Equine parkinsonism is a neurological condition of horses caused by repetitive ingestion of yellow star thistle (Centaurea solstitialis) and/or Russian knapweed (Centaurea repens). The condition is thought to resemble human parkinson's disease and is characterized by the impairment of the horse's ability to eat. One of the earliest signs is paralysis of the lips and tongue. Horses experience a reduction in jaw tone, which causes their mouth to remain partially open with their tongue hanging out. Sometimes the horse's upper teeth are exposed or hypertonicity of the facial and upper lip muscles. The effects of toxins are irreversible and prove to be ultimately fatal to the horse. They will become weak and emaciated, and eventually die from starvation.
Clinical signs in horses usually appear suddenly and unexpected, as they take at least 28 to 35 days for Russian knapweeed and 33 to 81 days for Yellow star thistle for symptoms to appear in horses. There is no specific treatment for the condition, and horses will not recover.