This disease is caused by the ingestion of the now common, exotic weeds Yellow Star Thistle and Russian Knapweed. The toxins in these weeds destroy specific structures within the brain (nigropallidal leukoencephalomalacia) resulting in abnormal chewing.

The signs of this condition are hard to mistake for anything else. When horses are offered feed, their lips move in an excessive, spastic and uncoordinated way. Their lips often form a grimace, while the tongue moves abnormally. Affected horses usually cannot eat or drink, and often appear anxious or confused. Most affected horses quickly lose body weight and need to be euthanized.

These weeds are usually unpalatable to horses, but may be consumed when there is little else for horses to eat. It usually takes quite a large quantity (20-40 lbs), consumed over weeks, for toxicity to occur.

Diagnosis is by signs, along with history of exposure to the plants. Diagnosis can be confirmed at post-mortem by examination of the brain. There are classic changes that can be seen in a particular part of the brain.

There is no known treatment once signs of disease are obvious.