Australian stringhalt

Stringhalt is characterized by an involuntary hyperflexion ranging from slight to severe of one or both hind limbs.

• The « classic » or idiopathic form, often affects only one hind limb, and is usually associated, among other causes, to a trauma on the dorsal side of the canon. It can appear after a few days, months , sometimes even years after the trauma occured.
• The « Australian form » wich is bilateral and due to distal axonopathy (the nerve fibres, axones are affected) – especially of the fibular nerves – which inhibit the myotatic reflex (muscle contraction reflex which occurs as a reaction to stretching the said muscle). Each of the hind limbs can be affected to a different degree.

It can be difficult to distinguish between the two forms of stringhalt just from the clinical signs. Only an epidemiological study will establish the difference.

This weed which thrives in dry conditions was thus incriminated. The drought in 2003 had in fact allowed the plant to proliferate in the South of France. These observations led to suggest a toxic etiology (intoxication from grazing flatweed). The plant’s toxicity seems however to vary depending on certain factors (synthesizing of secondary toxic metabolites following stress...)

Epizootic episodes of stringhalt on pastures where there is no flatweed have however been reported. Ingesting flatweed could therefore be an aggravating factor, but not the only factor. Consequently, several coexisting factors, such as mycotoxins, or nutritional deficiencies, have also been alluded to).

Hyperflexion of one or both hind limbs

The hyperflexion often appears suddenly and bilaterally. It occurs during voluntary movement, sometimes even at rest, with varying degrees of severity, frequency, and duration.

Huntington et al. (1989) suggested the following classification of the clinical signs following the severity of the condition:

Thus, depending on the degree of severity of stringhalt, hyperflexion varys from a mild exaggeration when flexing the hind limb, to a violent flexing movement where the horse strikes his abdomen with his fetlock.

Amyotrophy

In the most severe cases muscular atrophy (decrease in muscle volume) of the hind limbs due to neurological causes can become apparent.

Laryngeal hemiplegia

Australian stringhalt is occasionally associated with impairement of the laryngeal nerve (hemiplegia of the larynx) which leads to « roaring », suspected when during exercise the horse produces abnormal breathing noise.

Behavioural signs

Epidemiological clinical diagnosis is based on the symptoms, but to obtain a definite diagnosis the only sure method is an EMG (electromyography : medical technique analysing the neuromuscular system) of the lateral digital extensor muscle.

Since the precise cause(s) of the disease are still largely unknown, the treatment is sympyomatic (management of the clinical signs).

Restrict the horse’s ingestion of flatweed.

It is always better to prevent rather than cure ! Identifying and eliminating flatweed from the fields where the horses graze is the best way to limit the risk of poisoning through ingesting the plant in the grass or in hay.
Since the plant has a very developped root system, it is difficult to destroy it by hand, even when it is not largely present in the fields. The best way to get rid of it is to treat the pasture with a weed killer, or to turn the soil and reseed (when the fields are too impoverished).

When you have identified the presence of flatweed in your fields, move all the horses (whether affected or not) out of the fields where flatweed is present. Avoid letting the horses graze on poor soils during periods of drought.

Administer medication to reduce the clinical signs

Being restricted in his movements can induce stress in the affected horse (agitation, fear, depression, aggressivity…) and thereby worsen the clinical signs (stiffness…). Consequently, administering myorelaxant drugs which will act on the central nervous system, or tranquilizers (e.g. acepromazine) can prove beneficial.

Phenytoin (antiepileptic drug) can induce rapid albeit only partial regression of the stringhalt. It can be administered over several months. However there is no pharmaceutical form specifically for equids, which makes it quite unaffordable and only justified in cases where the horse cannot move 

Resorting to surgery

Myotenectomy (partial or total removal of a muscle) of the lateral digital extensor muscle, is more frequently resorted to in the case of classic stringhalt, and is the subject of some controversy since the mechanics of its action are not totally understood. However, according to an Italian study, it appears to give good results when used in the case of Austrlian stringhalt.

Be that as it may, spontaneous remission is frequent although long-winded, changing the horse’s environment remains the easiest and most effective treatment.