Unraveling the Secrets of Australian Stringhalt in Horses

Horses with Australian stringhalt jerk their hind legs up under their bodies, hyperflexing the hock and stifle joints with each step. An association has been determined between the movement disorder and ingestion of a perennial herb called false dandelion, cats-ear, or flatweed.

It was suspected that the plant, Hypochaeris radicata, contained some sort of unidentified neurotoxin that affected the long nerves in a horse’s body. However, some horses grazed pastures that contained false dandelion and never developed the problem, though they were probably nibbling on the plant along with other forage. It was also noted that outbreaks of Australian stringhalt seemed to occur most often during or after periods of hot weather with little or no rainfall.

Researchers at Massey University may be one step closer to figuring out why the toxin is dangerous at some times and seemingly harmless at other times. Suspecting that the plant might react to environmental stress, the researchers gathered samples of false dandelion and put half of them in a copper chloride solution to induce a stress response. The rest of the plants were not exposed to copper chloride.

Extracts taken from both treated and untreated plants were then combined with cultures of equine nerve cells. After 24 hours, the nerve cells exposed to extracts of the stressed plants showed evidence of damage, while cells exposed to extracts of the nonstressed plants showed only minimal changes. The researchers speculated that plants stressed by environmental factors such as high temperatures or drought could produce one or more compounds that are toxic to horses. This might be a new toxin produced by the plant only under stressed conditions, or a toxin that was always present but became more abundant or more concentrated during a drought. Further research may discover the specific compound as well as more information about its varying effects on grazing horses.