good-quality forages at all times when out at pasture. If access to pasture grass is limited, ensure horses have access to hay while turned-out.

Preventing Pasture-Associated Stringhalt (PSH)

Outbreaks of pasture-associated stringhalt (PSH) are more common in unmanaged pastures in which toxic weeds become dominant as grasses are overgrazed.

Desirable pastures contain multiple species of healthy forages. Horses prefer to consume a variety of forages and will ingest a mixture of grasses and weeds when given the option. ^[14]

Routine fertilization of healthy grasses with a non-toxic fertilizer can help to keep your pasture healthy.

When proper soil fertility is maintained, grasses out-compete weeds for resources. However, excessive grazing can damage pastures, allowing stress-resistant, toxic plants to grow in place of forages.

Controlling Toxic Plants

Regularly walk your horse’s pastures to inspect for toxic plants. If toxic plants are present, they should be fenced off from horses or removed from the pasture to avoid the risk of toxicosis.

Limit the growth of toxic weeds in your pasture by implementing a rotational grazing system that allows healthy pastures time to rest and grow.

Keeping horses off muddy pastures and providing horses with supplemental forage when fields are bare can also reduce weeds. ^[6][14]

H. radicata infestation is difficult to control. The taproots of the plant must be removed because they can resprout after being mowed or pulled.

Broadleaf herbicides can be used in the spring or fall. ^[6] These herbicides will not damage established grass species, making them ideal for pastures.

In cases of severe toxic plant infestation, field renovation is necessary. This process is costly but can prevent outbreaks of stringhalt in horses.

Diagnosis

Stringhalt is diagnosed based on an abnormal gait involving hyperflexion of one or two pelvic limbs at the walk and trot. ^[15]

Your veterinarian will diagnose your horse with stringhalt based on observation of clinical signs and a neurological examination. The diagnosis may be confirmed with electromyography which assesses how nerves fire as the horse moves, though is not commonly pursued.

Following the examination, the clinician will compare a horse’s clinical signs with a list of differential diagnoses to rule out other conditions and determine a treatment plan.

Your veterinarians may walk the horse’s pasture to look for evidence of toxic plants before diagnosing acquired stringhalt.

Case History

Clinical signs of stringhalt can appear suddenly or develop over time, so it is important to understand what normal behaviour looks like for the individual horse.

Horses can compensate for mild neurologic dysfunction, and abnormal behaviours may go unnoticed until they become severe. ^[3] Performance horses may meet expectations in their sport until their neurological condition progresses.

In some cases, idiopathic stringhalt is caused by injury or trauma to the body, including the back, neck or hindlimb.

Evidence of past trauma to the metatarsus or hock in combination with clinical signs could indicate idiopathic stringhalt. However, signs can develop in all breeds and any age, and there are few known risk factors for this condition.

Regularly observe your horse’s behaviour for early warning signs of neurological conditions. If you notice any sudden or gradual changes in your horse’s movement, contact your veterinarian for an examination.

Neurological Examination

A neurological examination is warranted if a horse shows signs of a neurological movement disorder or disease. ^[10]

Examinations may differ between veterinarians but can generally be broken down into four steps:

1. The mental status evaluation
2. The cranial nerve examination
3. The spinal reflex and muscle evaluation (muscle atrophy, skin sensation, etc.)
4. The gait and posture examination (walking, circling, backing, etc.)
5. Examination of the horse’s posture, limb placement, and movement

Most clinicians will conduct a lameness test to rule out hoof pain that can present as a stringhalt gait.

Differential Diagnosis

The clinical signs of stringhalt mimic those of several other conditions. A differential diagnosis must be made against other conditions that can affect movement and muscles in the horse, including:

• Hoof pain or lameness
• Upward Patellar Fixation
• Shivers
• Fibrotic myopathy
• Peripheral neuropathy
• Polysaccharide storage myopathy (PSSM)
• Equine protozoal myeloencephalitis (EPM)

Electromyography (EMG)

Electromyography (EMG) is a diagnostic tool used to determine the health of nerve cells that send signals from the brain to the muscles.

The nerves are stimulated with an electric current, and the speed of signal conduction is determined. ^[13]

Horses with stringhalt often have higher levels of electromyographic activity in affected muscles. ^[8]

Treatment

Consult with your veterinarian to determine an appropriate treatment plan if your horse is showing signs of stringhalt. Treatment may involve surgery, medication, or nutritional support.

Surgery

In progressive cases of idiopathic stringhalt, surgical intervention is recommended to eliminate the abnormal movement of the hindlimb.

Horses may undergo a lateral digital extensor myotenectomy, wherein the extensor tendon is removed alongside 7-10 cm of belly muscle. ^[15]

The success rate for myotenectomy varies greatly between individual cases. The likelihood of a normal gait following surgery is unpredictable, and the degree of improvement is variable.

A survey study in Australia found that over half of lateral digital extensor surgeries were successful in eliminating clinical signs of stringhalt. ^[12]

Some horses experience a recurrence of stringhalt symptoms after surgery. Many horses do not regain full normal movement, but surgery helps to reduce clinical signs so they can live comfortably.

Medication

Regular administration of anticonvulsant medication (phenytoin) is effective for decreasing clinical signs of PSH. ^[15][16]

When given a daily dose over the span of several weeks, many horses show significant improvement in gait abnormality and reductions in muscle spasms. ^[9] EMG recordings of horses given phenytoin show reduced spontaneous electrical activity in affected leg muscles.

Other muscle relaxants that could be used to treat stringhalt in the future include mephenesin and baclofen, although further testing is needed. ^[5]

Researchers have investigated the use of Botox (neurotoxin Botulinum type A) in horses with stringhalt. This drug is used to manage hypertonic or spastic muscular conditions in humans. ^[5][17]

Some case reports show improvements in horses with idiopathic stringhalt given regular Botox A injections. Further research is needed before Botox A can be used widely in practice.

Nutritional Support

As with all horses, those with stringhalt benefit from vitamin and mineral supplementation to ensure there are no nutrient deficiencies in their diet.

Vitamins and minerals are critical for optimal nerve function as several are involved in nerve signal transmission and maintaining healthy cells.

Thiamine (vitamin B1) may be helpful for horses with stringhalt. This vitamin supports nervous system health and is involved in neurotransmitter production and myelin synthesis. ^[5] Other B vitamins play a role in proper nerve function.

Antioxidant supplements are sometimes administered to horses with stringhalt to reduce injury to peripheral nerves. Antioxidant supplements include vitamin E, Vitamin C and dimethyl sulfoxide. ^[5]

Dimethyl sulfoxide (DMSO) is an antioxidant that can be administered intravenously, orally via nasogastric tube, or topically. With it’s purported anti-inflammatory properties, it remains a relatively common adjunctive therapy for musculoskeletal and neurologic conditions in horses. However, there is limited research on the safety of DMSO for long-term daily use.

It’s important to discuss which routes of administration are most appropriate for your horse and how long therapy should continue with your veterinarian. It is unlikely that horses will voluntarily consume DMSO and repeat topical application may irritate the skin over time. Options like vitamin E and vitamin C may be more feasible for long-term antioxidant support.

An equine nutritionist can help you formulate a feeding program to manage a horse with stringhalt.

Prognosis

The prognosis for horses with stringhalt varies depending on the type of stringhalt (acquired vs. idiopathic) and the individual case.

Horses with pasture-associated stringhalt tend to recover spontaneously within days or weeks. However, some horses take longer to heal and may not recover fully.

Laryngeal dysfunction resulting from PSH may be permanent if paralysis is present.

It is rare for affected horses to recover from idiopathic stringhalt without surgery, although some may see their condition improve over time. Relapses of stringhalt can occur during recovery.