Australian Stringhalt (AS), also known as pasture-associated stringhalt, is a debilitating neurological condition of horses characterized by an abnormal gait and involuntary extreme hyper (over) flexion of the hindlimbs when attempting to walk. AS can occur in individual horses sporadically or in outbreaks involving multiple horses turned out together in the same pasture.
AS occurs in horses worldwide. It was first reported over 120 years ago in horses in south-eastern Australia. The condition is one of three different forms of stringhalt that have been documented in horses; however the pasture-based or outbreak form of stringhalt is generally referred to as Australian stringhalt. Outbreaks of AS have been well documented in horses in the United States, United Kingdom, New Zealand, Australia, France, Chile, and Japan. Onset is often seasonal, with more cases diagnosed during dry conditions in late summer or fall. There has been a great deal of evidence from various research studies that associate the presence of a certain plant, commonly referred to as catsear (Hypochaeris radicata
), in pastures with horses that develop AS. In one study, it was reported that 65 out of 66 paddocks with horses that developed AS, contained catsear. However the particular toxin contained in the plant that causes AS has yet to be confirmed. It is suspected that it is associated with possible fungal contamination, related to mycotoxins similar to that seen in horses with ryegrass staggers.
Also known as flatweed, Hypochaeris radicata
is a common, yellow-flowered perennial herb that is found worldwide in horse pastures. It looks very similar to, and is closely related to dandelions (Taraxacum
genus); they are both part of the Asteraceae
plant family. It has a large, often branched tap root which allows it to be a very drought-tolerant plant, enabling it to survive during periods of long dry summers.
Horses with AS have such an extreme hindlimb hyperflexion such that the hindlimb may make contact under the belly when they try to take a step forward. The abnormal gait is caused by involuntary control of the hindlimbs. There may also be a relationship between Australian stringhalt and Recurrent Laryngeal neuropathy (RLN)