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. 
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.  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.
- 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.
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. 
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.  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. 
This condition usually occurs in an outbreak when horses are kept on poor-quality pasture during late summer and autumn.
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. 
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). 
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. 
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.  Horses will consume this weed if other forages are unavailable.
Other Toxic Plants
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.
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. 
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.
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.  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.
Stringhalt is diagnosed based on an abnormal gait involving hyperflexion of one or two pelvic limbs at the walk and trot. 
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.
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.
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.