Pasture-associated stringhalt: Contemporary appraisal of an enigmatic syndrome

Although described clinically for several centuries, stringhalt remains an intriguing and enigmatic condition. In pasture-associated stringhalt (PSH), the clinical sign of exaggerated, prolonged hindlimb flexion is associated with a peripheral neuropathy affecting the larger myelinated axons that is thought to be the result of exposure to a plant-derived neurotoxin. It is likely that multiple host and environmental risk factors interact to produce PSH and the precise aetiology has not yet been elucidated. Drought-affected, poor-quality pasture and the presence of Hypochoeris radicata (commonly referred to as Catsear, Flatweed and False Dandelion) are recognised risk factors. Affected horses are typically mature and taller animals are considered more susceptible. Most horses with PSH recover spontaneously if removed from the presumptive source of toxin; however, recovery can be prolonged, taking several years for some horses, and might be incomplete in occasional cases. A wide range of therapies have been attempted in horses with PSH including phenytoin, thiamine, taurine, infiltration of digital extensors with botulinum toxin and lateral digital extensor myotenectomy procedure. The efficacy of these treatments is uncertain since controlled trials have not been performed and the spontaneous recovery of most horses makes any response to treatment difficult to interpret.