However, the disease can occur when horses are allowed to graze infested pastures, especially those that lack adequate amounts of suitable green forage, or are fed contaminated hay over a period of time. Once the toxicity threshold has been reached, symptoms occur rapidly. Symptoms include fatigue, lowered head, an uncontrolled rapid twitching of the lower lip, tongue-flicking, involuntary chewing movements, and an unnatural open position of the mouth. Without intervention, affected horses are unable to eat or drink and eventually die from starvation or dehydration. The impact of yellow starthistle is not always negative.
Bees foraging on yellow starthistle flowers produce a flavorful high quality honey.
Several biocontrol agents have been introduced from the Mediterranean region to control yellow starthistle . These insects have yet to provide significant reduction in yellow starthistle populations in most areas. Yellow starthistle was introduced from Southern Europe. Malta starthistle is not known to cause chewing disease in horses and is used medicinally in Spain. Malta starthistle was introduced during the 1700's from Southern Europe.