Stringhalt is the involuntary hyperflexion of the hock joint when the horse moves and may involve one or both hind legs. The upward movement of the limb is exaggerated while the downward motion of the limb is normal. The upward movement varies in the intensity of expression from minimal to where the fetlock can contact the abdomen.
WHAT ARE THE DIFFERENT TYPES OF STRINGHALT?
There are two forms of the condition which appear to be geographically predisposed. One form affects isolated horses and usually affects only one limb. It occurs usually after injury to the limb and spontaneous recovery is rare. Some improvement may be seen with rest. It usually involves the lateral digital extensor muscle tendon unit. This type of stringhalt can occur anywhere in the world but is usually termed North American stringhalt. The other type of stringhalt is restricted to New Zealand and Australia. It can occur as an outbreak affecting multiple horses and usually affects both hind legs.
WHAT CAUSES THE OUTBREAKS OF AUSTRALIAN STRINGHALT?
It usually occurs in late summer and autumn where there is poor quality and or quantity of pasture due to reduced rainfall. In these times horses are exposed to more toxic weeds in the paddock. Feeding trials using the suspected weeds have failed to reproduce the condition and not all exposed horses contract the condition. It has been theorised that mould spores that grow on plants could, in fact, be the culprit. The peak time for the occurrence of stringhalt in Australia coincides with peak times for other plant related mycotoxic diseases. There are several common factors that are consistently noted. There is a strong association with the presence of the flatweed Hypochaeris radicata. Most outbreaks occur in late summer / autumn after prolonged dry spells with paddocks being mainly of poorer unfertilised soil types. A study by Rural Industries Research and Development Corporation in 2011 (Publication No.11 / 127) was conducted to identify possible fungal toxins in flatweeds. It had been previously hypothesised that mycotoxins associated with the flatweed may have been responsible for the onset of the clinical signs of Australia stringhalt. The findings of the study were that no evidence of mycotoxic alkaloids could be found. It would suggest that the toxic compounds come from the flatweed itself.
WHAT IS THE PATHOLOGY OF THE CONDITION?
It is known that there is toxic damage to the axon sheaths of the long peripheral nerves especially in the larynx and gaskin area of the hind leg. This leads to dysfunction in the larynx (upper throat area) and atrophy (wasting) of muscles in the gaskin area.
WHAT ARE THE CLINICAL SIGNS OF STRINGHALT?
The gait can be graded from very mild to severe in expression. It may be intermittent or occur with every step. It is usually more noticeable after rest and when backing up. Any breed can be affected. There are also signs of laryngeal paralysis with the Australian form of stringhalt.
WHAT IS THE TREATMENT FOR AUSTRALIAN STRINGHALT?
The majority of horses recover spontaneously with no treatment over a period of several weeks to a year. Improving soil fertility and pasture type would also be advised for long term management of the problem. Grazing of pastures that have a predominance of flatweeds should be avoided - especially after dry periods of time.
WHY IS MAGNESIUM USED TO HELP TREAT STRINGHALT?
There is no scientific evidence to support the theory that adding extra magnesium over and above normal daily requirements cures or prevents stringhalt from developing. It would appear that because magnesium is vital for the normal function of the nervous system that it has been assumed that it would help stringhalt cases. Low levels of magnesium in the body will produce hypo-magnasaemic tetany (grass staggers) which is a totally different pathology to toxic damage to a nerve as occurs in stringhalt. It may also be that because magnesium helps 'nervy' horses that it was assumed that it would help stringhalt. If supplementation with extra magnesium over a period of time also coincided with a spontaneous resolution of the stringhalt it could also be assumed that the magnesium 'cured' the condition.
Australian stringhalt is known to be caused by toxic damage to the axons of some of the peripheral nerves of the larynx and hind leg. It also tends to resolve spontaneously and no specific treatment has been scientifically validated to cure the condition. Supportive nursing care is required while the body's immune system deals with the toxin and allows healing of the affected nerves.