Yellow Star Thistle

Horses eat the plant over an extended period of time before symptoms appear. The ingested toxins actually cause a softening of parts of the brain and once symptoms appear, the animal will almost certainly die.

The horse will not be able to eat or drink, although he may attempt to do so. He may chew food only to spit it out when he cannot swallow, often acting as if something is caught in his throat. (Symptoms resemble the early onset of rabies, so you should consider rabies also as a possibility and not go reaching into the horse's mouth to look for obstructions.)

The horse may chew without having anything in its mouth, move his tongue and lips in unusual fashions, yawn frequently and assume unusual postures such as head hanging or pushing against solid objects.