Stringhalt is a layman's term used to describe a gait abnormality that involves exaggerated flexion of one or both upper pelvic limbs (including the hock and stifle). In severe cases, affected limbs may be elevated rapidly and violently strike the horse's abdomen, making normal ambulation difficult. Similar to the function of strings on a marionette, stringhalt can cause a horse to yank the hind legs up and halt them there momentarily before taking the next step.
See what this looks like:
Descriptions of stringhalt 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).
The condition not considered to be painful.
Abnormal gait characteristics are often exaggerated in nervous or agitated horses, or during backing.
There does not appear to be any breed or age predilection associated with Stringhalt. Gait characteristics can mimic those associated with shivers, fibrotic myopathy, intermittent upward fixation of the patella, and peripheral neuropathy (nerve degeneration) due to equine protozoal myelitis (EPM). Careful examination is necessary to differentiate Stringhalt from other diseases of clinical resemblance.
See a comparative disease chart HERE.
Most cases of stringhalt are attributed to peripheral nerve degeneration. Nerves providing motor innervation to the lateral digital extensor (LDE) muscle of the hindlimb appear to be targeted in cases of Stringhalt. Although the specific cause of nerve degeneration may be unclear in many cases, ingestion of toxic plants or injury (external or internal trauma) to the lateral digital extensor muscle and/or associated nerves is frequently implicated.
TYPES OF STRINGHALT
Veterinarians divide stringhalt into two major categories: "AUSTRALIAN" stringhalt is an often-transient disease resulting from toxicity, whereas "CLASSIC" (or atypical, depending on the geographic region) stringhalt has no known cause and rarely resolves on its own.
Stringhalt affecting multiple individuals as a consequence of ingesting toxic plants has acquired the common name of Australian stringhalt. It was originally described by veterinarians in Victoria in the late 19th century. Despite its name, Australian stringhalt can affect horses worldwide and has commonly been reported in the United States, New Zealand and Australia. It also has been more recently diagnosed in South America.
False dandelion (Hypochaeris radicata), also known as flatweed, has been incriminated as a major cause of Australian stringhalt. New cases often manifest in late summer or fall, frequently in horses grazing on poor-quality pastures abundant with weeds.
Although other plants including the common dandelion (Taraxacum officinale), cheeseweed or little mallow (Malva parviflora), vetch and sweet peas have been linked to Australian stringhalt, flatweed remains the primary indicator plant.
Not all false dandelion or other suspect weeds cause stringhalt, and many horses graze in "infected" paddocks without issue. Since toxicity of the same species of plant can vary from one location to another, the environment is thought to play a role in providing optimum conditions for producing the toxin.
Australian stringhalt targets larger nerve fibers; these are generally associated with the horse's pelvic limbs and larynx (voice box), both areas of which can be affected concurrently.
In most cases of Australian stringhalt, both pelvic limbs are afflicted with relative equivalency. In some cases the thoracic (fore) limbs are also debilitated. In horses exhibiting bilateral stringhalt a more central location of nerve damage (such as the spinal cord or brain) is considered to be more likely.
Laryngeal hemiplegia (or "roaring") may stem from recurrent laryngeal nerve damage resulting from the laryngeal component of Australian stringhalt. This issue may persist despite eventual resolution of gait abnormalities associated with the limbs.
Unlike Australian stringhalt, classic stringhalt does not typically occur in outbreaks; a specific cause is often not identified. Although clinical signs are similar, classic stringhalt more commonly manifests unilaterally (in one back limb) rather than bilaterally (in both back limbs).
Injury to the back, neck, and/or affected limb are the most commonly incriminated causes of classic stinghalt. Clinical signs associated with trauma-induced stringhalt often dissipate as injured tissue heals.
In cases lacking a history of trauma, however, causative agent(s) may be more difficult to discern.
Cervical arthrosis (degenerative joint disease in the neck) has been implicated in several cases of stringhalt. This issue can lead to upper motor neuron abnormalities which may correlate with the development of stringhalt.
Clinical signs often progress in cases of classic stringhalt, presumably as a result of increased age and/or exercise intensity.
Time. Some cases of stringhalt may resolve without treatment; spontaneous resolution usually takes a period of several months or more, however.
Once the source of the problem has been eliminated (e.g. once a horse has been removed from a pasture containing toxic weeds, for example) 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.
Medication. Sedatives may reduce anxiety and thereby improve the condition during the acute phase of Stringhalt. Muscle relaxants, anticonvulsants, and drugs that work directly on the central nervous system can be helpful for some horses with longer-lasting stringhalt.
The botulinum toxin "Botox", commonly used to smooth facial wrinkles in humans, is under investigation at the moment for its use in alleviating stringhalt symptoms.
Pasture Management. Affected horses should be removed from pasture and the pasture should be inspected for toxic plants known to cause stringhalt.
Surgery. Lateral digital extensor (LDE) tenectomy, a surgical procedure involving removal of a section of the lateral digital extensor muscle and tendon along the lateral aspect (outside) of the hock, can result in alleviation of clinical signs in many cases.
Since this muscle/ tendon combination is responsible for the exaggerated hindlimb movement associated with Stringhalt, its removal frequently results in immediate improvement in pelvic limb gait. Response to this procedure is variable, however, and can be difficult to predict preoperatively. Some horses do not exhibit any improvement pursuant to this procedure.
Horses with unilateral disease (affecting one leg) and those attributed to previous trauma generally have a better response to surgical intervention. Bilateral cases of stringhalt and those considered to be a consequence of central nerve pathology carry a poorer prognosis. Finally, more acute (recent cases) tend to carry a better prognosis than chronic (long-term) cases.