Also known as “Springhalt” and “Hahnentritt”. Stringhalt is an unusual disease that can affect adult horses of any breed. The syndrome is categorized by sudden, excessive and sometimes prolonged flexion of one or both hind limbs during attempted movement. In contrast to Shivering the limb is flexed slightly towards centre (adduction), can occur at gaits faster than a walk, and is uncommonly seen when backing. Stringhalt is graded, based on severity, from I through VI (examples of both are seen below).
Stringhalt is recognized in two major forms:
- A sporadic form that is usually limited to single hindlimb (unipedal stringhalt). This form occurs worldwide and is commonly preceded by trauma to dorsal aspect of the hock (tarsus) or top of the hind cannon bone (proximal metatarsus).
- An outbreak form of bilateral stringhalt is known as “Australian Stringhalt”. This form does occur elsewhere and has been described in New Zealand, Japan, Chile, and the United States. It is usually seen in poor-quality pastures during periods of “climatic stress”, usually late summer and fall. Not all horses that consume flatweed will develop Stringhalt. The signs typically involve both hindlimbs and can be symmetric or asymmetric. Distal limb muscle loss (atrophy) is obvious in chronic cases. Exposure to weeds has been made, all of which are present in Western Australia. In Western Australia there is a common association with flatweed (Hypochaeris radicata). These weeds are recognized by a basal rosette of leaves and yellow dandelion-like flower heads, positioned on top of long leafless stalks. Native to Europe, these are extremely common weeds of the southwest and can flower all year round, but most commonly in spring. A similar- appearing weed is the common dandelion (Taraxacum officinale), which can be difficult to distinguish from flatweed but the basal leaves have a more prominent “toothed” appearance, the stem is hollow, and there is only one flowerhead per stem. This confusion may have led some to suspect that common dandelions are also associated with Stringhalt. Another plant associated with stringhalt is the small flowered mallow (Malva parviflora). This is low growing annual plant that may reach 1 metre in height. It is native to the Mediterranean and produces pale mauve, pink or white petals in spring. In Western Australia the mallow is also widespread and can be found from Shark Bay to Eucla.
Stringhalt occurs in both outbreak and sporadic forms. Australian stringhalt describes the outbreak form of the syndrome and occurs predominately, but not exclusively, in Australia and New Zealand. In this form the disease is usually bilateral and can be severe and disabling. Sporadic stringhalt is more commonly in single hindlimb (unilateral).
In pasture-associated Stringhalt muscle atrophy commonly develops and approximately 1 in 5 will become roarers (laryngeal hemiplegia). The pathogenesis of stringhalt is not well understood. A neurotoxin produced by flatweed (scylloinositol) has been incriminated in “Australian” Strighalt. Alteration to components of the myotactic reflex (muscle spindle, afferent and efferent nerves) are likely involved.
Some cases improve over time, but sporadic cases frequently worsen. Tenectomy of the lateral digital extensor tendon may help and is occasionally curative. There is a report of improvement after repeated Botox (Botulinum toxin type A) into muscles of the distal hindlimb (lateral and long digital extensor muscles and the vastus lateralis muscles).
Suggested Reading:
El-Hage et al, 2019
Valberg and Baird, 2022
Tags: Neurology