Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Yellow Star Thistle or Russian Knapweed Toxicity

Synonyms: Chewing Disease, Nigropallidal Leukoencephalomalacia, Nigropallidal Encephalomalacia


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.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

Very Common
Less Common
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The prognosis is generally hopeless once a horse begins showing signs of this disease. Horses usually don't eat these plants but once they begin, it can become habit.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
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Questions To Ask Your Vet:
  • How can I prevent my horses from ingesting these weeds on pasture?

Know what these plants look like and whether they are common in your region. Inspect your pastures regularly and do not allow horses to graze these toxic plants.

If horses are exposed to these weeds in pasture, always ensure that they have plenty of other forage or hay supplementation. If you notice horses eating these plants, remove them from the pasture immediately.

Author: Doug Thal DVM Dipl. ABVP